Pressure Pattern Flying

A technical document describing the theory of pressure pattern flying. It contains formulae, diagrams, tables and text describing the techniques to be used. The last page is annotated 'Pressure Pattern Flying for Air Ministry orders.'

18 typewritten sheets

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MHattersleyCR40699-160506-060001

[underlined] PRESSURE PATTERN FLYING [/underlined]

[underlined] SECTION 1 – INTRODUCTION [/underlined]

1. Pressure Pattern Flying in the term applied to the use of atmospheric pressure distribution information as a direct aid to navigation.

2. In general there are two applications of this information :-

(a) to Constant or Single Heading Flights;

(b) to the determination of Pressure Position Lines.

3. The first of these is concerned with the attempt to find a minimum flight time between any two points. The track which achieves this does not always coincide with the Great Circle track between the two points, but is dependant [sic] on the winds existing at the required flight altitude. This in turn is dependant [sic] on the pressure distribution at this altitude.

4. By analysis of meteorological information re – the heights of various pressure levels at the destination and departure points, both for the start and the end of the flight, the mean efroot [sic] of the wind components at right angles to the aircraft heading can be calculated.

5. This mean cross wind effect is used to calculate a single heading for the whole flight. The aircraft has to be flown at a constant pressure height and to arrive at or near its destination it must maintain the single heading course throughout the flight.

6. All the requisite analyses and calculations are made pre-flight by careful study of the usual metorological [sic] forecast charts plus additional information obtained by Radio Sonde Ascent. By study of cloud depth and distribution, and icing index, plus the varying tail wind components along track, the most suitable pressure level for the flight is chosen and a flight plan is prepared which is dahered [sic] to throughout the flight. Ground speeds and E.T.A.’s, for XXX this flight plan are found by using the forecast winds in the normal manner. In other words, sing Loading flight technique will get the aircraft from A to be byt [sic] alone not provide any information as to the aircraft’s whereabouts between the departure and destination points.

7. The second of these applications is the determination in flight of position lines by the comparison of pressure altitudes and absolute altitudes as displayed by the ordinary pressure altimeter and the radar altimeter respectively. These take the form of a displacement at right angles to the single heading, the position line being drawn parallel to course. These position lines will be a small section of the track and therefore will be useless for checking ground speed; however they can be combined with any position line form [sic] a different source to give a fix.

8. This is the basic structure of Ptessure [sic] Pattern Flying and before going in to the theory of the subject it would be weel [sic] to revise some elementary meteorology. It will be seen that this revision is concerned almost entirely with the reason for, and the formation of, wind speed and direction.

[underlined] Isobars and Contours. [/underlined]

9. Atmospheric or barometric pressure is measured in millibars [sic]. A millibar [sic] being a thousandth part of a bar, a bar being a million dynes per square centimetre. Thus a millibar [sic] is simply a convenient meaure [sic] of “weight” per unit area based on the centimetre-gramme [sic] system. If the pressure at a place is a thousand millibars [sic] then the weight of the column of air above that place is such that it exerts a force of 1,000,000 dynes on each square centimetre.

[page break]

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10. Just as pressure at sea level gradually alters from place to place and from time to time, so pressure at any particular height will have a similar variation.

11. At any particular time, on a surface of constant height, lines can be dawn joining points of equal pressure. These are termed isobars and the arrangements of them will show at a glance where the pressure is haigh [sic] and low etc. Similarly at any particular time, on a surface of constant pressure (in mbs) lines can be drawn joining points of equal height. These as in geographical idiom, are called contour lines,

[underlined] Isallobaric [sic] Effect. [/underlined]

12. The rate at which the pressure at any one place is changing, may differ from the rate of change of pressure at any place. This rate of change is normally referred to in ‘millibars [sic] rise (or fall) per hour’, where ’43.2’ would mean the barometric pressure is rising 3.2. mb per hour.

13. Just as isobars show the pressure distribution so line can be drawn joining all places with like changes of pressure which will show areas of large or small change of pressure. These lines are called Isallobars.

14. From the arrangement of Isallobars a correction vector, can be deduced to modify wind speeds and direction determined by normal meteorological methods. This is known as the Isallobaric [sic] Component of the wind and is referred to as the Isallobaric [sic] Effect.

15. This Isallobaric [sic] component has a direction at right angles to the isallobars, and towards the area of fastest falling pressure, that is to say it ‘flows’ down the isallobaric [sic] slope.

16. For example, fig, 1 shows a selection of isallobars drawn at 1mb intervals, with the increasing pressure and the slowest decrease of pressure on the outside (the hgith [sic] ground) and fastest decrease of pressure in the middle (the low ground). The isallobaric [sic] component of the wind flows down the ‘slope’ in the centre:-

[drawing]

[page break]

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17. Given an area of rapidly changing barometric pressure, the isallobaric [sic] effect can be quite substantial in its influence on the prevailing wind in that area. But, conversely in an area of fairly constant or slowly changing pressure, the isallobaric [sic] effect is negligible for practical purposes.

[underlined] Geostrophic Force [/underlined]

18. The turntable effect due to the rotation of the eatth [sic] on its axis produces a force which will act at right angles to the direction of movement over the earth of any body. This force acts to the right (in the sense that you face in the direction of movement) in the Northern Hemisphere, and to the left in the Southern Hemisphere, and is called the Geostrophic Force.

19. It is a function of the angular movement of the earth on its axis, the latitude of the body, and the velocity with which the body is moving.

20. The geostrophic force acting upon any body at any place can be found from the formula:-

2wVsir/o

where w = angular velocity of the earth in radians per sec.

v = velocity of the body in feet per second.

/o = latitude.

21. The sin. Of 0o is zero, and the sin. of 90o is 1, therefore it is clear from the above formula that the geostrophic force is maximum at the poles and decreases with decrease of latitude until there is not geostrophic force at the Equator.

[underlined] Cyclostrophic Force [/underlined]

22. A body moving in a circular path has a force acting upon it in a direction away from the direction of the turn. This force is known as the centrifugal force.

23. The same effect is experienced by air which is trying to follow a circular path, in this case the centrifugal force is meteorologically termed the Cyclostrophic Force.

[underlined] Wind [/underlined]

24. To consider properly the be aviour [sic] of wind the atmosphere must be divided into two layers.

25. The first of these is known as the Turbulence of Friction Layer, in which the wind speed is affected, as would be any other body, by its actual contace [sic] with the surface, and aby the topography of that surface. The wind’s movements in this layer cannot be told accurately merely from the study of a synoptice [sic] chart, as the local features such as mountains, valleys, buildings, play too large a part in the determination of the winds speed and direction. The Friction Layer is that layer between the surace [sic] and a height of about 2000 above the surface.

26. Above the Friction Layer is the Free Atmosphere, which is self explanatory inasmuch [suc] as it is that part of the atmosphere in which the wind is no longer in contact with the ground, and therefore in which the wind speed and direction can be calculated reasonably accurately from a study of the synoptic chart. The Free Atmosphere is all that [unreadable words] sphere above a height of about 2000 feet.

27. Flights in the friction layer cannot be made using Pressure Pattern technique, and it is flights in the Free Atmosphere with which we are concerned.

[page break]

28. in the free atmosphere, air movement depends upon three factors:-

(a) The pressure distribution (para 10) at the level concerned

(b) The effect of the earth’s rotation, in other words the geostrophic force (para 18)

(c) The way in which the pressure distribution is altering, or the isalloearic [sic] effect (paras 11 to 18).

29. The isobars showing the pressure distribution may be looked upon as are the contours of an ordinary map – the low pressure area may be looked upon as “low ground”, and, the air tends to flow down the pressure gradient or “slope” from high to low. The force which causes this tendency as the Pressure Gradient Force, and it acts at right angles to the isobars in a direction from “high” to “low”.

30. The magnitude of this force is a function both of the density of the air “p” (rhe), and of the distance between the isobars or the pressure gradient “ [unreadable letter] “. The force is irrersely [sic] proportional to the distance between the isobars, that is to say the greater the distance between the isobars, the smaller the pressure gradient force.

31. With

P = pressure gradient

p = air density

Then

Pressure Gradient Force = P/p

32. In the Northern Hemisphere, the geostrophical force (para .18) acts at right angles to and to the right of direction of movement, therefore we have on our isobars:

[Fig. 2 diagram]

33. These two forces will produce a resultant air movement, in other words a new wind, which again will have a geostrophic force aeging [sic] at right angles to it:

[Fig. 3 diagram]

[page break]

– 5 –

and so on until the pressure gradient force is balanced by a geostrophic force equals to it and acting in an opposite direction, and the final direction of motion of the air, (i.e. the wind), is at right angles to these two force and parallel to the isobars:

[Fig. 4 diagram]

[underlined] Geostrophic Wind [/underlined]

34. This wind is called a geostrophic wind, and is the air movement which results from a balance between the pressure gradient force P/p and the geostrophic force (2wVsin/o), therefore:

P/p = 2wVsin/o

35. Therefore by simple transposition:

Geostrophic Wind V = P/2wpsin/o

which gives a very close approximation of the true wind.

[underlined] Gradient Wind [/underlined]

36. When the isobars are not straight lines, the wind must follow a curved path in order to remain parallel to the isobars, and is therefore subject to influence from the cyclostrophic force (para 22)

[circular drawing]

37. Therefore the geostrophic force G has the cyclostrophic force G to help it to balance the pressure gradient force P, which means that:-

P = G & C

Therefore with “low” or “cyclonic” curvature a wind calculated on the geostrophic scale, which assumes that the geostrophic force is doing all the work as it were, will be an over-estimation of the true wind speed.

[page break]

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38. Conversely with a “high” or “anti-cyclone” curvature:-

[circular drawing]

the force P acts down the pressure slope, G in an opposite direction, and the direction of movement W is again to the right of P. New G is acting [underlined] with [/underlined] P, and in this case

g = P & C

or

P = G – C

and the geostrophic scale this time will underestimate the true wind speed.

39. These last two are called Gradient Winds, and are the result of a balance between the pressure gradient force, the geostrophic force and the cyclostrophic force.

40. These last two examples also serve to show the direction of the wind in relation to the pressure. With your back to the wind, the lower pressure is on your left, in the Northern hemisphere.

41. In the Southern Hemisphere, because of the geostrophic force (para 18) the reverse applies.

[underlined] Equatorial Winds [/underlined]

42. At the Equator, the geostrophic force is zero (para 21), and therefore there is not balanced wind. In these regions the cyclostrophic force plays a large part in the determination of wind direction and speed, and the geostrophic wind equation does not apply within a region of 15o either side of the Equator.

[underlined] Pressure Altimeter [/underlined] (Mk XIV – Kollsman)

43. A pressure altimeter is actually a barometer which measures the weight or pressure of air above it. The Sensitive Alttimeter [sic] Mk XIV (Kollsman) assumes a standard atmosphere, in which the pressure at sea-level is 1013.2 mbs the temperature 15 C and in which the temperature decreases at a standard rat of 1.98 C per 1000 ft up to a height of 36090 ft, where the temperature remains constant at -56.5 C. This is the I.C.A.N. Law of Pressure Change with height.

44. With a datum, 1013.2 mbs, from which to start, and assuming this standard atmosphere, millibar pressure can be related to an equivalent height in feet. The needle on the pressure altimeter face moves against a scale so calculated that, although the altimeter is actually measuring pressure, this reading is given as an equivalent height in feet, which is called the Pressure Height in Pressure Pattern Flying.

[page break]

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45. It is because of the untrue assumption that the atmosphere obeys standard rules that a pressure altimeter reading must be corrected for temperature in order to obtain from it a reasonably accurate indication of height.

[underlined] Radar Altimeter [/underlined]

46. A radar altimeter measures height by timing the travel of a raido [sic] pulse form the aircraft to the surface directly beneath and back again to the aircraft. Such a method is not dependant on the vagaries of the atmosphere and gives an accurate height which is called the Absolute Height in Pressure Pattern Flying.

47. The most successful radar altimeter in use to date is the American SGR 718A.

48. The reader may wonder what some of these subjects have to do with Pressure Pattern Flying. It is hoped he will see, in the following sections, that all this knowledge is necessary in order that he may understand the theory, and also (at least equally important) so that he may recognise the limitations and errors of the system and the circumstances under which they occur.

[underlined] PRESSURE PATTERN FLYING [/underlined]

[underlined] SECTION 2 – THEORY [/underlined]

1. Section 1 of those notes has shown the possibility of the determination of wind speed by use of a geostrophic wind scale, which measures the wind speed as a function of the distance between the isobars, and the isobars are drawn for a constant level.

2. Pressure Pattern Flying demands a modification of this technique, and instead of using constant level with the lines indicating pressure, it uses constant pressure with the lines indicating the height at which the selected chart pressure is to be found.

3. For example, a chart may be drawn for a pressure of 700 mbs[?]., and at first glance it is similar to a chart of isobars. But the lines are labelled in feet instead of pressure, thus.

[diagram]

showing that point “A” the 700 mbs. Pressure level is at 9,400 feet, and at point “B” the pressure is 700 mbs at 9,800 feet. It follows also that a movement in [one letter deleted] in the direction from A to B is a travel towards an [one indecipherable word] of higher pressure (with very few exceptions).

4. A side view of such: a [one indecipherable word] from A to B at a constant pressure level of 760[?] mbs may clarify this last statement.

[diagram]

The aircraft is climbing to keep at the 700 mb level. The pressure at point X immediately below A is 700 mbs. (pressure at A) plus the weight of the column of air AX. The pressure at Y immediately below B is 700 mbs (pressure at, B) plus the wright of the column of air BY which is greater than the column of air AX. Therefore a slight in a direction A to B (i.e. X to Y) along a pressure level[?] which is gaining altitude is a movement towards higher pressure, considering the general case,

5. Pressure height lines drawn on a chart as in para 3 above are called [underlined] Contour Lines. [/underlined]

[page break]

[centred] – 2 – [/centred]

6. Consider, then, the path of an aircraft [two indecipherable words] across the isobars of a synoptic chart:

[diagram]

The wind is blowing along the isobars in a clockwise direction (Section 1, para. 40) as shown by the arrows, and CA is a vector representing the geostrophic wind speed and direction at A. Similarly F8 represents the geostrophic wind velocity at B.

7. CA can be split up into two components, one CD acting at right angles to the beading, which will cause the drift; and another CL parallel with the heading which will cause the difference between air speed and ground speed. With this latter we are not concerned, but will concentrate on the compontent[sic] of the wind CD, at right angles to the heading and causing drift.

8. Contour lines can be utilised to provide this component, which will be referred to hereafter as “Vn”, which means “Velocity normal (at right angles) to the heading”, and it is calculated using a contour wind equation instead of a geostrophic wind equation. Vn behaves similarly to a geostrophic wind, inasmuch as, with your back to the wind component Va, the lowest height is on your left (in the Norther[sic] Hemisphere)

9. A measure of the contour gradient is a measure of the magnitude of Vn, as the closer the contours the greater Vn. The path of the aircraft shown in para. 4 above is a measure of the contour gradient, or the Isobaric slope, the reason for this last term becoming obvious when it is remembered that the aircraft’s path shows the position of the 700 mbs level.

10. Successive differences between pressure height according to the I.C.A.N. Law as would be shown on a Kollsman altimeter set to 1013.2 mbs. (Section 1. Para. 44) and absolute height as would be given by a Radar Altimeter (Section 1 para.46,), would indicate the isobaric slope. The pressure altimeter would reas[sic] [reach?] a constant, because the aircraft is being flown at 700 mbs and the radar altimeter would show, in out[?] example, an increase of height with distance equivalent to the isobaric slope.

11. This may be clarified by a diagrammatic example.

[page break]

[centred] – 3 – [/centred]

12. The pressure height of 700 mbs., assuming a standard atmosphere, is 9,900 feet. Therefore the aircraft, (see para. 13) will be flown at such a height as to maintain a reading of 9,900 feet on the Kollsman altimeter. The radar altimeter will show an increase of absolute height with distance flown as the aircraft climbs up the isobaric slope, and will show the absolute height.

[diagram]

so that D1, the difference between radar altimeter height and pressure altimeter height is subtracted algebraically from the absolute height, and can produce either a positive or a negative e D. Similar readings at B produce a D2 (from readings taken at the second[?] position) of -100 feet.

14. [sic] The difference between D1 and D2 will be an indication of the [deleted] i [/deleted] isobaric slope and therefore of Vn, and is a function of the air distance flown between D1 and D2.

15. From information now at our disposal, it is possible to calculate Vn from a formula, and the reader will notice immediately the similarity between this formula and the geostrophic wind formula.

[equation] Vn = g(D2 – D1)

2W6sinØ[?] [/equation]

where

Vn = wind component normal to the heading[?], in knots.

D1 = absolute height minus pressure height at the first position.

D2 = absolute height minus pressure height at the second position.

S = air distance flown between D1 and D2, in nautical miles.

W & Ø see geostrophic wind formula.

g = Gravity acceleration in ft/sec/sec.

D1 is subtracted from D2 algebralcally [?], so that “(D2 – D1)”, may be either positive or negative.

16. In the Northern hemisphere

D2 – D1 is positive the drift is to port,

D2 – D1 is negative the drift is starboard.

The reverse applies in the Southern hemisphere.

17. In our example (para 13)

D2 – D1 = +400 therefore the drift is port, which may be verified from the wind direction shown in the diagram in para 6.

[page break]

[centred] – 4 – [/centred]

18. The Vn formula in para 15 can be simplified, in as much as w and g may be [one indecipherable word] as constants[?] and [equation] g over 2w [/equation] (to give Vn in knots) [one indecipherable word] 21.47.

Therefore substituting in the formula 21.47

[equation] Vn = 21.47 (D2 – D1) over [one indecipherable word] [/equation]

The latitude Ø is known by the navigator, and therefore the formula may be written thus

[equation] Vn = K(D2 – D1) over S

where K = 21.47 over sinØ[?] [/equation]

20. In this formula, the wind component is in knots or nautical miles per hour. The navigator need the Vn wind [underlined] effect [/underlined] for the time of flight between D1 and D2, this is achieved by multiplying each side of the equating by the number of hours “t” of flight between D1 and D2 thus

[equation] t x Vn = K(D2 – D1) x t over S [/equation]

t hours x Vn will give the wind effect Zn.

t hours x 1/S will give 1/True airspeed[sic].

21. Therefore the Ø formula finally appears at

[equation] Zn = K(D2 – D1) over TAS [/equation]

Where Zn = wind effect in nautical miles during flight from D1 to D2

TAS = True aispeed[sic] in knots.

[underlined] Pressure Pattern Position lines [/underlined]

22. A position line is obtained by [one indecipherable word] off the distance Zn normal to the true course and in a down wind direction from the six position of D2 with an air plot started from the DR position of D1. The calculation of (D2 – D1) will indicate in which direction Zn is acting. (para 16.)

23. In our example, (D2 – D1) was positive – therefore, our drift is to port, and the position line would be plotted thus

[diagram]

24. Zn may be calculated for any time interval between D1 and D2, but experiments have proved that, for the accurate results, this time interval must not be less than 25 minutes.

25. The course steered must be an accurate one, and the use of the Air Position Indicator will tend to eliminate any pilot error in this respect.

[underlined] Calculation of Dv[?]fit Angle. [/underlined]

26. Knowing Zn, and being able to calculate the track distance flown, it is a simple matter to use plane trigonometry to solve the drift angle.

[page break]

[centred] – 5 – [/centred]

27. Zn forms the side of a right angled triangle opposite to the dirft[sic] angle; the track distance flown, X, forms the hypoteneuse.[sic] Therefore

[equation] sin drift angle = opposite over hypoteneuse [sic]

sine[?] = Zn over X [/equation]

[diagram]

Where a = the drift[sic] angle

X = the track distance flown between D1 and D2.

28. Once again, as with K in para 18, it is a simple matter to construct tables [indecipherable letters] showing the drift angles for all combinations of Zn and X.

29. If track distance is not known, air distance may be used as X.

30. However, this will introduce a slight error if there is a large head or tail wind component of the wind parallel to the course, thus:-

[diagram]

Zn = wind effect normal to course.

B = drift angle calculated using air distance as X

d = true drift angle using X.

[underlined] Single heading flight [/underlined]

31. Single heading flight is an extension of the method of drift finding, and, as its name implies, enable calculationg[sic] of course to steer which will not be changed from departure to arrival at the destination.

32. The formula Zn = K(D2 – D1) over T.A.S.

holds good even when the pressure gradient does not change at a regular rate with distance. The formula can be used to calculate a mean wind effect between departure point and destination on a long flight at a constant pressure height, and hence the mean drift for the flight can be calculated using the formula in para. 27.

33. To calculate the course to steer on a single heading flight, the formula are used as follows:-

Zn = X(D2 – D1) over T.A.S.

When calculating the drift angle X is taken as the track distance of the complete flight.

34. Almost invariably, there will be a deviation from the Great Circle track, with [one deleted letter] a tendency to be blown around the centres of Highs and Lows.

35. A typical single heading track would compare with the Great Circle track as follows, with the wind arrows indicated for the Northern hemisphere Section 1, para 40) ½

[page break]

[centred] – 6 – [/centred]

[diagram] FIG 8

36. As a route example of Single Heading Flight, some extracts from figures of various flights between Prestwick and Gander may be of interest. The great circle distance of this flight is about 2,000 miles, and the flight normally takes about 12 hours.

37. In a total of about 6 flights, times ranging between two[?] minutes and 2 hours 17 minutes (depending upon the pressure distribution of each flight) were saved over the times which would have been taken had normal navigation been used to calculate the flight along the great circle track. In all cases, the single heading flight was shorter in time than the great sircle[sic] route; in no case was the plotted position at [?] the end of the single heading flight more than 17 nautical miles from the destination. In all cases, the single heading was simpler to calculate than the great circle route.

38. To avoid an area of very bad weather, the route can be split into two parts, and two separate single headings calculated.

[underlined] Advantages of Pressure Pattern Flying. [/underlined]

39. The advantages of pressure pattern flying are:-

(a) drifts may be calculated under blind flying conditions;

(b) drifts are mean drifts, therefore of greater value to the navigator than instantaneous drifts (except when drifts desired for the calculation of multiple drift wind velocity);

(c) position lines may be optional at any time, providing there is not less than 25 minutes between D1 and D2, and providing a constant course and pressure altitude have been maintained. This position line may be used with position lines from any other [one deleted word] source to obtain a fix.

[underlined] Disadvantahes[sic] if Pressure Pattern Flying [/underlined]

40. There are limitations to the pressure pattern flying technique, which prohibit its universal usuage, namely:-

(a) Position lines cannot be calculated in flight over a surface of an unknown height, because of the way in which absolute height is measured in flight (Section 1 para 47.)

(b) the technique does not work in low latitudes (between 15 N and 15 S), because the geostrophic wind equation upon which the technique is based is no longer accurate (Section 1, para 43)

[page break]

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(c) the technique cannot be used anywhere below 2,000 ft, in the friction layer, because in this region again the geostrophic wind equation is inaccurate (Section 1, para, .25)

(d) A constant pressure height must be maintained. The maximum allowance of deviation of the pressure height is [symbol] 200 feet between D1 and D2. The formulae used to calculate to Vn and Zn (paras. 15 and 17) assume a constant pressure height, and a deviation greater than [symbol] 200 feet from the constant pressure[deleted] height[/deleted] necessitates reduction of the readings to a common pressure height by use of further tables, and also introduces a source of error:

(e) The pressure pattern technique alone will not provide a fix en route, and gives no indication of distance travelled or of E.T.A. Use must be made of affiliated D.R. navigation to give positions and E.T.A’s.

[underlined] Sources of Error with pressure Pattern Flying [/underlined]

41. The sources of error are as follows:-

(a) with excessively curved isobars, and therefore an excessively curved wind path, the cyclostrophic component will be large the geostrophic wind equation will be inaccurate (Section 1, paras 36 to 38) & therefore the calculated Zn will be inaccurate.

(b) with rapid movement of the pressure system, for example a fast moving low pressure area or depression, the isallobaric effect (Section1 paras 11 to 17) will again cause the geostrophic wind equation to be inaccurate

(c) When calculating for Zn, it is assumed that the whole pressure system has remained static during the aircraft’s travel from D1 to D2. This is not strictly true (see (b) above, for example) and effects [sic] the truth of the resulting calculations.

[deleted] (d) [/deleted] NOTE. The errors caused by (a) (b) & (c) are generally small enough[deleted]t[/deleted] to be ignored.

(d) in an extreme case, a North South flight with a consequently large change of latitude, may have all its drift in the first half of the flight and no drift in the second half, so that using the mean latitude of the whole flight to find K will give a false result. This can be overcome by once again splitting the flight into two parts when calculating for single heading.

(e) obviously a bad meteorological forecast of D1 and D2 before beginning a single heading flight will cause subsequent error in the calculation of the drift angle, and therefore of the single heading itself.

42. Pressure Pattern flying certainly is not to be hailed as the answer to the navigator’s prayer, but none the less in its limited sphere of use it is revolutionary [sic] advance in the intricate technique of navigation [sic] an aircraft from A to B.

[page break]

[underlined] SECTION 3 – PRACTICAL APPLICATION [/underlined]

1. It is possible now to make use of the theoretical background and apply it practically in one of two ways. In these notes Single Heading Flight will be dealt with alone as the obtaining of pressure position lines demands the use of the radar altimeter.

[underlined] The Singe Heading Flight [/underlined]

2. Any aircraft, without any special equipment, can carry out a flight of this nature. Single heading flight entails the maintenance of a constant course (in effect a mean course) from departure point to destination. As we have a means of calculating the wind component at right angles to course and the drift to be applied, this course can be calculated before take off, provided that the heights of the upper pressure level can be forecast, with reasonable accuracy, between departure point and destination.

3. The technique holds godd [sic] even though the contour level between the two points in question is not changing regularly. Should the complete flight be divided into innumerable small sections, and the summation of resultant port or starboard Zn be calculated, then the same final Zn would be obtained as if the flight had been considered as a whole.

[underlined] Procedure for Calculation of Single Heading [/underlined]

4. Equipment and data required:

(a) Kollsman (Mk XIV) Altimeter

(b) Table I – K (latitude)

(c) Table II – drift angle (Zn against X)

(d) Table III – Conversion table – mbs to height in standard atmosphere

(e) [formula]

(f) [formula]

(g) Sine tables (preferably log tables)

(Unnecessary if table III carried)

[underlined] Met. Data Required [/underlined]

5. After deciding the pressure height at which the flight is to be made, obtain a forecast of actual height of this contour.

(a) over base at F.T.D.

(b) over destination at E.T.A.

[underlined] Example. [/underlined]

6. Pressure height selected, 700 mbs. Equals 9,900 ft in standard atmosphere.

Height over base (a) at E.T.D. 0300 hrs = 10400 ft

Height over dest. (b) at E.T.A. 1500 hrs = 9500 ft

Distance A – B 2400 n.m.

Track A – B 290 T

Mean latitude 50 N

True Airspeed 200K

[page break]

- 2 -

Treat departure point as D1 and destination point as D2, then

[calculation]

therefore the single course to steer between base and destingation [sic] will be 286.90T (to the nearest 10th of a degree)

8. It must be noted that the height of the aircraft must be maintained throughout the flight at the chosen pressure height of 9,900 (i.e. 9,900ft indicated on the pressure altimeter) to within 200 ft.

[underlined] NOTE: [/underlined] All the preceding notes are in effect a copy of the notes issued by E.A.N.S. On the subject of pressure pattern flying. These notes should be read in conjunction with A.P. 1699 (Sutcliffe – Meteorology for aviators)

The proof of the Vn formula can be obtained from the Wing Navigation Leader together with any other information which is required to clear any difficulties which may arise during your perusal of the above notes.

When radar altimeters are carried the procedure for obtaining pressure position lines will be stencilled out for the convenience of all navigators.

[page break]

[underlined] TABLE I [/underlined]

[Table of Latitudes]

[underlined] TABLE III [/underlined]

[underlined] CONVERSION TABLE [/underlined]

Feet to Millibars using U.S. Standard Atmosphere.

[conversion table]

[page break]

[blank page]

[page break]

[underlined] Pressure Pattern Flying [/underlined]

[underlined] For [/underlined] Artillary orders.

[underlined] SECTION 1 – INTRODUCTION [/underlined]

1. Pressure Pattern Flying in the term applied to the use of atmospheric pressure distribution information as a direct aid to navigation.

2. In general there are two applications of this information :-

(a) to Constant or Single Heading Flights;

(b) to the determination of Pressure Position Lines.

3. The first of these is concerned with the attempt to find a minimum flight time between any two points. The track which achieves this does not always coincide with the Great Circle track between the two points, but is dependant [sic] on the winds existing at the required flight altitude. This in turn is dependant [sic] on the pressure distribution at this altitude.

4. By analysis of meteorological information re – the heights of various pressure levels at the destination and departure points, both for the start and the end of the flight, the mean efroot [sic] of the wind components at right angles to the aircraft heading can be calculated.

5. This mean cross wind effect is used to calculate a single heading for the whole flight. The aircraft has to be flown at a constant pressure height and to arrive at or near its destination it must maintain the single heading course throughout the flight.

6. All the requisite analyses and calculations are made pre-flight by careful study of the usual metorological [sic] forecast charts plus additional information obtained by Radio Sonde Ascent. By study of cloud depth and distribution, and icing index, plus the varying tail wind components along track, the most suitable pressure level for the flight is chosen and a flight plan is prepared which is dahered [sic] to throughout the flight. Ground speeds and E.T.A.’s, for XXX this flight plan are found by using the forecast winds in the normal manner. In other words, sing Loading flight technique will get the aircraft from A to be byt [sic] alone not provide any information as to the aircraft’s whereabouts between the departure and destination points.

7. The second of these applications is the determination in flight of position lines by the comparison of pressure altitudes and absolute altitudes as displayed by the ordinary pressure altimeter and the radar altimeter respectively. These take the form of a displacement at right angles to the single heading, the position line being drawn parallel to course. These position lines will be a small section of the track and therefore will be useless for checking ground speed; however they can be combined with any position line form [sic] a different source to give a fix.

8. This is the basic structure of Ptessure [sic] Pattern Flying and before going in to the theory of the subject it would be weel [sic] to revise some elementary meteorology. It will be seen that this revision is concerned almost entirely with the reason for, and the formation of, wind speed and direction.

[underlined] Isobars and Contours. [/underlined]

9. Atmospheric or barometric pressure is measured in millibars [sic]. A millibar [sic] being a thousandth part of a bar, a bar being a million dynes per square centimetre. Thus a millibar [sic] is simply a convenient meaure [sic] of “weight” per unit area based on the centimetre-gramme [sic] system. If the pressure at a place is a thousand millibars [sic] then the weight of the column of air above that place is such that it exerts a force of 1,000,000 dynes on each square centimetre.

[page break]

– 2 –

10. Just as pressure at sea level gradually alters from place to place and from time to time, so pressure at any particular height will have a similar variation.

11. At any particular time, on a surface of constant height, lines can be dawn joining points of equal pressure. These are termed isobars and the arrangements of them will show at a glance where the pressure is haigh [sic] and low etc. Similarly at any particular time, on a surface of constant pressure (in mbs) lines can be drawn joining points of equal height. These as in geographical idiom, are called contour lines,

[underlined] Isallobaric [sic] Effect. [/underlined]

12. The rate at which the pressure at any one place is changing, may differ from the rate of change of pressure at any place. This rate of change is normally referred to in ‘millibars [sic] rise (or fall) per hour’, where ’43.2’ would mean the barometric pressure is rising 3.2. mb per hour.

13. Just as isobars show the pressure distribution so line can be drawn joining all places with like changes of pressure which will show areas of large or small change of pressure. These lines are called Isallobars.

14. From the arrangement of Isallobars a correction vector, can be deduced to modify wind speeds and direction determined by normal meteorological methods. This is known as the Isallobaric [sic] Component of the wind and is referred to as the Isallobaric [sic] Effect.

15. This Isallobaric [sic] component has a direction at right angles to the isallobars, and towards the area of fastest falling pressure, that is to say it ‘flows’ down the isallobaric [sic] slope.

16. For example, fig, 1 shows a selection of isallobars drawn at 1mb intervals, with the increasing pressure and the slowest decrease of pressure on the outside (the hgith [sic] ground) and fastest decrease of pressure in the middle (the low ground). The isallobaric [sic] component of the wind flows down the ‘slope’ in the centre:-

[drawing]

[page break]

– 3 –

17. Given an area of rapidly changing barometric pressure, the isallobaric [sic] effect can be quite substantial in its influence on the prevailing wind in that area. But, conversely in an area of fairly constant or slowly changing pressure, the isallobaric [sic] effect is negligible for practical purposes.

[underlined] Geostrophic Force [/underlined]

18. The turntable effect due to the rotation of the eatth [sic] on its axis produces a force which will act at right angles to the direction of movement over the earth of any body. This force acts to the right (in the sense that you face in the direction of movement) in the Northern Hemisphere, and to the left in the Southern Hemisphere, and is called the Geostrophic Force.

19. It is a function of the angular movement of the earth on its axis, the latitude of the body, and the velocity with which the body is moving.

20. The geostrophic force acting upon any body at any place can be found from the formula:-

2wVsir/o

where w = angular velocity of the earth in radians per sec.

v = velocity of the body in feet per second.

/o = latitude.

21. The sin. Of 0o is zero, and the sin. of 90o is 1, therefore it is clear from the above formula that the geostrophic force is maximum at the poles and decreases with decrease of latitude until there is not geostrophic force at the Equator.

[underlined] Cyclostrophic Force [/underlined]

22. A body moving in a circular path has a force acting upon it in a direction away from the direction of the turn. This force is known as the centrifugal force.

23. The same effect is experienced by air which is trying to follow a circular path, in this case the centrifugal force is meteorologically termed the Cyclostrophic Force.

[underlined] Wind [/underlined]

24. To consider properly the be aviour [sic] of wind the atmosphere must be divided into two layers.

25. The first of these is known as the Turbulence of Friction Layer, in which the wind speed is affected, as would be any other body, by its actual contace [sic] with the surface, and aby the topography of that surface. The wind’s movements in this layer cannot be told accurately merely from the study of a synoptice [sic] chart, as the local features such as mountains, valleys, buildings, play too large a part in the determination of the winds speed and direction. The Friction Layer is that layer between the surace [sic] and a height of about 2000 above the surface.

26. Above the Friction Layer is the Free Atmosphere, which is self explanatory inasmuch [suc] as it is that part of the atmosphere in which the wind is no longer in contact with the ground, and therefore in which the wind speed and direction can be calculated reasonably accurately from a study of the synoptic chart. The Free Atmosphere is all that [unreadable words] sphere above a height of about 2000 feet.

27. Flights in the friction layer cannot be made using Pressure Pattern technique, and it is flights in the Free Atmosphere with which we are concerned.

[page break]

28. in the free atmosphere, air movement depends upon three factors:-

(a) The pressure distribution (para 10) at the level concerned

(b) The effect of the earth’s rotation, in other words the geostrophic force (para 18)

(c) The way in which the pressure distribution is altering, or the isalloearic [sic] effect (paras 11 to 18).

29. The isobars showing the pressure distribution may be looked upon as are the contours of an ordinary map – the low pressure area may be looked upon as “low ground”, and, the air tends to flow down the pressure gradient or “slope” from high to low. The force which causes this tendency as the Pressure Gradient Force, and it acts at right angles to the isobars in a direction from “high” to “low”.

30. The magnitude of this force is a function both of the density of the air “p” (rhe), and of the distance between the isobars or the pressure gradient “ [unreadable letter] “. The force is irrersely [sic] proportional to the distance between the isobars, that is to say the greater the distance between the isobars, the smaller the pressure gradient force.

31. With

P = pressure gradient

p = air density

Then

Pressure Gradient Force = P/p

32. In the Northern Hemisphere, the geostrophical force (para .18) acts at right angles to and to the right of direction of movement, therefore we have on our isobars:

[Fig. 2 diagram]

33. These two forces will produce a resultant air movement, in other words a new wind, which again will have a geostrophic force aeging [sic] at right angles to it:

[Fig. 3 diagram]

[page break]

– 5 –

and so on until the pressure gradient force is balanced by a geostrophic force equals to it and acting in an opposite direction, and the final direction of motion of the air, (i.e. the wind), is at right angles to these two force and parallel to the isobars:

[Fig. 4 diagram]

[underlined] Geostrophic Wind [/underlined]

34. This wind is called a geostrophic wind, and is the air movement which results from a balance between the pressure gradient force P/p and the geostrophic force (2wVsin/o), therefore:

P/p = 2wVsin/o

35. Therefore by simple transposition:

Geostrophic Wind V = P/2wpsin/o

which gives a very close approximation of the true wind.

[underlined] Gradient Wind [/underlined]

36. When the isobars are not straight lines, the wind must follow a curved path in order to remain parallel to the isobars, and is therefore subject to influence from the cyclostrophic force (para 22)

[circular drawing]

37. Therefore the geostrophic force G has the cyclostrophic force G to help it to balance the pressure gradient force P, which means that:-

P = G & C

Therefore with “low” or “cyclonic” curvature a wind calculated on the geostrophic scale, which assumes that the geostrophic force is doing all the work as it were, will be an over-estimation of the true wind speed.

[page break]

– 6 –

38. Conversely with a “high” or “anti-cyclone” curvature:-

[circular drawing]

the force P acts down the pressure slope, G in an opposite direction, and the direction of movement W is again to the right of P. New G is acting [underlined] with [/underlined] P, and in this case

g = P & C

or

P = G – C

and the geostrophic scale this time will underestimate the true wind speed.

39. These last two are called Gradient Winds, and are the result of a balance between the pressure gradient force, the geostrophic force and the cyclostrophic force.

40. These last two examples also serve to show the direction of the wind in relation to the pressure. With your back to the wind, the lower pressure is on your left, in the Northern hemisphere.

41. In the Southern Hemisphere, because of the geostrophic force (para 18) the reverse applies.

[underlined] Equatorial Winds [/underlined]

42. At the Equator, the geostrophic force is zero (para 21), and therefore there is not balanced wind. In these regions the cyclostrophic force plays a large part in the determination of wind direction and speed, and the geostrophic wind equation does not apply within a region of 15o either side of the Equator.

[underlined] Pressure Altimeter [/underlined] (Mk XIV – Kollsman)

43. A pressure altimeter is actually a barometer which measures the weight or pressure of air above it. The Sensitive Alttimeter [sic] Mk XIV (Kollsman) assumes a standard atmosphere, in which the pressure at sea-level is 1013.2 mbs the temperature 15 C and in which the temperature decreases at a standard rat of 1.98 C per 1000 ft up to a height of 36090 ft, where the temperature remains constant at -56.5 C. This is the I.C.A.N. Law of Pressure Change with height.

44. With a datum, 1013.2 mbs, from which to start, and assuming this standard atmosphere, millibar pressure can be related to an equivalent height in feet. The needle on the pressure altimeter face moves against a scale so calculated that, although the altimeter is actually measuring pressure, this reading is given as an equivalent height in feet, which is called the Pressure Height in Pressure Pattern Flying.

[page break]

– 7 –

45. It is because of the untrue assumption that the atmosphere obeys standard rules that a pressure altimeter reading must be corrected for temperature in order to obtain from it a reasonably accurate indication of height.

[underlined] Radar Altimeter [/underlined]

46. A radar altimeter measures height by timing the travel of a raido [sic] pulse form the aircraft to the surface directly beneath and back again to the aircraft. Such a method is not dependant on the vagaries of the atmosphere and gives an accurate height which is called the Absolute Height in Pressure Pattern Flying.

47. The most successful radar altimeter in use to date is the American SGR 718A.

48. The reader may wonder what some of these subjects have to do with Pressure Pattern Flying. It is hoped he will see, in the following sections, that all this knowledge is necessary in order that he may understand the theory, and also (at least equally important) so that he may recognise the limitations and errors of the system and the circumstances under which they occur.

[underlined] PRESSURE PATTERN FLYING [/underlined]

[underlined] SECTION 2 – THEORY [/underlined]

1. Section 1 of those notes has shown the possibility of the determination of wind speed by use of a geostrophic wind scale, which measures the wind speed as a function of the distance between the isobars, and the isobars are drawn for a constant level.

2. Pressure Pattern Flying demands a modification of this technique, and instead of using constant level with the lines indicating pressure, it uses constant pressure with the lines indicating the height at which the selected chart pressure is to be found.

3. For example, a chart may be drawn for a pressure of 700 mbs[?]., and at first glance it is similar to a chart of isobars. But the lines are labelled in feet instead of pressure, thus.

[diagram]

showing that point “A” the 700 mbs. Pressure level is at 9,400 feet, and at point “B” the pressure is 700 mbs at 9,800 feet. It follows also that a movement in [one letter deleted] in the direction from A to B is a travel towards an [one indecipherable word] of higher pressure (with very few exceptions).

4. A side view of such: a [one indecipherable word] from A to B at a constant pressure level of 760[?] mbs may clarify this last statement.

[diagram]

The aircraft is climbing to keep at the 700 mb level. The pressure at point X immediately below A is 700 mbs. (pressure at A) plus the weight of the column of air AX. The pressure at Y immediately below B is 700 mbs (pressure at, B) plus the wright of the column of air BY which is greater than the column of air AX. Therefore a slight in a direction A to B (i.e. X to Y) along a pressure level[?] which is gaining altitude is a movement towards higher pressure, considering the general case,

5. Pressure height lines drawn on a chart as in para 3 above are called [underlined] Contour Lines. [/underlined]

[page break]

[centred] – 2 – [/centred]

6. Consider, then, the path of an aircraft [two indecipherable words] across the isobars of a synoptic chart:

[diagram]

The wind is blowing along the isobars in a clockwise direction (Section 1, para. 40) as shown by the arrows, and CA is a vector representing the geostrophic wind speed and direction at A. Similarly F8 represents the geostrophic wind velocity at B.

7. CA can be split up into two components, one CD acting at right angles to the beading, which will cause the drift; and another CL parallel with the heading which will cause the difference between air speed and ground speed. With this latter we are not concerned, but will concentrate on the compontent[sic] of the wind CD, at right angles to the heading and causing drift.

8. Contour lines can be utilised to provide this component, which will be referred to hereafter as “Vn”, which means “Velocity normal (at right angles) to the heading”, and it is calculated using a contour wind equation instead of a geostrophic wind equation. Vn behaves similarly to a geostrophic wind, inasmuch as, with your back to the wind component Va, the lowest height is on your left (in the Norther[sic] Hemisphere)

9. A measure of the contour gradient is a measure of the magnitude of Vn, as the closer the contours the greater Vn. The path of the aircraft shown in para. 4 above is a measure of the contour gradient, or the Isobaric slope, the reason for this last term becoming obvious when it is remembered that the aircraft’s path shows the position of the 700 mbs level.

10. Successive differences between pressure height according to the I.C.A.N. Law as would be shown on a Kollsman altimeter set to 1013.2 mbs. (Section 1. Para. 44) and absolute height as would be given by a Radar Altimeter (Section 1 para.46,), would indicate the isobaric slope. The pressure altimeter would reas[sic] [reach?] a constant, because the aircraft is being flown at 700 mbs and the radar altimeter would show, in out[?] example, an increase of height with distance equivalent to the isobaric slope.

11. This may be clarified by a diagrammatic example.

[page break]

[centred] – 3 – [/centred]

12. The pressure height of 700 mbs., assuming a standard atmosphere, is 9,900 feet. Therefore the aircraft, (see para. 13) will be flown at such a height as to maintain a reading of 9,900 feet on the Kollsman altimeter. The radar altimeter will show an increase of absolute height with distance flown as the aircraft climbs up the isobaric slope, and will show the absolute height.

[diagram]

so that D1, the difference between radar altimeter height and pressure altimeter height is subtracted algebraically from the absolute height, and can produce either a positive or a negative e D. Similar readings at B produce a D2 (from readings taken at the second[?] position) of -100 feet.

14. [sic] The difference between D1 and D2 will be an indication of the [deleted] i [/deleted] isobaric slope and therefore of Vn, and is a function of the air distance flown between D1 and D2.

15. From information now at our disposal, it is possible to calculate Vn from a formula, and the reader will notice immediately the similarity between this formula and the geostrophic wind formula.

[equation] Vn = g(D2 – D1)

2W6sinØ[?] [/equation]

where

Vn = wind component normal to the heading[?], in knots.

D1 = absolute height minus pressure height at the first position.

D2 = absolute height minus pressure height at the second position.

S = air distance flown between D1 and D2, in nautical miles.

W & Ø see geostrophic wind formula.

g = Gravity acceleration in ft/sec/sec.

D1 is subtracted from D2 algebralcally [?], so that “(D2 – D1)”, may be either positive or negative.

16. In the Northern hemisphere

D2 – D1 is positive the drift is to port,

D2 – D1 is negative the drift is starboard.

The reverse applies in the Southern hemisphere.

17. In our example (para 13)

D2 – D1 = +400 therefore the drift is port, which may be verified from the wind direction shown in the diagram in para 6.

[page break]

[centred] – 4 – [/centred]

18. The Vn formula in para 15 can be simplified, in as much as w and g may be [one indecipherable word] as constants[?] and [equation] g over 2w [/equation] (to give Vn in knots) [one indecipherable word] 21.47.

Therefore substituting in the formula 21.47

[equation] Vn = 21.47 (D2 – D1) over [one indecipherable word] [/equation]

The latitude Ø is known by the navigator, and therefore the formula may be written thus

[equation] Vn = K(D2 – D1) over S

where K = 21.47 over sinØ[?] [/equation]

20. In this formula, the wind component is in knots or nautical miles per hour. The navigator need the Vn wind [underlined] effect [/underlined] for the time of flight between D1 and D2, this is achieved by multiplying each side of the equating by the number of hours “t” of flight between D1 and D2 thus

[equation] t x Vn = K(D2 – D1) x t over S [/equation]

t hours x Vn will give the wind effect Zn.

t hours x 1/S will give 1/True airspeed[sic].

21. Therefore the Ø formula finally appears at

[equation] Zn = K(D2 – D1) over TAS [/equation]

Where Zn = wind effect in nautical miles during flight from D1 to D2

TAS = True aispeed[sic] in knots.

[underlined] Pressure Pattern Position lines [/underlined]

22. A position line is obtained by [one indecipherable word] off the distance Zn normal to the true course and in a down wind direction from the six position of D2 with an air plot started from the DR position of D1. The calculation of (D2 – D1) will indicate in which direction Zn is acting. (para 16.)

23. In our example, (D2 – D1) was positive – therefore, our drift is to port, and the position line would be plotted thus

[diagram]

24. Zn may be calculated for any time interval between D1 and D2, but experiments have proved that, for the accurate results, this time interval must not be less than 25 minutes.

25. The course steered must be an accurate one, and the use of the Air Position Indicator will tend to eliminate any pilot error in this respect.

[underlined] Calculation of Dv[?]fit Angle. [/underlined]

26. Knowing Zn, and being able to calculate the track distance flown, it is a simple matter to use plane trigonometry to solve the drift angle.

[page break]

[centred] – 5 – [/centred]

27. Zn forms the side of a right angled triangle opposite to the dirft[sic] angle; the track distance flown, X, forms the hypoteneuse.[sic] Therefore

[equation] sin drift angle = opposite over hypoteneuse [sic]

sine[?] = Zn over X [/equation]

[diagram]

Where a = the drift[sic] angle

X = the track distance flown between D1 and D2.

28. Once again, as with K in para 18, it is a simple matter to construct tables [indecipherable letters] showing the drift angles for all combinations of Zn and X.

29. If track distance is not known, air distance may be used as X.

30. However, this will introduce a slight error if there is a large head or tail wind component of the wind parallel to the course, thus:-

[diagram]

Zn = wind effect normal to course.

B = drift angle calculated using air distance as X

d = true drift angle using X.

[underlined] Single heading flight [/underlined]

31. Single heading flight is an extension of the method of drift finding, and, as its name implies, enable calculationg[sic] of course to steer which will not be changed from departure to arrival at the destination.

32. The formula Zn = K(D2 – D1) over T.A.S.

holds good even when the pressure gradient does not change at a regular rate with distance. The formula can be used to calculate a mean wind effect between departure point and destination on a long flight at a constant pressure height, and hence the mean drift for the flight can be calculated using the formula in para. 27.

33. To calculate the course to steer on a single heading flight, the formula are used as follows:-

Zn = X(D2 – D1) over T.A.S.

When calculating the drift angle X is taken as the track distance of the complete flight.

34. Almost invariably, there will be a deviation from the Great Circle track, with [one deleted letter] a tendency to be blown around the centres of Highs and Lows.

35. A typical single heading track would compare with the Great Circle track as follows, with the wind arrows indicated for the Northern hemisphere Section 1, para 40) ½

[page break]

[centred] – 6 – [/centred]

[diagram] FIG 8

36. As a route example of Single Heading Flight, some extracts from figures of various flights between Prestwick and Gander may be of interest. The great circle distance of this flight is about 2,000 miles, and the flight normally takes about 12 hours.

37. In a total of about 6 flights, times ranging between two[?] minutes and 2 hours 17 minutes (depending upon the pressure distribution of each flight) were saved over the times which would have been taken had normal navigation been used to calculate the flight along the great circle track. In all cases, the single heading flight was shorter in time than the great sircle[sic] route; in no case was the plotted position at [?] the end of the single heading flight more than 17 nautical miles from the destination. In all cases, the single heading was simpler to calculate than the great circle route.

38. To avoid an area of very bad weather, the route can be split into two parts, and two separate single headings calculated.

[underlined] Advantages of Pressure Pattern Flying. [/underlined]

39. The advantages of pressure pattern flying are:-

(a) drifts may be calculated under blind flying conditions;

(b) drifts are mean drifts, therefore of greater value to the navigator than instantaneous drifts (except when drifts desired for the calculation of multiple drift wind velocity);

(c) position lines may be optional at any time, providing there is not less than 25 minutes between D1 and D2, and providing a constant course and pressure altitude have been maintained. This position line may be used with position lines from any other [one deleted word] source to obtain a fix.

[underlined] Disadvantahes[sic] if Pressure Pattern Flying [/underlined]

40. There are limitations to the pressure pattern flying technique, which prohibit its universal usuage, namely:-

(a) Position lines cannot be calculated in flight over a surface of an unknown height, because of the way in which absolute height is measured in flight (Section 1 para 47.)

(b) the technique does not work in low latitudes (between 15 N and 15 S), because the geostrophic wind equation upon which the technique is based is no longer accurate (Section 1, para 43)

[page break]

- 7 -

(c) the technique cannot be used anywhere below 2,000 ft, in the friction layer, because in this region again the geostrophic wind equation is inaccurate (Section 1, para, .25)

(d) A constant pressure height must be maintained. The maximum allowance of deviation of the pressure height is [symbol] 200 feet between D1 and D2. The formulae used to calculate to Vn and Zn (paras. 15 and 17) assume a constant pressure height, and a deviation greater than [symbol] 200 feet from the constant pressure[deleted] height[/deleted] necessitates reduction of the readings to a common pressure height by use of further tables, and also introduces a source of error:

(e) The pressure pattern technique alone will not provide a fix en route, and gives no indication of distance travelled or of E.T.A. Use must be made of affiliated D.R. navigation to give positions and E.T.A’s.

[underlined] Sources of Error with pressure Pattern Flying [/underlined]

41. The sources of error are as follows:-

(a) with excessively curved isobars, and therefore an excessively curved wind path, the cyclostrophic component will be large the geostrophic wind equation will be inaccurate (Section 1, paras 36 to 38) & therefore the calculated Zn will be inaccurate.

(b) with rapid movement of the pressure system, for example a fast moving low pressure area or depression, the isallobaric effect (Section1 paras 11 to 17) will again cause the geostrophic wind equation to be inaccurate

(c) When calculating for Zn, it is assumed that the whole pressure system has remained static during the aircraft’s travel from D1 to D2. This is not strictly true (see (b) above, for example) and effects [sic] the truth of the resulting calculations.

[deleted] (d) [/deleted] NOTE. The errors caused by (a) (b) & (c) are generally small enough[deleted]t[/deleted] to be ignored.

(d) in an extreme case, a North South flight with a consequently large change of latitude, may have all its drift in the first half of the flight and no drift in the second half, so that using the mean latitude of the whole flight to find K will give a false result. This can be overcome by once again splitting the flight into two parts when calculating for single heading.

(e) obviously a bad meteorological forecast of D1 and D2 before beginning a single heading flight will cause subsequent error in the calculation of the drift angle, and therefore of the single heading itself.

42. Pressure Pattern flying certainly is not to be hailed as the answer to the navigator’s prayer, but none the less in its limited sphere of use it is revolutionary [sic] advance in the intricate technique of navigation [sic] an aircraft from A to B.

[page break]

[underlined] SECTION 3 – PRACTICAL APPLICATION [/underlined]

1. It is possible now to make use of the theoretical background and apply it practically in one of two ways. In these notes Single Heading Flight will be dealt with alone as the obtaining of pressure position lines demands the use of the radar altimeter.

[underlined] The Singe Heading Flight [/underlined]

2. Any aircraft, without any special equipment, can carry out a flight of this nature. Single heading flight entails the maintenance of a constant course (in effect a mean course) from departure point to destination. As we have a means of calculating the wind component at right angles to course and the drift to be applied, this course can be calculated before take off, provided that the heights of the upper pressure level can be forecast, with reasonable accuracy, between departure point and destination.

3. The technique holds godd [sic] even though the contour level between the two points in question is not changing regularly. Should the complete flight be divided into innumerable small sections, and the summation of resultant port or starboard Zn be calculated, then the same final Zn would be obtained as if the flight had been considered as a whole.

[underlined] Procedure for Calculation of Single Heading [/underlined]

4. Equipment and data required:

(a) Kollsman (Mk XIV) Altimeter

(b) Table I – K (latitude)

(c) Table II – drift angle (Zn against X)

(d) Table III – Conversion table – mbs to height in standard atmosphere

(e) [formula]

(f) [formula]

(g) Sine tables (preferably log tables)

(Unnecessary if table III carried)

[underlined] Met. Data Required [/underlined]

5. After deciding the pressure height at which the flight is to be made, obtain a forecast of actual height of this contour.

(a) over base at F.T.D.

(b) over destination at E.T.A.

[underlined] Example. [/underlined]

6. Pressure height selected, 700 mbs. Equals 9,900 ft in standard atmosphere.

Height over base (a) at E.T.D. 0300 hrs = 10400 ft

Height over dest. (b) at E.T.A. 1500 hrs = 9500 ft

Distance A – B 2400 n.m.

Track A – B 290 T

Mean latitude 50 N

True Airspeed 200K

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- 2 -

Treat departure point as D1 and destination point as D2, then

[calculation]

therefore the single course to steer between base and destingation [sic] will be 286.90T (to the nearest 10th of a degree)

8. It must be noted that the height of the aircraft must be maintained throughout the flight at the chosen pressure height of 9,900 (i.e. 9,900ft indicated on the pressure altimeter) to within 200 ft.

[underlined] NOTE: [/underlined] All the preceding notes are in effect a copy of the notes issued by E.A.N.S. On the subject of pressure pattern flying. These notes should be read in conjunction with A.P. 1699 (Sutcliffe – Meteorology for aviators)

The proof of the Vn formula can be obtained from the Wing Navigation Leader together with any other information which is required to clear any difficulties which may arise during your perusal of the above notes.

When radar altimeters are carried the procedure for obtaining pressure position lines will be stencilled out for the convenience of all navigators.

[page break]

[underlined] TABLE I [/underlined]

[Table of Latitudes]

[underlined] TABLE III [/underlined]

[underlined] CONVERSION TABLE [/underlined]

Feet to Millibars using U.S. Standard Atmosphere.

[conversion table]

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[underlined] Pressure Pattern Flying [/underlined]

[underlined] For [/underlined] Artillary orders.