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Air Fronts: Aircraft Manuals - RAF Pilot's Notes: A.P 2095, Pilot's Notes - General, 1943 - Part 1: General Flying Notes

A.P 2095, PILOT'S NOTES GENERAL, PROMULGATED BY ORDER OF THE AIR COUNCIL , 2ND Edition , April 1943. FOR OFFICIAL USE ONLY

PART I - GENERAL FLYING NOTES

PART I. NOTE A FLYING LIMITATIONS

1. Introductory.

(i) The Pilot's Notes for each type of aircraft lay down certain flying limitations which the pilot should observe in the flying of the aircraft. They state, broadly speaking, the demands which it is safe to make of the airframe. Non-observance of the flying limitations may lead to increased maintenance work, or in extreme cases to structural failure in flight.

(ii) In the fixing of these limitations there is of course a margin or factor of safety allowed. This factor for airframes varies according to the degree of confidence with which their strength and likely stresses can be predicted, but it is commonly around 2. This means, for instance, that a wing which is intended to withstand 4g should not break until 8g is imposed, but there is increasing risk of strain and failure as g rises above 4.

(iii) The flying limitations also involve questions of safe handling from the aspect of controllability.

(iv) In combat and in emergencies pilots must take risks with their aircraft, balancing one risk against another; limitations must be strictly observed only in so far as there' is no sufficient reason to exceed them.

2. Limiting Speeds.

(i) The diving speed limit may be determined by one of several considerations. Generally this speed is fixed by the strength of the tail and fuselage to withstand the down-ward pressure on the tail required to balance the rearward position of the centre of lift in a highspeed dive. (There is an increasing uncertainty in this calculation as the limiting speed becomes very high and - the factor of safety with a very high diving speed may in fact be less than supposed, but the pilot need not expect a tail failure if he exceeds the limit by a fair amount.)

(ii) Other possible reasons for fixing this limit are

(a) The possibility of the development of flutter of tail or rudder or wings, which would lead very quickly to a complete break-up;

(b) Aileron reversal, that is the loss of aileron control due to the ailerons twisting the wings to such an extent as to annul their effect in banking the aircraft ;

NOTE.-The pilot may also have to limit the speed in a dive to avoid exceeding the diving r.p.m. limitation if the propeller is one of the earlier types with insufficient range of pitch to control in the dive.

(iii) The speed limits on the lowering of flaps and under-carriage arise from calculations either of the strength of the parts to withstand the air forces or of the power of the operating mechanism. It should be noted that the figures quoted are limits based on such considerations, and not handling speeds recommended as the best speeds at which to perform these operations. The speeds quoted for other parts such as bomb doors and landing lamps are of the same nature.

3. Limiting Weights.

Pilot's Notes sometimes, but not always, quote two or three different weights, as :--

" Weight for take off and straight flying only . . .

Weight for landing and all forms of flying . . .

This practice has arisen with the military demand to carry more weight than aircraft were originally intended to carry. The meaning to the pilot is that the aircraft must be handled gently, banked moderately, and subjected only to small increase in g until the weight falls to the lower limit; and that, should it be necessary to make an early landing, load should first (if possible) be jettisoned to bring the weight within the landing limit.

4. Manoeuvres not Permitted.'

(i) Intentional spinning of operational aircraft is permitted only in the case of certain approved single-engine fighters within the limitations stated in the Pilot's Notes. (Normal methods will usually effect recovery - A.P.129 Ch. III).

(ii) Aerobatics are permitted normally on single engine air-craft and exceptionally on twin engine aircraft. They are prohibited on all aircraft at altitudes below 3,000 feet, unless specially authorised.

(iii) The following manoeuvres are prohibited on all aircraft:

(a) All flick manoeuvres (flick roll and half roll);

(b) All manoeuvres involving heavy inverted loading (bunt, outside loop).

(c) Inverted flying (other than the brief inversions occurring in rolls and loops) except on aircraft designed or adapted for the purpose (Part Il, Note E refers).

(iv) The reasons underlying these probihitions are partly considerations of strength and partly of control. Aircraft are designed to fulfil their operational role and not to perform manoeuvres of no operational value. The Pilot's Notes state whether spinning and/or aerobatics are permitted.

NOTE.-Aerobatics are manoeuvres which are not essential to normal flying. Permissible aerobatics (on approved types) include:--

Loop

Stall Turn

Inverted Glide (on approved training aircraft)

Slow Roll

Barrel Roll

Roll off top of Loop

Half Roll

(v) In the normal operational manoeuvres - turning, diving, corkscrewing - the controls must be used with due care according to the type flown. Harsh use of the rudder, and violent reversal especially, should be avoided at high speed - except in so far as coarse use is needed in the event of an engine failure.

5. Normal Acceleration or g

(i) The wings are subjected to an increased loading when they are set at an angle of attack greater than that appropriate to the I.A.S. in straight flight, the aircraft then moving in a curved path. The normal acceleration in the curved path is felt by the pilot as an increase in his weight (or increase of g), which is felt equally by the structure.

(ii) The maximum g is got by raising the angle of attack to the stalling angle, and the g which can be imposed is limited by the I.A.S. at the time. The greatest possible g at twice stalling I.A.S. is 4; at 3 times the stalling speed it is 9; at 4 times 16, and so on. If the stalling speed in straight flight (i.e. at 1g) is 100 m.p.h., then 16g can be imposed at 400 m.p.h. and so a fighter designed for 6g with a factor of 2 (i.e. to break at 12g) can easily be broken by coarse use of elevator.

(iii) The pilot, however, has a limited capacity for with-standing g; he is liable to "black out" within about 10 seconds at about 4 1/2g, and more quickly under higher g. It is clearly of no value to design aircraft to withstand g much beyond the capacity of the pilot, so sacrificing fighting quality in useless weight.

(iv) In the larger aircraft power of manoeuvre is deliberately sacrificed to carrying capacity in bombs, etc., and fuel for range. Whereas the small fighter is designed to be tougher than its pilot, the big bomber is designed to be less tough. In general, the g for which types are designed are :

Single-engine fighter .. 5 to 6

Twin-engine long range and night fighter .. .. 3 to 4

Reconnaissance .. 3 to 4

Heavy bomber .. .. 2 to 2 1/2

(v) The g imposed in a level correctly banked turn is moderate until a fairly large angle of bank is applied; it reaches 2 at 60 deg. of bank, and 3 at 70 deg.

6. Flying in Bumpy Air.

(i) "Bumpy" air imposes g on the airframe and the effect of either horizontal or vertical variations of the wind on the aircraft is proportional to the speed at which it is flying.

(ii) Speed should be restricted when flying in or near heavy cloud formations (especially cumulo-nimbus). For the larger aircraft speed should be limited to an economical cruising speed and the pilot should cruise away from the region if gustiness is severe.

(iii) As the effect of bumps may be added to g imposed by manoeuvres, g due to manoeuvres should he kept to lower limits in rough weather.

7. C.G. Limits.

(i) Flying Limitations properly include the most forward and the most aft permissible positions of the centre of gravity or C.G. of the aircraft. These positions will be found in the aircraft handbook Vol. I, Sect. 4, Chap' 1. They are not quoted in the Pilot's Notes because it is not normally necessary for the pilot to know them; the aircraft is flown at standard loadings at which the C.G. is within the safe limits.

(ii) It is, however, sometimes necessary to include in the Pilot's Notes some instructions on the use of fuel, the release of load, the disposition of crew, or the carriage of ballast in order to keep the C.G. within the limits, or to give the aircraft the best handling characteristics at the most important part of a flight.

(iii) Pilots may sometimes have occasion to carry non-standard loads and they must ensure that the disposition of load will keep the C.G. within the limits. The balance of loading will be maintained by the omission of a load equal to the additional load to be carried at the same distance from the C.G., or of a greater or less load at a correspondingly less or greater distance, or by carrying ballast on the same principle on the other side of the C.G. If the position of the C.G. is not known, it may be assumed for this purpose to be at one third of the root chord of the wing from the leading edge.

(iv) If these C.G. limits are not observed the aircraft may, in some condition of the flight, become uncontrollably nose or tail heavy. If the C.G. is too far aft it may become longitudinally unstable to an uncomfortable or even dangerous degree.

PART 1- NOTE B STABILITY AND 'TRIM!

1. Stability and the C.G.

(i) If the Centre of Gravity (C.G.) of an aircraft is in the right position fore and aft, the aircraft will be nicely stable in pitch; it will fly itself without attention by the pilot to the elevator controls; it will he pleasant to handle on the elevators. If the C.G. is too far forward, the aircraft will be too stable; it will fly itself, but it will be heavy on the elevators. If the C.G. is too far aft, the aircraft will be unstable; it will not fly itself but require constant attention on the elevator control.

(ii) The position of the C.G. changes somewhat as fuel or ammunition are used up, or bombs are released ; it varies with the different loads carried for different duties. It is the aim of the designer to keep the C.G. within suitable limits, however the aircraft may be loaded; but it is not practicable to avoid a considerable variation of C.G. Sometimes the co-operation of the pilot is required in using fuel tanks or releasing bombs in a particular order.

(iii) There is a tendency to add to the service loads of aircraft after the design has been settled, and the addition must generally be carried aft of the original C.G. So the C.G. tends to creep aft and to make the aircraft less stable.

(iv) The ideal position of the C.G. is not quite the same in all conditions of flight, and so an aircraft may become unstable in certain flight conditions only.

2. Stability and the Feel of the Control.

(i) The pilot has a right to expect that, if he trims an aircraft by means of the elevator tabs (so that he is neither pushing nor pulling on the control) and then pushes the nose down, he will need to push to hold the nose down; and, if the aircraft is stable, this will be so.

(ii) If, however, the aircraft is unstable, he will find that, having pushed the nose down, he must then pull to prevent it from going clown further. The feel of the control is reversed.

(iii) In the same way, the necessary movement of the elevator trimming tab is reversed, if the aircraft becomes unstable.

3. Stability and Trim in a Dive.

(i) If an aircraft is stable at all high speeds and trimmed in level flight, it will require an increasing push as it gathers speed to hold it in a dive. But the unstable aircraft will need an increasing pull to prevent it from going over onto its back.

(ii) If the push or pull becomes excessive, it may be lightened or annulled by trimming the aircraft nose or tail heavy. There is no objection to the use of trim tabs during manoeuvres so long as the pilot realises that he is using a very powerful control and operates it slowly and with care.

(iii) If the stable aircraft is trimmed into the dive it may need a heavy pull to get it out and it may be carefully re-trimmed (tail down) during the recovery. On the other hand, if it has not been trimmed into the dive, it will tend to come out too quickly and care is necessary to check this tendency.

(iv) The unstable aircraft, trimmed back to relieve the stick load in a dive, will tend to recover too quickly, needing a push to restrain it. If not trimmed into the dive, the pull will slacken during recovery.

(v) An aircraft may be stable, trimmed in the dive, although it was unstable in level flight. This aircraft will have to be pulled out of the trimmed dive; but when recovery has been started, a push will quickly become necessary to check the rate of recovery, because it has passed from a stable to an unstable condition of flight as its angle of incidence was raised. (Stability depends upon incidence and may change when g is applied by pulling back the control column, without the I.A.S. having changed.)

4. Stability in Turns.

(i) Too much longitudinal stability makes an aircraft heavy on the elevator in turns. Instability leads to a tendency for turns to tighten, against which the pilot must be on his guard to avoid stalling, blacking out, or overstressing the aircraft.

(ii) The aircraft may be stable in straight flight and become unstable in the turn for the reason explained in the previous paragraph 3 (v).

5. Two Forms of Instability.

(i) When an aircraft is longitudinally unstable it will commonly either dive or stall if left to itself. In the last two paragraphs the instability has been assumed to be of this kind.

(ii) An aircraft may, however, appear to be neutral or just stable judged by the action required to trim it at different speeds, by its behaviour in diving or turning, and yet show instability in a tendency to " hunt," to build up a pitching oscillation with a rising and falling I.A.S. But more usually this " phugoid " oscillation damps itself out and the aircraft is completely stable.

6. Stability with Fixed and Free Controls.

The longitudinal stability of an aircraft may clearly be considered in two conditions of flight, either with the controls held rigidly or with the controls left free. It is the second condition that decides how the pilot will like the aircraft, because it determines the nature of the control effort that he must make.

7. Lateral-Directional Stability.

(i) The lateral-directional stability of an aircraft is obtained by correctly proportioning fin area in relation to wing dihedral. It is not sensitive to C.G. position.

(ii) If the fin area is very large, the aircraft may tend to turn away to right or left, banking and dropping its nose, into a spiral dive. This form of instability is technically known as spiral instability. It is difficult to detect a mild spiral instability because it is almost impossible to trim an aircraft to fly itself straight for any length of time; stable or otherwise, pilot or autopilot must give continual attention to the controls to steer a straight course. This form of instability is not generally of practical importance.

(iii) If the fin area is too small the aircraft will tend to hunt or " wallow". The motion really involves yawing and crabbing as well as banking, but the banking is most noticeable and the pilot may speak of " lateral " instability. This instability may be bad enough to make the aircraft very tiring to fly.

(iv) If the fin area is much too small the rudder may feel unduly sensitive, the slightest rudder movement leading to a violent yaw. The modern trend towards large fins and rudders makes this instability unlikely and an aircraft with this defect could not be accepted for service use. The pilot might call this " directional" instability.

(v) Another form of " directional " instability is occasionally met in a quick flattish oscillation or " snaking", arising from some defect in the design of the rudder; and a "lateral" instability due to bad aileron design is conceivable.

(vi) A special kind of instability occurs at the stall when a wing drops. This is easily understood when it is realised that the motion of dropping is equivalent to an increase in angle of attack. Normally increased incidence brings increased lifting power, but when the wing stalls this ceases to be true and the reverse may occur with more or less violence, according to the stalling characteristics of the wing. Normally the ailerons have to fight against a strong resistance to banking, but at the stall the aircraft banks all too easily.

(vii) The handling of this instability at the stall is complicated by the fact that, while the ailerons may still be powerful for the work they have to do, they produce an adverse yawing effect and it is often better to use only the rudder to maintain an even keel, at the expense of loss of direction for the moment; for the dihedral effect of the wings is still effective in causing bank through yaw.

(viii) Pilots may sometimes find themselves in difficulty through inadvertently allowing a considerable sideslip to develop. This may lead to banking and dropping of the nose, accompanied by a tendency of the rudder to lock over - " stabilised yaw ". The prime necessity for regaining control is to get rid of the sideslip by quick, vigorous use of the rudder; but on aircraft with this tendency pilots should be on their guard against allowing much sideslip to develop.

NOTE.--The aircraft must not be expected to return to its original heading after a lateral or directional disturbance. It will return longitudinally or laterally after disturbance under the directing force of gravity, but it possesses no inherent sense of compass bearing.

8. Degree of Stability Desired.

(i) While a high degree of stability tends in itself to reduce manoeuvrability, it can be offset by nicely balancing the controls and the resulting aircraft may be pleasanter to handle than one which attains its manoeuvrability with less well balanced controls and a lower degree of stability.

(ii) Although slight instability is often tolerated, it is most desirable to have positive stability, especially in conditions of flight that persist for a long time. A main design problem is to avoid a longitudinal instability that would make the aircraft tiring to fly, or in extreme cases dangerous.

PART I--NOTE C - FLYING FOR RANGE AND ENDURANCE

1. The Best Speed for Greatest Range or Endurance.

(i) The figures below give the fuel consumed by a certain twin engine bomber when cruising at various indicated air-speeds, and these test results are exhibited graphically in the diagram alongside. The two sets of figures and the two curves give respectively the gallons used per hour and the gallons used for 100 air miles at 10,000 ft. Similar results are obtained for other aircraft. It will be seen that the lowest consumption per 100 miles is obtained at a considerably higher speed than that which gives the least consumption per hour, and that neither maximum range nor maximum endurance will be attained by flying as slow as possible.

(ii) The reason why the lowest consumption of fuel per hour does not occur when flying at the lowest possible speed, is to be found primarily in the way in which the drag of the wings varies with speed. To fly slowly the angle of attack must be large, and the drag of the wings increases rapidly as the angle of attack is increased. From this it comes about that least power is required from the engine at a considerably higher speed than the lowest possible, and so the fuel used per hour will be least when flying well above stalling speed.

(iii) To obtain the maximum air miles per gallon (or least gallons per air mile) the aircraft must be flown considerably faster than this; for so long as the increase in the miles covered in the hour is greater than in the gallons used in the hour the aircraft will do more miles per gallon.

For example, in the hour at I.A.S.: - we use: we travel which gives us :

120 m.p.h. 42 gall. 139 miles 30 gals/100 miles

140 m.p.h. 48 gall. 162 miles 29 1/2 gals/100 miles

(iv) It will be seen from these figures and curves that very little is lost in either range or endurance by flying 10 m.p.h. or so faster than the speed which gives the absolute minimum gallons in each case. So 145 m.p.h. may be taken as a good practical range I.A.S., while 120 m.p.h. is as slow as it is ever worth flying for maximum endurance. The range speeds recommended in Pilot's Notes are on this basis, i.e. about 10 m.p.h. above the absolute optima.

(v) The speed for maximum endurance is about four fifths of the speed for maximum range.

2. The Effect of Altitude on the Best Speed.

(i) The best I.A.S. for range is, in general, the same at all heights; for the drag is least when flying at the best angle of attack, which is the same at the same I.A.S. at every height (at the same load).

(ii) But when, at low altitudes, this I.A.S. can be obtained at minimum r.p.m. without using maximum weak mixture boost, a higher speed is usually better; in fact, the best speed may be the speed that requires the maximum weak mixture boost, if this speed is not very much greater than the best speed at higher altitudes. Advice may be found in the Pilot's Notes.

(iii) At the highest altitudes, when the normal best I.A.S. cannot be obtained at the weak mixture engine limits, the best speed is the highest obtainable on weak mixture.

(iv) The best speed for endurance is the same at all heights.

3. The Effect of the Load Carried on the Best Speed.

The best speed for range or for endurance is proportional to the square root of the all up weight; i.e. for a reduction in A.U.W. (e.g. by release of bombs or consumption of fuel) speed should be reduced by 5%. This adjustment can be ignored for short range fighters; for bombers it is good enough to use one speed for the outward and another for the homeward journey. Pilot's Notes indicate how the speed should be varied with the weight at which the aircraft is flying at the time.

4. The Effect of Wind on the Best Speed.

A wind which increases or decreases the ground speed at given I.A.S. has a big effect on the range, and it may seem that the aircraft should he flown faster into a head wind, and conversely; but in fact, unless the wind is very strong, it is not worth making any change. The following figures and diagram show the effects of a 50 m.p.h. head wind and a 50 m.p.h. following ("tail") wind on gallons per mile of the aircraft of the first paragraph.

It will be seen that the recommended speed of 145 m.p.h. is practically the best speed against the head wind, and that it is still quite a good speed with the tail wind. In stronger winds it would be worth while to make a change of about 10 m.p.h. - especially to increase 10 m.p.h. against a strong head wind, if it can be done while still using weak mixture.

5. The Effect of Wind on Range.

(i) The big increase in the fuel used per 100 ground miles against the 50 m.p.h. head wind will have been noted. It should also he noted that this increase (from 30 to 43 gallons at 145 m.p.h. I.A.S.) is not balanced by the reduction (from 30 to 23 gallons) when flying in the same wind in the opposite direction; there is a net increase from 60 to 66 gallons on the double 100 miles.

(ii) It may also be noted that these figures relate to one all-up weight. Actually fuel used per air mile is greater on the outward journey (heavy) than on the return journey (light), and the effect of a given wind will be most adverse when it is against the outward journey (heavy).

6. The Effect of Altitude on Range and Endurance.

(i) The air miles per gallon would be the same at the same I.A.S. at all heights if the engine used the same gallons per horse-power per hour. But, in fact, the specific consumption - the gallons used per horse-power per hour - varies with engine conditions. Air miles per gallon tend to be lower at the lower altitudes, and they fall away at the highest altitudes. Variation of air miles per gallon with height is, however, not large in general.

(ii) Account must also be taken of fuel consumed in climbing to the height, and - especially if the climb is not done on weak mixture - the fuel so used is only partially recovered in subsequent descent. But if the climb is made on weak mixture, range from take-off to touch-down will not vary much with the operational height.

(iii) On the other hand, maximum endurance falls the higher the aircraft is flown; for the power required to maintain level flight at the best I.A.S. increases because the aircraft must be flown faster in the rarer air. On this account the endurance decreases roughly as under:--

At 10,000 feet to 7/8 of that at Sea Level.

- 20,000 - 3/4 " "

- 30,000 - 3/5 " "

('This law may be somewhat modified by change of specific consumption as the power changes with change of height, especially with high-powered aircraft at low altitudes.)

7. The Effect of Weight on Range and Endurance.

The fuel used per hour and per mile would be proportional to the all-up weight if it were not for variation of specific consumption. In fact it is commonly found that range is rather less affected by the load carried, a change of 10% in A.U.W. causing a change of 6 to 7% only in the range. The effect on endurance is similar.

8. The Effect of Drag on Range and Endurance.

Range and endurance can be seriously affected by excrescences or holes that add to the drag of the aircraft, and anything the pilot can do to withdraw excrescences or reduce leakage of air in and out of the aircraft will improve his range and endurance. (If the drag is necessarily much increased the best I.A.S. may be appreciably lowered.)

9. Air Miles per Gallon in Climb or Descent.

(i) In a gentle climb on weak mixture, or in a descent under a fair amount of power, most air miles will be covered per gallon at the same I.A.S. as for level flight; but in a glide with little or no engine a rather lower speed will cover the greater distance (best gliding angle).

(ii) When climbing on a rich mixture it is best to gain height as quickly as possible in order to shorten the time spent in rich mixture.

10. The Effect of Air Temperature on Operational Ceiling.

A high atmospheric temperature affects performance in two ways:

(a) It thins the air at a given aneroid height and so increases the true speed at which the aircraft must be flown and the power required to fly it.

(b) It reduces the charge drawn into the engine and so reduces the power available.

For both reasons the operational ceiling for given r.p.m. and boost limits is reduced. To maintain the same ceiling the A.U.W. of an aircraft must be reduced 1,000 lb. per 30,000 lb. per 10° C. rise of atmospheric temperature, and conversely.

11. Handling the Engine for Maximum Economy of Fuel. This aspect is discussed in Part II, Note C. In brief:

(a) If there is a mixture control it must be WEAK .

(b) Within the engine limitations for weak mixture, use the highest boost and the lowest r.p.m. (provided that the generator charges at the r.p.m. used).

12. Handling the Aircraft for Maximum Endurance.

(i) The endurance at the recommended endurance speed will be about 15% greater than the endurance at the recommended range speed; but handling at the best speed for endurance may present some difficulty.

(ii) When flying on the absolute minimum of power for level flight the normal ability to manoeuvre disappears. Raising the nose will not cause the aircraft to climb, or check descent; and any considerable reduction of speed will have the reverse effect. Height will necessarily be lost on turns if the throttle is not opened slightly. (The recommended speed is higher than that which uses the absolute minimum of power, but it will still give little power of manoeuvre).

(iii) No attempt should be made to maintain an exact height; the necessary continual use of the throttle will increase consumption. Height gained or lost should be corrected from time to time as may be necessary by a slight resetting of the throttle.

(iv) Speed should be maintained within 5 m.p.h. up or down. Some aircraft, though stable at higher speeds, become longitudinally unstable at the best speed for endurance and it may not he practicable to fly some aircraft at this speed.

PART I. NOTE D. AIRCRAFT ICING.

1. Common Icing Conditions.

(i) Ice forms on aircraft most frequently when the air is at a temperature between -1 and -7° C. and contains large supercooled drops of water.

(ii) The normal rate of ice accretion in these conditions is about 1 inch in 10 minutes. A fall in the I.A.S. at given engine conditions in level flight, or a loss of height at given I.A.S., may be noticed when the thickness reaches 1 1/2 inch; the effect on performance only becomes serious when, at a thickness of about 2 1/2 inches, the ice begins to form jagged edges. It is normally possible to fly in icing conditions for 20 minutes or more without the accumulation of ice becoming dangerous.

(iii) The icing layer extends usually to a depth of about 3,000 ft. at a comparatively low altitude, and it may be climbed through without taking on much ice. The best practice is to climb quickly into air at -8° C. or lower. Alternatively, if the icing layer is sufficiently high, the pilot may descend and fly below it.

(iv) Icing may be detected by careful attention to height and I.A.S. when flying in likely conditions; or by icing of the windscreen. At night a torch can be used to examine windscreen and leading edges of wings.

(v) The slinger ring de-icer for the propeller (if fitted) should be turned on slightly in icing conditions and only turned on fully if it appears that ice on the propeller is causing vibration. Then, after a few seconds, increase r.p.m. to maximum for a few seconds, and then return r.p.m. and de-icer to normal.

(vi) As icing conditions are often accompanied by severe static electricity, especially in cumulus cloud and when hail is falling, aerials should be earthed and reeled in and sets switched off.

(vii) The pitot heater should he on.

2. Icing in Cumulo-nimbus Clouds.

(i) In cumulo-nimbus and large cumulus clouds, associated with cold fronts and squally, showery conditions, ice may build up quickly at temperatures down to -18°C., or even lower.

(ii) So far as possible these clouds should be avoided.

(iii) When it is not possible to avoid them by night they may be recognised from inside by turbulence, by rain, hail or snow. Icing conditions may extend through more than 5,000 feet in height, but the horizontal extent of the precipitation areas will generally be small and they will be flown through quickly. A quick climb to -18°C. can therefore generally be made without taking on too much ice.

3. Rain falling in Freezing Air.

(i) Occasionally rain falls into freezing air from milder over-lying air, often at a warm front, and an aircraft flying in these conditions will accumulate ice with dangerous rapidity.

(ii) The pilot should either turn back and avoid the rain, or fly up into the cloud and continue into air which is either above or much below freezing point.

(iii) These conditions are not likely to occur above 5,000 feet and they are rare.

4. Use of Pastes.

(i) Kilfrost wing and propeller de-icing pastes reduce the adhesion of ice so that it is more readily dislodged by centrifugal forces, aerodynamic forces, control movements, or vibration.

(ii) The efficacy of pastes depends upon proper smooth application (as advised in A.P. 1464/D173). Their use does not relieve the pilot of the need to avoid icing conditions as far as possible.

5. Further Information.

Reference may be made to:

A.P. 1931 Meteorological Handbook for Pilots and Observers - Chapter XXII.

M.O. 420 B Ice Accretion on Aircraft.

A.M. Pamphlet 138 Aircraft Icing.

PART I - NOTE E TRICYCLE UNDERCARRIAGES

1. Main Differences between Tricycle and other Aircraft.

(i) The wing incidence of the tricycle aircraft when resting on its three wheels is much less than that of the conventional aircraft and consequently :

(a) The forward view is generally better for taxying, for the initial take-off run and for the landing run.

(b) The ground attitude is one of low lift and low drag, which gives a good take-off acceleration but a poor retardation in the landing run; and effective braking is therefore particularly important.

(c) Since the normal landing is made with the nose-wheel but little higher than the main wheels (for reasons discussed below), the landing speed is higher and the landing run correspondingly longer.

(ii) The main wheels of the tricycle aircraft are aft of the centre of gravity, whereas in the conventional aircraft they are forward of the centre of gravity. Consequently:-

(a) Impact of the main wheels with the ground tends to pitch the tricycle aircraft forward, reducing wing incidence. The tricycle aircraft will therefore never " balloon off " and the reduced incidence after touch-down reduces the tendency to bounce over an uneven surface.

(b) The main wheels being aft of the centre of gravity, give directional stability to the aircraft when the wheels are on the ground, non-castoring wheels having an effect analogous to that of a fin or a rudder held central. So any tendency to swing, especially at high speed, is almost eliminated.

(iii) The nose-wheel of the tricycle allows the brakes to be applied as hard as necessary once the nose-wheel is on the ground without fear of nosing over.

(iv) To summarise :-The tricycle undercarriage makes landing markedly easier, provided that there is ample landing space, and the brakes are in good order. The correct handling is, however, somewhat different and certain special points which must be observed are discussed in the following paragraphs.

2. Ground Handling.

(i) Tricycle aircraft should be parked, especially in a restricted space, with the nose-wheel straight. If this is not done, a sudden unexpected turn may result from the next attempt to taxy forward and may lead to an accident.

(ii) When starting from rest, engines must not be opened up on one side only to produce a turn unless the nose-wheel is set for that direction of turn. To do so would impose a severe side load on the nose-wheel.

(iii) No aircraft should ever be turned on one wheel, since this practice causes severe tyre wear; but with the tricycle the attempt to do this also severely strains the nose-wheel. Always move forward a little before turning.

(iv) Care should be taken when taxying slowly, not to over-strain the nose-wheel by turning too sharply. The castoring range of the nose-wheel is limited.

(v) With toe-operated brakes, it is usually easier to taxy with controls (rudder) locked.

3. Take-off.

(i) The aircraft should be taxied straight a few yards before the throttles are opened in order to ensure that the nose-wheel is straight.

(ii) The nose-wheel should be kept on the ground until considerable speed has been gained. The control column should be central or slightly back. As the take-off speed is approached, move the control column steadily and firmly back until the tail drops and the aircraft leaves the ground. On some types considerable backward pressure may be necessary.

NOTE. - On rough or boggy ground the control column should be held further back to take load off the nose-wheel.

4. Normal Landing.

(i) It is important to check the brake pressure by gauge or by the feel of the brake pedals before coming in to land, since most tricycle aircraft cannot be stopped within the normal airfield without brakes unless the wind is very strong.

(ii) Approach and flatten out in the normal manner, preferably using a little engine.

(iii) Throttle back, and continue to move the control column back just too slowly to maintain a constant holding-off height. This allows the aircraft to sink slowly on to its main wheels in a slightly tail-down attitude. (Note that the landing should not be three-point, but main wheels first.)

(iv) Continue to move the control column back to prevent the aircraft from pitching forward suddenly and to keep some of the weight off the nose-wheel after it has made contact.

(v) The brakes must not be applied until the nose-wheel is on the ground, as this causes a violent pitch forward on to the nose-wheel.

(vi) The brakes should not be applied harder than is necessary, especially on runways, as this causes unnecessary tyre and brake wear; use the whole runway available whenever possible.

5. Slow Landing.

(i) The landing speed can be varied widely and depends upon the extent to which the tail is got down before the main wheels touch.

(ii) If a slow tail-down landing has to be made, it is particularly important to use the elevator control to lower the nose-wheel gently and not to apply the brakes till the nose-wheel is down.

(iii) Very tail-down landings should not be made on large heavy types.

6. Important Points in Landing.

The following must be avoided :

(a) Flying into the ground nose-wheel first.

(b) Landing with high rate of descent.

(d) Braking before the nose-wheel is down.

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