Some preliminary refresher notes
• The airspeed at which an aircraft stalls depends in part on the wing loading — the ratio of lift force generated to aircraft all-up mass expressed in units of 'g'. If a wing reaches the critical angle of attack under an aerodynamic load higher than 1g, the stalling speed will be higher than the normal 1g stall speed for that particular mass and wing configuration, and the effects of that accelerated stall are usually more pronounced than a 1g stall. An accelerated stall is not a 'high-speed' stall — the latter is one form of accelerated stall.
• Uncoordinated or cross-controlled flight: applying pressure to the rudder in one direction with opposite aileron applied is cross-controlling. This is, normally a rather sloppy way to fly but also a condition that can lead to an uncommanded roll if you inadvertently exceed the critical angle of attack — particularly in uncoordinated climbing or lower speed descending turns, such as in the approach to landing. It may also be the condition when a mis-rigged aircraft flies 'one wing low'. That being said, a planned and properly executed cross-controlled sideslip during final approach IS a normal and safe flight manoeuvre.
• Once established in a coordinated level turn, the lower inner wing has a slightly lesser airspeed and thus less lift than the outer wing, which produces a tendency for the outer wing to rise and the bank angle to increase. This requires the pilot to apply a slight opposite pressure to the control column which is known as 'holding-off bank'. This is quite normal and the pilot may not notice doing so because it is just part of maintaining the chosen bank angle throughout the turn. In a climbing turn, the outer wing has a slightly greater effective aoa than the inner wing and thus additional lift. Combined with its faster speed, this reinforces the tendency for the bank angle to increase and the need to hold-off bank.
• However, in a descending turn, the steeper path of the inner wing means that it will have a larger effective aoa than the outer — which may compensate, or over compensate, for the faster velocity of the outer wing. In order then to maintain the required bank angle, it may be necessary to apply a slight inward pressure to the control column; i.e. in a coordinated descending turn, the bank may be 'held on'.
• Should an aircraft be stalled inadvertently in a coordinated turn, both wings usually display the same progressive stall pattern — thus there should be no pronounced wing drop.
• When flying at speeds below 1.3 times Vs, the aileron moments are much less effective than at cruise speeds and larger aileron deflections are needed to bank the aircraft. There is always a tendency to be more forceful than necessary, thus overbanking the aircraft at a critical stage. The same applies to rudder effectiveness, so more coordinating rudder is required at slow speeds.
• In the following text, 'top/bottom rudder' refers to the relative position of the rudder pedals when turning; 'top' being the rudder pedal opposite the lower wing. Thus, if the aircraft is banked and turning to the left, then pressure on the right rudder pedal will apply top (or outside) rudder or pressure on the left rudder pedal will apply bottom (or inside) rudder. An excess of bottom rudder produces a skidding turn. Too much top rudder produces a slipping turn or may even halt the turn, thus producing a full sideslip. In a coordinated turn, there is just sufficient bottom rudder applied to keep the slip ball centred.
As a consequence of providing the centripetal force for a sustained turn, the lift force (i.e. wing loading) must be increased as angle of bank increases. Lift increases rather slowly up to a bank angle of 30° — where it is 15% greater than normal level flight loading — after which it increases rapidly, being 41% greater at a 45° bank angle. At this angle, the load on the airframe is 1.41g. The right-hand column in the following table shows the increase in stall speed, which is proportional to the square root of the wing loading. You can see that the percentage increase in stall speed is about half the increase in lift force.
|Bank angle||Cosine||g load |
The lift force increase in a constant-speed turn is provided by an increase in the lift coefficient [CL], which in itself is brought about by increasing aoa. Obviously, increasing CL implies an increase in drag and loss of height, or change in the rate of climb/descent, unless power is increased.
A rule of thumb for light aircraft with normally cambered wings is that each 1° aoa change — starting from 2° and continuing to about 14° — approximates to a 0.1 CL change, and each 0.1 CL increase/decrease at a constant airspeed represents a wing loading change of roughly 8%.
So, from the table above, a 30° bank angle in a sustained turn adds 2° to the basic aoa for the airspeed, a 45° bank angle adds 5° and a 60° angle adds 12°. The basic aoa for normal descending and climbing speeds in the circuit are probably around 6–8° and 6–10° respectively. Anything more than a moderate 30° banked turn decreases the safety margin between the effective aoa of some sections of the wing and the critical aoa.
Wing loading must also change with the payload carried, as does the stall speed and the performance speeds. If a recreational light aircraft is normally flown with just the pilot on board, the aoa associated with a particular calibrated airspeed is significantly less than when flying at the same airspeed with a heavy passenger and perhaps a full fuel load. For example, suppose the aircraft is normally flown with only the pilot on board and an all-up weight of 400 kg. But when flown with a heavy passenger and full fuel, then all-up weight increases to 540 kg. Then the wing loading increases by 35%, thus CL and the aoa for any particular CAS will be greater than the pilot is accustomed to — maybe 2° or 3° at low airspeeds — and much less at high airspeeds.
( The next section in the airmanship and safety sequence follows below and describes loss of control in an uncoordinated level turn )
If an aircraft is being held in a level turn at a particular bank angle with constant power, and excess bottom rudder is applied and held, the aircraft will rotate about the normal axis (yaw) in the direction of rudder deflection. Airspeed over the outer wing increases slightly while that over the inner wing decreases, thereby producing a lift differential; thus there will be a secondary roll effect that increases the bank angle. At the same time, the yaw increases fuselage drag and decreases airspeed — and thus lift — and the nose drops a little. This is an uncoordinated skidding turn, which often happens when the pilot tries to 'hurry' the turn with bottom rudder instead of increasing bank. We have a situation where the aircraft is overbanking with the nose yawing inward and downward.
If the pilot reacts by applying and holding opposite aileron to restore the required bank angle — i.e. holding off bank — then due to the downward deflection of the inner aileron, the outer 30% or so of the lower wing is flying at a much higher aoa than the corresponding section of the higher wing. (If equipped with flaperons, the whole lower wing would be flying at a higher aoa.) The lower wing will also be producing more aileron drag, so the inward and downward yaw will increase and there will be a tendency for the pilot to raise the nose by increasing control column back pressure. This increases aoa overall, while at the same time speed will continue to decrease because of the increased fuselage drag, unless power is increased.
The pilot is now 'pushing the design manoeuvring flight envelope'. Any consequent tightening of back-pressure on the control column to raise the nose (or any inadvertent back pressure applied when, for instance, looking at something of interest below you; looking over your shoulder; being distracted by fiddling with something in the cockpit; using the radio; or even any encountered atmospheric turbulence, wake turbulence or gust shear) may take the aoa of the inner wing past the critical angle. The aircraft loses its lateral stability (i.e. positive roll damping) and it is most likely that the lower wing will drop in an uncommanded roll, and thus become increasingly more deeply stalled than the upgoing wing — which may not be stalled or just partly stalled.
If that initial roll is not promptly recognised as a stall or partial stall and it is allowed to continue — or perhaps it is incorrectly countered with opposite aileron without first unstalling the wing(s) by easing forward on the control column — then the increasing aoa of the lower wing deepens the stall and causes greatly increased asymmetric drag. Additional yawing forces in the same direction as the lower wing come into play, the nose-down pitching moment increases and the nose drops further. This is the incipient spin condition where autorotation is about to commence, which will happen quickly and in some aircraft very quickly indeed. The result is the stall/spin fatality you hear about when an unwary pilot allows such to develop without sufficient height to recover; and of course you say 'How sad it is for the family' — while thinking — 'but I'm too wary to get caught by such a simple mistake! But you don't know how many times you have come within a hair's breadth of eternity without being aware of it.
If the cg is aft of the rearward limit, the amount of elevator deflection needed to bring the aircraft to the critical aoa is reduced; i.e. just a relatively small rearward movement of the control column may rotate the aircraft to the critical aoa. If MTOW exceeds the design limit and/or the cg is aft of the rearward limit then recovery from the initial stall may be impossible.
The rules to avoid such situations are:
• always maintain a safe speed near the ground consistent with the bank angle employed
• continually envisage the wing aoa; i.e. how it's flying
• keep the slip ball centred; i.e. never apply an excess of bottom rudder in an attempt to tighten any turn if height is below the safe recovery height (3000 feet agl perhaps) for a fully developed spin.
The height lost during a normal stall and recovery incident in a very light aircraft is probably between 50 and 250 feet depending on the aircraft type, the aircraft attitude at stall and the pilot's awareness. Loss of height in a stall/spin event is very much greater — perhaps 100–300 feet during the incipient stage, 200–400 feet to stop the autorotation and 300–500 feet during the recovery: a total of 600–1200 feet. This is why low-level stall/spin events are so deadly.
( The next section in the airmanship and safety sequence follows below and describes loss of control in an uncoordinated descending turn )
The precursors to a stall/spin event in a low-power descending turn are the same as those for such an event in a level turn: if an excess of bottom rudder is applied, the aircraft will be skidding. Unless some other factor is dominant, then whenever an aircraft is slipping or skidding in a turn, the wing on the side to which the rudder is deflected will usually stall before the other, resulting in a consequent instantaneous roll in that direction. At descent speeds, the aircraft is usually flying at a higher CL and thus higher aoa, than when on the downwind leg (for example) — so there is a reduction in available aoa margin before allowing for the additional aoa required for the turn.
The descending turn from base leg onto the final approach to landing is the most obvious place for a pilot to attempt to hurry a turn with rudder, because of the need to align with the runway. A tailwind component on base leg in a crosswind landing will increase the tendency to hurry the turn with rudder, as may other crosswind situations. If skidding, the excess bottom rudder is yawing the nose down and the tendency is to use elevator to keep it up, which is going to bring the aoa towards critical. Also, because of illusory ground reference cues, there may be a tendency to increase the rate of turn by applying additional bottom rudder whilst maintaining the bank angle with opposite aileron — 'holding off bank'; and you should never hold-off bank in a descending turn. If control column back-pressure is purposely or inadvertently applied, the aircraft may enter a cross-controlled stall where it is going to snap further into the bank and enter an incipient spin.
Apart from the turn from base to final, such stalls might occur on final when avoiding a bird strike; or attempting a late correction to an out-of-line crosswind approach; or any time when you try to hurry a turn with bottom rudder. Stalls on the final approach — caused by failing to increase power when raising the nose to stretch the approach or reduce a high sink rate — will be exacerbated if the aircraft is also slipping. Probably the most dangerous low-level descending turn is the turn-back following engine failure after take-off; see 'The turn-back; possible or impossible — or just unwise?
If flying cross-controlled when banked with an excess of top rudder — as in the sideslip manoeuvre, or a slipping rather than skidding turn — then if the aircraft stalls, the roll will probably be anti-spin; i.e. in the direction of the upper wing — towards an upright position — which is not quite so alarming and provides a little more time to react and reduce aoa.
( The next section in the airmanship and safety sequence follows below and describes loss of control in a low-level climbing turn )
As we saw above, the increased lift force in the turn is provided by an increase in aoa. Now what will happen if you are climbing at Vx (the speed for maximum climb angle) using maximum power and decide (because of rising terrain or other obstruction, an approaching aircraft or just natural exuberance) to make a quick 30° left turn using a 45° bank angle, while still maintaining the climb?
Even a 30° banked climbing turn at a Vx will produce an aoa of 14°. This is very close to the critical aoa and provides no margin for even minor turbulence, slight mishandling or inattention. Of course, climb performance will be degraded unless extra power is available, which is unlikely because full power is normally used for the climb until a safe height is reached. The aoa margin, which you should always have in hand to cope with such likely events, is 3 or 4°. This indicates that, when climbing at Vx, turns should not be contemplated. When climbing at Vy — the best rate of climb airspeed with aoa around 8° — until a safe height has been gained, turns should be limited to rate 1 (180° in azimuth per minute, requiring about 15° bank) to ensure an additional margin if wind/gust shear is encountered in the climb-out. When entering a turn during a full-power climb, the aircraft must slow because of the increased drag at the higher aoa with no excess power available to counter it, so the aircraft's pitch attitude must be reduced sufficiently to maintain safe airspeed.
- Coordinated climbing turn: if you do not keep a close eye on the ASI and the airspeed has decayed just a little, the general aoa at Vx could be around 12°. To initiate a 45° bank turn, wing loading and thus aoa must increase by 41%, which will take the aoa to 17°; i.e. past the critical stall aoa of 15° or 16°. Such full-power stalls in a coordinated climbing turn tend to result in the outer wing stalling first — because in a climbing turn, the outer wing has a slightly higher aoa than the inner — with a fairly fast outer wing and nose drop. The roll towards the outside of the turn would initially level the wings, but the increasing aoa of the down-going wing continues to accelerate the loss of lift and increases the drag on that wing. This is a particularly rapid action if the propeller torque effect is such that it also reinforces the roll away from the original direction of turn. P-factor may also cause the aircraft to yaw when flying with high power at high angles of attack. Such stalls are likely to result in a stall/spin event if corrective action is not taken as soon as the initial loss of roll stability — the uncommanded roll, or just a wing rocking warning — is apparent.
- Cross-controlled climbing turn: if the turn is skidding — i.e. with excessive bottom rudder applied —then the lower wing may stall first with the consequent roll into the turn because only one wing is stalled. This may be sufficiently pronounced to flick the aircraft onto its back. The propeller slipstream from a tractor engine will also be slightly asymmetric, as it supplies more dynamic pressure and thus lift to one wing while reducing the effective aoa. We will discuss cross-controlled climbing turns further when we look at illusory ground reference cues.
Although recreational light aircraft — with their low wing loading — normally display quite benign stall characteristics when slowly decelerated to stall speed in straight and level flight, they will exhibit quite different behaviour when a stall is initiated during an uncoordinated turn; and such is the usual unintentional stall situation. Under these circumstances, the height lost during the incipient spin plus recovery — i.e. before developing autorotation — may be 200 to 400 feet or more. Thus, a wing-dropping stall event is highly dangerous when occurring in the circuit pattern or in any other low-level flight situation.
( The next section in the airmanship and safety sequence follows below and describes a Standard recovery procedure for all stall types )
One standard recovery procedure is generally applicable to all stall events or attitude upsets in a three-axis aircraft, whether or not overbanked and/or overpitched — i.e. nose high/low — though this recovery procedure is not applicable to a fully developed spin, whether erect or inverted.
Stall recovery generally requires the following concomitant stages:
- Ease stick back-pressure to reduce aoa of the most stalled wing below critical — which immediately gets the aircraft flying and restores normal 3-axis control.
For any aircraft type, the amount of elevator deflection required to unstall the most stalled wing depends on many variables and may range from just an easing of back-pressure to a firm but smooth push towards the neutral position. All aircraft have their own handling idiosyncrasies and pilots must be aware of them. The nose should be positioned sufficiently below the horizontal to achieve safe flying speed while still well clear of the terrain. It's a matter of balancing height loss and proximity to terrain against a quick return to a safe flight speed. If the forward stick movement is both excessive and abrupt, the result could be an aoa movement below the zero-lift aoa, in which case there will be a reversed lift force on the wings that hinders recovery. This may be particularly apparent with trikes. The negative g due to the bunt could adversely affect some engines at a critical time.
In instances of extreme overbanking (past 60° or inverted) — where although the upset may be the result of a cross-controlled stall or perhaps wake turbulence — the inverted or near-inverted wing will not be stalled but the aircraft will be in an inverted descent. The forward control column movement is needed to reduce the angle of descent. However, there may be the possibility of an inverted stall if the control column is pushed into its extreme forward position.
Warning: never pull BACK on the control column as the initial response to a perceived stall or an overbanked nose-low attitude.
- Halt downward wing movement with rudder or centre the slip ball.
- Increase power smoothly, possibly up to maximum. The slipstream will also increase rudder and elevator authority, and aircraft stability, through its effect on fin and horizontal stabiliser; though if the aircraft is near the wings-vertical position — or is inverted — the throttle must be closed. In the recovery from a stall in a climbing turn, full power should be maintained unless the nose is pitched too far down.
- Roll the wings level with aileron so that all the lift force will be directed away from the ground. If inverted, choose the roll direction that provides quicker return to a wings-level attitude and, of course, the right way up.
Following the preceding actions: adjust power as necessary; if flaps were fully lowered then adjust by stages to take-off position; hold attitude until speed has built up to Vy (perhaps Vx if there are terrain problems); then ease into a climb to a safe altitude, where you can assess what went wrong. Never attempt to continue a landing approach after such an event; go around, allowing plenty of time to assess the environment before re-approaching.
If the aircraft is properly balanced (i.e. cg is within the limits for that all-up weight), any cross-controlled stall condition is readily countered. Of course if the pilot doesn't wait for the airspeed to build to a safe speed before again applying control column back-pressure, there will be a high risk of a secondary stall which may be very hazardous, depending on the height loss from the first stall.
A document titled 'Don't stall and spin in from a turn' expands the material presented on this page and is available in the 'Decreasing your exposure to aerodynamic risk' guide.
( The next section in the airmanship and safety sequence follows below and describes succumbing to illusory ground reference cues )
It is thought that some ground reference optical illusions may be a contributory factor in situations of loss of control near the ground. Such illusions can cause no problem in the circuit if the pilot confines external scanning to the intended flight path and checks for conflicting aerial traffic, while maintaining the appropriate instrument scan and a minimum safe flying speed. The latter is 1.5 times Vs, or perhaps as low as 1.3 times Vs in the latter part of a stabilised final approach as long as bank angle never exceeds 20°. Fixing the circuit pattern on particular ground reference points, rather than the landing strip (for example "turn downwind around the big tree"), may contribute to illusory ground reference cues.
When wind speed is reasonably high relative to aircraft speed, then the aircraft's drift with reference to the ground is very apparent to the pilot operating at lower levels, and particularly at short, difficult airstrips. The diagram above represents the ground track of an aircraft conducting a level 720° coordinated turn with constant speed and constant bank angle, such that in the second 360° turn, the aircraft would be encountering its own wake from the first 360° turn — assuming that the wake didn't sink below the flight path. The movement of the air mass in which the aircraft is borne is toward the west (with an easterly wind) and the turn is clockwise when viewed from above. When in the region above the red line, ground speeds will be lower; when below the red line, ground speeds will be higher. The separation of the tracks for each 360° is exaggerated for clarity.
When entering the south-west quadrant of the first 360°, the ground speed is initially high but reducing. The drift away from a central ground reference would provide the illusion of skidding out of the turn. Passing through the north-west quadrant, the skidding illusion will disappear as ground speed reaches the minimum. Ground speed starts to increase through the north-east quadrant. However, the increasing drift towards the reference point provides a very noticeable illusion of a slip into the turn. This reaches a maximum as the aircraft enters the south-east quadrant, where it abates as ground speed increases.
So, in a 360° coordinated level turn with constant speed and constant bank, the aircraft (and its wake) drifts downwind relative to the ground at the wind speed rate. The cockpit instruments will of course show a constant airspeed, bank angle and a centred slip ball. However, the reference cues seen by a pilot looking at the ground during a low-level turn indicate increasing and decreasing airspeeds, alternating with decreasing and increasing slip.
An unaware pilot may get into a difficult situation in the low-level circuit when an aircraft is turning 90° from crosswind to downwind (as in the progress through the SE quadrant of the diagram above), when drift cues create an illusion of slipping into the turn. At the same time, the increasing ground speed might suggest increasing airspeed. The reaction of an unwary pilot is to increase bottom rudder pressure. This will increase the bank angle and lower the nose. The pilot's reaction may well be to apply opposite aileron to reduce the bank, while increasing control column back-pressure to bring the nose up and possibly reducing power to reduce airspeed. Thus the aircraft is cross-controlled and flying at an aoa with little margin in reserve. This is coupled with decreasing airspeed, reducing lift and the aircraft sinking with a consequent increase in effective aoa. Under such circumstances, there is a likelihood of the aircraft stalling and snapping over. The downwind turn illusion seems to have more potential for error if the aircraft is climbing in a downwind turn.
Note: sometimes you may read material which purports that an aircraft loses airspeed and might stall when turning from crosswind to downwind because the aircraft is changing direction relative to the wind direction, which of course is nonsense. However, airspeed must decrease in the turn if power is not increased to counter the extra induced drag. Although an aircraft can only stall if the critical angle of attack is reached, a combination of aircraft inertia and a wind shear or turbulence event encountered in the turn could result in a stall (particularly if it is still climbing) or, more likely, a loss of height. If turning very close to the ground to follow a particular ground path (close to trees when stock mustering, for example) the increasing drift into the turn must be allowed for.
If you have doubts then imagine operating at 3000 feet agl with a 35 knot gradient wind and using the upper side of a layer of smooth stratus as the airfield surface for simulated circuits. Fly three squares, each leg of one minute duration, at 1.5xVs1 and 400–500 feet above the layer, making 30° balanced turns at the corners. Vary the alignment of each square by 30° in order to achieve near-crosswind alignment in at least one circuit. You will notice that 'ground-speed' does not vary in any part of each circuit (except for a small reduction in airspeed during every balanced turn), there are no drift illusions and nothing changes when turning from crosswind to downwind. You will also notice that in these conditions there is little turbulence.
Pivotal height or pivotal altitude is a term used by proponents of ground reference manoeuvres such as 'eights on pylons'. It is one particular height above ground at which, from the pilot's sight line, the extended lateral axis of an aircraft doing a 360° level turn (in nil wind conditions) would appear to be fixed to one ground point, and the aircraft's wingtip thus pivoting on that point.
Imagine an inverted cone with its apex sitting on the ground reference point and an aircraft flying around the periphery of its inverted base while maintaining a constant airspeed. The vertical distance from the reference point to the centre point of the inverted base is the pivotal height, and the distance from the edge to that centre point is the turn radius. The bank angle is formed between the outer wall of the cone and the radius line.
The pivotal height in nil wind conditions is readily calculated by squaring the TAS in knots and dividing by 11.3. So any aircraft circling at a speed of 80 knots would have a pivotal height (80 × 80 / 11.3) around 550 feet, no matter what the bank angle.
In other than still air conditions the pivotal height varies with the ground speed. If the wind was northerly and the aircraft was turning anticlockwise (viewed from above), then ground speed would be lower on the eastern side of the turn and higher on the western side. When in the northern quadrant the aircraft would be drifting towards the centre point, while in the southern quadrant it would drift away. Drift would not be noticeable in the eastern and western quadrants but changed ground speeds would.
At 70 knots ground speed, the pivotal height is reduced to 450 feet; at 90 knots it is about 750 feet.
(Thus an exercise requiring a continuous 360° balanced turn at constant speed around a ground reference point, whilst holding pivotal height, involves continually changing the height above ground so that the line of pivot around each point is held constantly — rather than maintaining a constant distance from the 'pylon'. The bank angle must also be changed constantly as the wind drifts the aircraft towards or away from the pivot point. It is not an easy exercise to do well, and requires an ability to manoeuvre accurately whilst including the ground reference point in the normal scan pattern. Usually two ground reference points, about five seconds apart, are included for a figure eight pattern — otherwise known as 'eights on pylons'.)
Now imagine two cones — the upper one is the inverted cone with the aircraft flying around the edge of its inverted base and below that is a second cone with its base on the ground and its apex connecting with the apex of the upper cone. The vertical distance from the ground through the cone intersection to the centre point of the inverted base is the aircraft height.
So when an aircraft is turning at pivotal height in nil wind conditions, the wingtip appears to be fixed to a single point in the landscape. But when at any height other than the pivotal height, the wing tip will appear to move across the landscape. When an aircraft is turning at a height greater than the pivotal height, which is the normal situation in flight, the wingtip appears to move backwards over the landscape — path A in the diagram. However, when an aircraft is turning at a height less than the pivotal height (thus close to the ground), the wingtip appears to move forward over the landscape — path B in the diagram.
Thus, when a turning and descending aircraft descends below pivotal height there is an apparent reversal of the wingtip movement from backward to forward, which is the reason pivotal height is sometimes termed reversal height. There is some thought that the reversal illusion may cause problems to unaware pilots during the final turn on approach to landing, because the turn may well pass through reversal height — at 50 knots ground speed, the reversal height is about 200 feet, at 60 knots it is about 300 feet and at 70 knots it is about 450 feet.
If the aircraft is in a banked turn below reversal height, and if the pilot looks down over the wingtip, she/he may get the impression that the aircraft is not turning and may then add additional bottom rudder so that the wingtip then appears to move backwards in the turn — the normal movement. This will cause a yaw and the aircraft's nose will slide down. The aircraft may then appear to be nose-low, and the pilot's reaction is to increase back pressure on the control column. Low speed, excessive bottom rudder and an increasing control column back pressure are the prerequisites for the aircraft to stall and roll toward the lower wing — an incipient spin entry. All pilots should be aware of this illusion and that wind drift will exacerbate it — the turn to final approach is probably the most important ground reference manoeuvre that recreational pilots regularly perform.
( The next section in the airmanship and safety sequence follows below and describes the effects of wind shear )
Air flow in the atmospheric boundary layer is normally turbulent to some degree but such turbulence does not significantly alter the aircraft's flight path. Bear in mind that what is a minor variation in flight path at a reasonable altitude may be hazardous when operating at slower speeds very close to the ground in take-off, landing, 'go-around' or perhaps cattle mustering operations. The velocity of near-surface winds is changing constantly; fluctuations in direction of around 20° and in speed around 25% either side of the mean occur every minute. In an unstable boundary layer, the rising air in thermals is accompanied by down-currents from the top of the layer, where the wind velocity approximates the gradient wind — i.e. the direction is backed by 20–30° from the wind at the surface, and the speed is greater. The descending air retains most of these characteristics when it arrives at the surface, thus the gust will back and increase in speed.
Except for the vortex turbulence from the wake of preceding aircraft — which is extremely hazardous to light aircraft at low levels because of its horizontal rotational properties — practically all turbulence hazardous to flight is a result of wind shear, a sudden "variation in wind along the flight path of a pattern, intensity and duration, that displaces the aircraft abruptly from its intended path and sufficiently that substantial control action is needed." The shear is the rate of change of wind speed and direction, and its effect on flight can range from inconsequential to extremely hazardous.
Vertical shear is the change in the (roughly) horizontal wind velocity with height; i.e. as the aircraft is climbing or descending.
Horizontal shear is the change in horizontal wind velocity (i.e. speed and/or direction — gusts and lulls) with distance flown.
Updraught, downdraught or vertical gust shear is the change in vertical air motion with horizontal distance.
Wind shear can derive from many sources — orographic, frictional, air mass instability, convective downbursts, wave disturbance and thermalic; for a full description see microscale meteorology and atmospheric hazards. The closer to the surface that the shear occurs, the more hazardous it is for aircraft, particularly for low-momentum aircraft. For an aircraft taking off or landing, the shear may be large enough and rapid enough to exceed the airspeed safety margin and the aircraft's capability to accelerate or climb. Thermals as such contribute relatively minor amounts of hazardous turbulence in temperate climates, but can produce very severe turbulence when flying in the superadiabatic conditions common to inland Australia.
Imagine an aircraft flying straight and level that suddenly encounters an area of substantial atmospheric downflow. Due to its inertia (which is a function of mass), the aircraft will momentarily maintain its velocity and flight path relative to the Earth. During that time the 'effective airstream' around the wings will no longer be aligned with the flight path but will have acquired a vertical component. The effective aoa, and consequently CL, will reduce. This produces a momentary reduction in wing loading, the airframe will experience a negative acceleration and the pilot will be restrained by the harness while the seat drops away from her/him. Following initial entry into the downflow, the inertial effects are overcome and the aircraft will restore itself to its trimmed angle of attack. Flight will continue normally, except that the new flight path will incorporate a rate of descent relative to the Earth and equivalent to the atmospheric downflow; i.e. drift now includes a vertical component — sinking air.
When the aircraft flies out of the downflow it will again momentarily maintain its flight path relative to the Earth. During that time, the effective airflow around the wings will no longer be directly aligned with the flight path but will have acquired a vertical component opposite to that at entry. The aoa, and consequently CL, will increase. This produces a momentary increase in wing loading, the airframe will experience a positive g load and the pilot will feel the seat pushing up before the aircraft is finally re-established in level flight.
A reversed sequence is applicable when encountering upflow; thus encountering changes in vertical flow causes momentary changes in aoa and wing loading, with some variation in the vertical profile of the flight path.
If an aircraft is flying straight and level and suddenly encounters a head-on increase in wind speed, then due to its inertia the aircraft will momentarily maintain its velocity (and flight path) relative to the Earth. Thus there will be a momentary increase in air velocity over the wings with subsequent increase in lift. The aircraft will rise until the inertial effects are overcome, then the aircraft restores itself to straight and level flight at an altitude a little higher than previously. Similarly, if the aircraft encounters a head-on decrease in wind speed, then lift will momentarily decrease and the aircraft will sink until the inertial effects are overcome.
Thus encountering changes in horizontal flow causes momentary changes in lift with consequent variation in the vertical profile of the flight path. The foregoing is just illustrative, because wind shear events are always a combination of speed variations and three-dimensional variations in direction.
Various scenarios have been outlined above where the aircraft could be flying with little margin between effective and critical aoa; it is on occasions like these that Murphy's Law springs into action. What can and will go wrong at those worst possible times is an encounter with wind shear that suddenly increases the effective aoa of the wing and instantly switches on a stall/spin event.
A more detailed document about coping with wind shear and turbulence' is available in the 'Decreasing your exposure to risk' guide.
( The next section in the airmanship and safety sequence deals with flight at excessive speed )