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Decreasing your exposure to aerodynamic risk
4. Don't land too fast in an emergency
Rev. 7a — page content was last changed 14 June 2012
AUTHOR'S NOTE. The Système International d'Unités [SI] basic units enable simplified calculations and are used for measurement in this and the Flight Theory Guide. The basic units used are:
In systems other than SI the kilogram is sometimes used as both a unit of mass and a unit of force (kgf). SI forces, including weight, are expressed in newtons [ N ]; 1 N is the force required to give a mass of 1 kg an acceleration of 1 m/s².
Acceleration or deceleration is expressed as metres per second per second [ m/s² ]. The nominal acceleration due to gravity [ g ] is 9.806 m/s² or, equivalently, 9.806 newtons of force per kilogram of mass.
So weight expressed in kg is normally converted to newtons when multiplied by g [ 9.806 ]; i.e. one kg weight = 9.806 N. For ease of mental calculation I have used a conversion factor of 10, where 1 kg weight = 10 N force.
In this document kinetic energy is expressed in newton-metres [ N-m ] rather than the equivalent joules.
Forced landings in recreational aircraft — due to engine/propeller failure or fuel starvation, exhaustion or contamination — are certainly not uncommon; but our pilots cope well and, in terms of injury, recreational aviation forced landings are generally uneventful. But occasionally something goes wrong. Light aircraft accident statistics from the US indicate that the most prevalent cause of a forced landing gone wrong is because the approach is too fast, leading to a heavy impact perhaps followed by a bounce and capsize.
Could this happen to you?
While flying at 3000 feet near Rochester, Vic in the company of two other aircraft, the pilot of the Corby Starlet reported that his engine had stopped.
Note: the kinetic energy of a body is due to its spatial motion and equals ½ mass × speed squared ( ½Mv² — I have used the uppercase M as the symbol for mass to distinguish it from the metre). In aviation when we discuss energy management the aircraft speed (in the equation KE=½Mv²) is that which is relative to the air; i.e. the true airspeed. For the purpose of measuring the work that has to be done to bring the aircraft to a halt on the ground — which equals the kinetic energy relative to the ground — the speed is not airspeed but the velocity that is the resultant of groundspeed and rate of descent. So, touching down into wind will make a big difference to the kinetic energy level of the horizontal component of the aircraft's velocity.
In nil wind conditions the kinetic energy of a 270 kg gross weight aircraft touching down at a speed of 30 knots (15 m/s) is ½ × 270 × 15 × 15 = 30 000 newton-metres [N-m or joules]. Whereas that of a 540 kg gross weight aircraft touching down at 45 knots (22.5 m/s) is 137 000 N-m, nearly five times greater. This underlines the fairly obvious expectation that very light aircraft landing at slow speeds have very much less kinetic energy to be dissipated.
Correct touchdown is the most important survival skill in a forced landing and the touchdown velocity is a critical factor. For example, if the 270 kg aircraft's ground speed was reduced by 7 knots (25% reduction) to 11.5 m/s, because of landing into wind, then the kinetic energy would be reduced by 40% to 18 000 N-m. On the other hand if that aircraft was landed downwind then ground speed would be 37 knots (18.5 m/s) and the kinetic energy to be subsequently dissipated would be 46 000 N-m — 2.5 times greater than landing into wind. The landing ground roll, on a smooth unobstructed surface, would also be about 2.5 times greater. So, there is a very significant advantage in landing into wind but perhaps other conditions, such as the clear landing distance available, may negate this.
Light aircraft accident statistics in the US indicate that the most prevalent cause of a forced landing gone wrong is because the approach is too fast and too high, leading to a hard touchdown followed by a bounce and capsize. This is probably because of a tendency to add a 'safety' margin (5–10 knots) to the optimum glide speed. The second most common factor is the natural tendency, when faced with some unexpectedly hostile terrain or the inability to clear an obstacle, to 'stretch' the glide distance by raising the nose — this may then lead to an uncontrolled impact in a most unfavourable attitude. Similarly when faced with an obstacle such as a powerline many pilots choose to pull up over it rather than taking a possibly safer path under it. Keep in the forefront of your mind, a controlled collision with an object is far preferable to an uncontrolled stall 50 feet above the surface — the latter generally results in total destruction.
The problem with always using the best glide speed for distanceFollowing power loss the importance of establishing the aircraft at the best glide speed for distance [Vbg], appearing in the aircraft flight manual or pilot's operating handbook [POH], is emphasised in training and in the text books. This emphasis is valid to the extent it provides a reasonably safe initial flight speed to attain and hold whilst ascertaining the situation, planning appropriate actions and subsequent manoeuvring into the final approach position. In simulated engine failure procedures Vbg is often used throughout the approach simply because it is safer to do so; but it may not be best practice for the real thing. See 'V-speeds' for an explanation of the various codes.
Note: the Vbg stated in the POH is for MTOW and should be decreased by half the percentage reduction in aircraft weight from MTOW — and of course the Vs1 and Vso stall speeds decrease in the same way. For example when there is no passenger in a two-place aircraft gross weight might be 16% below MTOW thus Vs1/Vso and Vbg (and Vmp below) are all reduced by 8%. So if the POH states Vs1 is 40 knots and Vbg is 60 knots but actual operating weight is 16% below MTOW then adjusted Vs1 and Vbg are 37 knots and 55 knots respectively.
There is often an impression that in an emergency the pilot should peg Vbg and stay with it otherwise the consequences may be dire. (This concept possibly pre-supposes that a reasonable landing site is always at extreme range and that Vbg is a fixed value.) What may not be mentioned is though Vbg provides the lowest glide angle (the flattest path and hence the longest air distance), it provides neither the lowest forward speed nor the lowest rate of sink i.e. the lowest kinetic energy. (The term 'rate of sink' is synonymous with 'negative rate of climb'.) Airspeeds lower than Vbg should generally be used when in the final approach stages in a real forced landing.
Vmp — the speed for minimum rate of sinkWhen close to a possible landing site, Vmp — the minimum power (i.e. drag × speed) or minimum rate of sink airspeed — is the speed that will provide the greatest time to survey possibilities. It is also the speed providing minimum kinetic (i.e. impact) energy conditions. The airspeed/sink rate polar curve diagram at the left is a generalised plot of the relationship between rate of sink and airspeed when gliding an erect light aircraft in still air with the propeller stationary (a windmilling propeller increases drag); it is essentially an inverted power curve. Stall point is shown at Vs1. Vmp is at the highest point of the curve. The best distance glide speed is ascertained by drawing the red line from the zero coordinate origin tangential to the curve (i.e. just touching); the point of contact is where the ratio of rate of sink to airspeed is at a minimum and Vbg is directly above that contact point. Also the angle between the red line and the horizontal is allied to the angle of descent and it is obvious that Vbg occurs at the smallest possible descent angle, though it can be seen that even in nil wind conditions Vbg is not a clearly defined point value; rather, it's the mid-point of a speed range for maximum glide distance.
It is apparent from the curve that any glide speed between Vmp and Vbg will provide a lower forward speed than Vbg, together with a slight reduction in rate of sink. Of course the glide path will be steeper, thus distance achieved from any particular height will be less than that achievable at Vbg. For example with Vbg of 60 knots (30 m/s) and a sink rate of 3 m/s an aircraft at a height of 60 metres would remain airborne for 20 seconds and travel forward 600 metres in nil wind. At Vmp of 50 knots (25 m/s) and a sink rate of 2.75 m/s the same aircraft would remain airborne for 22 seconds and travel forward 550 metres. (To convert feet per minute to metres per second divide by 200.)
At speeds greater than Vmp there is the possibility of converting glide momentum into height maintenance for a period. However, at Vmp or lower, there is no possibility of converting glide momentum into short-period maintenance of height; any control change will result in an increased sink rate.
In the diagram the Vmp is shown at around 1.2 times Vs1 and Vbg around 1.4 Vs1. The angle of attack at Vbg may be around 4–5° and perhaps 7–8° at Vmp. The increasing aoa at the sub-Vbg speeds reduces the safety margin between flight speed and stall speed so, at low altitudes, airspeed should only be reduced to Vmp in a stabilised approach after all significant manoeuvring is complete and surface obstructions are apparent. Descent at Vmp in poor visibility lessens impact a little if surface or obstruction contact is inadvertently made before flaring. In turbulent conditions the pilot must balance the possible safety of a higher airspeed against the higher impact forces brought about by that extra speed. We discuss the effects of low-level turbulence and wake vortices in 'Wind shear and turbulence'. Also pilots, particularly of low-momentum recreational light aircraft, should be aware that if a wing tip is first to make contact at low forward speed there is a possibility of cartwheeling.
The penetration speedMuch is said about the importance of maintaining the 'best gliding speed' during the descent but what is important is to maintain an optimum glide speed; a penetration speed that takes atmospheric conditions into account; for example, sinking air or a headwind. The gliding community refers to this as the speed to fly so that the ratio of rate of sink to ground speed is at a minimum. The normal recommendation for countering a headwind is to add one third to one half of the estimated wind speed to Vbg, which increases the rate of sink but also increases the ground speed so the ratio will again approach the minimum. For a tailwind, deduct one third to one half the estimated wind speed from Vbg, which will reduce both the rate of sink and the groundspeed, and of course there is a limit to any airspeed reduction. Bear in mind that, for safety, it is better to err towards higher rather than lower airspeeds.
To illustrate the speed to fly, the polar curve on the left indicates the optimum glide speed when adjusted for headwind, tailwind or sinking air. For a tailwind the starting point on the horizontal scale has been moved a distance to the left corresponding to the tailwind velocity. Consequently the green tangential line contacts the curve at an optimal glide speed that is lower than Vbg with a slightly lower rate of sink. This is the opposite for a headwind — shown by the purple line. For sinking air the starting point on the vertical scale has been moved up a distance corresponding to the vertical velocity of the air. Consequently the pink tangential line contacts the curve at a glide speed higher than Vbg.
Kinetic energy increases exponentiallyYou may find pencil and paper a helpful back-up from here.
The kinetic energy [KE] of a body is due to its motion and equals ½ mass × speed squared [½Mv²], thus as speed changes linearly KE changes exponentially.
For example a 540 kg aircraft with a stall speed of 42 knots CAS might have a Vmp around 50 knots CAS, so in nil wind conditions the KE when touching down at 50 knots (about 25 m/s) is ½ × 540 × 25 × 25 = 169 000 newton-metres [N-m]. Vbg for the same aircraft might be 60 knots CAS (near enough to 30 m/s) and touchdown KE at that speed would be ½ × 540 × 30 × 30 = 243 000 N-m; a 44% increase in energy at touchdown because of a 20% increase in speed. The distance required to bring the aircraft to a safe stop is directly proportional to the touchdown energy, as is the impact energy, if the aircraft and occupants come to a premature halt.
The pilot of a low-momentum recreational light aircraft is exposed to much less KE. For example consider a 270 kg aircraft with a stall speed of 28 knots, Vmp 34 knots (17 m/s) and Vbg 40 knots (20 m/s). At Vmp touchdown KE = ½ × 270 × 17 × 17 = 39 000 N-m, while at Vbg touchdown KE = ½ × 270 × 20 × 20 = 54 000 N-m. The aircraft weight is half that of the heavier aircraft but impact KE is one-fifth.
Kinetic energy is substantially affected by wind velocityUsually when we discuss in-flight energy management the aircraft speed (in the equation KE=½Mv²) is that which is relative to the air — the true airspeed. However, for the purpose of measuring impact energy or the work that has to be done — i.e. the energy expended — to bring the aircraft and occupants to a stop, the speed is not true airspeed but the velocity resultant of ground speed and rate of sink; thus touching down into wind with a low sink rate will make a very favourable difference to energy level.
If the 270 kg aircraft's Vmp ground speed is reduced by 6 knots (18% reduction) to 28 knots (14 m/s) by landing into a 6-knot wind then KE is reduced 33% to 26 500 N-m. In the same conditions if that aircraft was landed downwind then ground speed would be 40 knots or 20 m/s, and the KE to be subsequently dissipated would be 54 000 N-m — the possible impact would be twice as great as landing into wind — and 6 knots is just a pleasant light breeze. The figures for the 540 kg aircraft landing at Vmp with a 6-knot headwind and tailwind are 131 000 N-m and 212 000 N-m respectively; quite a difference from the nil wind impact of 169 000 N-m even with just that light breeze. (It also underlines the fairly obvious expectation that low-momentum recreational light aircraft landing into wind at minimum speeds don't have a lot of energy.) There is a very significant advantage in a low-speed into-wind forced landing, but other conditions — such as clear landing distance available — may modify this.
A worse case is when the aircraft touches down both fast and downwind. For example if our 540 kg aircraft touches down somewhat fast, say 65 knots CAS, with a 6-knot tailwind then the KE at touchdown is ½ × 540 × 35.5 ×35.5 = 340 000 N-m; twice the energy at 50 knots in nil wind conditions. It also reinforces the point that, with the ever-present possibility of engine stoppage or degraded performance, it's a silly decision to take off downwind, no matter how long the distance available — unless it's a one-way downhill strip.
But let's look at the case where only the pilot is on board, our aircraft weight is 16% below MTOW at 454 kg, and the pilot lands at a Vmp reduced by 8% to 46 knots into a 6-knot headwind. Then the KE at touchdown is ½ × 454 × 20 ×20 = 91 000 N-m, a very significant decrease from the previous 340 000 N-m.
High density altitude adds kinetic energyDensity altitude also affects KE because TAS is about 1.5% higher than CAS for each 1000 feet of density altitude. For example the density altitude at Armidale, New South Wales (elevation 3500 feet) with temperature of 30 °C would be around 6000 feet, which means that the TAS will be about 9% greater than CAS. So using the preceding example of 540 kg touching down at 65 knots CAS, the TAS at 6000 feet density altitude would be 71 knots. So adding the 6-knot tailwind for a touchdown groundspeed of 77 knots the KE is then ½ × 540 × 38.5 ×38.5 = 400 000 N-m. The density altitude adds 60 000 N-m; quite a lot of energy that you need to be aware of. Always bear in mind that Australian climatic conditions are significantly warmer than the latitude 40°– 45° N climate on which the International Standard Atmosphere (and consequently airspeed indicator dial calibration) is based. In summer day temperatures the airfield density altitude would be from 2000 feet to 3500 feet greater than the airfield elevation. See The Civil Aviation Safety Authority declared density altitude charts. You must expect that TAS is significantly greater than IAS.
Doing the KE calculations for the aircraft you flyKinetic energy calculation is easy if you first halve airspeeds and windspeeds to convert from knots to metres per second and express the operating weight (mass really) in kilograms; i.e. KE= ½ operating weight × groundspeed squared — the result is in newton-metres or joules if you prefer.
Be well preparedThere is some element of chance in every emergency landing (Murphy's Law suggests that what can go wrong will go wrong, and at the worst possible time), but being well prepared and keeping cool (but not so cool that you freeze up) are by far the most important factors in deciding the outcome. A safe outcome greatly depends on placing occupant safety before airframe loss, knowing your aircraft, and on fully controlling the approach and landing/crash. The latter depends, firstly, on carefully flying an approach (having selected the best readily attainable landing site) that finally minimises the forward speed and the sink rate at a nose up/wings level touchdown; thus minimising impact angles and better distributing the initial impact forces. It is best to dissipate excess energy as drag while still in the approach by using full flaps or side-slipping; both together if acceptable, though some aircraft are downright dangerous if side-slipped with full flap. Secondly, plan the direction of the subsequent ground travel so the remaining energy is substantially dissipated before the occupant enclosure hits something large and unyielding, or a barbed wire cattle fence, or the aircraft overturns. In short; you must plan and totally control the flight all the way into the potential crash.
So what final approach speed should be chosen?Comment from CAR 35 engineer Dafydd Llewellyn: "It is often difficult to hold a steady speed on short final, even if there is no wind shear; it is not commonly appreciated that most aeroplanes have considerably degraded longitudinal stability in the landing configuration. Firstly, they have a greatly increased pitching moment coefficient due to flaps etc and secondly, they are close to or below the minimum drag speed — and both of these effects tend to make them speed-unstable. This is reflected in FAR 23.175, which requires a positive stick-force gradient in the landing configuration, with power off or only with sufficient power to maintain a 3 degree angle of descent. Try it with more power that this, and you will often find — especially if the cg is somewhat aft — that the thing cannot be trimmed to any given speed; it's actually negatively stable (this is the reason such care is needed in a baulked landing). If disturbed in speed, by turbulence for example, it will continue to slow down or speed up. The stick force versus speed gradient may be positive below this power, but it's usually pretty small, so the thing is a lot less stable than in cruise. This is why it's so critical to keep an eye on the ASI, in those phases of flight, and almost certainly the root cause of the high proportion of accidents that occur in the landing and aborted landing phases. Most pilots are not aware of this."
After considering the trade-off between adequate controllability, margin from stall, and excess kinetic energy the optimum speed over the fence is probably 1.3 times Vso (the stalling speed in the landing configuration at the particular operating weight); it will be 3– 4 knots faster than the corresponding Vmp. Add no more than 5 knots in gusty conditions and resist any compulsion to add any additional 'safety' margin. Make sure Vso and the approach speed are both determined in terms of CAS rather than IAS, as the ASI may have significant, but not comparable, position error corrections at Vso and 1.3 × Vso.
Flare with care!In the final approach, the aircraft aoa may be somewhere between 5° and 8°, and the flare to arrest the rate of descent may raise aoa close to critical, and the increased drag will slow the aircraft quickly — thus a rapid increase in rate of sink will follow. It is usually essential that the aircraft is flared gently, smoothly and at the height appropriate for a consequent near-stalled or fully stalled touchdown — but see 'Alighting in tree tops'. In some aircraft the loss in slipstream (compared with that from an idling engine in a normal landing) may significantly reduce the elevator authority and thus the stick must be pulled back further to flare successfully. If possible, correct crosswind drift before touchdown so that side-impact forces are reduced.
(The term 'dead-stick landing' — an 'in-word' used to describe a forced landing following complete loss of power (or a training exercise where the engine is shutdown on approach) — originated during World War I. It is thought to describe the decreased elevator authority following loss of the slipstream in some of those early aircraft. Nowadays the use of the term is deprecated — no one describes a normal sailplane landing as 'dead-stick'.)
The possibilities of landing safely downslope may range from difficult to impossible. A strong headwind may make a downslope landing feasible though it is difficult to judge the degree of slope until you are close to the surface and thus committed.
Moderate upslope is good if the pre-touchdown flare is well judged. There is a much greater change in the flight path during the flare; for example if the upslope has a one-in-six gradient (about 10°) and the aircraft's glide slope is 6° then the flight path has to be altered by 16° so that the aircraft is flying parallel to the upslope surface before final impact. A higher approach speed is needed because the increased wing loading during the very pronounced roundout/flare (a turn in the vertical plane) increases stall speed. If the wind is upslope then a crosswind landing may be feasible.
Load factorsThe KE of a 540 kg aircraft touching down at 45 knots groundspeed is 137 000 N-m. If, in a normal landing on a prepared airstrip, the aircraft is uniformly decelerated to stop 100 metres from touchdown then the deceleration force — the total forces applied to stop the aircraft — is KE/distance (N-m/m) = 137 000/100 = 1370 newtons.
The deceleration forces place a load on the aircraft and the airframe transfers a load to its occupants. It is usual to compare such load factors in terms of the non-dimensional 'g ratio' calculated, in this case, by dividing the uniform deceleration force by the aircraft's weight in newtons (its mass in kg multiplied by the acceleration of gravity — close to 10 metres per second per second) which, in the example, would be 540 × 10 = 5400 N. Thus the horizontal deceleration factor is 1370/5400 = 0.25g — just a slight load which probably wouldn't register with the occupants; it can also be seen that the aircraft is decelerating at the rate of 2.5 m/s² (i.e. 10 × 0.25). If the family sedan is brought to a controlled stop under heavy, sustained braking the occupants would be unlikely to experience more than a 1g deceleration.
If the aircraft, under uniform deceleration, came to rest in 10 metres then the deceleration force is 13 700 newtons and the load factor is 13 700/5400 = 2.5g. But if uniformly brought to a halt in 5 metres by landing in dense, light scrub then the deceleration force is 27 400 newtons and the forward deceleration load factor is 27 400/5400 = 5g. Of course the aircraft's velocity at impact includes a vertical component, but we will look at that later.
It is unlikely that in the early stages of ground travel, after a planned and controlled forced landing approach, a light aircraft would slam head-on into a large unyielding object, such as a large tree trunk or a very large boulder. On the other hand, it is also unlikely that an aircraft will be uniformly decelerated — the surface conditions may be such that varying impact loads (from contact with brush, saplings, stumps, roots, stones, holes, furrows) are intermittently applied to the airframe and occupants from near touch-down until coming to a stop, making it impossible to control direction, or even keep feet on the rudder controls. These multiple impacts result in a series of peak deceleration loads applied for very short periods, probably a few hundredths of a second, and felt as severe jolts; many of these will have a sideward load component.
A note of caution. A firm touchdown with no float in ground effect is the aim but if you are forced into dissipating excess airspeed by holding off half a metre above the surface and the undercarriage strikes a rock or stump then the consequences are likely to be more traumatic than if you had pegged it down earlier at the higher speed and then run on into the object. Ground-assisted deceleration is better than ground effect float. The consequences may also not be good if you are holding off and pull back on the stick to avoid tripping over an obstruction. So if the terrain is cluttered with unavoidable obstructions of that nature then it may be best to place the main wheels on the ground earlier even though the velocity, and thus kinetic energy, is higher. If the distance between relatively high obstructions is less than the wing span try to steer a course that will equally distribute the impact forces on each wing so that the cockpit enclosure is not spun around into something unyielding. Of course if landing on a clear surface the aircraft will slow faster with its wheels on the ground than if held in ground effect, but the faster the speed at touchdown the greater the possibility of bouncing.
Airmanship is about making and implementing the wisest choice in such difficult situations. For example, when faced with an obstacle such as a rural powerline many pilots might choose to pull up over it rather than taking the possibly safer path under it. That natural tendency, when faced with some unexpectedly hostile surface or the inability to clear a previously unseen obstacle, to 'stretch' the glide distance by raising the nose excessively, may lead to an uncontrolled impact in a most unfavourable attitude. A controlled collision is far preferable to control loss 50 feet above the surface — the latter generally results in severe injury or worse. It is probably better to put it between obstacles that are closer together than the wingspan, than to stretch the glide and then drop-in nose first. Protect the occupant zone by sacrificing the wing structure. It is best to avoid higher-speed impact with a strong, barbed-wire fence by ground looping, if possible.
The following is an extract from a detailed incident report by a Boorabee pilot who did everything right when the engine packed up:
"... the positioning and timing seemed to come together almost at a crawling pace, but it must have been just a few seconds. The turn onto final had to be made at low level so I made a definite intent to ensure good speed into the turn. Turned onto the final approach high enough to clear the barbed wire fence and fast enough to have full control and touched down beyond the fence parallel with the ploughed furrows ... recall pushing the nose down just enough to ensure longest distance possible for ground roll as the dirt paddock would retard the motion a lot faster than flaring and easing onto the ground halfway up the paddock ... noisy and bumpy ride with underside of pod sliding along top of furrow ... ground looped to halt the aircraft when getting close to the end fence and into cross ploughing ..."
Tailwheel aircraft have an advantage over nosewheel aircraft on rough ground. The tailwheel is likely to be pulled over obstacles but even if it is knocked off, the aircraft remains stable and is converted into a true 'taildragger' with its built-in arresting effect. On the other hand recreational light aircraft nosewheel structures are not very strong and if a nosewheel can't be held off then it tends to be pushed into holes and may not ride across or over obstacles — the consequences may be loss of the nosewheel strut and of aircraft ground stability. In the worst case the aircraft nose may dig in and the aircraft flip onto its back; in which case ensure you are in an aircraft where the design includes a structure that rests on itself rather than the occupants heads, and there is an escape route from the inverted cockpit. Some aesthetically pleasing bubble canopies with unobstructed views may be death traps; steel roll-cages/bars or high-wing aircraft are safer. The accident/incident reports indicate a surprising number of aircraft end up inverted following a forced landing or other landing mishap, but certainly for the high-wing aircraft the damage to the airframe is generally not total and injuries are low. If your heart is set on a low-wing or mid-wing aircraft first figure how you and your passenger will escape when it's inverted.
Energy absorptionFrom the foregoing it is evident that very little distance is required to bring the aircraft to a safe halt IF the kinetic energy can be dissipated uniformly during ground travel. For example the occupants of the 540 kg aircraft touching down at 45 knots and uniformly brought to a stop over 20 metres would experience about 1.25g deceleration. (In the days of heavy piston-engined aircraft conducting carrier landings the arresting load was 2–3g, which was not uncomfortable when well strapped in.)
So where there is no clear, open space to land the aircraft, more or less normally, then an option is to choose an area where the vegetation is of sufficient height and density to absorb much of the kinetic energy and retard the aircraft. If that vegetation is weaker than the aircraft structure so much the better, but the primary consideration is occupant safety so energy absorption by sacrifice of non-vital aircraft structure — i.e. all that outside the occupant zone — is warranted. The requirement of course is to set up the touchdown so the aircraft is moving in a direction where the vital structure is unlikely to slam into an unyielding obstruction at speed.
High and dense crops, sugar cane, brush and light scrub all provide good energy-absorbing properties and good cushioning is provided if the aircraft is put down in the proper nose-high attitude so the impact forces have more spread over the aircraft's under-surfaces, rather than just catching at the undercarriage and overturning the aircraft. But even an unfavourable impact angle may not be particularly dramatic; e.g. here is an extract from a forced landing incident report: "The Jabiru impacted the sugar cane in a 20 degree left wing low attitude and came to rest upright after sliding 20 metres."
Impact forces are less if you touchdown at Vmp or a little higher and then run on into obstructions at the far side of a clearing rather than stall/spin at the near end. The aircraft structure will withstand longitudinal impact forces much better than concentrated lateral impact forces (such as side-swiping a tree trunk), so generally avoid touchdown with substantial drift or slip towards the lower wing, unless you are in a position where the impact loads will be widely spread, as in the cane field landing above.
'Alighting' in tree tops is certainly extremely hazardous and always results in total aircraft write-off. But if the aircraft is flown into a selected, dense crown in a reasonably nose-high attitude (and into wind) — so that some of the initial impact is absorbed by the under-surfaces of the fuselage, tailplane and wings — then the hazard to occupants may be reduced. It is important that the aircraft is not stalled above the tree crowns, because of the possibility of the nose and/or wing dropping into the crown before impact; rather, it should be flown into the canopy at the minimum sink glide speed. The greatest hazard may come from a subsequent slide, of the fuselage remains, from the tree.
Easier said than done, but certainly the aircraft must be flown all the way into the crash. The following is a summary of an accident report; the aircraft was a Skyfox CA22, the pilot had 16 000 hours experience and rescue was fast:
'While on cruise at 1100 feet agl the engine failed completely. The pilot set the aircraft for a forced landing into heavily timbered terrain and transmitted two mayday calls. The second call was answered and he gave details of his situation and position. He then maintained control of the aircraft until it touched the top of the tree canopy where he flared steeply, as the aircraft entered the trees, to present the underside of the aircraft for speed reduction and impact damage minimisation.
The following is a report from a Jabiru passenger in another treetop alighting. The pilot, using a runway downslope advantage, took off toward the north with a five to eight knot south-east wind. The pilot had just turned crosswind at about 350 feet agl to avoid a noise-sensitive residential area when the engine died. There was no clear area within gliding distance — only a full expanse of mature eucalyptus trees some 20 to 40 feet high. The pilot lowered the nose to maintain 65 knot best glide speed and turned the aircraft slowly into wind for the landing. The passenger (an RA-Aus CFI along for a ride home) takes up the story.
"The trees were rapidly getting closer. They changed from a mass blur to individually defined trees and I prepared myself for the worst. The pilot tried the engine again unsuccessfully, then turned fuel and magnetos off. He started making a mayday broadcast, but before the transmission was complete, there were sounds of trees and aircraft breaking bits off each other as first contact was made.
Here is a witness comment in an RA-Aus double fatality report:
"The take-off on the 1200 metre runway was sluggish and the engine was misfiring. (Fuel was later found to contain a substantial amount of diesel!) The Murphy Rebel continued climbing and turned toward the east then right again onto a low downwind leg. In the mid-downwind position a loud bang was heard and the aircraft then descended flatly ... it appeared that the aircraft had a slow forward velocity but a high rate of descent when it struck ... came to rest 16 metres from the initial impact ... cockpit area and engine bay badly damaged by fire ... the reason for the inadequate airspeed in the forced landing may have been false horizon effect as the aircraft was approaching rising ground."
What is most concerning, from a perusal of the accident/incident reports, are the occasions where a forced landing precursor has been the engine displaying ample warning of a problem before take-off or while in the circuit area (as described above) but the pilot seems to have been hoping that it would fix itself and opted to press on — why? What could possibly be gained?
What this means is that it is possible that a normal category aircraft, touching down at 45 knots, running into something sufficiently yielding (for example scrub and small saplings) and decelerating at 9g will come to a halt over a distance of just 3 metres (during a time of one second) with the occupants only suffering body bruising from properly fitting harnesses and perhaps some minor injuries to the legs and arms; provided the occupant zone remains reasonably intact and nothing intrudes into it. Of course, the rest of the aircraft itself will not come out of it so well. Many of the top-end recreational light aircraft fit into that FAR Part 23 'normal' category. By the way, rocket-deployed aircraft emergency parachute recovery systems generally aim for a maximum descent rate of six metres per second (1200 fpm).
Momentum and occupant safetyThe airframe density per cubic metre of finished structure is generally homogenous but the engine, fuel and occupant bodies have higher densities — thus a higher momentum (momentum = M × v) than the rest of the aircraft — and should all be properly restrained. The engine by very strong mountings, particularly if mounted behind the occupants; and the occupants by an adequate seat/restraint system so that the core fuselage structure, engine and occupants all decelerate at the same rate even though a considerable part of the aircraft may be sacrificed along the way. Re-read the preceding description of alighting into tree tops. There should be no loose objects in the aircraft — they will become a harmful missile.
If the adult body is properly restrained, human organs and their attached blood vessels, will cope with transverse deceleration loads very much greater than 20g — applied for short periods. However the spinal column has a much lower tolerance to downward deceleration loads; i.e. loads applied parallel to the spinal column. In this aspect the skeletal structure is much weaker than the aircraft under-structure and downward deceleration loads may result in serious spinal injury — thus the importance of minimising the vertical velocity at impact.
Great care must be taken with child restraint systemsInfants cannot be carried in ultralight aeroplanes and it is most unwise to carry small children (say under 15 kg) as a passenger; there is no satisfactory restraint system. They cannot extricate themselves and they cannot go for help. Children weighing between 15 and 25 kg should use a government-approved child restraint system [CRS]. The US approved types have 'This restraint is certified for use in motor vehicles and aircraft' printed on them. The CASA advisory publication CAAP 235-2(1) contains more information on CRS standards.
Children over 25 kg and 145 cm tall might be restrained safely within a normal fully adjustable four-strap (preferably five-strap) seat harness. A booster cushion might be used if they don't quite make the height. Safest approach is to never carry children who are not old and strong enough to extricate themselves safely (and unaided) from the harness and a wrecked cockpit.
Personal protection equipmentPerusal of the accident/incident reports for 3-axis aeroplanes shows that rather few pilots or passengers were wearing head and face protection at the time and no doubt some are now wondering why they chose not to wear personal protection. Using the correct type of helmet and inner energy-absorbing pads will provide considerable protection from serious head injury if the occupant zone be deformed, intrusions occur or the restraint system fails. Helmets also reduce the chances of being knocked unconscious in a wreck that subsequently catches fire.
There are quite a few high quality sport aviation helmets available, though they are not cheap. Two helmets with face vizors and an intercommunication facility may cost $1500 to $2000. Children must always wear an appropriate helmet and liner. Why do parents who won't allow their child to ride a bicycle in the backyard without a helmet take them flying with no such protection?
Generally the structure of the under fuselage does not incorporate a crush zone to provide some occupant protection from spinal injury, so it is advisable that aircraft with a retractable undercarriage should be landed with the gear down. That will absorb quite a lot of vertical load before collapsing. Some aircraft seats may be designed as a deforming, load-absorbing system in which case it is important that nothing is stowed beneath the seat. In a low-wing aircraft the pilot/passenger seats are probably directly over the main spar — which is obviously built not to collapse — so, if there is no crushable structure between the seats and the spar, the occupants' spinal columns will be directly exposed to the full vertical deceleration. In that case ensure the seat includes something similar to a body-conforming, energy-absorbing, 3-inch thick seat cushion laminated from three layers of Confor urethane foams or similar.
Nearly all light aircraft have a fixed undercarriage; there may be a problem with some low-wing aircraft fitted with in-wing fuel tanks if the collapse of the undercarriage causes penetration of those tanks.
In a fixed, high-wing aircraft (excluding minimum aircraft) the overhead structure provides a crush zone sufficient to allow exit room from the cockpit if the aircraft pitch-poles onto its back. Also the cockpit is fitted with doors that can generally be forced open when the fuselage is inverted; even when rolled-up into a ball, as seen in this Cessna Skyhawk image.
In some low-wing, bubble cockpit canopy aircraft the cockpit area may be the weakest part of the fuselage structure. If such aircraft are involved in a forced landing where the aircraft nose digs in, the occupant zone may distort sufficiently to allow failure of the occupant restraint system; also there have been cases where, during a rapid deceleration, the outward buckling at the cockpit allows the momentum of the rear fuselage to swing itself over the cockpit enclosure. There may be insufficient roll-bar or other structure — except possibly the vertical stabiliser — to prevent crushing of the cockpit canopy in a capsize. Even when a strong roll-bar bow is incorporated, if capsize occurs, unaided exit of the pilot/passenger may be near impossible until the aircraft is lifted. If the aircraft is fitted with a canopy that can be jettisoned in flight, make sure the front canopy bow cannot drop down during the jettison process and scalp the occupants. All occupants should wear safety helmets in aircraft with bubble-type canopies and consider stowing a suitable pry bar/escape axe/fireman's axe in the cockpit.
The parachute canopies are circular with a central vent (quite unlike a parachute wing), have a diameter around 12 metres for a 544 kg aircraft or 10 metres for a trike, and the length of the harness and lines from the aircraft to the canopy rim would be around 15–20 metres. So, the aircraft may be oscillating on quite a long arm. This oscillation will be greatly increased in gusty conditions as the canopy has a lot less inertia than the aircraft — as powered parachute pilots will be aware.
On deployment of the parachute the aircraft may initially experience a deceleration around 3—5g depending on the aircraft's speed, so it is advisable that four-point occupant harness systems are fitted. From activation, it will take perhaps two or three seconds for the parachute to fully open then another four or five seconds for the aircraft to stabilise in the appropriate attitude (wings level and perhaps slightly tail-down to provide additional energy absorption). The aircraft would descend at a target maximum rate around 6 metres per second (1200 feet per minute), at which vertical velocity the aircraft will impact the surface. The undercarriage system is probably designed to absorb energy equivalent to around 3g. The balance of the kinetic energy would have to be absorbed by collapse of the undercarriage and other structural crushing. The horizontal velocity at impact will be the wind velocity near ground level.
Depending on aircraft weight, speed and parachute type the loss of height from activation to stabilised descent is likely to be 100–300 feet if deployed when the aircraft is in a reasonably level attitude, so deployment is best activated above that height. However, in emergency conditions the aircraft is not usually in a reasonably level attitude, quite the reverse — it may be steeply banked and nose pitched down, even inverted, so the safe height may be much greater than 300 feet. For tractor-engined aircraft the rocket deployed recovery system is usually installed in the fuselage with the rocket's ascent path slanted at a rearward angle to the aircraft's longitudinal axis. But for a trike, it may be deployed sideways or at 45° to the longitudinal plane; so, there is much to be considered when estimating safe height for deployment. If the aircraft is not established in the appropriate attitude, with the minimum vertical velocity at impact, it is likely that damage will be severe, a combination of the wind velocity and, for example, a nose-down attitude could capsize the aircraft and perhaps drag it a short distance. In an emergency situation below a minimum height the only feasible action may be to activate the recovery system.
It would not be the usual practice to deploy a recovery parachute in a normal forced landing, but in a limited space it might be used successfully as a braking parachute if deployed just after touchdown when the aircraft's momentum is low. (When a parachute is deployed above the aircraft it acts as an 'air anchor' and the aircraft's momentum will tend to swing the aircraft upwards which, when near the surface, may then follow with a tail-slide into the ground.) After use the complete system must be returned to the manufacturer's agent for restoration; substantial cost will be involved.
Safety pins should be disengaged before take-off and re-engaged after landing; in a low-level in-flight emergency there will be no time available to fiddle with safety pins.
The rocket propellant is quite stable; however, it is possible that the ignition system can be activated accidently if the airframe is distorted in a forced landing or a ground accident. An armed rocket is a serious safety risk to anyone attending the site of an accident, so hazard identification and warnings must be provided on the external surfaces of the aircraft.
Passengers must be fully informed on both the operation of the system — should the pilot suffer inflight incapacitation — and the dangers of inadvertment activation.
Read the CASA bulletin AWB 25-003 'Inadvertent Activation of Rocket-Deployed General Aviation Recovery Device (GARD) During Maintenance'.
The next article in this series discusses the worst situation for engine failure — that occurring soon after take-off.
'Decreasing your exposure to risk' articles
| Introduction and contents | Recent RA-Aus accident history | Don't fly real fast | Don't stall and spin in from a turn |
| Don't land too fast in an emergency | Engine failure after take-off | The turn back: possible or impossible — or just unwise? |
| Wind shear and turbulence |
Copyright ©2007–2012 John Brandon [contact information]