Coping with emergencies
Deceleration forces
Rev. 9c — page content was last changed 10 December 2009
consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au
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Coping with emergencies |
Deceleration forcesRev. 9c — page content was last changed 10 December 2009 consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au |
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:
Acceleration or deceleration is expressed as metres per second per second [ m/s² ]. The acceleration due to gravity [ g ] is 9.806 m/s². In systems other than SI the kilogram is sometimes used as both a unit of mass and a unit of force, particularly weight. SI forces, including weight, are in newtons [ N ]; 1 N is the force required to give a mass of 1 kg an acceleration of 1 m/s². 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. In this document kinetic energy is expressed in newton-metres [ N-m ] rather than the equivalent joules. |
2.1 Kinetic energyThe safe outcome of a forced landing depends greatly on controlling the landing, which then depends on an approach that minimises the vertical velocity and the forward (ground) velocity at the selected touch-down position. This is followed by a ground run that dissipates all kinetic energy and minimises the risk of the aircraft hitting something large and unyielding. The final approach speed that will provide these minimum energy conditions is Vmp — the minimum power or minimum rate of sink airspeed — Vmp, being lower than the normal approach speed, reduces the margin between flight speed and stall speed.Note: the kinetic energy of a body is due to its spatial motion and equals ½ mass × speed squared ( ½mv² ). 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 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 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 then leads 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 the 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. |
2.2 Deceleration forcesAs stated above the kinetic energy of a 540 kg aircraft touching down at 45 knots (22.5 m/s) is 137 000 N-m. In a normal (good) landing on a prepared airstrip the initial touchdown produces only slight loads on the aircraft and its occupants. Then aerodynamic drag, surface/wheel friction and judicial use of brakes slow the aircraft at a uniform rate without significant loads being applied — and without deviation in the landing path. In our example if the aircraft is uniformly decelerated to halt 100 metres from touchdown then the deceleration force — the force needed to halt the aircraft — is the kinetic energy/distance (N-m/m) = 137 000/100 = 1370 newtons.Load factorThe deceleration forces place a load on the aircraft and its occupants, and it is usual to express the load factor in terms of 'g'. This is readily calculated by dividing the uniform deceleration force by the aircraft's weight in newtons which, in the above example, is 540 × 10 = 5400 N. Thus the horizontal deceleration factor is 1370/5400 = 0.25g — just a slight load in a normal landing.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 it is uniformly brought to a halt in 5 metres by landing in,say, dense scrub then the deceleration force is 27 400 newtons and the forward deceleration load factor is 27 400/5400 = 5.0g. Of course, the aircraft's velocity includes a vertical component but we will look at that later. It is unlikely that in the early stages of the ground run,an aircraft will be decelerated uniformly — the surface conditions may be such that varying impact loads (from contact with vegetation, stones, animals, plough furrows etc.) are intermittently applied to the airframe and occupants from touchdown until coming to a halt. This makes it difficult to control direction, or even keep feet on the rudder bar. These multiple impacts result in a series of peak deceleration loads applied for very short periods — probably a few one hundredths of a second and felt as severe jolts — and many of these will have a sideward load component. A note of caution. A firm touchdown with no float is the aim, but if you are 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 put it down earlier at the higher speed and then run on into the object. The consequences may also not be good if you are holding off and pull back on the stick to avoid striking 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. 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. 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 braking effect. On the other hand, the nosewheel tends to be pushed into holes, may not ride across or over obstacles, and 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 pitch-pole on to its back; in which case ensure you are in an aircraft where the structure rests on itself rather than the occupants heads, and there is an escape route from the cockpit. 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 the ground roll. For example, the occupants of the 540 kg aircraft touching down at 45 knots and uniformly brought to a halt over 20 metres would experience not much more than 1g forward deceleration. (In the days of piston-engined aircraft conducting carrier landings the arresting load was 2–3 g.)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 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 prime consideration is occupant safety so energy loss by sacrificing non-vital aircraft structure (i.e. 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. Dense crops, sugar cane, brush and light scrub all provide good energy-absorbing properties . Some 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 underside surfaces. The emphasis is on controlling the landing; impact forces are less if you touchdown and then run on into obstructions at the far side of a clearing rather than stall into the trees at the near end; as noted, the aircraft structure will withstand longitudinal impact forces much better than lateral impact forces. 'Landing' in tree tops is certainly hazardous and always results in airframe breakup. But if the aircraft is flown into a selected spreading, dense crown in a nose-high attitude (and into wind) so that much of the impact is absorbed by the underside surfaces of the fuselage, tailplane and wings, then the major hazard to occupants is probably caused by the fall of the occupant zone from the tree tops. It is important that the aircraft is not stalled into the crown of trees, because of the possibility of the nose dropping before impact. Rather it should be allowed to 'mush' down into it with a slightly higher vertical velocity than that appropriate for a surface landing. Easier said than done but, for a copybook demonstration by a recreational aircraft pilot , read 'Engine failure after take-off' in the January – February 2002 issue of Flight Safety Australia. Deceleration effect of sink rateIn a forced landing there is normally no power available to vary the rate of descent or arrest sink, but it is extremely important that the downward component of the aircraft's velocity at touchdown be minimised. So in the last stages of the approach, after all manoeuvring is completed, the airspeed should be reduced to Vmp — the glide speed that provides minimum sink. A high sink rate at touchdown in rough terrain can result in a crash rather than a controlled landing. If the descent rate of a 540 kg aircraft was 500 fpm (2.5 m/s) and this was not arrested in the flare before touchdown, then the kinetic energy of the vertical component of the aircraft's velocity would be 11% (2.5/22.5) of 137 000 = 15 000 N-m. If the undercarriage (which held the aircraft 0.5 m above the ground) collapsed and the downward movement was arrested in 0.5 m then the downward deceleration force is 30 000 newtons and the downward load factor is 30 000/5400 = 5.6 g.Aircraft design regulationsThe Federal Aviation Regulations [FAR] Part 23 lays down some crashworthiness requirements for 'normal' category light aeroplanes to give each occupant every reasonable chance of escaping serious injury when the occupant experiences forward loads up to 9g and sideward loads up to 1.5g. So, in theory, if a certificated aircraft touches down under control and decelerates at a constant 9g forward, the occupants should escape serious injury — provided the lapbelts and shoulder harnesses are used properly. FAR Part 23 also has a 6g downward load requirement.This means that it is possible a normal category aircraft touching down at 45 knots, running into something sufficiently yielding (e.g. scrub or saplings) and decelerating at 9g will come to a halt over a distance of just 3 metres (during a time of 0.8 seconds). The occupants would only suffer body bruising from the harness 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 will not come out of it so well. Many of the top-end recreational aircraft fit into that FAR Part 23 normal category. Minimum ultralights and powered parachutesAt the very light-weight, low-speed (55 knots maximum level flight speed) end of the recreational aircraft spectrum are the home-built single-seat minimum aircraft, the airframes of which are often constructed from aluminium tubing and sailcloth. The design, the structural integrity and the impact resistance of such aircraft will certainly not provide the 9g occupant protection required of the type-certificated aircraft but their kinetic energy, when touching down into wind, is very low — in the range 10 000 to 18 000 N-m. If such an aircraft was uniformly decelerated over 5 metres to a stop, the force would be less than 1.5g. Powered parachutes are in the low end of this minimum category. |
2.3 Occupant safetyAirframe density per cubic metre of structure is generally homogenous. But the engine, fuel and occupants have higher densities — thus a higher momentum (momentum = m × v) than the rest of the aircraft. They should all be properly restrained — the engine by very strong mountings; particularly if mounted behind the occupant zone, and the occupants by an adequate seat/restraint system so that the main fuselage structure, engine and occupants all decelerate at the same rate, even though some parts of the aircraft are sacrificed along the way. There should be no loose objects in the aircraft — they may become a harmful missile.Properly restrained, the human body has coped 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 understructure, and downward deceleration loads may result in serious injury; thus the importance of minimising the vertical velocity at impact. Sling seats, being prone to tearing, do not offer much protection in high vertical load conditions. Use of the correct type of protective helmet will provide some protection from serious head injury if the occupant zone be deformed, intrusions occur or the restraint system fails. Occupant restraint systemThe pelvis/hip girdle is the strongest part of the body structure and the body's centre of gravity lies between the hip bones. The better occupant restraint systems usually consist of a welded steel tubing seat with an aluminium pan and webbing back or a fully moulded fibreglass seat. There should be a body-conforming, energy-absorbing, 3-inch thick seat cushion laminated from three layers of Confor urethane foams; a lapbelt angled to hold the hips into the internal corner of the seat and a shoulder harness system to prevent forward/sideways movement of the upper body. Shoulder harness systems are usually an adjustable webbing strap over each shoulder, which clips to the lapbelt; the shoulder harness may incorporate an inertia reel system. A single diagonal belt, as used in the family sedan, might be used in a minimum aircraft. The following should be noted:
Crush zonesIn theory the distance over which the aircraft is brought to a halt is the distance over which the aircraft's centre of gravity travels. Thus an energy-absorbing crush zone or deformable structure in the front of the occupant zone, even if it adds just half a metre to the distance, increases the occupant stopping distance and reduces the deceleration forces on the occupants.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, which 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 susceptible to full vertical deceleration. The great majority of light aircraft have a fixed undercarriage; there may be a problem with some low-wing aircraft fitted with wing fuel tanks if the undercarriage collapses and penetrates the tanks.  In high-wing aircraft (excluding trikes, powered 'chutes and minimum aircraft) the overhead structure usually provides a crush zone that allows room to exit from the cockpit should the aircraft pitch-pole onto its back. Also, the cockpit is fitted with doors that can generally be forced open when the fuselage is inverted. In many low or mid-wing aircraft fitted with a bubble-type cockpit canopy there is no structure, except possibly the vertical stabiliser, to prevent crushing of the cockpit canopy in a capsize. If the aircraft capsizes the pilot/passenger may not be able to exit until the aircraft is lifted.
Structure integrity and impact resistanceFrom the foregoing it is evident that aircraft with low kinetic energy near stall speed only require a low degree of occupant protection from impact forces and intrusions into the occupant zone, apart from the restraint system. As the kinetic energy at the aircraft stall speed increases, then the aircraft structural integrity and impact resistance must be engineered to provide higher standards of protection. |
2.4 Aircraft emergency recovery parachutesSome factory-produced aircraft are now fitted with rocket-deployed aircraft parachute recovery systems as standard equipment. Builders/owners of homebuilt aircraft often choose to add systems which could be spring deployed, mortar deployed or rocket deployed. The parachute recovery systems are primarily intended for use following events such as mid-air collision, catastrophic structural failure, pilot incapacitation, engine failure over difficult terrain or water, unrecoverable or low-level spin, and disorientation/loss of control in IMC. They are generally very effective in such situations. In the case of inflight fire parachute deployment should be delayed as long as possible in order to limit the hang time. The parachute systems are not intended for use in a normal forced landing event except possibly as a braking 'chute in a tight squeeze (see below).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, and 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 potentially 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. The next module in this 'Coping with emergencies' guide deals with forced landing procedures. |
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Things that are handy to know
Cockpit or occupant zone? The 'cockpit' term is a doubtful description of the occupant zone in some recreational aircraft types. The most significant being the structures suspended from the wings of powered parachutes and trikes. But many three-axis aeroplanes don't have any form of cockpit; consider the Breezy where the pilot and passenger are seated in tandem — on top of a truss — out in the open. The Drifter is similar except the pilot and passenger are sitting on top of a boom, generally with just a fibreglass nose semi-enclosure for airflow diversion. At the other end there are the side-by-side cabin aircraft, where the front seats are usually referred to as the cockpit area because both seats have access to controls and instruments. The rest of the occupant zone is the passenger cabin. Of course only two seats are allowed in RA-Aus registered aircraft but the three-door Jabiru fuselage, such as for the J230 model, is designed for 4 occupants. |
Groundschool – Coping with emergencies
| Guide contents | Knowing the aircraft | Deceleration forces | Forced landing procedures |
| Overcoming aircraft control failures | Procedure when lost | Safety and emergency communication procedures |
| Aviation distress beacons | Understanding SAR services |
| Comfort and survival in a remote environment | ERSA emergency and survival procedures |
Copyright © 2004–2009 John Brandon [contact author]