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Kinetic energy, crash worthiness and airbags

This thread discusses this article, kindly provided to us by its author RobertL18C.

There is a correlation between the ability of a structure to absorb and dissipate Kinetic energy and the risk of injury in a crash.

Here the old adage of hitting the softest, cheapest thing at the slowest possible speed applies. Conversely in aviation it is also worth keeping in mind the Northrop chief test pilot’s comment on the Piper Cub: that it is a strong, simple, safe airplane that can fly slowly enough to just barely kill you.

Using the speed between stall with landing flaps and Vref (approach speed), and a no wind assumption, the following Kilo Joules of energy need to be dissipated for typical GA types. I also use 90% of MAUW as the assumption for mass. Ke formula is 1/2*M*V^2. For consistency Mass is in Kg, and Velocity in m/s. If you halve the speed in knots you get a close approximation to m/s.

The Ke increases exponentially across different types. The aforementioned Cub comes in at 227 Kilo Joules. Modern LSA Cubs can be fitted with airbags and 26G seats, which suggests that in nearly all cases a controlled crash should be survivable.

The Ke, using my assumptions for a controlled forced landing, for typical GA types is listed below:

CE-172N 648KJ
CE-182P 1,014KJ
CE-210N 1,959KJ
PA-34 2,580KJ
PA-31 4,803KJ

My thesis is that somewhat clunky fixed gear singles with good slow speed characteristics, through simple physics, have a strong built in advantage in terms of crash worthiness and survivability. Conversely, I am quite sceptical of the multi engine piston as a species. In fact MEPs have a much higher fatality rate per accident, and since they exhibit average, to above average accident rates overall, their decline as a species (except in the training role), may be Darwinian, and not just down to economics.

I am not sure there is crash test dummy data around, but the 182 (or PA28-235) at 1,000 KJ, seems to mark a sweet spot between Ke, structure and survivability.

Last Edited by RobertL18C at 09 Oct 08:00
Oxford (EGTK), United Kingdom

A microlight with mtow 450 kg and stall speed of 65 km/h has a ke of 85 kJ. (Assumed 20 knot landing speed).

I am not sure I follow you though. Absorbing energy is one thing, but what kills you is too much acceleration, isn’t it?

ENVA ENOP ENMO, Norway

but what kills you is too much acceleration, isn’t it?

I’d expect that structure parts going through your head will kill you first…

LSZK, Switzerland

…or deceleration. Hence brake pads and tyres getting hot as the Ke is dissipated via wheel braking.

A controlled forced landing on a long runway is probably survivable even in a large aircraft, but commercial air transport still suffer injury/fatalities in over runs when the structure meets a ditch or wall.

Fixed Gear tractor engine single engine aircraft have relatively modest Ke to dissipate, and quite a lot of structure to absorb it – assuming you did not lose control and spin or spiral dive in.

Oxford (EGTK), United Kingdom

…here is a link to an interesting US Defence article on crash worthiness

http://www.dtic.mil/dtic/tr/fulltext/u2/a218436.pdf

Oxford (EGTK), United Kingdom

I’m not sure your reasoning using the aircraft mass is valid. The question for survivability is not the amount of energy the aircraft has to dissipate (the aircraft is going to die anyway ), but the amount of energy your body has to dissipate. A heavy car can be more crashworthy than a light one because it takes longer for the heavy car to dissipate its energy (i.e. stop) and thus that time will be longer for your body, too.

Heavier aircraft are at a disadvantage because their stalling speed tend to be higher. (In particular twins as they don’t have the 59 knot limitation.) On the other hand a heavier structure can protect you better in case of a crash.

ESKC (Uppsala/Sundbro), Sweden

Airborne as I am a fair weather cycling commuter agree entirely with the premise that how crash worthy a structure is, is an important factor.

Modern cars have become passive battering rams, however this technology does not translate to aircraft, with the possible exception of Agricultural Application where some serious thought is given to crash worthiness on impact.

Semi monocoque aluminium skin structures, with a fibre glass nose in some cases, are of limited benefit above a certain speed of impact.

If I understand your suggestion, the mass of the aircraft becomes irrelevant above a certain speed of impact?

Certainly stronger, heavier aircraft structures improve survivability while resulting in a higher Ke due to higher mass. One example, although have not tracked down the NTSB file on this, is a SE Commander ploughing into a hill side and the occupants surviving.

Oxford (EGTK), United Kingdom

My suggestion is just that the kinetic energy of the aircraft itself (less occupants) is irrelevant to the survivability of a crash.

ESKC (Uppsala/Sundbro), Sweden

I hit a fence in a Jodel DR1050. The engine hit a post and broke it, stretching the wires. The adjacent posts were pulled before the wing hit them. If it had been a pusher, there would have been serious injury to me and my passenger. A very light aircraft might have come to dead stop on the post, with more accelleration to the occupants.
A Vickers Viscount ran out of fuel and landed in small fields surrounded by dry-stone dykes. It had the inertia to just crash through them. A Jodel would have stopped dead on the first dyke, with injuries.
AAIB report Memory failure regarding Viscount, it didn’t hit walls.

Last Edited by Maoraigh at 09 Oct 19:56
Maoraigh
EGPE, United Kingdom

Good comments, I will do some more reading.

Beyond the black box by a Dr Bibel gets good reviews – most accident books look at CRM, human factors – apparently this book focuses more on the physics.

Oxford (EGTK), United Kingdom
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