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In this video of someone making a homemade electric aircraft, at around 19:44 into the video, the creator shows his estimates of survival chances if he stalled and crashed from various altitudes, a screen grab is below.

survival rate from various altitudes

Is this accurate, broadly speaking? 5% from 200ft seems very low to me, I thought it'd be more like 50% at least.

By the way, the reason he enters 100% over 1000ft is on the assumption a parachute is installed. Now, I would personally never put 100% probability to... well, anything.

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  • $\begingroup$ This kid looks super smart, and an amazing job doing what he clearly loves. But I really wouldn't put much stock into a back-of-the-envelope guestimate of his survivability. You fall from 49ft in that, you're just as dead as 51ft. $\endgroup$
    – Jamiec
    Jan 9 at 11:46
  • $\begingroup$ @Jamiec you think a 49ft fall would kill you? $\endgroup$
    – Cloud
    Jan 9 at 12:20
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    $\begingroup$ Thats roughly the 4th floor of a building, would you fancy your chances jumping from there? It's going to hurt, a lot, if not kill you. Now wrap yourself in wood/metal/electric motors and go again. $\endgroup$
    – Jamiec
    Jan 9 at 12:50
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    $\begingroup$ Also, your question you say "5% from 200ft seems very low to me, I thought it'd be more like 50% at least." - Would you jump from the 25th floor of a building and really expect to live 50% of the time? I think you're underestimating just how high 200ft is! $\endgroup$
    – Jamiec
    Jan 9 at 14:09
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    $\begingroup$ Great video and outstanding build. It seemed his point (compared to the 80 knot plane) was being very low and slow (25 knots) gave more chances of survival nearer the ground. 1000 feet may have been his estimate of minimum survival altitude with parachute deployment (time) for both aircraft. $\endgroup$ Jan 9 at 22:11

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So much depends on the airframe how hard it hits the ground, the type and extent of structure and mass between you and the surface, and the kind of restraints you are wearing. Peter's talking about the thing coming apart and the survivability of a free fall drop from a vertical distance. We really don't take that into account because the event is so rare. In the real world, the main concern is survivability following a stall spin close to the ground, or a forced landing into a confined space that forces you to crash into objects while still moving.

So much depends on the airplanes. I tow gliders in a Piper Pawnee ag plane (with the hopper tank removed).The Pawnee has the cockpit quite a ways aft, so most of the mass is ahead of you, with a steel tube fuselage that's rather tank-like. There are stories of Pawnees stalling and spinning into the ground and the pilot, strapped tightly into a 4 point harness and a cockpit that remains pretty much fully intact even at that level of impact, walking, or at least limping, away.

At a glider club I was in there was a kid landing a Schweitzer 1-26 that entered a stall spin on a very low turn to final. Only going 30 mph, and starting the spin from maybe 30 ft in the air, the thing more or less hit the ground nose down with a bonk, and he climbed out with nothing more than a sore back.

Shoulder harnesses are critically important. People who don't wear them in crashes often die from head injuries in otherwise perfectly survivable crashes.

If you hit the ground at a decently low speed and the structure between the seat back and your feet remains intact, and you're wearing a shoulder harness, you will probably live, and very well may get away with only leg injuries from the front part of the cockpit folding up.

Ultralights are like the 1-26, and a lot of accidents are survivable just because the energy is so low at impact and as long as you keep your head and torso out of the instrument panel with shoulder restraints, you'll likely live.

Hit the ground while spinning in something faster and heavier, if it's been able to fully develop, and unless you're in a flying tank like a Pawnee, you're probably going to get hurt pretty bad, and maybe killed. But in accident studies, the number one factor that stands out on surviving or not, is whether or not you're wearing a shoulder harness. the vast majority of ground contacts, while the airplane is under control at the time, while wearing a shoulder harness, are survivable.

Hence the advice to strap in tight and fly it all the way into the crash.

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Altitude is not a measurement in "survivability", and I don't think that the guy on the linked video was actually trying to say it was, in the general sense at least.

What he was saying, if I understood correctly, was that at low level he might be OK, but unless he got high enough to deploy a hand operated parachute, that he was toast. So it was stay low or climb quickly though a zone which would not afford him time to deploy his parachute.

Surviving a crash is more about your position, your restraints, and limiting deceleration forces. If you would like a long read on human crash survivability then NATO Science and Technology Organization have produced a paper on Human Tolerance and Crash Survivability.

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  • $\begingroup$ "...limiting deceleration forces." As the old saying goes, it's not the fall that kills you, it's the sudden stop at the end. $\endgroup$
    – CGCampbell
    Jan 10 at 17:13
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    $\begingroup$ For a more thorough paper on crash survivability I recommend the "Aircraft Crash Survival Design Guide" (link), of which the linked report is just a small excerpt. $\endgroup$ Jan 10 at 17:39
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His reasoning is that his 30 mph ultralight flies so slowly that his forward speed will be survivable if not much of a vertical speed is added. Then he compares this to a composite 80 mph design where forward speed alone is already enough to possibly get killed in a crash even with the fuselage around you and wearing a restraint.

And that is about right.

However, altitude is a poor metric for survivability. A better is the ISS (injury severity score) which is a numerical score which is derived from an individual's three most severely injured body regions. Many high-velocity impacts resulted in low injury- severity scores, and many low-velocity impacts had high degrees of injury severity (see plot below). For survival, body posture and the energy absorption capabilities of the ground must be taken into account.

ISS over impact velocity

Plot of ISS of falls over estimated impact velocity. Impact velocity was calculated form the height of the fall. Source: Figure 9 from "Study of Impact Tolerance Through Free-Fall Investigations", Highway Safety Research Institute, The University of Michigan, Ann Arbor, Michigan.

Another measure of injury severity is OAIS (Overall Abbreviated Injury Scale). The OAIS is based on the Abbreviated Injury Scale (AIS) which is in use internationally as a method of quantifying injury severity in auto accidents. Plotting this measure over impact velocity (see below, from the same source), there is a good correlation for head impacts age 10 and older and a weak correlation for impacts other than to the head, age 10 and older. No other correlations are evident for impact velocity as an indicator of injury.

OAIS over velocity plot

Again, from Study of Impact Tolerance Through Free-Fall Investigations:

If the potential indicators selected were good indicators, one would expect to see a positive relationship between the independent and dependent variables — for example, an increase in impact velocity resulting in an increased indication of injury. There is no such indication in the majority of the plots.

Given that Peter Sripol flies over fields and meadows (as opposed to concrete surfaces), his reasoning is correct. But still, it will hurt, even if he crashes from 10 ft of altitude. And his expectation of the plane coming apart is not unreasonable, too, when we consider that he did his load test first and then added more wingspan.

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  • $\begingroup$ Fig. 10a may support your point better than fig. 9 because it uses OAIS not ISS, and includes points for which OAIS=6, i.e., fatalities. If fig. 9 plots only survived falls, then it doesn't directly address survivability. $\endgroup$ Jan 10 at 18:05
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    $\begingroup$ @CamilleGoudeseune Thank you, I will revise the answer tonight. I only wanted to make the point that fall height doesn't determine severity of injury or survivability. The last half of the linked document contains some grim examples ... $\endgroup$ Jan 11 at 8:25
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    $\begingroup$ @CamilleGoudeseune Oops - some ISS scores are also fatal. In the examples there are two fatal cases with OAIS 5 and ISS 26 only, another with even ISS 16. Text corrected. $\endgroup$ Jan 11 at 20:41

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