Safety Implications of Kinetic Energy

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Safety Implications of Kinetic Energy

Postby drseti » Sat Feb 07, 2015 7:54 pm

I've stated elsewhere that the weight and stall speed restrictions on Light Sport Aircraft are far from arbitrary, but rather engineered to keep within non-fatal limits the Kinetic Energy which must be dissipated in the event of a takeoff or landing accident. It is my argument that, as some (probably unknown) fraction of the total KE will be absorbed by the occupants in the event of such an accident, the lower the total KE, the less likely that a given accident will result in fatalities. I hypothesize that fatality rate should be directly correlated with total KE, although the exact relationship may or may not be linear.

Given the above hypothesis, I was dismayed (purely on the grounds of safety) by the possibility that the FAA may grant Terrafugia a requested further increase in both weight and stall speed, while remaining within the SLSA category. Here are the kinetic energy numbers I get for the LSA limit vs. the Terrafugia request:

LSA, 1320# and 45 kts yields 160.8 kiloJoules of kinetic energy
Terrafugia 1800# and 54 kts yields 315.7 kiloJoules of kinetic energy

If my KE hypothesis is correct, and the relationship is linear (not sure it is, but we have to start somewhere), this suggests that the fatality rate for takeoff and landing accidents in a fat and fast Terrafugia may end up being roughly twice that of the rest of the LSA fleet.

For those who wish to crunch the numbers themselves for these and other scenarios, I have created a Kinetic Energy Calculator excel spreadsheet, and posted it to:
http://avsport.org/spreadsheets/KEcalc.xls
Just change the numbers in the red cells to your heart's content, and see the results instantly.

Happy KE calculating!
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
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Re: Safety Implications of Kinetic Energy

Postby snaproll » Sun Feb 08, 2015 1:49 pm

Thanks Paul... When I grow up, I want to be just like you.... Hey, at 65, I'm still not ready... VR.. Don

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Re: Safety Implications of Kinetic Energy

Postby drseti » Sun Feb 08, 2015 8:07 pm

Thanks, Don. In fact, when I grow up (unlikely to ever occur, given that it hasn't yet happened at 68), I want to be just like your dad!
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

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Re: Safety Implications of Kinetic Energy

Postby snaproll » Sun Feb 08, 2015 10:58 pm

drseti wrote:Thanks, Don. In fact, when I grow up (unlikely to ever occur, given that it hasn't yet happened at 68), I want to be just like your dad!

Yes Paul... I think we both have the same wish - and are still flying Young Eagles at 93 years young (94 in June). His Skycatcher is still doing well for him also. VR.. Don

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Re: Safety Implications of Kinetic Energy

Postby rezaf_2000 » Tue Feb 10, 2015 2:43 am

60 hours of flight and counting

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Re: Safety Implications of Kinetic Energy

Postby rezaf_2000 » Tue Feb 10, 2015 3:22 am

I stand corrected, this is even more interesting.
http://www.dtic.mil/dtic/tr/fulltext/u2/a082512.pdf

It shows how much acceleration can human body take in aviation related accidents. The human body can tolerate certain amounts of acceleration or deceleration per time period.

Yes, the lower kinetic energy means there is less of that energy to be dissipated through airframe structure deforming, and impacting the environment like the sod and trees. But, apparently, the quality of how this energy is dissipated is also very important. The longer it takes, the better, since it results in less acceleration.
http://hyperphysics.phy-astr.gsu.edu/hb ... r.html#cc1

According to theory then, if you're going to crash, it's better to find a field or something that has lower chance of the airplane getting stuck against a rigid object. This theory tells us that trees are bad. Soy beans or corn fields are good. Sudden stopping is bad, tumbling along is good.

That last point might also imply that high wing aircraft are safer, since they can tumble without immediately exposing the passengers' head to the ground.

The report mentions that most fatal accidents are head injuries. I guess wearing helmets could be an effective safety move! But aside from military pilots, I've never seen any other GA pilot to wear a helmet.
60 hours of flight and counting

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Re: Safety Implications of Kinetic Energy

Postby MrMorden » Tue Feb 10, 2015 9:29 am

rezaf_2000 wrote:But aside from military pilots, I've never seen any other GA pilot to wear a helmet.


You need to hang out with more RV pilots. :twisted:
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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drseti
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Re: Safety Implications of Kinetic Energy

Postby drseti » Tue Feb 10, 2015 10:29 am

Of course, Reza, there are many factors that influence accident survivability. But the one that is most easily controlled by the regulators is kinetic energy. After all, FAAis not going to mandate that all pilots wear helmets, or that all accidents impact something soft!
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

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Re: Safety Implications of Kinetic Energy

Postby drseti » Tue Feb 10, 2015 10:35 am

From that report:

When an aircraft impacts the ground, deformation of the ground absorbs some energy. This is an uncontrolled variable since the quality of the impacted surface usually cannot be selected by the pilot.


As a deterministic engineer, I prefer to concentrate on what we can control.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US


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