Safety Implications of Kinetic Energy
Posted: 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!
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!