As a former DPE who used to test Terminal
Velocity Descent to students of Commercial LTA certificates, I can attest that
the rate can range from 700 fp minute to as much as 1300 fp minute. If
the canopy streamers then all bets are off after 1500 fp minute = 25fps.
I had one applicant ask what would happen if….., and only quick action
saved both our lives.
The FAA no longer allows Terminal Velocity
Descent maneuvers. Something about Old Bold Pilots.
John Cox
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf
Of Mark Sletten
Sent: Thursday, August 16, 2007
6:19 PM
To: lml@lancaironline.net
Subject: [LML] Re: Apologies to
the farmer's daughter
I believe the analogy describing the impact by
referring to falling or jumping from a particular height doesn’t account
for the attitude of a BRS-equipped aircraft on impact with the ground following
chute deployment. Since the aircraft descends in a nose-low attitude, the nose
hits first (assuming level ground) absorbing a great deal of the impact energy
-- the rest of the airframe would rotate downwards behind the nose until coming
to rest (upright hopefully). Assuming the main gear hits after the nose, this
would dissipate even more energy as it (the landing gear) is designed to absorb
landing energies.
In short, landing under a BRS would be more like two
smaller impacts instead of a single large one.
During jump training in the USAF we learned to do the
same thing with our bodies; the Air Force calls it a Parachute Landing Fall
– I don’t know what the other services call it. Essentially you
land into the wind – feet first – then roll forward while rotating
your body so as to dissipate the impact energy. Instead of keeping your feet
and legs rigid so as to absorb the full impact – as the Cirrus would if
it pancaked flat onto the ground – you roll sideways allowing your knees,
hips and shoulders to take their share of the beating. If done properly, one
can easily jump from 8 feet without (permanent) injury.
Mark
From:
rtitsworth [mailto:rtitsworth@mindspring.com]
Sent: Wednesday, August 15, 2007
5:47 PM
To: lml@lancaironline.net
Subject: RE: [LML] Apologies to
the farmer's daughter
Rob, etal,
FYI: 25fps ~= 17
mph. About like running (full sprint) into solid wall. Perhaps
survivable, by OUCH.
More math…
Gravitational
acceleration is approx 32ft/sec^2 (drag-less freefall at the Earth’s
surface)
Doing some algebra to
solve for the time to reach 25 fps results in: (25/32)^.5 = 0.88 seconds
Doing some integral
calculus (not shown) results in a distance traveled of approx 7.5 feet
Thus, 25 f/s is about the
impact speed from falling off the top step of an 8’ step ladder
Be careful when cleaning
your gutters out there J
Or another analogy (in
case you haven;t fallen off the top of a 8’ step ladder lately), terminal
velocity from the top of a 34” bar stool would be approximately 12.5 fps
(note: 1/2 speed = 1/4 of the impact energy).
Rick