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Most light airplane initial training is
accomplished is aircraft that are underpowered or just adequately powered.
Aircraft that fall in to that category do not pose unsafe flight
characteristics with abrupt throttle movement. I suspect when
the Lancair IVP ways initially designed, 350 HP not 750 was assumed
to be the max. I believe Brent is correct in his analysis of the
problem. Is the airplane unsafe? I don't think so. Does
the airplane require additional operational considerations and training?
Absolutely!
There are more considerations than excess
horsepower and torque at play here. In addition there is
turbine lag. In any jet engine 70% of the thrust is produced in the
last 25% of the RPM. In the late 60's Unites Airlines was made
painfully aware of this fact at SLC. A B727 crashed short of the
runway. The crew transitioning from piston airplanes pushed the power up
from idle to arrest the decent rate. On that particular engine it can take
as long as 7 seconds for the engine to accelerate. In this case by the
time the engines came up to speed the aircraft had hit the ground. In more
modern jet engines the problem still exists. It has been
mitigated by a flight idle (vs. ground idle) mode in the fuel
controller. Depending on the airplane, flight idle is activated
by extension of the flaps or gear. This feature does not exsist in
the 601. The Walter 601 is a free turbine which means there is no
mechanical link between the powered turbine and the turbine that drives the
propeller. The propeller is "free wheeling" and is controlled by the
governor. Thus the propeller RPM does not have a direct relationship with
the speed of the engine. Keeping that in mind, just because the propeller
is turning at max RPM (2080 in the
Walter) turbine lag is not eliminated. We can imagine a scenario with
a aircraft on short finial, power at Idle, prop 2080 rpm. The aircraft
slows and descends below the intended glide path. A small correction is
added and nothing happens. The natural reaction is to add more
power by advancing the throttle. The situation compounds as ground
starts to rise rapidly more throttle movement is applied and as the nose is
pitched up airspeed slows. At some point the power turbine comes up to
speed and now we have horsepower and torque. Lots of horsepower and
torque. The correct reaction would be to retard the throttle,
but as pilots our first job is to fly the airplane and not hit the
ground. At that points every body is a passenger to the point of
impact. It is imperative that turbine pilot understand this
concept, train accordingly and stay ahead of the airplane.
Power available. Our community has suffer
numerous stall spin accidents in the IVP. The power and turbine
lag exacerbates this problem. It is critical to use only the
power required for these maneuvers and recoveries. The problem is
not specific to our airplanes. In more complex airplanes, aircraft
systems help mitigate the effects of excess power. In the B777 (with
auto throttles) with the first push of the Go Around button the
engines are programmed to produce just enough power to result in a 2000 feet a
minute rate of climb. Push the button a second time and you get the
benefit of 90,000 pounds of thrust from each engine. The bottom line is training on the use of the throttle and
understanding that this is a different animal than a piston powered
airplane. In the case of the stall recovery the aircraft is in the
most vulnerable condition. High angle of attack and low power
setting. A perfect setup for the use a more power than that amount
required. Again training and education is the solution to the
problem. In my airplane I had initially decided not to put a AOA indicator
in the airplane. In light of our safety record I have I have
purchased an AOA with an aural warning. Part of my 'memory items" will be
to think throttle movement when I here "Stall, Push". Slow airspeed will
always reqire something less that full power.
----- Original Message -----
Sent: Friday, August 12, 2005 7:14
PM
Subject: [LML] Torqued to death
During a recent post crash investigation of a
Walter 601 powered (not a Lancair) aircraft I was having difficulty
understanding what caused a left roll just before the crash on short final.
The roll had an average rate of 46 degrees per second and corresponded with a
power increase. Given the 745 available horsepower (near sea level) and
the 2,067 RPM prop speed I calculated an engine reaction torque of 1,893 foot
pounds, roughly the same as putting a 200 pound weight about mid span on the
aileron. Factor in that the airspeed was only 120 KIAS and you get that queasy
feeling in the pit of your stomach.
The data shows that plane was on
short final when the pilot let the plane get a little low and slow. He drops
the nose slightly but doesn't accelerate because of the drag of the prop
and/or he is behind the lift curve. He punches the throttle and
pulls the nose up. The added load on the left wing due to the torque
reaction either stalls that wing or overwhelms the aileron authority and the
plane begins an uncontrolled left roll. Five terrifying seconds
later the plane has rolled 260 degrees and the right wing is pointing
straight down. This is the last data point and the plane crashes, with two
fatalities, less than 5 seconds later.
To all you LIV Turbine guys out
there, remember this well. The Walter is a LOT of motor for those little
wings. Low and slow is not a place you want to be and putting in the power is
not always the solution.
I would be interested to know what
happens (at 10,000 feet) if you slow to ~120 KIAS and then apply full
power. Do you still have any aileron authority?
Regards
Brent
Regan
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