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All would be alive
if this aircraft had a “Low
Thrust Detector”
Reprinted from Aviation Safety, April 15, 1990
A
14,500 hr ATP Air Taxi pilot has a partial engine failure and retards
the wrong throttle. Killing
himself and four of six passengers!
Puzzle Palace
It's
drilled into a pilot during multiengine training and check flights that
an engine failure on takeoff must be handled quickly and methodically.
One point given special emphasis is the Importance of feathering the
propeller to reduce drag. Most piston twins will neither climb nor maintain
altitude with the prop on the 'dead' engine wind-milling.
But
to accomplish this critical task the pilot first has to determine which
engine is the dead one. It's easier said than done. In a textbook
scenario. the pilot will by reflex be holding full rudder to counter the
yaw produced by asymmetric thrust from the good engine and by drag from
the bad one and use the mnemonic 'dead foot: dead engine' to Identify
the one that's failed.
It
works just fine most of the time when an instructor or check pilot
simulates an engine failure. But it isn’t always that simple in the
real world, where a pilot probably Isn’t expecting an engine problem,
where an aircraft being bounced around in turbulence can make
identification of the dead engine a conundrum, where a propeller
over-speed could fool a pilot into thinking a perfectly good engine has
given up the ghost.
Which
One?
It was
still dark at 6 a.m. when the pilot taxied his Piper Chieftain and six
passengers to take off from Runway 1 at Kenai Municipal. The weather
there was cruddy VMC, with scattered clouds at 300 feet, a measured
1,500 overcast and six miles of visibility in light rain and fog. The
flight was cleared direct to Anchorage. IFR at 3.000 feet.
Shortly
after takeoff, the pilot radioed, “Central One has lost an engine and
we're circling for one. “Witnesses saw the aircraft turn onto downwind
for Runway 1 and begin descending from an altitude of only 300 to 600
feet. One of the surviving passengers would later recall hearing the
left engine backfire loudly on takeoff. The other survivor would say he
heard a sound he thought was an engine blowing up but could not say for
sure which one it was.
The Chieftain descended
wings-level into trees
and then crashed inverted into a house about a half mile west of the
airport, where it exploded and burned. There were two people inside the
house, but they escaped with minor injuries. Only two of the seven
people aboard the aircraft survived.
The
flaps were found nearly fully lowered, and NTSB's report indicates that
investigators explored the possibility that the pilot may have taken off
with full flaps. The Board concluded, however, that he most likely took
off without flaps but lowered them at the last moment in an attempt to
cushion the impact.
The
investigation also disclosed that although (he Lycoming TIO-540 engines
were producing power when the aircraft crashed. both of them had
mechanical problems. The left engine had severely worn camshaft lobes, a
condition that would sap some power. However, neither NTSB nor Textron-Lycoming
was able to determine how much actually would have been lost.
Cracked
Head
The right
engine was in worse shape. NTSB said there was a pre-existing crack in the cylinder head that
stretched halfway around the barrel of the number three cylinder. This
crack opened up during the ill‑fated takeoff, causing the intake
manifold to separate and vent the compressed air supplied by the
turbocharger. The Board determined that this rendered the right engine
capable of producing only 193 of its maximum rated 350 horsepower.
It
appears, then, that although the left engine was down on power, it was
in fact producing substantially more power than the right engine. The
Board believes, however, that the pilot, like one of the surviving
passengers may have thought it was the left engine that was backfiring
and purposely reduced power on the left engine substantially “in
a mistaken effort to control the engine and lessen the damage."
With the right engine able to produce only
55 percent of its maximum rated power and the left engine throttled
back, the aircraft was no longer capable of maintaining altitude. NTSB
said.
The
Board's theory is supported by the position of the rudder trim tab. With
the right engine producing some power and the left one throttled back,
the Chieftain would have yawed to the left and the pilot would have
needed right rudder to counteract it. The trim tab was found fully
deflected to a position that would have helped the pilot maintain right
rudder.
NTSB
concluded that the cracked cylinder head on the right engine was a
probable cause of the accident. In its own formal terms, the Board also
determined that during the few seconds that elapsed between the pilot's
takeoff announcement and the Chieftain's descent, the pilot was
presented with a puzzle he was unable to solve. |