Cougar Accident Raises Questions Over
“ 30 Minute Rule”
Cougar Helicopters Flight 491 lost tail
rotor drive about a minute before it
crashed into the ocean off Newfoundland
on March 12, killing 17 of the 18 people on
board. The aircraft’s flight data recorder
had stopped working but investigators
were able to piece together information
from the health and usage monitoring
system (HUMS) and flight control
computer. Canada’s Transportation Safety
Board (TSB) says that, at the time of
failure, the pilots were making a controlled
descent to land in the water after the loss
of MGB oil pressure, and were at about
500 feet above sea level. Three seconds
afterward, the pilot cut off the engines to
attempt an engines-off landing.
“It’s showing us that there was control remaining and the guys were doing
their best to get the helicopter down on
the surface,” said Mike Cunningham,
TSB investigator-in-charge. The main
rotor was working throughout
the descent. Fellow investigator Allan Chaulk explained to
Rotor & Wing that following
the gearbox oil loss, heat was
transferred to the tail rotor take-off pinion gear. “The gear rotates
at six to seven times the speed
of other gears in the assembly,
and it wasn’t able to sustain that
without lubrication.”
“It’s at the top of the gearbox
and the first thing to fail,” he
continued. Photos on the TSB
website ( http://tsb.gc.ca) show
the teeth of the pinion gear
completely stripped.
The interval between the loss
of transmission oil and contact
with the surface was “
approximately 10 minutes.” The investigation determined the helicopter struck the water at a “
moderate” speed. The pilot may
have also flared before meeting
the surface. Despite this, the
helicopter hit the water with a
significant impact—reportedly
20G—nose upward and banked
slightly to the right.
The S-92 wreckage shortly after being recovered from
the ocean off Newfoundland. The flight data recorder
was found to be inoperative.
Transportation Safety Board of Canada
All flotation devices failed as the
helicopter hit the three-meter seas. “The
damage done ... during the impact was so
significant that ... it might be unreasonable
to expect that this (flotation) system could
have even operated,” said Cunningham.
“And would it have made any difference?
If it did, is pretty hard to say.”
“One of the bags was ripped right out
of its housing,” he continued. This failure
is still under investigation.
Under Part 29 certification rules
for rotorcraft, failures that result in the
loss of lubrication to the drive train
must allow for 30 minutes of flight time
after the crew is presented with the cockpit warning. However, a clause notes
that, if a failure mode is determined to
be “extremely remote,” the 30 minute
rule no longer applies.
In an accident update communiqué,
the TSB stated: “the investigation has
revealed that, even though the Sikorsky S-92A MGB was certificated to
meet requirements of (FAR 29), there
is a perception in some areas of the
aviation community that the MGB can
be run in a dry state—that is, without
lubricating oil—for 30 minutes. FAR 29
does not require run-dry operation of a
gearbox to meet the 30-minute ‘
continued safe operation.’
“Based on the applicable guidance
material at the time of certification, the
lubrication failure modes of interest were
limited to the failure of external lines, fittings, valves, and coolers. This practice
was consistent with industry experience,
which had found that loss of lubrication
tended to be associated with external
devices. Therefore, the possibility of a
