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U.K. Air Accidents Investigation Branch
The teeter stops,
shown in the
photographs below,
had split because
of mast bumping,
which occurs when
rotor blades “flap
downwards to an
extreme angle and
strike the mast,”
the AAIB says.
The accident flight began at 0958 local time,
when the helicopter left Manston Airport for a
flight to Fenland Airfield, about 240 km (130
nm) northwest.
The pilot told air traffic control, as the R22
passed east of Cambridge Airport at 1118, that
the helicopter was flying at 1,400 ft. That was the
last radio transmission received from the pilot.
The helicopter continued northwest, toward Fenland,
and disappeared from radar about 1125.
Witnesses on the ground southwest of the
accident site said they saw the helicopter rapidly
pitch and roll left and heard a “pop, as if it was a
paper bag you banged in your hands,” the report
said. Two witnesses also said that they saw objects
separate from the helicopter before it fell,
inverted, to the ground.
The wreckage was found in a field 2 nm (4
km) southwest of Ely. The helicopter was inverted
and there were few ground marks, indicating
that the helicopter had little horizontal speed
just before the impact. Both main rotor blades
had separated from the hub and were found several
hundred meters from the main wreckage.
One tail rotor blade also had separated and was
not found, but the other was still attached. There
was no indication that the main rotor had struck
the tail boom, the accident report said, noting
that “tail boom separation following main rotor
contact has been a characteristic of a number of
R22 in-flight structural failures.”
Accident investigators said that weather
conditions at the time of the accident included
visibility of 30 km (19 mi), few clouds below
25,000 ft and a light, westerly wind that was
FLIGHTSAFETY.ORG | AEROSAFETYWORLD | MAY 2013
CAUSALFACTORS
not considered strong enough to have generated
low-level turbulence. They said either wake
turbulence or a need for a sudden maneuver to
avoid another aircraft was highly unlikely.
The accident helicopter was manufactured
in 1988; at the time of the accident, the airframe
had been in operation for 6,407 hours and the
engine, for 1,595 hours. The last maintenance
check was a 50-hour check completed Dec. 6,
2011 — 28 flight hours before the accident. No
significant defects were found, the report said.
Maintenance records included no mention
of any disturbance of the pitch control links
during the year before the accident and showed
that the last “known disturbance” of the rotor
system occurred in April 2010, when the main
rotor blades were removed for replacement of
the spindle bearings.
Examination of the wreckage found damage
to the main rotor that showed that the main
rotor blades had “flapped to extreme up and
down angles prior to separation,” the accident
report said.
This condition — known as main rotor divergence
— has several causes in helicopters such as
R22s, which have “teetering, two-bladed rotors,”
the report said, citing low-g maneuvers, low-rotor
rpm, turbulence and “large abrupt control inputs.”
The report said it was “possible that a combination
of low rpm, an abrupt control input
and low-g caused the main rotor divergence.
… If carburetor ice caused a loss of rotor rpm,
this would have triggered the low rpm audio
warning, and this warning sounds like the stall
warning in some light fixed-wing aircraft. The
response of a fixed-wing pilot to a stall warning
is often to push forward on the controls to unstall
the wing. This would be an inappropriate
response from the pilot in these circumstances
but understandable given that the vast majority
of his flying was in fixed-wing aircraft.”
The report said that low rotor rpm “could
explain why the pitch link failed in the way that
was observed” — with the separation of the no.
1 pitch link and indications that the attached
bolts had failed because of overload resulting
from the application of considerable force.
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