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CAUSALFACTORS
FAA and EASA
certification
requirements … are
less stringent than
equivalent military
requirements.
In addition, the report said, “the forward
deflection of the cyclic, leading to a low-g flight
condition, could explain the rapid roll, but only
if the witnesses were mistaken and the roll was,
in fact, to the right.”
The report also cited previous studies,
including a 1996 study by the U.S. National
Transportation Safety Board (NTSB), that linked
“large, abrupt control inputs” to mast bumping. 1
NTSB Study
The report noted the NTSB’s 1996 recommendation
calling on the FAA to require helicopter
manufacturers to “provide data on the response
of helicopters to large, abrupt cyclic inputs as a
part of the certification process.”
The FAA implemented part of the recommendation
by modifying Advisory Circular
(AC) 27.661, Rotorblade Clearance, to require
manufacturers to conduct a blade flapping
survey.
“However, the AC did not define what the
control deflections should be or what the rate of
input should be,” the AAIB report said. “It specified
that margins should be determined, but it
did not specify what the margins should be.”
The NTSB eventually closed out the recommendation
and characterized the FAA’s response
as “acceptable,” but the AAIB said that the NTSB
action was influenced by the decline in the
number of accidents involving main rotor loss of
control in the mid-1990s.
“The NTSB attributed this to the increased
training and experience requirements imposed
by the FAA,” the report said. “However, since
the 1996 NTSB study, there have been at least a
further 16 fatal R22 accidents involving loss of
main rotor control.”
Precise causes of many of these accidents
have been difficult to determine, the report said,
“because the pilot’s control inputs leading up to
the divergence are rarely known.”
Nevertheless, the report added that some of
the 16 accidents probably resulted from a loss
of rotor rpm that followed a power loss “without
the pilot lowering the collective quickly
enough. In the R22, the pilot must react to a loss
of power by lowering the collective in less than
about 1.5 seconds in the cruise, or one second in
the climb, to prevent rotor stall.”
As a result, EASA has begun a study of the
effect of increasing the required reaction times,
the report said.
Handling Qualities
The report said that handling qualities are
another probable factor in a number of R22
crashes involving loss of main rotor control.
“Only light control forces are required to
apply full cyclic deflection in the R22, making
it easy inadvertently to enter a low-g situation
or to make an abrupt and rapid control input
leading to rotor stall and mast bumping,” the
report said.
FAA and EASA certification requirements,
which have changed little in recent decades, are
less stringent than equivalent military requirements,
the report said.
The document noted that a 2005 study by
the U.S. National Aeronautics and Space Administration
Ames Research Center included a
recommendation calling on helicopter manufacturers
to “explore the feasibility of designing
a low-cost, lightweight stability augmentation
system, which would also provide benefits for
the reduction of low-speed and hovering helicopter
accidents.”
In addition, the AAIB report said that a
stability augmentation system “would provide
some control force feedback, thereby making
large abrupt cyclic inputs less likely, as well as
recovering the aircraft to a safe attitude should
the pilot release the cyclic control.”
Safety Notices
Robinson Helicopters included a series of safety
notices in the R22 Pilot’s Operating Handbook
that discussed, among other topics, the dangers
of failing to maintain rotor rpm and the importance
of avoiding a rotor rpm stall.
“As the rpm of the rotor gets lower, the
angle-of-attack of the rotor blades must be
higher to generate the lift required to support
the weight of the helicopter,” one safety notice
36 | FLIGHT SAFETY FOUNDATION | AEROSAFETYWORLD | MAY 2013