AS350 Squirrel Fatal Accident
A Eurocopter AS350 Squirrel been flown by a pilot whose
licence and Type Rating had both expired crashed into trees while engaged in
low-level high-speed flight close to the pilot’s home. The pilot and his
three passengers died on impact.
The accident resulted in an exhaustive investigation by The Air
Accidents Investigation Branch and the publication of the AAIB findings in
their Report EW/C2007/09/06 that is included in AAIB Bulletin 2/2009. This
Report covers 32 pages in the Bulletin and contains a wealth of information
that is of vital interest to all owners and operators of the Eurocopter AS350
and similar helicopters.
The following comments and opinions are those of
the Gremline Technical Editor and do not seek to represent those of AAIB or
anyone else. They are based on the findings of the AAIB Report, which source
is gratefully acknowledged.
The AAIB Report highlights a characteristic of the Eurocopter AS350
flight control system that may not be fully understood by all pilots and
operators, and this lack of understanding may have led to the accident that
caused the death of two adults and two children. There are other possibly
contributing factors covered in the Report that deserve careful attention
from all aircraft pilots, particularly from helicopter pilots, dealing with
the hazards of low flying by pilots who have not had the benefit of specific
specialist instruction in this very demanding skill.
The AAIB Report provides a detailed explanation of the “Flight control
servo transparency phenomenon” with particular reference to the AS350 series
of helicopters. This phenomenon is also sometime known as “jack stall” but is
termed “servo transparency” or “control reversibility” by Eurocopter. No
matter what it is called it is of the utmost importance that all AS350 pilots
fully understand how this phenomenon may occur, what immediate action is
demanded and how best to avoid entering that corner of the flight envelope
where the phenomenon may be encountered. The topic of “servo transparency”
does not lend itself to being précised so no attempt to do so will be made
It appears from eyewitness evidence and from a
video recording recovered from the wreckage that the helicopter was been
flown in what I would describe as an exuberant manner at very low level
immediately before the impact. It seems likely that the pilot lost control at
a height and attitude that made recovery impossible. He probably lost control
because he encountered “servo transparency.” Eurocopter had issued Service
Letter SL 1648-29-03 and Rush Revision 3A to the AS350B2 Flight Manual
providing owners and operators with more information about servo
transparency. The Flight Manual for the accident aircraft did not include
Rush Revision 3A, issued in 2004. The revisions, and other information, are
also available directly from Eurocopter via their internet site. Owners and
operators may register to receive this material without charge. The pilot of
the accident aircraft had not used this facility.
The fact that the pilot’s licence and Type Rating had both expired may
have no bearing on the accident. He also failed to keep the required
up-to-date records of his flying hours. These lapses may have been oversights
or may reflect an attitude towards regulations.
The AAIB Report includes remarks about helicopter low flying that are repeated
here as sound advice to ALL helicopter pilots.
is a complex and often unforgiving activity that demands not only skill and
knowledge, but also discipline and sound judgement. Low level flying is
inherently high risk, increasing the aircraft’s exposure to hazards and
reducing the pilot’s options in the event of an aircraft malfunction. An
engine failure at low height in a wooded valley would leave the pilot of a
single-engined helicopter (like the accident aircraft) with little or no
chance of landing safely. The risks associated with low level operations are
well known by agencies like the military, who are required to operate there.
To address and minimise the risks, military pilots are subject to rigorous
selection, and extensive training in low level flying techniques, and are
required to maintain currency in the environment.
There are also sensitive environmental issues concerning helicopter
operations, particularly as helicopters often operate closer to the general
public than many other aircraft types. Military and commercial operators
place great emphasis on lessening the environmental impact of low level
helicopter operations. The CAA produced a leaflet in their ‘Safety Sense’
series covering many aspects of helicopter airmanship, including
environmental considerations. Readers of the leaflet are urged to read the
‘Codes of Conduct’ produced by the British Helicopter Advisory Board (BHAB)
and available on their website.
The BHAB’s main objective is to promote the use of helicopters
throughout the country and to bring to the attention of potential users the
advantages of using or owning a helicopter. It is also concerned that
helicopter operations are conducted safely and responsibly, and proper
attention is paid to environmental issues. The first point in the BHAB’s Code
of Conduct is:
‘ALWAYS FLY AS HIGH AS POSSIBLE consistent with the weather
and other factors. This will reduce your projected noise at ground level, and
also give you more scope to find a suitable landing site in the event of an
GREMLINE STRONGLY URGES EVERY OWNER AND OPERATOR OF AS350
HELICOPTERS TO READ AND FULLY UNDERSTAND AAIB REPORT EW/C2007/09/06 BEFORE
THEY NEXT APPROACH THEIR HELICOPTER.
Text and Photographs © 2009 Gremline & Hill House
Publications, unless otherwise stated.
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