CH139308
Jet Ranger
02
July 2002
Southport, MB
The Standards Officer was conducting
a proficiency check ride on one of the instructors from
the Basic Helicopter School in Southport. The focus
of the flight was to assess the instructor's proficiency
in autorotations. The crew successfully completed a
number of straight-ahead and 500-foot turning autorotations,
but the aircraft struck the ground during the landing
portion of a 250-foot turning auto. Both crew members
received serious back injuries. The aircraft sustained
"A" category damage.
The winds at the time of the accident were variable
in strength and direction but within the limits indicated
in School Orders. Crews operating in the area reported
having to add throttle to cushion some landings and
to adjust the entry point on downwind due to strong
winds aloft. The crew of the accident aircraft experienced
problems with airspeed control on some of their autorotations,
overshooting on several (both pilots' first attempt
at the 250-foot turning auto). These facts point to
the likelihood of a decreasing performance wind shear
as the aircraft descended from circuit altitude to the
ground. Unfortunately there is no wind recording equipment
at the autorotation training area.
The accident manoeuvre was the Instructor's second attempt
at the 250-foot turning auto. The Instructor was sitting
in the right seat and flying right hand circuits. The
entry was normal, but during the turn to final the instructor
used considerable bank and backpressure to expedite
the turn. This bled off the airspeed to below the '60
knot' ideal. Although the requirements of the '100 foot'
check were met, the aircraft was on the low end of parameter
acceptance (low and slow). The Instructor commenced
the flare at 50-60 feet AGL. As the nose of the aircraft
was pulled up for the flare both pilots stated that
the airspeed dropped off quickly and an excessive descent
rate developed. The Instructor was somewhat startled
by the aircraft reaction and did not immediately initiate
the overshoot. The Standards Officer took control at
30-40 feet and applied throttle and then collective
("low level save"). This did not seem to have
any effect and therefore he concentrated on getting
the aircraft level prior to impact.
It is possible that the transition out of the turn (low
and slow) and into forward autorotation may not have
been "clean enough". This would have left
less time to develop a steady forward autorotative glide
prior to flaring. With low airspeed, the descent rate
would be higher than desired. At the commencement of
the flare, the rate of descent notably increased coincident
with a marked decrease in airspeed. It is perhaps at
this point that the aircraft entered a zone of decreasing
performance shear. It is possible that these two factors
(glide and shear), in combination, created conditions
where the flare would be unable to effectively reduce
the rate of descent.
The investigation also examined the possibility that
Vortex Ring State (VRS) may have been a contributory
factor during the landing phase. For this accident,
the steep descent and/or the sudden increase in rotor
thrust during the power recovery attempt may have combined
to create conditions for VRS to occur. However, the
rotor must be generating significant lift for VRS to
develop fully, and that would have occurred only after
collective and throttle application. These occurred
too close to the ground for VRS to develop sufficiently
to have had material effect. It is unlikely that fully
developed VRS was a factor in the accident however;
it is possible that the application of power during
the 'low level save' put the aircraft into the incipient
stage of VRS, thereby reducing the effectiveness of
the overshoot attempt.
As an interim measure, the entry altitude for the low
level turning autorotation was raised from 250 feet
above ground to 350 feet above ground to allow more
time for the set-up of the sequence. DFS further recommended
that:
a.a formal review of the policy for autorotation
training be conducted. The resulting policy must ensure
that pilots have the skills and knowledge to preserve
life and limb during helicopter emergencies requiring
autorotation. It should also maximize the potential
for saving the aircraft in such an emergency, but only
to the extent that it does not unnecessarily jeopardize
aircraft or crew in training.
b. as a part of the above review, the
possibility of establishing wind variability limitations
for autorotation training be investigated .
c. the feasibility of employing wind and
video recording equipment at 'Grabber Green' be investigated.
d. more emphasis be placed during Supervisory
and Proficiency Checks on low level save techniques
and recognizing the parameters when a low level save/overshoot
is required.
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