In February 2005, the Robinson Helicopter Company advised the
Australian Transport Safety Bureau that it had modified the R22
helicopter type seat structure design to strengthen the seatbelt
anchor points for both seats.
The circumstances of the accident are consistent with a loss of
control due to insufficient main rotor RPM being maintained, and
incompatible control inputs from the instructor and the student
following the initiation of the simulated engine failure by the
instructor. The reported actions by the instructor indicate that he
was attempting to recover the situation and allowing the student to
follow him through on the controls. The student also recalled
attempting to manipulate the helicopter's controls during the
descent. It was unlikely that the instructor could have maintained
effective control of the helicopter with both pilots manipulating
the controls. Procedures for clarifying who is in control at all
times, should be established and followed.
The helicopter manufacturer warned that to recover lost main
rotor RPM, the pilot must immediately roll on throttle and lower
the collective simultaneously. Both pilots reported that they could
not lower the collective to the full down position. The activation
of the low rotor RPM warning horn during most of the descent
confirms that the collective was seldom in the full down position.
The instructor reported attempting to increase the throttle
position, but it felt like the student had frozen on the throttle.
There were no defects found in the examination of the helicopter
that would have explained why the collective was not able to be
lowered to the full down position or the throttle increased. The
manufacturer cautions that once the main rotor RPM decreases below
80%, pilots may not be able to recover control even if the flight
controls are correctly positioned. Both pilots recalled seeing the
rotor RPM needle in the vicinity of 80% during the descent. The
student's recollection suggested that the rotor RPM may have
reduced to below 80%.
The investigation was unable to resolve the differences between
the statements by the instructor and the student with reference to
the way in which the throttle was reduced.
The seat structures are designed to deform during a high G
vertical impact, reducing the load transmitted to the seat occupant
and increasing survivability. However, in deforming, the seat
structure loses significant strength. In this case, the seat
structure lost sufficient strength to allow the left anchor point
of the left seat lap belt to tear free, increasing the risk of
injury to the seat occupant.
An instructor and student were conducting a training flight from
Caloundra aerodrome in a Robinson Helicopter Company Model R22
(R22) helicopter, registered VH-HBI. The weather was fine with a
light north-west wind.
The instructor reported that shortly after the helicopter
reached the intended initial cruising altitude of 1000 ft, and over
a suitable area that was clear of other traffic, he reduced the
throttle setting to idle to simulate an engine failure. The
instructor said the purpose of the exercise was to test the
student's alertness and ability to enter and maintain a stabilised
autorotation. He intended to terminate the practice engine failure
by introducing engine power at about 500 ft. During interview, the
instructor said that he rolled the throttle off quickly but
gradually. In a subsequent letter, he advised that he slowly
reduced the throttle setting to idle. The instructor advised that
he did not announce the simulated failure to the student, and that
he had previously initiated 'unannounced' engine failure exercises
to the student as part of the student's training.
The instructor reported that the student correctly applied right
anti-torque pedal and pulled the cyclic control rearwards, but did
not lower the collective lever. He also reported that the low rotor
RPM warning horn came on within a few seconds and the airspeed
reduced rapidly to between 30 and 40 kts. The instructor said that
the helicopter then started shuddering and he lowered the
collective as far as it would go and pushed the cyclic forward. He
said that, as they approached 500 ft he attempted to increase
engine power, but it felt like the student had 'frozen' on the
throttle and he was not able to rotate the twist grip. The
instructor said that the low rotor RPM horn was on for most of the
descent. He said that the rotor RPM was just above the horizontal,
around 83%, and the engine RPM was at idle on the engine and rotor
RPM indicator (see Figure 1). He said that he did not advise that
he was taking control of the helicopter, but provided 'spoken
instruction, supported by directive pressure on the controls',
because he wanted the student to feel the control movements.
Although the instructor recalled applying up collective to cushion
the helicopter for landing, the pilots were not able to terminate
the helicopter's descent.
The helicopter impacted tidal mudflats near the northern end of
Bribie Island in a nearly level attitude with a high rate of
descent and low main rotor RPM. The floor of the helicopter
sustained significant deformation and physically trapped the
student's feet. Both occupants of the helicopter sustained serious
injuries during the accident.
An examination of the wreckage did not identify any defect that
would have prevented normal operation of the helicopter prior to
the accident. The collective was able to be moved to the full down
position and there was no indication of any restriction.
The student was unable to recall much of the sequence of events
during the occurrence. He said that the instructor did not mention
the possibility of unannounced engine failures during the flight.
In an interview with the ATSB, he said that the instructor had
rolled off the throttle. Later, in a letter he said that the
instructor snapped the throttle off very rapidly. The student
described the correct response to a simulated engine failure, but
said that on this occasion, the collective did not feel like it
went all the way down. The student recalled seeing the engine RPM
and rotor RPM needles below the horizontal position on the engine
and rotor RPM indicator (see Figure 1), when the helicopter was
descending through about 700 ft. The student said that during the
descent the collective was about half way up and although he tried
to push it down, it didn't feel like it moved.
The helicopter manufacturer advised that deformation of the
seats provided additional absorption of vertical energy beyond that
required for certification. Both crew seat structures significantly
deformed during the impact with the right seat being more affected.
The right seat had a plastic first aid kit under it, which had been
crushed. The left anchor point of the left seat lap belt had torn
free of the seat pan. The rivets attaching the anchor point had
stretched, but not separated, however the sheet metal had failed,
allowing the anchor point to come free.
The helicopter was operating with a valid maintenance release,
and had accrued 387 hrs total time in service. The instructor had
accumulated approximately 925 hrs total helicopter experience, of
which about 158 hrs was in the R22. He had completed his instructor
rating with the same operator, and had a total of about 105 hrs
instructional time, all in R22 helicopters. The student pilot held
the equivalent of a helicopter student pilot license. He had
accrued about 94 hrs in the R22, of which about 16 hrs was in
command.
The R22 Pilot's Operating Handbook stated that during an
autorotation (prior to the flare), the collective should be
adjusted to maintain the rotor RPM in the green arc between 97% and
104%, or approximately 90% to obtain maximum glide distance.
Section 4, page 10 of the R22 Pilot's Operating Handbook
stated:
CAUTION - During simulated engine failures, a rapid
decrease in rotor RPM will occur, requiring immediate lowering of
the collective control to avoid dangerously low rotor RPM.
Catastrophic rotor stall could occur if the rotor RPM ever drops
below 80% plus 1% per 1000 ft of altitude.
The engine RPM and rotor RPM needles would have been in the
horizontal position when the respective RPM values were 80% (see
Figure 1). The low rotor RPM warning horn was designed to activate
when the rotor RPM was less than 97%. The warning horn did not
activate if the collective was in the full down position.
The R22 Pilot's Operating Handbook also included three Safety
Notices pertinent to this accident.
The first, titled 'Surprise throttle chops can be deadly',
stated:
The student may freeze on the controls, push the wrong
pedal, raise instead of lower the collective, or just do nothing.
The instructor must be prepared to handle any unexpected student
reaction.
The second Safety Notice, titled 'Fatal Accidents caused by Low
RPM Rotor Stall', stated:
No matter what causes the low rotor RPM, the pilot must
first roll on throttle and lower the collective simultaneously to
recover RPM before investigating the problem. It
must be a conditioned reflex.
The third Safety Notice, titled 'Practice autorotations cause
many training accidents', stated:
As the aircraft descends through 100 feet AGL, make an
immediate power recovery unless all of the following conditions
exist:
- Rotor RPM in middle of green arc,
- Airspeed stabilized between 60 and 70 KIAS [knots indicated
airspeed],- A normal rate of descent, usually less than 1500 ft/min,
- Turns (if any) completed.
This Safety Notice also stated:
Practice autorotations continue to be a primary cause
of accidents in the R22 and R44. Each year many helicopters are
destroyed practicing for the engine failure that very rarely
occurs.
A review of the ATSB database identified 18 accidents between
1985 and 2003 that occurred during autorotation training in
Robinson R22 helicopters.
Most flight instructor manuals emphasise the importance of
establishing and using procedures that at all times identify which
pilot has control of the aircraft. For example, the Flight
Instructor Guide - Helicopter (1995) issued by Transport
Canada stated:
CONTROL OF AIRCRAFT
2. There should never be any doubt as to who has control of the
aircraft. ...:(d) when the student has control, you must not "ride" the
controls. Your student may feel that you are taking control and
this could lead to a dangerous situation. Additionally, you may rob
your student of the feeling of accomplishing the manoeuvre
independently. This is particularly difficult during critical
manoeuvres, such as full-on autorotations, when there is little
time available to the instructor to correct errors. This procedure
must be adhered to at all times.