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 halfway 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.