Accident Details
On 13 January 2007 at 1021 UTC, a Robinson R22 Beta, registration G-UNYT, was involved in an accident at Hollis Farm, Tupton, Derbyshire. The helicopter was on a private flight with one pilot and one passenger. The pilot, aged 46, held a Private Pilot's Licence and had 2,697 hours total flying experience, of which 1,500 were on type. The aircraft, manufactured in 1989, was powered by one Lycoming O-320-B2C piston engine. The pilot sustained minor injuries; the passenger was uninjured. The aircraft was destroyed.
Flight History
The flight was a 10-minute local flight returning to its base at Hollis Farm, a private landing site. The wind was from the west at 20–25 kt. The helicopter approached over an adjacent field, entered a hover, and began manoeuvring towards the landing site. As the helicopter turned, the pilot, who was also an instructor on this type, observed the low rotor rpm warning light illuminate and heard the associated horn. The helicopter descended and struck the ground with the front of the skids, then nosed over and came to rest inverted. Both occupants vacated without difficulty; the pilot re-entered the cockpit to turn off the engine ignition and the battery master switch.
Aircraft and System Information
The maximum permitted gross weight for the helicopter is 622 kg. The basic weight of G-UNYT was 400 kg. The pilot reported his weight as 82 kg, the passenger as 80 kg, and fuel weight as 33 kg, giving a total weight of 595 kg.
Normal rotor rpm is between 97% and 104%. A low rotor rpm warning (horn and light) activates at or below 95%. A governor system assists the pilot in controlling rotor rpm. The governor controller, a solid-state analogue circuit box, is mounted behind the left seat. An electrical rpm signal from the right magneto is sent to the governor controller, which compares it against a datum and applies corrective input forces to the throttle via a friction clutch that can be overridden by the pilot. The governor is active above 80% engine rpm and can be switched on or off via a toggle switch on the right seat collective control.
The collective pitch control lever is to the left of the pilot; a motorcycle-type twist grip throttle is on the forward end. The collective lever is connected to the main rotor blade pitch change mechanism by control rods. A mechanical linkage correlates collective pitch angle to engine throttle.
Examination and Findings
The pilot inspected the impact site and noted from ground marks that the left skid had slid approximately 2 ft and then dug into a hole, which he concluded caused the helicopter to nose over. He commented that he believed the low rotor rpm warning was due to a 'partial governor failure', which he had experienced previously on another R22.
The wreckage was examined by the AAIB and a representative of the helicopter manufacturer. The engine governor system was inspected. The maintenance manual indicates that most governor problems are caused by the magneto tachometer contact assembly being out of adjustment or faulty. The sensor connection to the right magneto was checked and appeared satisfactory.
The throttle connection was checked over the full range of movement and found to be functioning correctly. Friction checks were within maintenance manual limits. The pilot stated that he had experienced no previous problems with the governor system on G-UNYT. External examination of the engine showed evidence of power at impact. The governor was selected 'ON', carburettor heat was selected partially towards 'HOT', and the mixture control was full rich.
No cause could be found for a loss of rotor rpm. No faults were found in the governor system. The helicopter manufacturer and other operators stated that the governor system is extremely reliable and there is no history of 'partial governor failures'. Previous governor problems had not been experienced on G-UNYT, so a failure, although not impossible, appears unlikely.
When a helicopter enters a hover, the pilot raises the collective lever, and the governor system responds by opening the throttle to maintain rotor rpm. It is necessary to remain relaxed on the twist grip to allow the governor to operate. If the throttle is gripped too tightly, the governor actuator can be prevented from moving the throttle.
In a still-air hover, the required power is reduced due to ground cushion. This effect reduces as wind increases. At the time of the accident, G-UNYT was close to its maximum weight. In a hover with a wind of 20–25 kt, it is possible that there was a negligible power margin. Thus, the low rotor rpm warning could have been triggered if the collective lever was raised in response to a descent and, without more power available, the rotor rpm would start to decay.