Dual controls a hazard in ex-Army Scout
PUBLISHED: 11:18 23 June 2011 | UPDATED: 14:04 10 October 2012
The helicopter was an ex-Army Air Corps Westland Scout, recently refurbished and placed on the civilian Register. The pilot, a part owner, had completed his conversion training the day before and planned to ferry the Scout from Thruxton, Hampshire via Halton to its base at Welshpool
The helicopter was an ex-Army Air Corps Westland Scout, recently refurbished and placed on the civilian Register. The pilot, a part owner, had completed his conversion training the day before and planned to ferry the Scout from Thruxton, Hampshire via Halton to its base at Welshpool.
With the pilot were a pilot's assistant, who was an experienced fixed-wing pilot, and a helicopter ground-handler. They left Thruxton with the fixed-wing pilot in the left front seat.
Approaching the Chiltern Hills and cruising at 1,500 feet and ninety knots, they met severe turbulence. The helicopter pitched forward and to the right--almost at the same time the pilot noticed increasing loads on the cyclic pitch control.
Believing that the increased control loads were the result of a hydraulic failure, the pilot switched off the hydraulic system that provides servo assistance to the flying controls. The helicopter immediately and violently pitched down and to the right to the point where the pilot felt that the main rotor blades reached the vertical plane.
The control loads were extremely heavy and the pilot considered that his only hope of regaining control of the helicopter was to restore hydraulic power to the flying controls. After some difficulty in locating the switch, the pilot managed to re-establish hydraulic power and immediately the helicopter violently pitched up and to the left.
There was a bang followed by a loud mechanical noise, so the pilot began autorotation towards the only available field, which was immediately below him. At about fifty feet he applied collective pitch control to reduce the rate of descent and cushion the landing. As he did so the helicopter started to rotate and yaw; he immediately lowered the collective to stop the rotation and the helicopter hit the ground heavily. The pilot shut down the engine and the occupants got out unaided; one of the two passengers was seriously injured and collapsed not far from the wreckage. The Scout was beyond economic repair.
The pilot states that he misinterpreted the increased cyclic stick loads for a hydraulic failure and he did not check the HYD caption on the standard warning panel before selecting manual control--the violent manoeuvre and control difficulties after switching off the hydraulic power are consistent with manual selection having been accomplished in a manoeuvre above the maximum speed-the subsequent pitch up when hydraulic power was reinstated is consistent with aft pressure being applied to the cyclic when the reinstatement took place.
The uncontrollable yaw during landing was probably the result of low forward speed aggravated by automatic application of engine power as the collective lever was raised with the engine governor still engaged.
As to the cause of the initial increased cyclic load, military operators were concerned at the possibility of inadvertent interference with the flying controls when carrying passengers in the front seat, and it was common practice to remove the left seat controls when the helicopter was not being used for instruction. The possibility that a map might have impeded movement of the cyclic control was tried during a flight test in a similar aircraft and it was found that as the cyclic control was pulled back against a folded map the cyclic loads increased and were very similar to those described by the pilot. Also, a map could lodge under the edge of the trim switch, although both these causes seem unlikely in this accident. However, pilots should be aware of the hazards of dual controls in the Scout--especially the override facility of the left cyclic trim switch--and should brief their passengers accordingly.