Lessons Learned: Transient System Malfunction?
What caused this incident? Was a transient system malfunction responsible for the collective lever “jumping” upward and the aircraft lifting into the air?
On March 15, 2025, a private aircraft, Westland Wasp HAS1, G-CMBE, with a 1 Rolls Royce Nimbus 10301 turboshaft engine, was in Marston Doles, Warwickshire, UK, with a crew of two, a 66 year-old captain and a student pilot, carrying out pre-flight checks on what may have turned into a transient system malfunction.
Prior to the probable transient system malfunction, one of the checks required the deselection of the hydraulic system to ensure mechanical flying control continuity and freedom of operation in the correct sense. As the instructor selected the hydraulic system off, the collective lever ‘jumped’ upward and the aircraft lifted into the air. The instructor was unable to regain control and the helicopter struck the ground and rolled over. The cause of the abnormal collective movement could not be positively determined but a transient system malfunction could not be ruled out.
History of the flight
An instructor was preparing for a routine instructional type rating flight with an experienced rotary wing pilot. They were running through the post engine start pre-flight checks with the rotors running. One of these checks required the hydraulic system to be cycled off, to carry out an assessment of the flight control forces without hydraulic servo assistance. When completed the hydraulic system is selected back on for flight. Immediately the instructor selected hydraulics off, the collective lever forcefully lifted taking the instructor by surprise.
The helicopter became airborne to a height of 5 or 6 ft and ”lurched” forward and to the right towards a nearby hangar door. The instructor was unable to select the hydraulic system back on but immediately attempted to regain control. The helicopter moved back away from the hangar and tipped rearwards causing the tail rotor to contact the ground. As this happened the main rotor hit the ground and the helicopter rolled onto its side. The instructor then shut down the engine and made the aircraft safe, after which he extricated himself and assisted the student to vacate the cockpit. The helicopter sustained severe main, tail rotor and structural damage during the accident.
Final notes
He also consulted other Wasp and Scout helicopter pilots. In their experience, during this particular check, the collective lever was known to move upwards, but never more than a few centimeters. Its movement can easily be arrested by hand pressure. This matched the instructor’s experience when regularly carrying out or demonstrating this check.
The abnormally forceful upward movement of the collective lever, immediately after the hydraulic system was selected off, and the post-accident position of the servo jack, suggest the possibility of a transient system malfunction.
Content/image source/credit: AAIB (Air Accidents Investigation Branch): “AAIB investigation to Westland Wasp HAS1, G-CMBE,” published August 14, 2025.
