History of the Flight
The aircraft, a DH82A Tiger Moth (registration G-AOES), was being flown by the pilot from the rear cockpit with the front cockpit vacant. During a local flight, the pilot conducted aerobatic manoeuvres including barrel rolls, a loop, a wing-over, and steep turns. A friend in another aircraft was flying nearby and the two remained in radio contact. On returning to Charity Farm Airstrip, the aircraft positioned low downwind to allow a model aircraft to land. As the pilot levelled from his descent, he suddenly realised his controls had jammed. The aircraft was just above tree top height. He informed the other aircraft of the jammed controls, but his next recollection was being in a dazed condition with the aircraft on the ground. The other pilot alerted Birmingham Approach, who called emergency services. The pilot, wearing a military helmet, suffered facial cuts from contact with the lower edge of the visor, which he considered 'very sharp'. He was taken to hospital and later diagnosed with a cracked vertebra and internal bruising to his groin.
Examination of the Aircraft
The aircraft came to rest beside a hedge and ditch bordering the airfield. The engine bay and front cockpit were severely crushed. The control columns in the front and rear cockpits are connected by an aileron torque tube and elevator push rod, enclosed in a wooden compartment on the fuselage floor. The compartment top is covered with plywood with two circular holes for the control columns, covered by leather gaiters to prevent loose objects from entering. On this aircraft, the front control column and gaiter had been previously removed to facilitate egress of a parachutist; the rear gaiter was torn. The front column stub tube protruded above the plywood decking. The rear control column had been pushed forward during impact. The front closure member and a section of plywood decking around the cut-out were pushed off. This could have occurred if an object, such as the harness Quick Release Fitting (QRF), became trapped between the cut-out edge and the stub fitting. Crushed wood fibres were observed on the internal edge of the forward left arc of the cut-out. The pilot noted that if the front harness lap straps fell onto the cockpit floor, the QRF could lodge in the gap between the stub fitting and the circular cut-out, potentially impeding elevator and/or aileron movement.
Potential Restriction of Control Column
The pilot commented that even with the front lap straps fastened, the shoulder harness adjustment was insufficient to keep the QRF clear of the control column base. He had secured the front harness by tying the shoulder straps around the lap straps, but the 'slippery nature' of the webbing may have allowed this to become undone. He suggested a restraint like an elasticated luggage strap could secure the harness when the front seat is unoccupied. BAE Systems stated that the certificated build standard of the DH82A included a hinged control box cover (part H19901) to prevent loose articles from causing control restrictions when a control column is removed; no modification to remove such covers has been approved. The pilot also noted that even with the gaiter in place, slack leather could allow loose objects to fall into gaiter folds and impede control movement.
Survivability Issues
The aircraft was equipped with 'Z type' harnesses, similar to those in de Havilland Chipmunk aircraft, using existing attachment points for Sutton harnesses originally fitted. The rear seat shoulder straps were attached at a single point to a transverse cable bolted to fuselage longerons behind the pilot's head/shoulders. This cable failed due to overload during impact, allowing the pilot's head to contact the instrument panel. The harness and remaining attachments held. The pilot wore a non-approved lap strap on top of the harness for additional security during aerobatics. The source notes that original Sutton harnesses were designed with a military seat parachute and not for crash restraint; without a parachute they do not meet original Air Ministry requirements. The impact parameters appeared similar to a previous accident (G-AOBJ on 20 August 1997). In that accident, the transverse cable failed similarly; G-AOBJ had Sutton harnesses made of canvas webbing, and the lap strap had failed, causing the pilot to be unrestrained. Following that accident, Safety Recommendation 98-40 was made to the CAA, recommending mandatory action to equip de Havilland Moth series aircraft with improved modern harnesses. The CAA accepted, requiring the Type Design Authority to issue a Service Bulletin limiting Sutton harness life or introducing modern harnesses. BAE Systems issued Technical News Sheet CT (Moth) No 33 Issue 1 in March 1999, establishing an installed life of 9 years from initial fitment or replacement within 150 hours or 3 years from issue date if fitment date unknown. The TNS notes that the safety harness fails to meet Air Ministry Design Leaflet E3 when military seat parachutes are not used, and recommends owners consider fitting improved harnesses made from modern materials.
Discussion
The accident occurred following reported jamming of the flying controls at a low height. The evidence strongly suggested that the jam had occurred as a result of the QRF of the unsecured front harness in the empty front cockpit (from which the column had been removed) having fallen into the gap between the front control column base stub fitting and the surrounding aperture, from which the protective leather gaiter had been removed previously.