What happened
On 3 December 2000, a Piper PA-2SB-200-2 Cherokee Arrow II, registration G-BKCB, was conducting a private flight near Thruxton Airfield with the pilot and three passengers. While flying in clear air at approximately 4,200 feet, the aircraft suddenly deviated from its straight and level flight path. Witnesses observed the aircraft rolling and pitching violently, with some describing the motion as aerobatic. During this period, structural debris, including a significant portion of the left wing, was seen to separate from the airframe. The aircraft subsequently entered a descent, losing control and impacting trees near Warren Farm, Lambourne, Berkshire. The impact resulted in four fatalities.
The investigation
The AAIB investigation focused on determining the cause of the sudden structural failure. Detailed examination of the wreckage revealed that the outer section of the left wing had failed in a downward direction. Investigators found no evidence of pre-existing structural defects, corrosion, fatigue, or material flaws in the wing or the flight control systems. Furthermore, there was no evidence of atmospheric turbulence or any external factor that would have necessitated an evasive manoeuvre.
To understand how such a failure could occur at or below the design manoeuvre speed (Va), specialists conducted finite-element analysis and aircraft simulations. The study explored the possibility of a "checked" manoeuvre—a rapid, large-scale control deflection followed immediately by a reversal. The analysis demonstrated that while the aircraft was structurally capable of handling single-axis inputs at these speeds, a simultaneous and rapid reversal of both pitch and roll controls could generate enough stress to exceed the material's yield strength and cause the wing to fail in download.
Findings
- The primary cause of the accident was a structural failure of the left wing caused by excessive downward loading.
- This loading was likely produced by unintentional and unusual control inputs involving simultaneous and rapid movements in both the pitch and roll axes, followed by a rapid reversal.
- The investigation considered a scenario where a passenger, perhaps reaching for a handhold, inadvertently moved the control yoke, which the pilot then attempted to correct with a violent recovery manoeuvre.
- No technical failures of the autopilot or pitch trim systems were identified as the cause of the uncommanded movement.
- The aircraft was flying at an airspeed near its design manoeuvre speed, but the investigation highlighted that flying at or below this speed does not protect the airframe from the stresses of extreme, multi-axis control reversals.