What happened
On 14 May 2009, a Pitts S-12 Special, registration G-PXII, was performing a private aerobatic training session over White Waltham Airfield in Berkshire. The pilot was attempting a 'cobra' maneuver, a complex sequence involving reducing speed to below 80 mph at idle power before applying full throttle to pitch the aircraft into a near-vertical attitude.
During the maneuver, witnesses observed the aircraft pitch upward almost vertically. At the peak of this movement, the engine was heard to cough. Following this, the aircraft failed to recover to a safe flight path; instead, it pitched into a downward descent. The aircraft struck the ground with a high rate of descent near the threshold of the grass Runway 29. The impact caused the aircraft to bounce, resulting in the destruction of the landing gear, propeller, and engine cowling, as well as damage to the wings and fuselage. The pilot sustained serious injuries.
The investigation
The investigation examined the mechanics of the maneuver and the performance of the Ivchenko Vedeneyev M-14P engine. The pilot noted that while he intended to perform the maneuver at a height of 750 to 850 feet, this specific attempt began at approximately 600 feet.
Investigators consulted other pilots familiar with the M-14P engine, including a co-owner of the aircraft. These pilots confirmed that the engine can exhibit a slow response when the throttle is opened rapidly from idle. Furthermore, the investigation looked into the aerodynamic characteristics of the aircraft, noting that at speeds below 40 mph, the ailerons may lack sufficient authority to maintain roll control, particularly if power is insufficient during the climbing phase.
Findings
- The aircraft entered the maneuver at an altitude of approximately 600 feet, which provided insufficient margin for recovery.
- Slow engine acceleration following a rapid throttle opening likely caused a momentary loss of power during the climb.
- This lack of power led to a rapid loss of airspeed, resulting in insufficient flight control authority to prevent the aircraft from entering a downward flight path.