What happened
On 7 July 2014, a Percival P66 Pembroke C Mk1, registration G-BNPH, was conducting a private flight from RAF Waddington to MOD St Athan. During the approach to Runway 26, the pilot encountered turbulence and windshear, though the landing itself was initially described as normal. However, the aircraft landed slightly left of the runway centerline and began drifting toward the left edge of the runway.
In an effort to correct this drift, the pilot applied full right rudder and attempted to use gentle braking. This action resulted in a rapid rightward yaw of approximately 70 degrees. After observing that the area ahead was clear of obstacles, the pilot allowed the aircraft to depart the paved surface and move onto the grass. The aircraft was eventually brought to a halt parallel to the runway. There were no injuries and no damage to the aircraft or surrounding structures.
The investigation
The investigation focused on the mechanics of the aircraft's braking system and the pilot's inputs. The Percival P66 Pembroke C Mk1 utilizes a pneumatic braking system acting on the two main wheels. Control of braking effort is managed via a lever on the main control yoke, while brake pressure is distributed differentially to the wheels based on rudder pedal deflection. Consequently, applying full rudder directs the majority of braking force to the corresponding wheel.
Findings
- The initial drift toward the left edge of the runway was caused by an unexpected change in wind direction immediately after touchdown, which caused the aircraft to weathercock.
- The pilot applied more asymmetric braking force than intended due to the high sensitivity of the braking system when the rudder is at full deflection.