What happened
On 15 June 2017, a Cessuna 510 Citation Mustang, registration OE-FHK, was parked at London Biggin Hill Airport for a training flight. The flight was intended to be a type rating training session for a co-pilot, overseen by a commander acting as a Type Rating Examiner.
Prior to the flight, the aircraft had been moved to its final parking position. Because no ground personnel were immediately available to assist, the commander decided to personally remove the wheel chocks. The aircraft was parked on a slight slope, and the commander signaled to the co-pilot to engage the parking brake. The co-pilot applied the brake pedals and pulled the parking brake handle, signaling to the commander that the action was complete.
After the commander removed the chocks and moved away to place them on the ground, the aircraft began to roll forward. The commander observed the aircraft moving toward a fuel bowser that had stopped approximately two metres in front of the aircraft. Despite the co-pilot's attempts to apply the brakes, the OE-FHK did not stop until it struck the bowser. The collision caused dents to the forward fuselage and the left wing's leading edge, and distorted the cabin access door hinge. There were no injuries to the two crew members on board.
The investigation
The AAIB investigation focused on the functionality of the braking system and the procedures followed by the crew. The investigation established that the aircraft's hydraulic system was powered by an electric pump and an accumulator. While the parking brake is designed to trap hydraulic pressure to maintain a locked state, this pressure can dissipate if the pump is not active.
It was found that the battery switch had not been set to the 'batt' position, meaning the electric hydraulic pump was not running. The investigation revealed that the co-pilot had not been instructed to ensure the battery was active before attempting to set the parking brake. Furthermore, the commander had assumed that sufficient pressure would remain in the accumulator to hold the brakes without active pump power, unaware that pressure could dissipate within 30 minutes of the power being disconnected.
Findings
- The aircraft rolled because the hydraulic system lacked sufficient pressure to maintain the parking brake due to the electric pump not being active.
- The co-pilot applied the parking brake without the battery switch being in the 'batt' position, which prevented the hydraulic pump from maintaining system pressure.
- The commander removed the wheel chocks without ensuring the parking brake was effectively secured by active hydraulic pressure.
- There was a lack of specific guidance in the pilot training manual regarding how long the accumulator could maintain pressure or how many brake applications were possible without the pump running.
- The aircraft operator's procedures did not explicitly cover the scenario of flight crew removing chocks themselves in the absence of ground personnel.