What happened
On 22 April 2011, a Beagle B121 Series 1 Pup, registration G-AXPM, was involved in an accident at Panshanger Aerodrome, Hertfordshire. The aircraft had undergone a 50-hour maintenance check the previous day. During the pre-flight inspection, the pilot performed the required manual propeller rotation. On the second rotation, the engine unexpectedly fired and immediately accelerated to full power.
As the engine ran, the aircraft broke free from its wing tie-downs. The aircraft accelerated in a westerly direction, striking the wingtip of a parked Cessna 3 and subsequently crashing into an earth embankment approximately 200 metres from its original parking spot. The impact left the aircraft nose-down in a ditch, causing significant damage to the propeller, landing gear, wings, and engine. There were no injuries to the pilot.
The investigation
Investigators discovered that during the previous day's maintenance, an engineer had used an incorrect key to operate the ignition switch. Because the maintenance organisation could not locate the correct key, the engineer used a spare key, which allowed the switch to be left in the RIGHT magneto position while the key was removed.
During the pre-flight inspection, the pilot observed the switch through the cabin door. Due to parallax error and the absence of a key in the switch, the pilot incorrectly perceived the switch as being in the OFF position. Furthermore, the aircraft's approved flight manual checklist required the pilot to set the throttle to fully OPEN and the mixture to FULL RICH during the external inspection, a configuration that would allow the engine to run at full power if the ignition were live.
Findings
- The engine started because the propeller was rotated while the right magneto was live.
- The ignition switch was left in the live position because an incorrect key had been used during maintenance, allowing the key to be withdrawn while the switch was still active.
- The pilot's perception of the switch position was obscured by parallax error and the lack of a key in the switch.
- The aircraft's pre-flight checklist design created a hazardous condition by requiring a throttle and mixture setting that would facilitate an uncommanded start if the ignition system were live.
Safety action
Following the investigation, amendments to the approved flight manuals for all series of the Beagle B121 Pup were prepared and promulgated to aircraft owners in September 2011.