Incident Overview
On 26 March 2014, a Bombardier CL600-2B16 (registration N664D) operated by an undisclosed commercial air transport operator was conducting a post-maintenance check flight at Biggin Hill Airport, Kent. The flight, which carried three crew members and no passengers, ended with all four main wheel tyres deflating and damage to the main wheels and brake units. No injuries were reported.
Background and Flight History
The check flight was required to perform an airborne test of the aircraft's air-driven generator (ADG) as part of its scheduled maintenance programme. The aircraft had previously flown to Biggin Hill for maintenance, but the ADG test had not been completed prior to landing, necessitating a dedicated check flight.
During the flight, the commander acted as handling pilot. After takeoff from Runway 03, the crew maintained the takeoff flap setting of 20°, as required for the ADG test. On the downwind leg, they took the main generators off-line to simulate failure conditions, causing the ADG to deploy. The test confirmed correct deployment and functioning. However, the aircraft was not returned to its normal electrical configuration.
On the base leg, the pilots selected wing flaps for landing, but they remained at 20°. The commander continued the approach with a reduced flap setting at approximately 160 kt. The aircraft touched down at an estimated 150 kt. The commander selected reverse thrust, but only the left thrust reverser deployed. The aircraft decelerated slower than expected, and the commander applied maximum wheel braking, bringing it to a stop about 120 m from the runway end. The landing distance available was 1,550 m.
Air Traffic Control reported seeing smoke from the tyres during the landing roll and alerted the Airport Fire and Rescue Service, who attended the scene. The pilots were unable to taxi the aircraft off the runway; subsequent inspection revealed that all four main wheel tyres had ruptured, and the main wheels and brake units on the left side were damaged.
Technical Context
The aircraft operating manual includes a detailed procedure for the ADG in-flight check, which states: "Normal electrical power must be restored before final landing approach is commenced." The procedure involves manually deploying the ADG, turning off main generator switches, verifying ADG functionality, and then restoring normal power by switching generators back on and pressing a power transfer override switch. The manual also lists systems unavailable when operating on emergency power, including flaps, ground spoilers, nosewheel steering, and brake anti-skid. Additionally, a note warns that if the ADG is the only electrical power source for landing, the manual deployment handle should be pulled to keep the DC essential busbar powered by batteries.
Crew and Engineer Reports
The commander reported that roles were not clearly established before takeoff; the pilots assumed the engineer on the jump seat would guide them through the check, while the engineer considered his role primarily observational. The commander later acknowledged that the crew should have referred to the Quick Reference Handbook rather than relying on the engineer. The engineer noted that he did not see the flight crew read the ADG test procedure, and that after the generators were switched off, the flaps did not move when selected. He believed the co-pilot announced the abnormality twice without response from the commander. The engineer expected the approach to be discontinued, but it was not.
Recorded Data
The Cockpit Voice Recorder was not isolated promptly after the incident, and the recording was overwritten. The Flight Data Recorder showed the flight lasted 271 seconds from liftoff to touchdown. Airspeed on short final was 172 kt, reducing to about 159 kt at touchdown. The left thrust reverser first indicated unlocked at 153 kt; the right thrust reverser remained stowed throughout the landing roll.
Maintenance Organisation Findings
The maintenance organisation conducted its own investigation and recommended that full briefings be given to flight crews undertaking maintenance check flights, regardless of whether the crew claimed familiarity with the procedure.