A Bombardier BD700 Global Express was on approach to London Luton Airport, Runway 26, after an uneventfully positioning flight, onboard 3 crew members.
Bombardier BD700 Global Express (Sourced from public domain)
ATC instructed the crew to maintain a speed of 160 knots (184 mph / 296 kph) on the approach up to four miles from the runway. At four miles from the runway full flaps was selected and the speed was reduced to Vref+5 (Reference speed + 5 knots). of 121 knots (139 mph / 224 kph) The approach was reported as stable by the commander, with ATC providing the crew with the latest wind, 210º at 15 knots, resulting in a crosswind component of ~11 knots. The aircraft continued its approach with the autopilot and autothrottle engaged, the Flight Management System gave a steady speed command of 121 knots to the autothrottle system. In the last 5 seconds before touchdown, the speed decayed down to 114 knots, while the aircraft pitch varied from +6º to +2º (normal pitch attitude for landing ~+4º), in the last three seconds before touchdown the pitch increased to +8º. During the approach, the bank angle of the aircraft varied up to 10º to either side of the wings level.
At a height of 210 feet (~62 meters) above the runway, the left-wing dropped 10º from level, followed by a 5-second roll to 8º right wing down. This rolling motion continued up to a fraction of a second before touchdown when the left wingtip contacted the runway surface. During the final stage of the approach, the autopilot was disconnected below 10 feet radio altitude and the autothrottle was disconnected below 4 feet radio altitude After touch down the rollout was normal and the aircraft taxied to the ramp where it was guided by a marshall to its parking position. The marshaller noticed that the left-wing was damaged, and informed the crew after the engines were shut down. An inspection revealed damage to the following parts of the wing;
Trailing edge flap
Flap track fairings
A Bombardier Global Express with light wingtip damage after a tip strike. (on a different aircraft as discussed in this blog, AAIB Bulletin: 10/2014)
An investigation was launched by the Air Accident Investigation Branch and several observations were made;
The aircraft flight manual states that the minimum altitude for autopilot use ‘for a precision approach (Category I or II ILS) is 50 ft AGL’. (The Global Express is not equipped with an autoland capability}
The approach chart states; Due to the sloping terrain in the approach area of ILS Cat II operations, the rate of radio altimeter height reduction prior to the threshold will be approximately double the normal rate.
Were the latter might be the reason for, what appears, the late disconnection of the autopilot. The full report into the tip strike is available by clicking here.
Due to the geometry of the aircraft, the supercritical wing has a 35º sweep and the span is so great (95 feet / 29. 5 meters) that the wingtips extend far backwards causing the tips to get closer to the ground when the aircraft is pitched up.
Diagram showing the effect on wingtip clearance in relation to aircraft pitch. (source AAIB Bulletin: 10/2014)
The table below shows the difference between the degrees of bank angle that will result in a tip strike with the aircraft static on the ground and with the wings producing lift.
(source AAIB Bulletin: 10/2014)