What happened
On 10 December 2013, a Gulfstream 550, registration HZ-A6, was conducting a commercial charter flight from Riyadh to London Stansted. During the approach to Runway 22, the aircraft encountered fog, with the runway visual range (RVR) reported at approximately 300 to 325 metres, which was below the required CAT I minimum of 550 metres.
While the aircraft initially captured the localiser and glideslope using autopilot and autothrust, the approach became unstable. At approximately 1,600 feet, the autopilot and autothrust were disengaged, and the commander began hand-flying the aircraft. During this phase, the aircraft's speed exceeded the flap limiting speed, triggering an overspeed warning. The aircraft subsequently flew above the glideslope, prompting air traffic control to advise the crew of the deviation.
At an altitude of approximately 691 feet, the go-around mode was selected, though the reason for this selection remained undetermined. This mode change inhibited the Enhanced Ground Proximity Warning System (EGPWS) glideslope alerts. The crew continued the approach visually, but the aircraft eventually descended significantly below the glidepath. At 19 feet above the ground, the left landing gear and the underside of the left wing struck the Runway 22 ILS localiser monitor aerial and the Runway 04 localiser aerial array. The aircraft touched down 109 feet short of the runway threshold.
The investigation
The AAIB examined the aircraft's flight data recorder (FDR) and cockpit voice recorder (CVR), as well as the maintenance history and the airfield equipment. Investigators analyzed the flight path, noting that the aircraft's descent below the glideslope was the direct cause of the impact with the antenna structures. The investigation also looked into the functionality of the EGPWS, concluding that while the system was operational, the selection of go-around mode prevented the glideslope warnings from alerting the crew to the dangerous descent.
Findings
- The approach became destabilised due to the crew attempting to correct the flight path at a late stage rather than executing a go-around.
- The aircraft descended below the glideslope, leading to the collision with the ILS localiser equipment.
- The RVR at the time of the approach was below the 550 m minimum required for a CAT I approach.
- The selection of go-around mode inhibited the EGPWS glideslope warnings, meaning the crew did not receive automated alerts regarding the vertical deviation.
- The crew continued the approach based on visual references to the runway and approach lights, despite the approach parameters being outside company-defined stability limits.