What happened
On 26 November 2008, an Airbus A320-232, registration G-EUUR, was operating a commercial passenger flight from London Heathrow to Glasgow. During the approach to Runway 23, the aircraft was being vectored by air traffic control (ATC) under instrument meteorological conditions.
While the aircraft was on a base leg, the air traffic control officer (ATCO) intended to provide a new heading to intercept the localizer. However, the controller recorded the heading on the ATC strip without transmitting the instruction to the flight crew. Believing the instruction had been communicated, the controller cleared the aircraft to descend to 2,000 ft. The flight crew, unaware of the specific heading change, accepted the descent clearance.
As the aircraft descended, a strong south-westerly wind pushed its ground track into an area of higher terrain. This triggered a GPWS mode 2 "terrain terrain pull up" warning. The pilot flying responded by disconnecting the autopilot, applying full thrust, and pitching the aircraft up significantly. The aircraft climbed to 5,000 ft, and the minimum radio altitude recorded was 959 ft.
During a subsequent descent to 4,000 ft, the aircraft's track again entered an area where the terrain minimum safe altitude (MSA) was 4,500 ft, though the crew remained unaware of this second breach.
The investigation
The AAIB investigation utilized flight data monitoring, radar, and ATC voice recordings, alongside interviews with the involved parties. The investigation examined the controller's actions, the flight crew's response, and the effectiveness of the Minimum Safe Altitude Warning (MSAW) system at Glasgow.
Findings
- The controller failed to follow the standard "write while you talk, read while you listen" procedure, resulting in a heading instruction being written but not spoken.
- The flight crew's decision to use an "open" descent mode contributed to a high rate of descent that, combined with rising terrain, triggered the GPWS alert.
- The controller issued a descent clearance without verifying the aircraft was in compliance with unit procedures regarding terrain clearance.
- The MSAW system at Glasgow was not available for immediate deployment due to technical issues with false warnings from non-controlled traffic.
- The controller's attempt to recover the approach by directing the aircraft toward the ILS centerline, rather than initiating a go-around, led to the terrain conflict.