What happened
On 7 February 2012, a Boeing 737-8AS, registration EI-ENT, was conducting a scheduled passenger flight from Shannon Airport to London Gatwick Airport. During the descent, air traffic control (ATC) informed the crew that an ILS approach was unavailable and provided radar vectors for a visual approach. The crew accepted a shortened approach track, which required them to intercept the final approach path earlier than originally planned.
To comply with ATC speed constraints and manage the shortened track, the crew used flaps and speed brakes to increase their descent rate. While the aircraft was configured with landing gear down and flaps at 15°, the crew was managing several simultaneous tasks, including adjusting the flight management computer to reflect a 3° glidepath on the navigation display.
During this process, the aircraft continued to descend through the intended glidepath. At approximately 1,000 ft, the crew received a terrain caution from the EGPWS, heard a call from ATC noting the aircraft appeared low, and observed the PAPI lights. The pilots immediately disconnected the autopilot and autothrottle, adjusted the rate of descent, and successfully re-established the correct glidepath before landing without further incident.
The investigation
The AAIB examined the flight data and crew statements to determine why the aircraft deviated from the vertical profile. The investigation focused on the autopilot mode settings and the impact of the modified approach instructions on the crew's performance. The investigation established that the aircraft was operating in the LVL CHG mode, which maintains speed by adjusting pitch and reducing thrust to idle, rather than following a vertical navigation path.
Findings
- The crew was managing a high workload due to the shortened approach, the requirement to meet speed constraints, and routine cockpit duties.
- The pilots failed to engage the VNAV mode on the autopilot, which would have automatically maintained the 3° glidepath.
- Increased cockpit workload prevented the crew from noticing that the VNAV mode had not been properly engaged.
- Hazy weather conditions meant the PAPI lights were not visible until the aircraft was at a low altitude, contributing to the delay in recognizing the deviation.