History of the Flight
The aircraft, a Boeing 737-8AS registered EI-ENT, was descending into London Gatwick Airport after a scheduled flight from Shannon Airport. The pilots anticipated conducting an ILS approach to Runway 08R. During the descent, ATC advised that an ILS approach was unavailable and that the aircraft would receive radar vectors for a visual approach. The pilot flying (PF) programmed the flight management computer so that the vertical navigation (VNAV) path information on the navigation display (ND) would correspond to a 3° glidepath.
ATC vectored the aircraft onto the final approach track earlier than expected, which placed the aircraft above the intended 3° glidepath. The pilots accepted an early turn onto the final approach with a reduced number of track miles to touchdown. The aircraft was flown with the autopilot engaged and LVL CHG mode active during the descent. The pilots assessed that using flap and speed brake would allow them to regain the correct glidepath and achieve a stable approach.
Shortly after, ATC requested a speed reduction. By approximately 4,500 ft, the aircraft was configured with landing gear down and flaps at 15°. The PF selected lateral navigation (LNAV) to intercept the final approach track. At 2,200 ft, the aircraft was established on the final approach track and approaching the 3° glidepath from above, descending at 1,500 ft/min. The pilots noticed the VNAV indication but omitted to engage VNAV mode. The aircraft continued to descend through the glidepath.
Alert and Recovery
At approximately 1,000 ft, the Enhanced Ground Proximity Warning System (EGPWS) generated a terrain caution, and the pilots observed the Precision Approach Path Indicators (PAPIs). ATC informed the crew that the aircraft appeared slightly low and asked if they were visual. The PF disconnected the autothrottle and autopilot, reduced the rate of descent, and re-established the correct approach angle. The aircraft continued the approach and landed without further incident.
Both pilots reported that due to haze, they could not see the PAPIs until the time of the terrain alert. They stated that because they could see the ground and runway throughout the approach, there was no risk of ground collision.
Analysis
The pilots accepted a shortened approach, resulting in the aircraft being above the ideal approach path. They used a combination of flap and speed brake to increase the rate of descent, aiming to re-establish the approach path and comply with the ATC speed constraint. The combination of the shortened approach track, the requirement to regain the correct glideslope, and other routine cockpit duties increased crew workload. Although the pilots noticed the VNAV path information on the ND, the increased workload distracted them from engaging the VNAV mode on the autopilot, which would have allowed the aircraft to follow a 3° glidepath. The aircraft remained in LVL CHG mode and descended through that glidepath. The pilots were alerted to the situation by a combination of the ATC call, the terrain caution, and seeing the PAPIs.