What happened
On 1 June 2019, a Boeing 737-86N, registration I-NEOT, was conducting a commercial passenger flight to Bristol Airport. While on approach to Runway 27, the aircraft was flying an unstable approach following a shortened routing offered by air traffic control. The aircraft was traveling significantly above the target speed and was below the required vertical flight path.
During the approach, the tower controller, noticing the aircraft's position, instructed the crew to perform a go-around. Upon initiating the maneuver, the aircraft initially climbed; however, it then entered a descent that lasted for over 30 seconds. The aircraft reached a minimum radio altitude of 457 ft, triggering EGPWS alerts. The crew and the controller eventually realized the aircraft was not climbing as expected, at which point the crew corrected the flight path. The aircraft was subsequently vectored for a second approach, which was completed without further incident.
The investigation
The AAIB investigation established that the aircraft was unable to utilize VNAV mode due to excessive speed, resulting in the use of LVL CHG mode. Because the aircraft was in this mode, the crew had not updated the Mode Control Panel (MCP) to the missed approach altitude of 3,000 ft, leaving it set at the approach minima of 1,000 ft.
When the go-around was initiated, the autopilot disconnected, and the flight director commanded a pitch up. However, because the MCP altitude remained at 1,000 ft, the flight director eventually commanded a descent to capture that altitude. When the crew finally updated the MCP to 3,000 ft, the vertical mode reverted to V/S (vertical speed) mode, which captured the existing rate of descent. This caused the aircraft to continue descending until the crew manually intervened to establish a climb.
Findings
- The approach was rushed and unstable because the crew accepted a shortened routing that left insufficient distance to descend and reduce speed.
- The aircraft was flying significantly above the procedure speed and below the required vertical path.
- The altitude selector on the MCP was not updated to the go-around altitude, which led the flight director to command a descent to 1,000 ft during the maneuver.
- Neither crew member initially noticed the aircraft was descending during the go-around.
- The aircraft's landing gear warning horn and EGPWS alerts were triggered during the descent.