Introduction
On 11 September 2021, a Boeing 737-8K5 (registration G-FDZF) operated by a commercial air transport company experienced a serious incident during a go-around at Aberdeen Airport. The aircraft was on a scheduled passenger flight from Palma de Mallorca to Aberdeen with 67 passengers and 6 crew on board. The incident came to the attention of the AAIB on 13 September 2021, after which an investigation was initiated with assistance from the operator, the US National Transportation Safety Board, and the aircraft manufacturer.
History of the Flight
After departing Palma at 1047 hrs, the flight approached Aberdeen for a radar-vectored CAT I ILS approach to Runway 34. At 1230 hrs, the crew contacted Aberdeen Radar. At 1235 hrs, while descending through 5,100 ft amsl, ATC advised that a search and rescue helicopter on the ground might take priority, requiring a possible discontinuation of the approach with a climb straight ahead to 3,000 ft amsl. At 2,600 ft amsl, the radar controller instructed the crew to break off the approach, climb to 3,000 ft, and turn left onto heading 270°. Eighteen seconds later, at 2,250 ft amsl, the aircraft began climbing and turning. However, as it approached 3,000 ft amsl, the aircraft began to descend before the flight director could transition from ALT ACQ to ALT HOLD. The aircraft descended to a minimum altitude of 1,780 ft amsl (1,565 ft agl) with a peak descent rate of 3,100 ft/min. The tower controller noticed the descent on radar and alerted the radar controller, who instructed the crew to climb. The crew initiated a recovery, and the aircraft began climbing again, reaching a speed of 286 kt (selected speed was 200 kt). The event occurred in IMC. After stabilizing, the aircraft was vectored for another approach and landed without further incident.
Flight Data and Go-Around Mode
Recorded flight data from the operator's flight data monitoring (FDM) system, along with radar and R/T recordings, were analyzed. The aircraft's Cockpit Voice Recorder had been overwritten. The Boeing 737-800 is equipped with dual autopilots, and automatic go-arounds are only available from a dual autopilot approach. During the go-around, the autopilot was disconnected when the TO/GA mode was activated. The landing gear was retracted, thrust increased to the full go-around N1 limit, and pitch increased. As the aircraft approached 3,000 ft, pitch decreased and autothrottle reduced power. Flaps were retracted from flap 15 to flap 5, and subsequently to flap 1 and flap 0 during the descent. The aircraft failed to meet the criteria for the flight director to transition to ALT HOLD. The recovery began when the crew responded to ATC instructions. The autopilot was re-engaged after passing through 3,000 ft.
Crew Recency
Both pilots had experienced significant periods without flying in the preceding 18 months. The commander had flown 10 flights in the previous month, while the co-pilot had completed only two flights with a trainer seven days before the incident, this being his fourth flight in nearly 11 months. Both had completed numerous simulator sessions. The investigation did not establish a link between this incident and lack of recent line flying, but considered it a possibility. The bulletin notes that go-arounds from intermediate altitudes are not practiced frequently and can provoke errors.