On 11 September 2021, a Boeing 737-800 (G-FDZF) being operated by TUI Airways on an international passenger flight from Palma to Aberdeen in day IMC was instructed to discontinue an ILS Cat 1 approach to runway 34 to destination but instead of climbing to and maintaining 3000 feet QNH as cleared, when it approached that altitude, a rapid descent began at an increasingly high speed towards terrain before a still high speed recovery climb was commenced at just over 1500 feet agl. The whole excursion occurred with the aircraft in cloud throughout. Once returned to the previous cleared altitude, further ATC instructions were followed and the remainder of the flight was uneventful.
A Field Investigation was carried by the UK Air Accident Investigation Branch (AAIB). Relevant recorded data was recovered from the operators OFDM service provider but that on the CVR had been overwritten as a result of the aircraft continuing in service prior to the AAIB being notified of the event. ATC radar and voice recordings were also available. It was noted that the 56 year-old Captain, who was PF during the excursion, had a total of 15,490 hours flying experience which included 1,524 hours on type. Similar experience and age information for the First Officer was not recorded.
When descending through 5,100 feet QNH five minutes after establishing initial contact with Aberdeen Radar to establish on a Cat I ILS approach to Runway 34, the flight was informed that it was possible that their approach may have to be discontinued due to the need to prioritise an imminent SAR helicopter departure in which case a climb ahead to 3000 feet could be expected.
Shortly after becoming fully established on the ILS at 3,000 feet QNH, with the AP and A/T engaged, the landing gear down and flap 15 selected, the radar controller instructed the flight to break off the approach, climb back to 3,000 feet and turn left onto 270°. After 12 seconds, the AP was disengaged and the A/T, which remained engaged, increased engine thrust to 97% N1, then after another six seconds, the gear was retracted and the aircraft, now at 2,250 feet QNH, began to climb towards the cleared altitude and started the left turn towards the required radar heading. However, as the aircraft which was being manually flown approached 3,000 feet, the A/T began to reduce thrust and the aircraft pitch decreased as the FD began to command the level-off. As the aircraft passed a recorded 2,850 feet, the flaps were retracted from 15 to 5 and on reaching a recorded 2,930 feet then began to descend. It briefly levelled at 2,650 feet and the flaps were retracted to Flap 1 and then, almost immediately, to zero after which the descent was resumed. Further heading instructions were passed by ATC whilst the aircraft continued descent, reaching a minimum altitude of 1,780 feet QNH (1,565 feet agl) before a climb was re-established. The rate of descent was recorded as peaking at 3,100 fpm as the aircraft passed 2,160 feet QNH.
The TWR controller saw from his radar repeater in the VCR (visual control room) that the aircraft was descending unexpectedly and contacted the radar controller to advise him which led to him instructing the flight to climb to 3,000 feet. This instruction was read back “at about the same time as the climb was initiated”. During the recovery, the airspeed initially continued to increase reaching a maximum of 286 KIAS whereas the speed which had been selected when the approach was discontinued was 200 KIAS and the maximum permitted speed in the airspace involved was 250 KIAS. As the aircraft finally reached 3000 feet, the AP was re-engaged and the remainder of the flight was uneventful. It was noted that there had been no EGPWS activation during the unintended descent.
The fact that the radar controller was unaware of the high speed and high rate descent which was occurring instead of returning to and remaining at 3000 feet was attributed by him to an assessment that “it was reasonable for the crew to complete a turn to the left and climb back to 3,000 feet QNH” whereas “had the aircraft been significantly lower, it would have been more appropriate to instruct the crew to conduct a standard missed approach” by continuing straight ahead and climbing back to 3,000 feet. He also stated that he had observed aircraft Mode ‘C’ return approaching 3,000 feet before beginning to communicate with two recent departures that required instructions.
It was noted that Aberdeen ATC had two non-mandatory alerting systems with activation thresholds which it was considered might have been reached. In the case of the MSAW system, it was found that when the aircraft descended to 1,780 feet QNH, it had not reached a position which would have put it “within 23 seconds of breaching 750 feet QNH” which was required to trigger a stage 1 system alert. In respect of the second such system, a radar-based ‘Descent Rate Monitor’, which was set to activate if a rate of descent of 2,500 fpm or more was detected below 3,000 feet QNH in controlled airspace, QAR-derived data indicated that although the aircraft had been descending at more than 2,500 fpm for approximately nine seconds, no alert was generated. This was explained when it was found that the detection process involved was based on successive six second radar sweeps of aircraft altitude to the nearest 100 feet and it was concluded that this would have meant that the derived vertical speed had not exceeded 2,500 fpm.
The annotated flight path showing the unintended descent. [Reproduced from the Official Report]
Why It Happened
It was considered that having pressed the TO/GA switches once for the go around, the crew would have expected the A/T to select thrust for a rate of climb of between 1,000 and 2,000 fpm but since the aircraft was above 2,000 feet agl, the thrust was (unexpectedly for the crew) advanced to TO/GA. This sudden large increase in thrust resulted in a significant pitch up and a correspondingly rapid rate of climb but when thrust was almost immediately reduced, the pitch attitude reduced. This pitch change was then exacerbated by flap retraction from 15 to 5 and since the aircraft had not reached the 3000 feet ‘ALT HOLD’ altitude, the AFDS remained (passively) in ‘ALT ACQ’ mode.
Completion of the remaining two stages of flap retraction then resulted in a further decrease in pitch attitude which, as the aircraft was descending, led to the speed increasing without regard to the 200 KIAS selected speed. At the same time, the crew were being given heading changes to get the flight onto downwind and taken together, it was clear that the situation rapidly led to both pilots becoming overloaded. Acknowledging and actioning heading instructions “could have distracted the First Officer from his monitoring tasks” whilst the Captain, flying manually, was having to manoeuvre the aircraft in roll during a very dynamic period of pitch change. Overall, it was concluded that the crew had been “unaware of the descent for a significant period” until the point at which the radar controller was alerted to the descent and called them.
The Investigation considered whether the crew might have been “affected by a somatogravic illusion as the aircraft accelerated, but although this could not be completely dismissed, an analysis of the OFDM data showed it was unlikely (with) any nose-down force on the controls during the initial part of the go-around being most likely due to the aircraft being out of trim” so that the pitch up caused by the unexpected large thrust increase being countered by the Captain pushing forward on the control column. It was noted that there had been no further nose-down inputs as the aircraft then accelerated during the descent.
It was already becoming clear that the fact that the decline in passenger air travel during the COVID 19 pandemic had led to most pilots flying significantly less than usual had been presenting challenges to the retention of normal pilot currency and skill levels with similar challenges affecting controllers. Nevertheless, the aircraft operator involved had rearranged their normal simulator recurrent training system explicitly to allow their pilots to maintain their skills in both the normal and emergency phases of flight whilst undertaking considerably less line flying duties than usual. However when presented with a two engine go around not commenced at the procedure DA, a procedure which is not often practised in simulators anyway, the circumstances encountered had evidently overloaded both pilots who had then been “unable to retain their situational awareness” during manually controlled flight and in particular had not responded to pitch and configuration changes by manually trimming the aircraft.
It was considered that whilst this go-around “should have presented little problem to the experienced crew”, the combination of less than average flying in the recent period (and very little flying in the case of the First Officer) could reasonably have led to both the aircraft not being managed effectively by the PF Captain and a monitoring failure on the part of the PM First Officer.
The formally-stated Conclusion of the Investigation was as follows:
The flight was instructed to go-around by ATC. After initially climbing towards the missed approach altitude, the aircraft began to descend. The descent continued for 57 seconds reaching a minimum of 1,565 feet agl before the aircraft was recovered to a climb. A combination of an unexpected large increase in thrust when the go-around was initiated, instructions from ATC to fly a heading and a lack of manual pitch trimming or flap configuration caused the crew to become overloaded, allowing the aircraft to descend unnoticed for a significant period. Both pilots had experienced significant periods away from flying the aircraft type during the pandemic.
Safety Action taken prior to completion of the Investigation was noted as having included, but not been limited to, the following:
- Conducted an extensive review of pilot recency-related safety events and additional company restrictions were introduced to safely manage pilots through a period of reduced flying.
- Reminded their Boeing 737 pilots that above 2,000 feet agl, a push of the TO/GA switches will provide full go-around thrust.
- Advised their non-Boeing 737 pilots of the incident.
- Planned to include enhanced go around training in the next recurrent simulator cycle and improve awareness and understanding of the AFDS system in GA mode and the potential consequences of exposure to two engine go-around events.
Aberdeen ATC introduced MATS 2 procedural changes in respect of aircraft being broken off from an approach after passing the final approach fix (FAF) which required them to be instructed only to make a standard missed approach unless there were “overriding safety considerations, or alternative instructions have already been issued” with headings only allocated once an aircraft is level at the missed approach altitude.
Boeing amended the FCOM in respect of the first push of the TO/GA switches at or above 2000 feet agl as follows:
If pushed at or above 2,000 feet RA (or below 1,500 ft if both RA’s have failed) with glideslope engaged or the flaps down: the A/T (if armed) engages in N1 mode and advances thrust towards the full go around limit. The A/T Engaged Mode annunciation on the FMA indicates N1.
The Final Report was published on 18 August 2022. No Safety Recommendations were made.