Description

On 2 September 2019, a Boeing 777-300ER (9V-SWD) being operated by Singapore Airlines on a scheduled international passenger flight from Shanghai Pudong to Singapore Changi failed to climb as expected when the autopilot was engaged soon after takeoff in night VMC and a series of EGPWS ‘DON’T SINK’ cautions were followed by a ‘PULL UP’ activation. The autopilot was disconnected and climb restarted but when it was re-engaged, the aircraft again began to descend and it was only then realised that the FMS had been mis-programmed. Once corrective action was taken, the remainder of the flight was without further event. 

Investigation

An Investigation was commenced by the Singapore Transport Safety Investigation Bureau (TSIB) and was based on QAR data provided by the aircraft operator and interviews with the flight crew. Relevant CVR data were overwritten.
The 55 year-old Captain, who was acting as PF for the flight, had a total of 18,369 hours flying experience including 11,085 hours on type and the 29 year-old First Officer had a total of 1,744 hours flying experience, all of it on type.

What Happened

It was established that the pre-flight preparation had been somewhat more involved than normal (discussed below). It was also learned during the subsequent Investigation that whilst the Captain intended to engage the AP soon after the takeoff “because he was mindful of the need to convert altitude clearances given in metres to feet and of the potential weather to the east of the airport”, the First Officer did not recall this plan being communicated to him.    

On completion of pushback and engine start, clearance to taxi to the holding point for the expected departure runway 35R was given and an uneventful taxi and takeoff followed. Passing approximately 360 feet agl with landing gear retraction completed, the Captain called for the AP to be engaged. According to the subsequent statement of the First Officer, he noted that on passing 400 feet agl, the FMA (Flight Mode Annunciator) indication on the PFDs had changed from ‘HOLD / LNAV / TO/GA’ to  ‘SPEED / LNAV / VNAV PTH’ and he had accordingly called out ‘SPEED / VNAV PTH’ twice. In his subsequent account of the departure, the Captain stated that he had not heard this call and the First Officer did not pursue seeking a response.

Soon after the aircraft had climbed above 500 feet agl, both pilots noticed an increasing speed trend indication on their PFDs. The Captain reported having “believed that the aircraft had crossed 1,000 feet agl” and so called for flap retraction which the First Officer actioned in stages as instructed. However, neither pilot had looked at the PFD altitude indication and recorded flight data showed that in fact after the AP was engaged, the aircraft had climbed from to 750 feet agl before descending back towards 500 feet agl. Although unrecognised by both pilots at that time, this was due to an incorrect flight altitude constraint for the first SID waypoint entered into the FMC during pre flight preparations. 

During the first stage of flap retraction, an EGPWS ‘DON’T SINK’ Caution was annunciated which was reported to have startled both pilots. Nevertheless, the Captain stated that he had “not rushed into reacting to the caution as he had deemed the flight was stable and had decided that flap retraction should be completed”. Although he had not noticed the loss of height on his PFD, he stated that he had noticed a tailwind on his ND and this had led him to say to the First Officer that the EGPWS Caution “could be due to the aircraft decreasing its pitch as a result of the tailwind”. The First Officer stated that although he did not think so, he had not commented to that effect.

Recorded flight data showed that nine seconds after the first EGPWS “DON’T SINK’ Caution had been annunciated and whilst the flaps were still being retracted, a second such Caution occurred followed by a third after a further nine seconds. Both pilots then reported having “realised that the aircraft had levelled off and they needed to reinitiate a climb”. In fact the aircraft had descended back to around 500 feet agl and oscillation between 480 to 500 feet agl was the cause of the continuing EGPWS ‘DON’T SINK’ Cautions.  

The Captain’s initial action to restore a climb was to keep the AP engaged and select FLCH mode on the MCP but he then replaced this two seconds later with the VNAV mode. He stated that he had thought that he had resolved the issue as the ‘DON’T SINK Caution had not recurred. However, a fourth EGPWS ‘DON’T SINK’ Caution occurred nine seconds after VNAV was re-selected and this time was followed by an EGPWS ‘PULL UP’ Warning. In response to this, the Captain disconnected the A/P and manually increased engine thrust and began to climb the aircraft at a pitch attitude of 15° which led to the ‘PULL UP’ Warning ceasing.

As the aircraft passed approximately 1,600 feet, the Captain called for the AP to be re-engaged. However, neither pilot had yet worked out why the unexpected descent had occurred and with the VNAV PTH mode still active, at about 1,780 feet agl, the aircraft then began a second pitch down to regain the inadvertently programmed target altitude of 500 feet. Only then did the First Officer notice on his ND that there was a “250/0500” speed/altitude constraint set for the first waypoint PD062 and realise that it was this altitude constraint that had been causing the aircraft to attempt to again descend to 500 feet agl when the AP VNAV mode had been re-engaged. He immediately alerted the Captain and pushed the altitude selector button on the MCP to delete the programmed speed/altitude constraint which fully resolved the flight path control issue and the flight was thereafter completed without further event.

The incorrectly input 500 feet flight altitude constraint for the new first waypoint in the FMC which caused the unintended descent and high speed terrain proximity each time the AP was engaged was introduced inadvertently by the Captain during pre flight preparations without being detected in the required cross check. The Captain subsequently informed the Investigation - in considerable detail - that his pre flight preparation had been rather more involved than usual due to a combination of procedural issues arising from one of the ADDs which was subject to MEL procedural modifications and the intermittent functioning of both CPDLC and ACARS.

He had delegated the external pre-flight check to the First Officer and whilst they were away from the flight deck for this purpose, he had manually input the SID for the expected runway 34L departure into the FMS noting that the first waypoint of this SID - the ‘HSN’ VOR - had a pre-programmed speed/altitude constraint of ‘250/1970A’ which meant that the speed should not be more than 250 KIAS and the altitude should be at or above an altitude of 1,970 feet at that position.

Following the First Officer’s return to the flight deck, the Captain advised him that the departure runway would now be 35R rather than the 34L for which his FMC SID data had been originally entered into the FMC. The consequence of this was that the SID now went to the ‘HSN’ VOR via another intermediate waypoint - PD062 - which had no speed/altitude constraint. Instead of amending the initially input active route, the Captain used the ‘ROUTE COPY’ function to duplicate the initial data and then manually amended its SID information by adding the additional waypoint. Whilst subsequently briefing the First Officer, the Captain noticed that the new first waypoint of the updated SID was not showing a speed/altitude crossing constraint (since there was none). Whilst recognising that this was not abnormal, the Captain stated that as he “preferred to have the speed constraint explicitly displayed, he decided to input a speed constraint of “250/0500” for this waypoint (but left off the ‘A’ suffix). In accordance with the cross-checking requirement, the First Officer “observed the Captain’s inputs and accepted them as correct” and the Captain then activated the flight path and selected it to ‘execute’. The insertion of the speed constraint without the suffix ‘A’ and the unusually rapid engagement of the AP resulted in the aircraft entering a descent at high speed as it sought to return to and then maintain 500 feet after passing the new first waypoint. 

Discussion

In seeking to explain the unexpected vertical profile followed, the observations made included the following:

• The aircraft type FMC was programmed to only display a speed constraint when there was an associated procedural altitude constraint at a waypoint. 
• The Captain was reportedly aware that since the FMC was automatically programmed to restrict the aircraft speed to not more to 250 KIAS below 10,000 feet, there was no need to insert such a speed restriction for any waypoint below that altitude.
• There were a number of instances in this occurrence where the flight crew did not comply with the operator’s SOPs. These included the following:
  •     the Captain manually input the anticipated SID into the FMS without the First Officer being present to cross check his actions as required. 
  •  The Captain failed to activate the amended FMC SID until after adding the constraint. Had it been activated before selecting the SID, the FMC would have automatically displayed the predicted speed and altitude of all the waypoints when the SID was selected whereas instead the new initial (PD062) waypoint was presented without speed/altitude constraints which prompted the Captain to make his inadvertently erroneous manual entry and overwrite the stored SID requirement for the waypoint.
  • When the Captain did not acknowledge the First Officer’s call of “SPEED / VNAV PTH” on seeing the VNAV engaged on the FMA, a response which is required, the First Officer did not challenge him to do so.
  • The Captain’s response to the EGPWS ‘PULL UP’ Warning was prompt but not fully in accordance with the applicable procedure which required disengaging the A/T and pitching the aircraft to 20°.  
• There were a number of instances during the build up to the event and the event itself which indicated sub-optimal CRM. These included the First Officer not challenging the Captain for a required response or when he considered that an explanatory remark was not necessarily correct or an action unnecessary. They also included the Captain apparently omitting to advise the First Officer of his intention to engage the AP as soon as the landing gear was up and the First Officer’s failure to notice the Captain’s omission of the suffix ‘A’ (for ‘above’) from his late change to the first FMC SID waypoint.

In general, it was considered that the First Officer had been insufficiently assertive and should have applied the escalation technique taught in CRM training to alert the Captain to his concern regarding any lack of response noting that “good communication can serve to maintain a shared understanding and situation awareness”.

• The fact that both pilots were able to simultaneously lose critical situational awareness (aircraft altitude) when confronted by an unexpected EGPWS activations directly related to a low altitude condition at night.
• The extent to which the Captain had been subject to significant stress which had contributed to his lapses in FMC programming and decision-making during the flight was unclear but the potential for a series of individually minor matters he had encountered during the pre flight preparation to have cumulative effect on aspects of his performance could not be discounted.
• The Captain’s autoflight management performance in response to the initial series of EGPWS Cautions and the eventual ‘PULL UP’ Warning and his delay in taking control was considered contrary to FCTM guidance that “when automation does not perform as expected, the flight crew should reduce the level of auto-flight and identify and resolve the condition and that the original level of auto-flight should only be resumed after they have regained proper control of the flight path and performance level”. It was considered that “since he had achieved an above-average grade and been considered good in the area of auto-flight management during his line and base checks [...] it might be desirable for the operator to review its autoflight management assessment programme to ensure that it is robust”.   

The formally stated Conclusions of the Investigation were as follows: 

• The EGPWS “DON’T SINK” events encountered by the flight crew were the result of the FMC having registered a speed/altitude constraint of “250/500” for the waypoint PD062. The Captain had inadvertently keyed in “250/0500” instead of the intended “250/0500A”. 
• As to the activation of the EGPWS “PULL UP” warning alert, the investigation team was unable to determine the reason of the activation. However, the aircraft manufacturer suspected that the warning could be as a result of the combination of the low radio altitude, descent rate and flight path angle required to capture the 500 feet altitude constraint that had been entered into the FMC. The Captain did not identify that there was an abnormal condition with the auto-flight system despite the series of “DON’T SINK” alerts over a period of relatively short time. 
• This occurrence also revealed many instances where the flight crew did not comply with the operator’s standard operating procedures, where the flight crew lost situational and flight mode awareness, and where the flight crew’s crew resource management performance in terms of communication had not been optimal.   
• The Captain had to deal with a number of issues prior to the departure. Each of these issues, by itself, would have been just a minor issue for the Captain. However, together, these out-of-the-normal pre-flight circumstances might have perturbed the Captain. The extent of any perturbation could not be ascertained, and the investigation team could only suspect that they could have added up to some significant stress on the Captain and contributed to his lapses in FMC programming and decision-making during the flight. 
• Notwithstanding that the Captain’s training and assessment record did not indicate any issues in the area of auto-flight management, there might be room for improvement on the part of the operator to ensure that its auto-flight management assessment programme is robust.

Safety Action known to have been taken during the course of the Investigation was noted as having included the following:

• Singapore Airlines has enhanced its recurrent flight crew training by including more emphasis on automation mode awareness and mode management and the proficient and judicious use of the AP, increasing the number of multiple and randomised scenarios employed to enhance the response to startle effect and requiring all simulator instructors to ensure correct and timely FMA callouts and proper use of the FMC.
• Boeing has agreed to consider modifying the FMC software so that it is possible to input a speed constraint during climb and descent phases of a flight without having to also input an altitude constraint.

The Final Report was published on 12 April 2021. No Safety Recommendations were made.

Related Articles

• Situational Awareness
• Startle Effect
• Terrain Avoidance and Warning System (TAWS)
• Response to a "PULL UP" Warning
• Autopilot
• Flight Management System
• Authority Gradients
• Crew Resource Management (CRM)