A single missing letter in the aircraft’s computer caused it to stop climbing and led to multiple activations of its ground proximity warning system
The crew of a Singapore Airlines Boeing 777 inadvertently stopped a climb at 500ft after take-off from Shanghai/Pudong in 2019, after the captain mistakenly keyed in the wrong altitude constraint in the jet’s flight management computer, a report by the country’s Ministry of Transport has found.
On September 2, 9V-SWD (c/n 34569) – a 777-300ER delivered to the operator in December 2006 – was scheduled to fly between the Chinese city and Singapore. On departure from runway 35R, the widebody’s autopilot was engaged after gear retraction when passing approximately 360ft.
The aircraft then continued to climb through 500ft and the pilot in command (PIC) and first officer (FO) noticed the increasing speed trend arrow. At this time, the captain believed the jet had crossed 1,000ft and called for flap retraction. The FO then began this action.
According to the quick access recorder (QAR), the 777 did not attain 1,000ft. Instead, after the autopilot was engaged, it climbed from 360ft to 750ft before descending back towards 500ft.
During the initial stage of flap retraction, a “Don’t sink” caution alert from the enhanced ground proximity warning system (EGPWS) sounded. The crew was startled but nevertheless, the PIC did not rush into reacting as he deemed the flight to be stable and put it down to a decrease in pitch caused by a tailwind.
Nine seconds after the first alert, a second “Don’t sink” was activated. At this time, the flaps were still being retracted and the flight crew was in the process of troubleshooting the first warning. This was followed by a third caution a further nine seconds later. The pilots then realised that the aircraft had levelled off and they needed to initiate a climb.
The PIC then engaged the flight level change mode on the autopilot but two seconds later reselected VNAV mode. The crew believed they had resolved the issue because the EGPWS call-outs ceased, but according to the QAR, no appreciable change in altitude resulted.
A fourth “Don’t sink” alert was activated about nine seconds after VNAV was reselected. Shortly after, a more serious “Pull up” warning alert sounded. In response, the PIC disconnected the autopilot and manually pushed the thrust levers forward to increase power, and pitched the aircraft up into a climb.
When they passed 1,600ft, the captain called for the autopilot to be reengaged and the FO carried out this action. The jet then pitched down to regain the previously programmed target altitude of 500ft. At this time, the FO noticed on his navigation display that there was a “250/0500” speed/altitude constraint set for the next waypoint.
The first officer alerted the captain that they needed to cancel the constraint and the FO pushed the altitude selector button on the autopilot to delete the programmed speed/altitude constraint.
After doing so, there were no more issues with the aircraft climbing and it proceeded to its destination without further incident.
Investigators found that the error in keying in altitude constraints was made by the captain when changing the take-off runway and corresponding instrument departure in the jet’s FMC.
Originally assigned runway 34L, the parallel 35R was eventually allocated to the flight instead. This required changing the departure routing in the FMC and the first waypoint – identified as PD062 – did not have any speed or altitude constraints associated with it.
According to the Ministry, the captain preferred to have speed constraints explicitly displayed so he decided to enter 250kts as shown on the charts. The computer required an altitude to accompany the speed so he chose to input “500A”, which would mean that when crossing PD062 the jet should be at or above 500ft.
When keying in this information, the captain inadvertently typed “500” and omitted to include the “A”, which neither crew member spotted.
The inquiry noted that the flight crew had been experiencing a number of pressures prior to take-off including minimum equipment list considerations, bad weather, and issues obtaining and downloading route information via ACARS and the controller-pilot datalink connection.