What happened
On 2 September 2019, a Singapore Airlines Boeing 777-300ER, registration 9V-SWD, departed from Pudong Airport in Shanghai, China. During the initial climb phase, the aircraft's Enhanced Ground Proximity Warning System (EGPWS) issued three consecutive "DON'T SINK" caution alerts.
In an attempt to resolve the alerts, the flight crew briefly engaged the Flight Level Change (FLCH) mode. However, after reverting to Vertical Navigation (VNAV) mode, the crew encountered a more severe "PULL UP" warning alert. The pilot-in-command responded by disengaging the autopilot, increasing engine thrust, and pitching the aircraft upward to climb. The warnings ceased, and the flight continued to its destination without further incident.
The investigation
The investigation focused on why the EGPWS triggered these alerts and the circumstances leading to the flight crew's actions. Investigators examined the flight crew's pre-flight workload, which was significantly high due to several simultaneous factors: bad weather, departure delays, the need to manually coordinate aircraft transport, and complex maintenance requirements involving a deferred engine bleed system item. Additionally, the crew had to manage intermittent data link connectivity and manually input flight routes into the Flight Management Computer (FMC).
Technical analysis of the FMC inputs revealed that the pilot-in-command had intended to program a speed and altitude constraint of 250 knots and at least 500 feet (250/0500A) for a specific waypoint. However, due to a typing error, the altitude constraint was recorded as exactly 500 feet (250/0500). This caused the VNAV mode to command a descent to capture the 500-foot altitude, triggering the proximity warnings.
Findings
- The primary cause of the "DON'T SINK" alerts was an incorrectly programmed altitude constraint in the FMC, where the aircraft was instructed to descend to 500 feet.
- The pilot-in-command's workload was heavily impacted by multiple concurrent stressors, including weather delays, manual route entry, and complex MEL coordination.
- The crew demonstrated a lack of situational awareness regarding flight mode changes, specifically failing to recognize that the VNAV mode was executing the erroneous altitude constraint.
- There was a breakdown in Crew Resource Management (CRM), as the First Officer did not perform a thorough cross-check of the FMC inputs due to time pressure.
Safety action
Following the investigation, the operator implemented several safety improvements, including:
- Sharing lessons learned with all flight crews across the fleet.
- Reinforcing Standard Operating Procedures (SOPs) regarding the requirement for both pilots to be present during FMC inputs.
- Enhancing recurrent simulator training to focus on automation mode awareness, managing the "startle effect," and proper use of the autopilot.
- Consulting with the aircraft manufacturer regarding potential software updates to allow speed constraints without mandatory altitude inputs.