What happened
On an oceanic crossing from London to Montreal, an Air Canada Boeing 747-238 (ACA003) experienced a period of deteriorating airspeed after leveling off at flight level 360. While the aircraft was cruising, the flight crew was engaged in various tasks, including a passenger announcement by the captain. During this time, the aircraft's airspeed decayed from a target of Mach 0.84 down to approximately Mach 0.75.
As the airspeed dropped, the aircraft's pitch increased, and the crew felt a slight vibration through the airframe. In an attempt to recover the lost speed, the crew increased engine thrust to maximum continuous thrust and began a descent. This descent caused the aircraft to drop below its assigned altitude and pass through flight level 350, where a second Air Canada Boeing 747-400 (ACA857) was cruising. This resulted in a loss of separation between the two aircraft, which was detected by the Scottish Oceanic En Route air traffic control centre via a separation monitoring alert.
Initial attempts by the crew and controllers to communicate were hindered because the aircraft was at the edge of the VHF radio range. However, the crew of ACA857 observed the descending aircraft visually and via TCAS, and proactively moved to a different track to maintain separation and avoid wake turbulence. Communications were eventually re-established, and the aircraft was re-cleared to a lower altitude.
The investigation
Investigators examined the flight data, radar records, and crew reports to reconstruct the sequence of events. Although the digital flight data recorder (DFDR) provided limited information due to an acquisition error, it confirmed the reduction in airspeed and the change in the aircraft's pitch angle.
The investigation also looked into the aircraft's performance capabilities. While the Boeing 747-238 was capable of maintaining flight at the assigned altitude under the prevailing conditions, the investigation focused on the crew's monitoring of the aircraft's performance near its operational limits. The investigation also noted that the crew's initial inability to contact air traffic control delayed the notification of their change in altitude.
Findings
- After reaching a cruising altitude that was at or near the aircraft's performance limits, the crew failed to adequately monitor the aircraft's performance, allowing the airspeed to drop to an unsafe level.
- The crew's attention was diverted by tasks unrelated to monitoring flight performance, such as passenger announcements and position reporting.
- To regain necessary airspeed, the crew was forced to descend, which directly caused the loss of separation with the following aircraft.
- Communication difficulties prevented air traffic controllers from being notified of the aircraft's descent until after the separation had already been lost.