Warning flags distracted crew from correct monitoring
An Atlas Air Boeing 747 captain has been criticised by investigators for carrying out a badly managed descent that became dangerous, then for failing to report it because the aircraft had recovered a normal profile by the time it reached final approach.
The incident, which happened late last year, came to light because people living below the London Stansted approach path knew the aircraft was too low and reported it. The 747-200 freighter had flown from Chicago non-stop and was carrying out a normal radar-vectored descent to intercept the instrument landing system (ILS) for runway 05 at London Stansted, according to a UK Air Accidents Investigation Branch incident report on the 12 December 2004 event.
The aircraft, which had been on a heading of 180° at 3,000ft (915m) altitude, was told to turn left on to 020° by the controller, who apologised for requesting a late turn. During the turn, the controller cleared the aircraft to descend from 3,000ft to 2,000ft, and further descend when locked on to the ILS.
The crew acknowledged this clearance correctly, says the report, which observes that normal procedure would have been for the captain to set altitude select (ALT SEL) on the master control panel with the cleared altitude, which would have ensured the autopilot levelled the aircraft at 2,000ft. This did not happen, but the details of what did occur were not recoverable from the flight data recorder (FDR) because it had been overwritten during subsequent flights.
The captain said he thought he had selected 500ft/min (2.54m/s) vertical speed, but radar traces recovered by the AAIB showed that the actual rate of descent averaged 1,570ft/min and a descent angle of 5.4°, which ensured the ILS glideslope could not be captured on that profile.
Meanwhile, the co-pilot reported that the ILS indicator showed two warning flags, and the crew admit they became fixated with “troubleshooting” the cause. As a result, descent through 2,000ft was not noticed until the aircraft emerged from the cloudbase at 900ft with about 6nm (11km) to go to the runway, when the captain recognised the situation because he had the runway precision approach path indicators in sight.
He levelled the aircraft, carried out the pre-landing checks, and began a normal ILS descent from about 2nm on final approach, from which the crew made a safe landing.
The flight engineer and first officer made special submissions to confidential reporting systems, but the captain did not file a report to the company about the incident, mentioning only the issue of the warning flags, for which no fault was found.
David Learmount / London
Source: Flight International
