Metal structures beneath the holding area for runway 28 at London City Airport can cause aircraft compass deviations of up to 97deg left and right, according to the UK Air Accidents Investigation Branch (AAIB).
The Branch took the magnetic anomaly readings during its investigation of a 31 October 2006 incident in which a Raytheon Hawker 800XP, which was flying the standard departure from London City, called for air traffic control assistance to return there because its heading reference systems (HRS) remained unreliable for some time after take-off.
The aircraft returned to land safely, but the AAIB investigation found there had been other related events at London City affecting a Cessna C560 Citation XL, Fokker 50s, and two other Hawker 800s.
The local magnetic field anomaly in the runway 28 holding area at London City is caused partly by buried metal structures that are the remnants of the airport site's history as a dockside loading area for ships. These include buried railway lines and the massive below-ground sections of what used to be bollards that ships tied up to.
But the main disturbance to the earth's magnetic field at that point, says the AAIB, was caused by the tubular metal sleeves surrounding the vertical concrete piles that support the holding loop. Research company Qinetiq found that these had been sections of oil pipeline that had been rendered highly magnetic by induction resulting from years of uni-directional mineral oil flow.
The AAIB recommends that the UK Civil Aviation Authority require London City Airport to "mitigate the effects of the magnetic anomaly in the loop hold".
The CAA says it has accepted the board's recommendations, and London City chief executive Richard Gooding says work to comply has already begun. This consists of cutting away the metal sleeves from the concrete piles and is expected to be completed in a few weeks.
Source: Flight International
