Magnetic interference was responsible for “significant navigation problems” a NetJets Hawker 800XP experienced on takeoff from London City Airport (LCY) 15 months ago, according to a UK AAIB report released last month.

On Oct. 31, 2006, the fractional airplane was holding at Mike for a departure from Runway 28 on an empty positioning flight from London City Airport to Brussels when the crew saw that the attitude and heading reference system (AHRS) and the heading on both primary flight displays (PFDs) were giving a red flag indication. The pilots considered this a “known fault” at LCY (thought to be attributable to metal in the taxiway pilings). When the airplane was lined up for departure, the red flags disappeared without any further action on the part of the crew.

After departure the crew could not select either heading selector bug and there was a 60-degree discrepancy between the PFDs. The standby instrument indicated a heading difference of 15 degrees from the PFD1 and both flight directors were unusable. As indicated in the Quick Reference Handbook the crew selected AHRS1 to drive both sets of flight instruments, but it was another 10 minutes before the system functioned normally. They took radar vectors to return to the airport.

The AAIB investigation of the incident revealed a number of similar occurrences beginning in January 2000. Early occurrences were blamed on the pilots’ failure to comply with assigned routings, and in September 2003 London ATC observed that the failure to follow the correct SID was becoming a regular event.

A number of reports from late 2004 referred to heading problems after departure from Runway 28, some leading the crew to make an urgency call and report that navigation equipment was unusable for up to 30 minutes after departure.

During the investigation an AAIB inspector took a number of readings on foot with a handheld compass and found that substantial magnetic anomalies gave needle deviations of up to 60 degrees. Further investigation by a specialist survey team measuring deviation from magnetic north at a height of approximately four feet above ground level showed that in some locations the deviation could be as great as 93 degrees.

Built on the site of a boat dock, the airport sits on a number of steel structures. When the runway was built, all rail lines, buildings and access roadways were removed, except those rails along the dockside. In addition, the large bollards that were used to tie up the ships were cut off at ground level, leaving a substantial amount of metal below.

The investigation found that operators have experienced similar problems at Stockholm Arlanda, Houston International and New York La Guardia. Although these events at LCY and other airports appear to be the result of local magnetic anomalies, there are no requirements– national or international– to assess and mitigate them. As a result, the AAIB has recommended that ICAO amend Annex 14 to emphasize the importance of ensuring that no airport infrastructure is allowed to significantly alter the earth’s magnetic field at aircraft holding areas. This recommendation has been repeated to the UK CAA and to the European EASA.

LCY airport management has issued a notam warning pilots about the possibility of magnetic disturbances affecting the heading reference system.