What happened
On 31 October 2006, a Raytheon Hawker 800XP, registration CS-DRQ, was preparing for a non-scheduled commercial flight from London City Airport to Brussels. While positioned at holding point Mike, the crew observed red flags on both Primary Flight Displays (PFDs), indicating that the Attitude and Heading Reference System (AHRS) had failed and heading information was unreliable. The crew believed this was a known issue related to magnetic interference from the taxiway area.
After taking off from Runway 2HD, the crew discovered they could not control the aircraft's heading via the autopilot, as the heading selector bugs failed to respond to rotation. A significant discrepancy of 60 degrees was noted between the two PFDs, and the standby instrument showed a heading 15 degrees less than the first PFD. Additionally, flight director flags appeared on both displays. Following the procedures in the Quick Reference Handbook (QRH), the crew attempted to reset the AHRS source. After approximately 10 minutes of flight, the crew decided to return to London City Airport, using radar vectors to navigate. The aircraft landed safely without further incident.
The investigation
An investigation by the AAIB and a magnetic survey conducted by engineers from QinetiQ revealed that the holding area for Runway 28 contained severe magnetic anomalies. The survey identified several sources of interference, including the 68 steel-encased concrete piles used to support the taxiway, remnants of flame-cut iron bollards, and the magnetic signature of the reinforced concrete and old railway lines. These anomalies were strong enough to cause compass deviations of up to 97 degrees.
Findings
- The primary cause of the navigation difficulties was strong magnetic anomalies located within the Runway 28 holding area.
- The magnetic field density in certain areas was so intense that it prevented the aircraft's magnetic compass from rotating properly.
- The aircraft's flux valve correction system, which realigns the compass at a rate of only 3 degrees per minute, was unable to compensate for the rapid change in magnetic field as the aircraft moved away from the anomaly.
- The crew's decision to depart without completing the full AHRS realignment procedure, due to pressure from approaching landing traffic, exacerbated the heading error.
Safety action
- The AAIB issued several recommendations to the CAA and ICAO to ensure airport infrastructure does not significantly alter the local earth's magnetic field density in departure holding areas.
- A recommendation was made for the CAA to require London City Airport operators to include specific briefings regarding these magnetic anomalies in their Category C briefings.
- The airport operator issued a NOTAM to increase pilot awareness of the potential for compass disturbances when using the Runway 28 hold.