This from the BBC website
Pilots warned of magnetic threat
A plane was forced to turn back in October 2006
London City Airport has warned pilots their instruments may be affected by magnetic interference from metal structures found below the runway.
A report was carried out after an aeroplane was forced to turn back when its autopilot system failed.
Railway lines, and other metal structures left from the days when the airport was a dock, were found to be causing "significant interference".
A spokeswoman said action would be carried out "wherever necessary".
An investigation was launched by the Air Accident Investigation Branch (AAIB) after a jet was unable to follow a standard departure route, because of an autopilot problem, after taking off on 31 October 2006.
Magnetic fields
The Raytheon Hawker plane, which had two crew onboard, had to turn around and land back at the east London airport 10 minutes after take-off.
A survey was carried out which revealed a series of metal structures beneath the runway. They were found to be causing significant interference to the magnetic fields on which aircraft compasses and some navigation equipment rely. The airport has been ordered by the AAIB to address the problem.