What happened
On December 17, 1997, a DC-9-83, registration OH-LPE, was conducting an approach to Rovaniemi Airport. The flight, operating as FIN427 from Helsinki, was initially flying at FL330 before being cleared to lower altitudes. During the approach to runway 21 using the ILS-B procedure, the aircraft encountered heavy winds and turbulence.
As the aircraft progressed along the localizer, the captain observed the ILS glide slope indicating a downward trend. The co-pilot, who had tuned the ILS frequency on his side of the cockpit, observed a conflicting indication. Due to this discrepancy between the two cockpit displays, the captain decided to initiate a go-around. During this maneuver, the aircraft's altitude dropped to approximately 1600 ft, well below the required 2400 ft altitude specified for that point in the approach. The radar controller confirmed the low altitude, noting the aircraft was at 1600 ft when it should have been higher.
The investigation
The investigation examined the aircraft's flight data recorder, radio communications, and the functionality of both the aircraft's avionics and the airport's ground-based navigation aids. Investigators checked the maintenance history of the DC-9-83 and found no prior or subsequent issues with the ILS receiver. Ground-based ILS equipment at Rovaniemi was confirmed to be in good working order through both technical inspections and flight calibrations.
The investigation also considered the impact of the heavy winds and the potential for electromagnetic interference from portable electronic devices (PEDs) used by passengers, which can cause uncommanded fluctuations in sensitive navigation instruments.