Preliminary German investigations indicate that an Estonian Air Boeing 737-500 escorted to Hamburg by fighter aircraft had lost the area navigation (RNAV) capability from its flight management system.

Flight OV102 was en route from London Gatwick to the Estonian capital Tallinn on 13 November when the aircraft suffered the malfunction, although the carrier confirms the aircraft retained its normal back-up navigation capabilities.

But loss of the RNAV function can reduce the accuracy of the aircraft's flightpath, so the crew opted to divert because the 737 was passing through congested airspace, Estonian Air confirms. Within European airspace aircraft must be able to perform RNAV to a specific level of accuracy.

Two fighters provided support as the twinjet diverted to Hamburg where it landed without incident at 21:25.

The German accident investigation agency BFU says that the 737 appears to have lost its RNAV capability. RNAV allows an aircraft to fly more efficient routes, because aircraft equipped with it no longer have to fly directly along tracks overhead ground-based navigation aids. Using data from the navaids and its own inertial reference systems processed by the FMS, the pilots are presented with the cleared track on their navigation display.

"At the moment we are in the phase of the investigation where we are trying to collect all the information," says the BFU. "We are waiting for information from air traffic control and from the technical side."

The agency adds: "The weather was also not good in Tallinn. Hamburg's weather was much better."

None of the 75 passengers on board was injured. The 737 involved is a 12-year-old aircraft, registered ES-ABC, owned by International Lease Finance. Estonian says that the aircraft was still in Hamburg two days later while technical specialists removed the affected system for inspection.

Source: Flight International