The investigation into the LOT Boeing 767-300 gear-up landing at Warsaw is focusing on the role of circuit-breakers, after the twinjet's undercarriage failed to deploy following a hydraulic leak.
The aircraft had departed Newark, bound for Warsaw, on 1 November when, during flap and gear retraction, the engine indication and crew alerting system (EICAS) warned of a malfunction in the central hydraulic system, which was suffering a loss of pressure.
Having opted to continue the flight to the Polish capital, the LOT crew carried out a quick-reference handbook procedure which included use of alternate landing-gear extension.
However, the gear failed to extend and the aircraft was vectored to a holding pattern while combat aircraft were sent to observe the state of the undercarriage. The crew attempted to use gravity extension to lower the gear but this was unsuccessful, and diminishing fuel levels forced the crew to consider a gear-up landing.
The 767 landed on Runway 33 with its gear retracted, about 1h 15min after declaring the initial emergency. It came to a halt about 2,000m (6,550ft) along the strip, where Runway 33 and Runway 29 intersect.
Preliminary inspection of the aircraft traced the central hydraulic system problem to a fluid leak from a damaged hose in the starboard main landing-gear.
But it also revealed that while the individual circuit-breaker for the alternate landing-gear extension motor was "on", an overarching battery bus circuit-breaker - which protects several systems including the alternate landing-gear extension motor - was "off".
Polish investigation authority PKBWL said this "off" position "was not recorded or indicated" either by the EICAS or the flight-data recorder of the 767.
Suspicion over the role of the circuit-breaker in the accident heightened during a test carried out as the aircraft was being recovered from the runway using cranes and air bags.
After connecting a ground-power unit to the 767, and switching the battery bus circuit-breaker on, investigators successfully used the alternate landing-gear extension system to lower the undercarriage - enabling the aircraft to be towed to LOT's technical facilities.
"Functional tests of the entire electrical installation of the alternate landing-gear extension system are being conducted," said PKBWL.
None of the 221 passengers and 10 crew members was injured.