Polish investigators are recommending amendments to checklists to help pilots cope with alternate undercarriage deployment failure, following the gear-up landing by a LOT Boeing 767-300ER.
While the inquiry has attributed the failure to a pulled circuit-breaker, it has yet to determine the probable reason why the breaker was in this state.
The aircraft had suffered loss of pressure in the central hydraulic system after departing Newark on 1 November 2011, a problem subsequently traced to a damaged hose in the main landing-gear.
But having chosen to continue the flight to Warsaw the pilots found that the undercarriage would not deploy during the alternate landing-gear extension procedure.
Examination of the aircraft's electrical system found the suspect circuit-breaker - affecting the battery bus and, in turn, the alternate gear extension system - to be in the 'off' position.
Extensive testing of the electrical system, including X-rays of the circuit-breaker, did not reveal any faults.
In an interim report on the accident Polish investigation authority PKBWL has concluded that the circuit-breaker's 'off' status is the "most likely cause" of the alternate gear-extension failure.
But it adds: "A separate issue is an explanation of what was [or] could have been the reason that, at that time, the circuit-breaker was in the 'off' position."
While the reason remains undetermined the inquiry indicates that the breaker could have been inadvertently tripped, because it is recommending that breakers in certain areas be physically protected against accidental contact with shoes, luggage or cleaning equipment.
PKBWL also points out that the circuit-breaker's status may be noticed by the crew only if there is a need for activation of the systems it protects.
It is recommending that checklists be modified to instruct pilots to check the appropriate circuit-breakers in case the alternate gear-extension system fails.
While the LOT crew managed to perform a successful gear-up landing on Warsaw's runway 33, with no injuries to the 231 occupants, the inquiry points out that the quick-reference handbook did not contain any guidance for dealing with complete failure of the primary and alternate landing-gear systems. PKBWL is advising development of a checklist for handling such a situation.
LOT opted not to repair the aircraft (SP-LPC) but instead auction its components.