Aviation experts said Wednesday that a LOT Polish Airlines flight from Newark, New Jersey, made an emergency landing in Warsaw last year because of technical problems with the Boeing 767 and inadequate guidance in its cockpit handbook.

None of the 231 people aboard were injured when the plane landed on its belly on Nov. 1, 2011, after its landing gear failed. Its pilot, Capt. Tadeusz Wrona, has been hailed a national hero.

In its preliminary opinion, Poland's State Commission for Investigation of Air Accidents said the aircraft's main landing gear discharge system failed due to a broken hydraulic hose, and the backup system also failed, probably because its circuit breaker was accidentally in the off position.

The commission said the cockpit checklist did not include guidance on what to do with a malfunction of the alternative landing gear system, or if the landing gear could not be discharged. The experts said they have recommended that Boeing revise the checklist and that Warsaw Frederic Chopin airport improve procedures for relocating plane passengers after an emergency evacuation.

Boeing said it would not comment until the commission makes its final report.

A statement emailed to The Associated Press said, "Boeing is committed to the safety of our airplanes and the people who fly on them" and that the company is "providing technical assistance" to Poland's commission.

LOT declined to immediately comment about the commission's preliminary finding, except to say that the plane that had the problem was built in 1997. The airline is trying to sell the plane as is — with a lower chassis damaged by the emergency landing.

The commission did not say when it will release its final report about the causes and circumstances of the accident.