TOKYO (AP) — Japan’s fleet of hybrid-helicopter military aircraft have been cleared to resume operations after being grounded following an accident last month.

A V-22 Osprey tilted and hit the ground as it was taking off during a joint exercise with the U.S. military on Oct. 27. An investigation has found human error was the cause.

The aircraft was carrying 16 people when it “became unstable” on takeoff from a Japanese military base on Yonaguni, a remote island west of Okinawa. The flight was aborted and nobody was injured, Japan’s Ground Self Defense Forces (GSDF) said at the time.

In a statement on Thursday, the GSDF said the pilots had failed to turn on a switch designed to temporarily increase engine output during take off, causing the aircraft to descend and sway uncontrollably.

Chief Cabinet Secretary Yoshimasa Hayashi said an internal investigation determined that the accident was caused by a human error, not by “physical or external factors.”

He said the fleet of more than a dozen V-22s would resume flight operations from Thursday after a review of safety and training measures.

It was the first major incident involving Japan’s V-22s since November 2023 when a U.S. Air Force Special Operations Command Osprey crashed off Japan’s southern coast killing eight people.

The fleet only resumed flight operations earlier this year, but the use of the V-22 remains controversial, particularly in Okinawa where residents have questioned its safety record. The small southern island is home to half of about 50,000 U.S. troops based in Japan.