Pilot error was the cause of a helicopter crash in April, when a CH-53E Super Stallion crashed in Yuma, Arizona, an investigation found.

Five people were aboard the helicopter at the time of the crash: two pilots and three aircrew. No injuries or deaths resulted from the crash.

At the time of the crash, the helicopter was flying too low and it banked too hard to the right, according an abridged version of the investigation, which Marine Corps Times obtained through a Freedom of Information Act request. 

The CH-53E was one of two helicopters practicing a defensive maneuver at the time of the crash, according to the investigation. A Russian Mi-24 Hind helicopter had just flown between the two U.S. helicopters when the CH-53E made a sharp right turn and hit the ground.

The minimum altitude for the maneuver was 100 feet, but the CH-53E was 55 feet off the ground at the time, the investigation found. The CH-53E also went into a 73-degree right bank — much more than the helicopter’s limit of 60 degrees.

“The aircraft impacted the ground, bounced and then impacted the ground a second time,” the investigation found. “After initial impact the aircraft began to spin to the right and after the second impact the aircraft began a roll to the left coming to rest nearly completely inverted.”

One of the three enlisted Marines on the helicopter’s aircrew was thrown from the CH-53E while still connected to the aircraft by his gunner’s belt and was “bounced and dragged” until he was able to release the latch on his belt, according to the investigation.

The investigation found that the helicopter had flown below the minimum for the maneuver 22 times and had exceeded the CH-53E’s bank limit 12 times leading up to the crash. The pilots said they only recalled banking too hard a couple of times and they thought they had only gone below the minimum altitude once.

Neither of the helicopter’s pilots were scanning the aircraft’s instruments immediately before the crash, so they did not know that the CH-53E was only 55 feet off the ground, the investigation found. One of the pilots said he “used the horizon and muscle memory” to determine how high the helicopter was flying.

The pilot, whose name was redacted from the investigation, “demonstrated a fundamental lack of aerodynamics and aviation knowledge by believing that he could accurately perceive the aircraft state, without looking at flight instruments,” the investigation found.

Moreover, the pilots thought that the bank angle warning would continue to sound until they righted the aircraft, the investigation found. In fact, the warning “Bank Angle, Bank Angle” only sounds once. The investigation recommended CH-53E’s software be updated so that the warning continues to sound until the CH-53E’s bank falls below 60 degrees.

The investigation also found that the CH-53E was suitable for flight despite some technical problems. The aircraft had a hydraulic leak that was not a factor in the crash. The leak was not properly noted in the helicopter’s maintenance form.

The helicopter’s radar altimeter had also malfunctioned during an April 3 flight, according to the investigation. The altimeter’s needle stuck several times, but the problem was not properly documented. Prior to the crash, the CH-53E crew did not discuss how they would respond if the problem recurred.

Six people involved with the mission could face administrative or disciplinary action, according to the investigation’s findings. Their names were redacted from the report.