A report into the fatal May 2 crash of a WC-130H four-engine turboprop operated by the Puerto Rico Air National Guard has concluded that an inadequate response to an engine failure by the onboard crew led to it losing control of the aircraft, although a number of other contributing factors have also been attributed to the cause of the incident.

The improper application of left rudder in response to an engine failure resulted in a skid below the minimum controllable airspeed for three-engine operation, the report from the U.S. Air Force Aircraft Accident Investigation Board said, resulting in a left-wing stall and the WC-130H’s departure from controlled flight.

The weather-surveillance aircraft was assigned to the guard’s 156th Airlift Wing located at Muñiz Air National Guard Base in Puerto Rico and was travelling from Savannah/Hilton Head Airport in Georgia to the 309th Aerospace Maintenance and Regeneration Group at Davis-Monthan Air Force Base in Arizona. It crashed some two minutes after takeoff, killing all nine of the crew onboard.

The aircraft was traveling to the so-called "boneyard" at Davis-Monthan AFB to be retired and had been at the airport in Georgia for approximately a month so that work could be carried out on the aircraft by 156 AW personnel, namely scheduled work on the fuel cells and unscheduled work on engine number one, which ultimately malfunctioned.

The report said it is likely that one of the pilots became confused and unsure of what action to take, evident in the varied left and right rudder inputs throughout the mishap flight, the banking of the aircraft toward an inoperative engine, the failure to gain airspeed, and the failure to retract the flaps.

“Additionally, the board president found, by a preponderance of the evidence, the MC’s [mishap crew's] failure to adequately prepare for emergency actions, the MC’s failure to reject the takeoff, the MC’s failure to properly execute appropriate after takeoff and engine shutdown checklists and procedures, and the mishap maintainers’ failure to properly diagnose and repair engine number one substantially contributed to the mishap,” the report explained.

Therefore, while the response of the crew to the engine failure and the subsequent series of events were deemed the cause of the crash, a number of other concerns were also contributing factors, according to the report.

Notably, the report says that on the ferry flight to Savannah from Puerto Rico on April 9, an RPM issue with the same number one engine was identified and put forward for repair, but the maintenance team did not use a tachometer when troubleshooting the engine malfunction on either of two occasions when engine runs were performed, which is a direct violation of maintenance technical orders.

The second engine run determined that the system achieved 99 percent RPM, which the maintainers deemed to be good enough according to the investigation board, as the aircraft was heading for the boneyard anyway.

“Since the mishap maintainers failed to properly diagnose and repair engine one, the MA [mishap aircraft] was cleared for flight with a significant malfunction in the valve housing,” the report says, adding that when the engine speed dropped below 94 percent, the bleed valves opened as designed to prevent a compressor stall.

“This substantially reduced engine power and required the MC to take action to shut down engine one. Had engine one functioned properly, the engine shutdown and in-flight emergency would not have occurred.”