A 2017 accident involving an RQ-4B Global Hawk that crashed in California was caused by issues with one of its navigational data devices or navigators, according to an Air Force report released Wednesday.

The Air Combat Command Accident Investigation Board report states that the unmanned aerial vehicle, part of the 12th Reconnaissance Squadron from the 9th Reconnaissance Wing at Beale Air Force Base, California, was conducting a ferry flight on June 21, 2017, from Edwards Air Force Base, California, back to Beale at the time of the accident. Issues with the navigational data device caused the Global Hawk to enter “an unusual flight altitude and experience airspeeds that exceeded structural limitations, from which the aircraft could not recover.”

RQ-4 Global Hawk Photo: Northrop Grumman
RQ-4 Global Hawk
Photo: Northrop Grumman

At the time of the accident, the aircraft was being operated by an aircrew of contractors from Northrop Grumman [NOC], which builds the RQ-4 line. Northrop Grumman did not reply to a request for comment before deadline.

The report noted that the RQ-4B aircraft has four navigators and operates with two “on” and two “off” while in a normal configuration. The mishap was due to one of the two in-use navigation systems – a Litton LN-100G navigator — producing “erroneous navigational data” that was not detected by the other aircraft’s navigation system. That caused the aircraft to roll into a “nearly inverted position” and “enter a dive that resulted in excessive airspeed.” The Global Hawk subsequently broke up during flight and crashed in “an unpopulated and rugged area” between Lone Pine and Mount Whitney, California. 

The Accident Investigation Board president Col. Jeremy Thiel also found that disabling the other two navigation systems after takeoff “substantially contributed to the mishap,” which were deemed to be operating normally at the time of the accident. The report noted that the aircraft typically operates with two navigators on and two off in a “normal configuration,” but due to the hierarchical precedence of the system, if all systems had been enabled, one of the other navigators’ data would have been selected for flight controls.

There were no reported fatalities, injuries or damage to civilian property, though the loss of the aircraft is valued at $79 million, the report said. The aircraft left a debris field from Lone Pine until its point of impact at Mount Whitney, including parts of the Inyo National Forest, but initial environmental analysis did not establish any significant impact to the forest or wildlife.

Portions of the wreckage have been recovered since the accident, according to a statement from Air Combat Command. The Air Force is working with the U.S. Forest Service for the remainder of the wreckage cleanup, it said.