Accident Overview
On July 10, 2021, at approximately 1254 mountain standard time, a Beech C-90 turboprop airplane, registration N3688P, was destroyed when it crashed near Wikieup, Arizona. The aircraft was operating as a public use firefighting airplane in support of the Bureau of Land Management, conducting aerial reconnaissance and supervision over the Cedar Basin fire. The pilot and Air Tactical Group supervisor on board sustained fatal injuries.
Flight History
The airplane had been on station for about 45 minutes, performing multiple orbits over the fire area at approximately 2,500 ft above ground level. The last ADS-B data point recorded the aircraft at an altitude of about 2,300 ft agl, an airspeed of 151 knots, and in a descent, located about 805 ft east southeast of the accident site. No distress call from the airplane was overheard on the radio.
Witness Observations
A witness reported observing the airplane in a steep dive toward the ground. The aircraft impacted the side of a ridgeline in mountainous desert terrain. Another witness saw the left wing falling to the ground after the aircraft had struck the terrain. The left wing separated outboard of the nacelle and was found about 0.79 miles northeast of the main wreckage; it did not sustain thermal damage.
Wreckage and Damage
The main wreckage was mostly consumed by a post-crash fire. Debris from the accident was scattered over an area of several acres. The left wing's outboard section had separated from the aircraft in flight.
Probable Cause
The failure and separation of the left wing’s outboard section due to a fatigue crack in the lower spar cap. Contributing to the accident was the operator’s decision to repair the wing spar instead of replacing it as recommended by the aircraft manufacturer. Also contributing to the accident was the failure of the Non-Destructive Testing inspector to detect the fatigue crack during inspection.