What happened
A McDonnell Douglas DC-10, modified for use as a firefighting air tanker, was operating under an exclusive-use contract with the State of California. During the mission, standard operational procedures required a lead airplane to fly the intended flight profile ahead of the air tanker to identify obstacles and hazards.
On this specific mission, the original lead airplane and its specially trained pilot had swapped out for a backup lead airplane and pilot to allow for refueling. The backup lead airplane completed the flight profile prior to the air tanker joining the formation. While the captain was flying the profile, the DC-10 flew at a lower altitude than the lead aircraft and entered a left turn, resulting in the left wing impacting multiple trees.
The investigation
Data from the digital flight data recorder showed that the aircraft entered a 35-degree left bank. During this maneuver, vertical acceleration changed from 0.8 to 1.4 G's, which was consistent with normal loading during a banked turn.
Findings
While the flight crew was experienced in operating the McDonnell Douglas DC-10, they possessed limited experience specifically related to fire suppression. The crew had gained most of their retardant drop experience, totaling over 100 hours using water for drops, during the aircraft's certification testing. There were no fatalities reported in this incident.