Air Tractor AT-802A N836MM accident near Emmett, Idaho

Probable cause

The pilot’s descent below surrounding terrain and his delayed decision to initiate a climb which resulted in impact with terrain. Contributing to the accident were terrain and lighting conditions that affected the pilot’s ability to accurately assess terrain clearance.

NTSB analysis

A flight of three airplanes departed to conduct firefighting operations; the accident airplane was number two. After establishing contact with the Air Tactical Group Supervisor (ATGS), the pilots were briefed on the target description and identified the steepness of the drop and the rising terrain on the exit. After the first airplane completed its pass, the ATGS requested that the accident airplane adjust his flightpath to the left to reinforce the line closer to the burned vegetation. The accident pilot confirmed his gate was armed, on the correct line and was subsequently cleared to proceed and drop the fire retardant. Witnesses reported that the airplane crossed over the intended target ridge, descended into the valley, and passed the proposed drop area without releasing any retardant. No mechanical failures or malfunctions were revealed with the firegate system, and it was undetermined why the accident pilot did not accomplish the retardant release. The accident airplane initiated a climb; however, it impacted terrain about 40 ft below the opposing ridge line.

A video provided by a witness showed the accident airplane descend into the valley, without releasing any retardant. The airplane did not begin its climb out of the valley until 3 seconds before impact with rising terrain. The NTSB’s performance study determined that there was an insufficient amount of time and distance for the airplane to gain enough attitude to clear the rising terrain.

The pilot had orbited the area at least 3 times from about 2,000 ft above the ground before initiating the drop run. This would have provided an opportunity to assess ground features, including terrain, that could aid in conducting the run. However, environmental conditions in the area at the time may have made terrain features more difficult to discern from the lower altitude of the drop run because of the uniformity of the vegetation on the ridges and flat light conditions; and would have made accurate assessment of height above and distance from terrain more difficult. This may have led the pilot to believe he had greater clearance from terrain than what was available. Had the pilot initiated a climb at the GPS point prior to the impact location, there would have been sufficient altitude to clear the rising terrain. However, the airplane continued to descend into the valley and the pilot initiated the climb too late for the airplane to clear terrain.

Postaccident examination of the recovered airframe and engine did not reveal evidence of any mechanical anomalies that would have precluded normal operation.

Text from the NTSB. Open the NTSB docket for WPR20LA315 for the full record.