N836MMAir Tractor AT-802A · 2020

On September 22, 2020, about 1830 mountain daylight time, an Air Tractor AT-802A, N836MM, was substantially damaged when it was involved in an accident near Emmett, Idaho. The pilot was fatally injured. The airplane was operated as a public aircraft for the purpose of firefighting.

Witnesses conducting firefighting operations adjacent to the accident site reported that the accident airplane, a single engine air tanker (SEAT), descended and made an approach similar to the previous SEATs that were dropping fire retardant. The witnesses said the airplane passed over the top of the ridge and descended into the valley as seen in figure 1; however, the pilot did not drop the fire retardant as the previous SEATs had done. A witness stated he heard a brief application of engine power as the airplane began to ascend over rising terrain directly in front of the airplane’s flightpath. The airplane subsequently impacted rising terrain approximately 40 feet below the ridgeline.

Figure 1: View of accident airplane descending below a ridge line. (Source: a screen capture from witness video)

A video provided by a witness captured the accident sequence. The recording showed the airplane descend over an intermediate ridgeline and into a valley as seen in figure 2. About 3 seconds later, the airplane momentarily returned to level flight before it pitched upward to a nose-high attitude and impacted terrain.

Figure 2: View of accident airplane descending into valley. (Source: a screen capture from a witness video)

The supervisor of Air Tactical Group reported that, prior to the accident airplane being cleared to maneuver, he briefed all SEAT pilots on the target description and asked them to set for coverage level 3. He also discussed the steepness of the drop and the rising terrain on the exit. He requested that the accident airplane reposition to the left and reinforce the [retardant] line closer to the black (burned vegetation). After the pilot of the accident airplane confirmed his gate was armed, he was cleared to make the drop and was on the correct line. The accident airplane crossed over the target ridge, descended into the valley, and passed the intended drop location without releasing any retardant. The accident airplane initiated a climb but was unable to clear the opposite ridge and impacted terrain.

GPS data recovered by the National Transportation Safety Board (NTSB) Vehicle Recorder Division showed that the airplane departed from Ontario Municipal Airport in Ontario, Oregon at 1824. The accident airplane climbed to an altitude between 5,000 and 5,500 ft mean sea level (msl) while en route to the fire location. The accident airplane reached the fire/accident location at about 1837 and began a series of left-hand circles over the area. The accident airplane descended below 5,000 ft at 1843 and slowed from about 140 kts of calibrated airspeed to about 120 kts. The airplane remained on a northeasterly heading and continued to descend until the GPS point prior to impact with terrain was recorded at 1845:44, at an altitude of 3,235 ft msl, about 900 ft southwest of the accident site as seen in figure 3.

Figure 3: View of accident airplane GPS track. Points after “impact” are from the airplane wreckage as it moved down the far side of the hill.

Six months prior to that accident flight, the pilot was evaluated by the Office of Aviation Services (OAS), Western Regional Office and carded as a Level II SEAT pilot. The Level II endorsement permits the pilot to perform missions in the fire traffic area (FTA).

According to the United States Department of the Interior, Office of Aviation Services, Level II pilots must inform the aerial supervisor that they are level II prior to entering the FTA. When no aerial supervision is on scene, they must notify the incident commander. If no aerial supervisor or incident commander is present, they are required to notify other aircraft on the fire and/or broadcast in the blind that they are a level II.

The airplane was manufactured in 2020 and was equipped with a trotter controls fire-retardant dispersal system (GEN III).

The flight manual supplement for the trotter controls fire-retardant dispersal system (GEN III) states, “DO NOT FLY UPSLOPE TO DUMP ON A FIRE. MAINTAIN SUFFICIENT ALTITUDE TO CLIMB AWAY IF DUMP IS ABORTED.”

The operator reported that, during a prior mission, the pilot experienced an issue with the dispersal system. When the pilot attempted to dump the fire retardant nothing was jettisoned and he elected to go-around for another attempt. During the second attempt, again nothing was jettisoned. However, the pilot elected to utilize the red salvo button on the fire controller and the load fully dispersed. Subsequent communication with the manufacturer revealed a correction for the issue, which the pilot performed on August 21, 2020; the correction comprised disconnecting one cable from the firegate computer, which addressed the issue, and no further anomalies were noted.

The airplane was manufactured in 2020 and was equipped with a trotter controls fire-retardant dispersal system (GEN III).

The flight manual supplement for the trotter controls fire-retardant dispersal system (GEN III) states, “DO NOT FLY UPSLOPE TO DUMP ON A FIRE. MAINTAIN SUFFICIENT ALTITUDE TO CLIMB AWAY IF DUMP IS ABORTED.”

The operator reported that, during a prior mission, the pilot experienced an issue with the dispersal system. When the pilot attempted to dump the fire retardant nothing was jettisoned and he elected to go-around for another attempt. During the second attempt, again nothing was jettisoned. However, the pilot elected to utilize the red salvo button on the fire controller and the load fully dispersed. Subsequent communication with the manufacturer revealed a correction for the issue, which the pilot performed on August 21, 2020; the correction comprised disconnecting one cable from the firegate computer, which addressed the issue, and no further anomalies were noted.

Examination of the accident site by a Federal Aviation Administration inspector revealed that, the airplane impacted the southwest side, about 40 ft below a ridgeline. The wreckage was found in foothills containing small brush and grass. There were no trees, power lines, or towers in the vicinity of the wreckage. The wreckage debris path extended from the initial impact point over the top of the ridgeline into a small ravine. A ground scar consistent with impact from the left main landing gear was observed on the southwest face of a ridge, followed by a ground scar that contained the firegate doors. The rear firegate cowling, firegate vent tube, and battery box drain were found near the initial impact location and are consistent with the underside of the airplane impacting the ridge in a nose up attitude. The main aircraft wreckage came to rest near the top of the ridge approximately 120 yards northeast of the initial ground scar on a heading of 040°. Evidence of a small grass fire was observed near the main engine wreckage. No heat or fire damage was observed on the airplane. All major structural components of the airplane were located throughout the wreckage debris path.

Postaccident examination of the recovered airframe and fire-retardant dispersal system revealed that the cockpit and fuselage structure were mostly intact.

The fire gate was separated from the fuselage and exhibited significant impact damage. The forward fire gate bulkhead was separated from the fuselage. The fire gate gearbox remained attached to the forward fire gate bulkhead. The gearbox was tested using a hand tool and the gearbox rotated from stop to stop. The electric motor was separated from the gear box and was impact damaged. The fire gate torque tube was separated from the gearbox and exhibited upward bending about mid span. The aft fire gate bulkhead separated from the fuselage. The emergency dump linkage was heavily damaged but remained attached to the aft fire gate bulkhead.

Mechanical continuity of emergency dump handle was established from the cockpit control to the gate, with two fractures in the linkage tubes consistent with impact. The linkage position at the fire gate was consistent with the closed position.

Electrical continuity from the battery buss and main circuit breaker was established to the firegate electric motor. The motor drive was heavily damaged as was the electric motor. Electrical continuity from the motor contactor and EDUMP limit switches was established. The limit switches were tested to work properly.

Flight control continuity was established throughout the airframe from the cockpit controls to all of the primary flight controls. Separations in the control tubes were observed consistent with impact damage and recovery operations.

The Federal Aviation Administration's Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B, provides information and guidance in a section titled " Spatial Disorientation and Illusions” which stated in part, “Spatial disorientation specifically refers to the lack of orientation with regard to the position, attitude, or movement of the airplane in space.”

The Wright State University provides information and guidance in an article titled, “Visual Perspective Illusions as Aviation Mishap Causal Factors,” which states, while flying in low-level environments, the “masked terrain illusion” can readily occur when pilots are forced to rely upon fixed horizontal spatial cues instead of a real horizon. Distant ridgelines often appear as reliable horizontal cues even though they are fixed or false horizons with azimuths that typically fall several degrees above the true horizon and are quite prominent in the pilot’s field of view. Unfortunately, if there is a slightly lower ridgeline in front of a distant ridgeline being used as a horizontal reference, the front ridgeline can blend in visually (i.e.; become masked) with the more distant reference and may not become visually discernible until the aircraft reaches a point very close to the forward terrain.

Figure 4: Still images from video highlighting the ridgeline.

Airplane Performance Study

An airplane performance study was completed based on a witness video of the airplane and its impact with terrain and the recovered data recovered from a Garmin Aera 660, a touchscreen portable GPS. The video captured the last 9 seconds of flight as the airplane passed over the hillside from which the witness was filming, descended into the valley, began to climb, and then impacted the terrain of the next hillside. The rest of the video captures the wreckage of the airplane continuing over the ridgeline of the hill.

The GPS data shows that that the accident airplane at about 1824 to conduct firefighting operations and climbed to an altitude of about 5,500 ft msl. The airplane reached the area of the accident at about 1837 and began a series of left-hand circles over the area. The airplane descended below 5,000 ft at 1843 and slowed from about 140 kts of calibrated airspeed to 120 kts. The end of the GPS data shows that the airplane descended into a small valley and was climbing when it impacted terrain. The wreckage then continued over the top of ridge and down the other side. The elevation of the impact point was approximately 3,220 ft. The elevation of the top of the ridge above the impact point was approximately 3,260 ft. The last flying GPS data point was at 1845:44, when the airplane was at a recorded altitude of 3,235 ft. The path between 1845:39 and the final flying GPS point is consistent with the accident video which showed the airplane descending and passing low over the 3,277 ft ridgeline. The final GPS point before impact shows the airplane was about 900 ft from the approaching terrain. The video indicates that the airplane did not begin climbing until about 3 seconds before impact. This is consistent with the elevation of impact location (3,220 ft) being lower than the GPS point as the airplane was still descending.

The performance study determined that the airplane was likely between 14,000 and 15,000 lbs, at the time of the accident and its rate of climb can be estimated to be about 700 ft/min or roughly 12 ft/s. Since the airplane had been climbing for 3 seconds at the time of impact, its minimum altitude in the valley was no more than approximately 3,184 ft. According to the performance study, had the airplane been climbing at the GPS point prior to impact, it should have been able to clear the approaching terrain. However, the airplane continued to descend into the valley and the final climb began too late for the airplane’s reported climb rate to clear the terrain. Additionally, the witness video showed the airplane descending into the valley and not beginning to climb until 3 seconds before impact. The performance study determined that 3 seconds was an insufficient amount of time and distance for the airplane to gain enough altitude to clear the approaching terrain.