N402RE

On August 11, 1999, approximately 1500 mountain daylight time, an Air Tractor AT-402A, N402RE, owned and operated by Ray Edmiston and doing business as Aerial Sprayers, Inc., was substantially damaged when it collided with terrain following a loss of engine power during final approach to landing at Carrera Airpark, Mead, Colorado. The airline transport certificated pilot, the sole occupant aboard, was not injured. Visual meteorological conditions prevailed for the local aerial application flight being conducted under Title 14 CFR Part 137, and no flight plan was filed. The flight originated at 1400.

According to the pilot, he had performed an aerial spray of a field with a combination of insecticides (Lannate and Asana) and was returning for landing at the privately-owned airstrip. He stated that while on a final approach to landing with full flaps applied, "partial power" was still applied to the engine. At approximately 100 feet above ground level (agl), he reported that he increased the propeller control to high rpm and the engine began to vibrate, followed by white smoke exiting from the exhaust duct. According to the pilot, he attempted to apply power to the engine but it "did not respond to more power application." With diminutive altitude and speed remaining, he "flared to [a] full stall lag" in a field of 6-foot tall corn stalks 150 yards short of the runway. Upon impact with the ground, the aircraft sustained damage to the left wing and wing spar, horizontal stabilizer and elevator, landing gear and propeller.

On September 9, 1999, the aircraft's engine was disassembled and examined at the facilities of Covington Aircraft Engines in Okmulgee, Oklahoma. The engine's fuel filters were removed and the fuel basin was observed to be full. The fuel was clear and free of contaminates. The oil filter was removed and exhibited no unusual smell or color. The power section screen was removed and no foreign material was detected. The P-3 filter was examined and appeared to be in serviceable condition.

The engine was separated at the 'C' flange. A visual inspection of the hot section revealed no apparent physical damage to the compressor turbine blade and disc assembly face, and did not exhibit any indications of rubbing against the power turbine stator baffle inside bore. In addition, no evidence was found indicating compressor turbine blade tip rub on the segments of the small exit duct assembly. No external damage was found on the gas generator case, inlet case, accessory gearbox case or the mounting flange for the high pressure fuel pump.

The primary propeller governor and the over-speed governor were removed from the power section. The accessories and their respective drive gears were found to rotate normally. The power section stator housing assembly was removed from the power section. Burn marks were observed on the power turbine disc assembly rear bore outer diameter and the power turbine stator baffle forward bore. The power turbine disc assembly was removed and the blades were intact and displayed evidence of heavy tip rub. The power section was rotated by hand and no sounds of gear rubbing were noted. The power turbine shaft housing was removed and no discrepancies were found.

On October 26, 1999, the engine was reassembled and run in a test cell at Covington Aircraft Engine's test facilities. With the primary governor installed, a series of test runs were accomplished. The engine was operated at various engine speeds typical of approach power settings. The dynamometer speed was rapidly advanced to maximum engine rpm (2200) while governor output pressure was observed, and pressure readings were normal. The engine's overspeed governor was also installed and again readings were normal. During both the engine disassembly and test run, no abnormalities or discrepancies were found that would have interfered with normal engine operation.

Following the accident, an examination of the airframe was performed. Each of the propeller blades were bent in the reverse direction (see attached photographs). The power lever in the Air Tractor AT-402A is designed with a beta mode in which the pilot can manually pull the throttle rearward with corresponding negative blade angles. According to an engineering representative with Air Tractor, it is possible for a pilot to bring the throttle back into beta mode when the aircraft is potentially faster and higher than necessary to land. According to an engineering representative with the engine manufacturer, Pratt and Whitney, once the throttle is pulled back into beta mode, it takes approximately 3 to 6 seconds for the engine to respond to increased engine power input.

According to the Airplane Flight Manual (AFM) for the Air Tractor AT-402A under 'Approach and Landing - Normal-Empty Hopper,' the instructions state:

1. Prop Lever "P" - Full forward (Reduce power a little first). 2. Start Lever "S" - Flight Idle Position (68 to 70% Ng). 3. Power Lever - Adjust to provide required rate of descent. Do not move power lever below the idle stop position. Check Beta light - OUT.

Only after landing , the AFM states under the 'Landing (Normal)' section :

1. Power Control Lever - As desired during landing roll. If reverse thrusting is necessary, the thumb latch on top of the power lever must be pushed forward and the power lever moved slowly aft until the Beta Light is observed ON. Reverse thrust must be selected as necessary by continued aft movement of the power lever (Keep control stick fully aft). Insure Torque and/or ITT limits are not exceeded.