Air Tractor AT502 accident near Olney, Texas, August 29, 2019

The NTSB's narrative final · quoted from the NTSB record

The pilot was conducting a cross-country business flight of the newly manufactured airplane when during initial climb the airplane’s thrust began to oscillate every 1-2 seconds between what the pilot perceived as idle and full engine power. He was unable to resolve the malfunction and a forced landing was made in a rough pasture. The airplane sustained substantial damage to the fuselage, wings, and empennage during the forced landing. The investigation determined that a combination of the propeller design and the airplane’s nose-high angle while on the ground likely resulted in trapped air in the propeller’s external piston cavity. Air is present in the piston cavity of new or overhauled propellers before installation, is introduced during maintenance when a propeller is separated from an engine, or replaces the oil that gradually drains from the external piston cavity while the engine is not operating. Trapped air in the external piston cavity can act as an unintended spring in the system and confuse the interaction between the propeller and governor, which can result in an undamped oscillation event. While the airplane is operated on the ground, it is unlikely that all trapped air is purged from the external piston cavity when the propeller is cycled between maximum speed and feather. The pilot stated that he cycled the propeller 3 times before takeoff. The airplane manufacturer’s test pilots typically cycle the propeller 4 to 5 times before takeoff in a newly-manufactured airplane. Although the airplane flight manual states to feather the propeller twice before takeoff, it does not address propeller surge behavior, how to avoid it, or how to respond when it occurs during flight. The airplane manufacturer’s chief test pilot stated they do not change the engine power lever in the event of a propeller surge; they slightly retard the propeller speed lever. Implementing a small, less than ¼ inch, aft movement of the propeller speed lever changes the propeller speed 10-20 RPM and stops the propeller surges. However, this slight reduction of propeller speed as a resolution to a propeller surge is not discussed in the airplane flight manual. The airplane manufacturer stated that although their flight test department has used a slight aft change of the propeller control lever to resolve propeller surge events, they have not conducted a formal evaluation to determine how or why this resolves the propeller surge event or if it could be implemented in an effective way that also ensures pilots do not inadvertently exceed the engine’s maximum torque limitation.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.