Air Tractor Inc AT-802 N802JF accident near Hickory Valley, Tennessee

Probable cause

A partial loss of engine power due to a leak in the fuel control unit bellows, which resulted in the fuel valve moving to minimum fuel flow.

NTSB analysis

The pilot of the aerial application airplane reported that he departed his base airport to spray fields about 6 miles away. He had made multiple passes over the target fields when he noticed the turboprop engine’s fuel flow was lower than he had set. He pushed the power lever forward and the engine began to lose power. The pilot then pushed the power and condition levers to the forward stops in an attempt to regain power, but the engine continued to “roll back.” He jettisoned the aerial application payload in the hopper and initiated a forced landing. The airplane impacted a hay field and bounced on a slight uphill section before coming to rest. The airplane’s fuselage and engine mounts were substantially damaged during the off-airport landing.

Review of data downloaded from the engine monitoring system showed that about 10 minutes after takeoff, the fuel flow began to decrease from about 82 gallons per hour to about 11 gallons per hour over 24 seconds. Examination of the engine’s fuel control unit (FCU) found the bellows exhibited corrosion and was leaking between the bellows and end cap. A review of maintenance records showed that a service bulletin (SB) was issued by the FCU manufacturer that called for the replacement of the bellows due to its age; however, the SB was not required by FAA airworthiness directive and was not complied with at overhaul or during any subsequent repair. The engine manufacturer reported that a leak in the bellows would have allowed the bellows to expand, subsequently moving the FCU fuel valve to the minimum fuel flow output as observed in the engine data recorded and as described by the pilot, preventing further power modulation. Had the actions prescribed by the SB been complied with, it is likely that the bellows would have been replaced before they began leaking and the loss of engine power occurred.

The operator reported that the airplane was mainly used for spraying and fertilizing. It is possible that the corrosive chemicals used during those operations contributed to the corrosion of the bellows.

Text from the NTSB. Open the NTSB docket for ERA24LA375 for the full record.