N6126U – AIR TRACTOR INC AT-502A – 2024-01-10 NTSB Accident Report

On January 10, 2024, about 1010 Pacific standard time, an Air Tractor AT-502A airplane, N6126U, was substantially damaged when it was involved in an accident near Stratford, California. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight.

The pilot reported that the airplane was being repositioned to another location to apply fertilizer and that the hopper was empty. During takeoff the airplane failed to develop full power. At about 200 ft agl, the engine rolled back to near idle. The pilot’s attempt to vary power lever inputs did not affect engine power output. Unable to maintain altitude, the pilot maneuvered the airplane between two houses into a grove of small pistachio trees and soft dirt. The gas generator compressor turbine (CT) was still running when the pilot got out of the airplane. He shut off the fuel and secured the engine. The airplane sustained substantial damage to the wings.

The fuel tanks in both wings were breached. Fuel was observed in the surrounding areas under the airplane. Power control continuity was established from the cockpit controls to the engine. The high-pressure pump filter and last-chance filter were removed: visual examination revealed that both contained fuel and were clean. A fuel sample was obtained from the fuel farm and no contaminants were found. The engine was shipped to the engine manufacturer for additional examination and testing. No preaccident mechanical malfunctions or failures with the airframe that would have precluded normal operation were observed.

Examination of the engine at the engine manufacturer’s facilities revealed the power section, of the engine showed buckling and rotational internal contact of the exhaust casing which was consistent with the accident sequence to the power turbine section. An alternate power section was provided by the manufacturer and was installed on the gas turbine section of the engine to facilitate an engine operational check.

The engine was installed in the test cell and started normally. Following a short dwell period at ground idle speed to raise the oil temperature, the engine slowly accelerated to approximately 35,000 rpm (95% Ng). The test cell plenum pressure change confirmed that the compressor bleed-off valve (BOV) closed during engine acceleration. Subsequent review of the data showed the BOV was slow to close. The test cell T5 temperature (inter turbine temperature) readings began to fluctuate, and the engine was shut down for troubleshooting. Engine testing was interrupted due to a slave PS wire harness malfunction and ended to prevent the risk of overtemperature.

Review of the engine run test data and hesitation heard during the engine sweep was consistent with a compressor BOV malfunction. Subsequent bench testing of the BOV revealed air leakage around the piston and P3 feed gasket. Bench test pressure check of the BOV revealed an abnormal amount of air leaking around the piston. Disassembly of the BOV revealed a piston ring set was stuck within the piston groove, consistent with an abnormal air leak and irregular function. Abrasive and adhesive wear was observed on the piston rings and piston groove, consistent with the environmental debris and/or by the twist observed on the BOV metal piston ring.

During the disassembly of the BOV, two assembly part numbers were vibro-peened by hand on the BOV cover, with neither of the two-part numbers having a strike through.

A review of the maintenance records revealed that the BOV had been disassembled, cleaned, inspected and repaired in April of 2012 by Dallas Airmotive, which is a designated overhaul facility for Pratt & Whitney Canada. This BOV was not installed on the engine at delivery.