History of Flight
On December 27, 2016, at approximately 1553 Pacific standard time, an Air Tractor AT-602, registration N602L, was substantially damaged when it landed short at Willows-Glenn County Airport (WLW) in Willows, California. The commercial pilot, who was also the owner and operator, was not injured. The flight was conducted under 14 Code of Federal Regulations Part 91 in visual meteorological conditions without a flight plan. The cross-country flight departed Medlock Field Airport (69CL) in Davis, California at about 1527, destined for WLW.
According to the pilot, the airplane had just completed a 100-hour inspection and was carrying 160 gallons of JET A fuel. During takeoff, the airplane seemed sluggish, and the engine did not produce more than 97% power, but the pilot observed no exceedance of the turbine inlet temperature or other abnormal indications. During the 25-minute flight, the pilot initiated a straight-in approach to runway 31 and slightly reduced the throttle. Immediately, a "pop" noise was heard, and airspeed decreased. Advancing the throttle to full power did not restore engine power or airspeed, and the airplane began sinking rapidly. Further attempts to recycle the throttle were unsuccessful. As the airplane approached the south end of the airport, the pilot deployed wing flaps, and the main landing gear touched down on a gravel road beyond a ditch. The tailwheel struck the ditch, causing the airplane to bounce, nose over, and come to rest inverted.
The pilot later reported that he did not complete an engine run-up or power assurance check before the flight, which is not required by the manufacturer. He also suggested the "pop" sound might have come from a hunting party in the area.
Postaccident Examination
A Federal Aviation Administration (FAA) inspector found substantial damage to the rudder and left wing spar. An unknown quantity of fuel was in the left main fuel tank. The fuel and fuel filter appeared uncontaminated, and fuel lines and strainer were unobstructed.
Aircraft Information
The airplane was manufactured in 1999 and registered to the pilot's family in 2010. It was powered by a Pratt and Whitney PT6A-65AG 1,300 shaft horsepower turboprop engine. The most recent 100-hour inspection was completed on December 22, 2016, at which time the airframe had 6,537 total flight hours and the engine 4,974.5 hours. The airplane flew 0.4 hours between that inspection and the accident. The engine's previous hot section inspection was on May 1, 2014, at 4,078 hours.
Tests and Research
An initial examination by a Pratt and Whitney Canada representative revealed carbon debris in the oil filter, but the fuel filter and high-pressure fuel pump filter were clean. A clear liquid resembling JET A fuel was in the fuel bowl.
The compressor bleed valve was examined. The manufacturer stated that a missing valve seat can cause a permanent air leak, preventing the engine from accommodating power demands. During postaccident inspection, the bleed valve's internal piston fell out, indicating it was not secured. The valve seat was absent, and the two screws that normally secure the seat were finger tight and protruded, preventing proper seating. The seat was later found among components of a previously submitted core unit at the overhaul facility.
Maintenance Facility
The Director of Maintenance (DOM) at Grower's Air Service, which performed the 100-hour inspection, reported that the facility had only serviced two PT6A-65AG engines, including the accident engine. The DOM stated that he had trouble accessing the manufacturer's manuals and assumed the bleed valve procedures were similar to those for PT6A-34A and PT6A-60 engines, which they regularly service. He was unaware that the PT6A-65AG bleed valve had a removable seat. The mechanic who installed the overhauled bleed valve was also unaware. The maintenance facility did not perform a required bleed valve closing check or an engine performance check after the bleed valve replacement.
Engine and Component Examinations
Further examination at the manufacturer's facility showed that the accessory gearbox rotated freely. A small corner of a 2nd stage compressor blade had separated, and several 3rd and 4th stage blades had trailing edge damage. The prop flange, reduction gearbox, and power turbine blades rotated freely. Some rotational scoring was on the power turbine shroud. The left side exhaust case was damaged.
Four fuel nozzles (positions 5, 9, 10, and 12) had abnormalities, but the manufacturer stated these would not have affected power. The fuel control unit (FCU) had idle fuel flow values above pre-delivery limits, indicating adjustments by the operator or maintenance facility. A test at simulated cruise showed normal fuel flow initially, but it exceeded limits by a few pounds per hour. The FCU was adjusted to production standards. The fuel pump and fuel-to-oil heat exchanger tested within limits.
Engine Surge
According to the engine manufacturer, a "pop" sound is characteristic of an engine surge. The compressor bleed valve opens at low speeds to prevent compressor stall. If the bleed valve setting is improper, the valve will not close correctly, and an engine surge can occur. The missing bleed valve seat would result in improper bleed valve function, which could lead to compressor stalls.