N602L

HISTORY OF FLIGHTOn December 27, 2016, about 1553 Pacific standard time, an Air Tractor AT-602, N602L, was substantially damaged when the airplane landed short at Willows-Glenn County Airport (WLW), Willows, California. The commercial pilot was not injured. The airplane was owned and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the cross-country flight that departed Medlock Field Airport (69CL), Davis, California at approximately 1527. The personal flight was destined for WLW.

According to the pilot, he was returning the airplane to WLW after a recent 100-hour inspection with 160 gallons of JET A fuel onboard. The airplane seemed sluggish during takeoff and the engine did not produce more than 97% power, but the pilot did not observe an exceedance of the engine's turbine inlet temperature or any further abnormal instrument indications. At the conclusion of the 25-minute flight, the pilot initiated a straight-in approach to runway 31 and slightly reduced the throttle. Immediately, he heard a "pop" noise, and observed a reduction in airspeed. When the pilot advanced the throttle to full power the engine power and airspeed did not change, and the airplane began to sink rapidly. Further attempts at recycling the throttle were unsuccessful in restoring engine power. As the airplane approached the south end of the airport, the pilot deployed the wing flaps, and the main landing gear touched down on a gravel road beyond a ditch. The tailwheel impacted the ditch and bounced, and the airplane nosed over and came to rest inverted.

The pilot later reported that he did not complete an engine run-up or power assurance check before the accident flight, which is not a required item by the airplane manufacturer. The pilot also reported that the "pop" sound may have been from a hunting party that was in the area at the time of the accident.

Postaccident examination by a Federal Aviation Administration (FAA) inspector revealed substantial damage to the rudder and left wing spar. Additionally the inspector reported an unknown quantity of fuel in the left main fuel tank. The fuel and fuel filter did not appear to be contaminated, and the fuel lines and fuel strainer did not appear to be obstructed. AIRCRAFT INFORMATIONAccording to FAA records, the airplane was manufactured in 1999, and registered to the pilot's family in 2010. The airplane was powered by a Pratt and Whitney PT6A-65AG, 1,300 shaft horsepower turboprop engine. The aircraft logbooks showed that the airplane's most recent 100 hour inspection was completed on December 22, 2016 at which time the airframe had accumulated 6,537 total flight hours and the engine had accumulated 4,974.5 total flight hours. The airplane accumulated 0.4 hours between that inspection and the accident. The engine's previous hot section inspection was completed on May 1, 2014 at a total time of 4,078 flight hours. AIRPORT INFORMATIONAccording to FAA records, the airplane was manufactured in 1999, and registered to the pilot's family in 2010. The airplane was powered by a Pratt and Whitney PT6A-65AG, 1,300 shaft horsepower turboprop engine. The aircraft logbooks showed that the airplane's most recent 100 hour inspection was completed on December 22, 2016 at which time the airframe had accumulated 6,537 total flight hours and the engine had accumulated 4,974.5 total flight hours. The airplane accumulated 0.4 hours between that inspection and the accident. The engine's previous hot section inspection was completed on May 1, 2014 at a total time of 4,078 flight hours. TESTS AND RESEARCHAn initial examination of the airplane was completed under the supervision of the NTSB Investigator-in-Charge by a representative of Pratt and Whitney Canada who was an airframe and powerplant mechanic. The oil filter, fuel filter, and filter to the high pressure fuel pump were inspected. The oil filter displayed some carbon debris, while both the fuel filter and filter to the fuel pump were free of contamination. A clear liquid was observed in the fuel bowl that exhibited an odor and appearance that resembled JET A fuel.

Compressor Bleed Valve

The purpose of the compressor bleed valve is to prevent a compressor stall at low gas generator speeds. The unit is comprised of a piston that is pushed into the closed position by P3 air and moved to the open position using P2.5 air from the inter-stage compressor area. At low power settings, P3 pressure is lower than the P2.5 pressure, which causes the bleed valve to position itself to the open condition. As compressor speed increases, P3 pressure rises faster than the P2.5 pressure, which causes the bleed valve to move towards the closed condition.

The speed at which the valve closes is both a function of P3 pressure and surface area on which P2.5 pressure is pushing on the piston. P3 pressure is dependent on the restriction at the outlet of the cavity where it is drawn. The surface area on which P2.5 air pushes depends on the inside diameter of the bleed valve seat.

According to the engine manufacturer, the piston will normally extend to the closed position against the valve seat in response to a power demand from the operator. The piston is equipped with a carbon ring that is normally compressed between the piston and the piston guide collar. If the valve seat is not installed, the piston can travel beyond its normal range, which allows the carbon ring to expand and come out of its groove. This will then prevent the valve piston from moving within its guide collar. The absence of the valve seat will cause a permanent air leak around the piston, which will allow P2.5 pressure to escape the engine. This condition can prevent the engine from accommodating any additional power demand and/or result in significantly slower accelerations.

At the time of the accident airplane's recent 100 hour inspection, the bleed valve was replaced with an overhauled unit from the bleed valve overhaul facility, Prime Turbines. The original bleed valve had been submitted to Prime Turbines a few weeks earlier as a core unit.

During the post accident examination, when the bleed valve was removed from the engine, its internal piston fell out, indicating that it was not secured within the valve body, as it was supposed to have been. An inspection of the bleed valve showed that the valve filter was intact and free of contamination. A comparison of the accident bleed valve with the manufacturer's illustrated parts catalogue revealed that the valve seat was absent and the two screws that normally secure the seat to the valve body were installed finger tight and protruded from the flange of the valve body, preventing proper seating when the valve was installed on the engine. In response to a query by the investigation, a representative of bleed valve overhaul facility discovered the valve seat among the components that comprised the previously submitted core unit.

According to the section of the engine maintenance manual entitled "Compressor Bleed Valve – Maintenance Practices," the bleed valve seat must be removed from the core unit and remain with the gas generator assembly after the bleed valve has been removed. Once the overhauled bleed valve arrives, the previously retained seat should be installed on the replacement bleed valve and tightened to 5 to 6 inch lbs before the unit is installed on the engine.

Maintenance Facility

The Director of Maintenance (DOM) to Grower's Air Service, the service facility, who also signed off the airplane's most recent 100 hour inspection, reported that his facility started working on PT6A-65AG engines about 2 years ago. During this time period, the company only serviced 2 PT6A-65AG model engines, including the engine from the accident airplane. Initially, the DOM stated that they completed the engine inspection in accordance with the manufacturer's maintenance manual (MM), which was revised in March 2016. Normally, the maintenance facility DOM will review the applicable service manuals prior to engaging in a particular maintenance activity. However, the DOM had trouble accessing the manuals through the manufacturer's website, and decided not to review the bleed valve guidance prior to the maintenance on the accident engine because he assumed that the procedures were similar to those prescribed for the PT6A-34A and PT6A-60 engines, which the facility regularly services. According to a PT6A-34A diagram, the bleed air valve is not equipped with a calibrated seat similar to the bleed valve assembly on the PT6A-65AG engine as it contains a metering orifice in the P3 air inlet.

The DOM stated that he removed the old bleed valve from the engine, and that another mechanic installed the overhauled unit. When asked about the valve seat by the investigative team, the DOM stated that he was not aware that the bleed valve on the PT6A-65AG had a removable valve seat, and was certain that the mechanic who installed the overhauled bleed valve on the engine was not aware of that removable valve seat either.

At the conclusion of the inspection, the line mechanic completed two engine runs to dynamically balance the propeller; however, the run-up power settings were not recorded by the maintenance facility. According to the maintenance manual, a bleed valve closing check procedure is required after the unit is installed to verify the gas generator speed at which the valve closes. This value is charted using the gas generator speed and plenum pressure. This procedure was not completed by the maintenance facility.

The engine maintenance manual includes guidance for completing an engine performance check to measure engine torque, fuel flow, gas generator and gas generator speed at various temperatures and specified power settings. This check is only recommended by the engine manufacturer at regular intervals and...