N766BFAyres S2R-G10 · 1999

On July 21, 2019, about 0705 eastern daylight time, an Ayres S2R-G10 airplane, N766BF, was substantially damaged when it was involved in an accident near Breckenridge, Michigan. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 agricultural application flight.

The pilot stated that the accident occurred during his second flight of the day, and that he encountered no issues during his first flight. Before departure, the airplane was loaded with about 60 gallons of Jet-A fuel in the fuel tank and 325 gallons of fungicide and water in the hopper. The pilot stated that shortly after takeoff on runway 27, about 100 ft above the ground, he observed a sudden increase in exhaust gas temperature (EGT) above the engine’s limitation. He immediately reduced the throttle, but the engine had a total loss of engine power. The pilot turned on the igniters to restart the engine; however, the low altitude of the airplane required an immediate forced landing in a cornfield. He lowered the flaps and leveled the wings for landing. The main landing gear collapsed when it contacted the corn. After the accident, the pilot pulled the emergency fuel cutoff, and turned off the igniters, generator, and battery.

Data downloaded from the airplane’s engine monitor established that there was a sudden and total loss of engine oil pressure shortly after takeoff that resulted in a loss of control of the propeller blade angle with a corresponding decrease in engine speed and increase of EGT, as shown in figure 1. The exhaust gas temperature increased from 535°C to 850°C during the 10 seconds after the loss of engine oil pressure. After the accident, the engine N1 stabilized at about 44% for about 5 seconds until the fuel flow and fuel pressure simultaneously dropped to zero, consistent with the pilot closing the emergency fuel shutoff valve.

Figure 1. Plot of Engine Monitor Data

The engine, a Honeywell TPE-331-10-511M, serial number P42076C, was last inspected during a 100-hour inspection completed on July 19, 2019, at which time it had accumulated 16,857.5 hours since new and 603.9 hours since its last overhaul. The airplane had accumulated an additional 2 engine starts, 4 landings, and 1.74 hours since the 100-hour inspection.

According to the maintenance documentation, the engine oil cooler was replaced during the 100-hour inspection. The oil cooler is remotely mounted below and aft of the engine in the nacelle to provide a good source of ambient air flow in which to transfer the heat from the oil cooler. Flex-hoses carry hot engine oil from the engine scavenge pump to the oil cooler and then return cooled oil to the engine oil tank.

The engine, a Honeywell TPE-331-10-511M, serial number P42076C, was last inspected during a 100-hour inspection completed on July 19, 2019, at which time it had accumulated 16,857.5 hours since new and 603.9 hours since its last overhaul. The airplane had accumulated an additional 2 engine starts, 4 landings, and 1.74 hours since the 100-hour inspection.

According to the maintenance documentation, the engine oil cooler was replaced during the 100-hour inspection. The oil cooler is remotely mounted below and aft of the engine in the nacelle to provide a good source of ambient air flow in which to transfer the heat from the oil cooler. Flex-hoses carry hot engine oil from the engine scavenge pump to the oil cooler and then return cooled oil to the engine oil tank.

The accident site was documented by a Federal Aviation Administration (FAA) airworthiness inspector on the day of the accident. The FAA inspector reported that the airplane was found upright in a cornfield about 1,400 feet from the end of the runway, as shown in figure 2. The 70 ft long impact path through the corn was on a 200° magnetic heading. There was no evidence of a fire. The fuselage, cockpit, and empennage remained intact. Both wings remained attached to the fuselage with impact-related damage to the leading edges and wing skins. The flaps were found fully extended. Both main landing gear collapsed during impact. Fight control continuity was established from the cockpit controls to all control surfaces. Engine and propeller control continuity was established from the cockpit controls to their respective engine components. The upper half of the hopper was undamaged, but the hopper gate and all associated spray equipment were damaged by ground impact. All four propeller blades remained attached to the propeller hub, were bent aft around the engine cowl, and exhibited minor chordwise scratching.

The FAA inspector who was at the accident site stated that on the day of the accident he was assisted by the airplane mechanic who replaced the engine oil cooler 2 days earlier. The mechanic told the FAA inspector that there was still engine oil present in the tank after the accident, but the FAA inspector acknowledged that he did not personally verify the presence of oil in the tank or see the mechanic check the oil level.

A few weeks after the accident, the FAA inspector assisted the engine manufacturer investigator with separating the engine from the airframe. At that time, the engine oil tank did not contain any oil. The oil tank was dented and scratched in several locations consistent with impact damage but otherwise remained intact and was not breached. Additionally, the oil transfer hoses and fittings associated with the oil cooler were found intact and tight.

Engine disassembly at the manufacturer’s factory did not reveal any evidence of a mechanical malfunction or failure that would have precluded normal operation. Normal oil wetting and wear signatures were observed throughout the engine. There was no evidence of an oil leak within the engine; however, there was oil observed on the external surfaces of several engine accessories. The oil on the external surfaces did not originate from the engine case, engine accessories, or the reduction gearbox.

Examination of the on-scene photos taken on the day of the accident by the FAA inspector established that there was engine oil present on several components inside the engine nacelle. Engine oil was observed on the engine rear frame, the exhaust duct, the firewall/battery box, the oil cooler just below the exhaust duct, and on the oil cooler transfer hoses, as shown in figure 2. The on-scene photos show engine oil pooled on the ledge of the battery box frame, consistent with escaped oil being distributed around the inside surfaces of the nacelle. There was also engine oil wetting that extended to the top of the exhaust duct and the engine rear frame, consistent with eductor operation with the engine operating at a high-speed. There was additional oil wetting on the center of the engine exhaust pipe, at the same plane as the oil cooler, which gradually became less apparent near the mid-point of the engine rear frame. The internal surface of the eductor mixing duct was oil wetted, consistent with engine exhaust drawing escaped oil particles from the nacelle through the annulus and into the mixer duct, as shown in figure 3. Additionally, the fuel control unit and fuel pump cases were oil wetted, consistent with escaped oil being distributed around the inside of the nacelle, as shown in figure 4.

Figure 2. Engine Exhaust and Oil Cooler at Accident Site (Source: FAA)

Figure 3. Eductor Interior Surface (Source: Honeywell)

Figure 4. Fuel Control Unit and Fuel Pump Assembly (Source: Honeywell)

The airframe portion of the engine oil system, consisting of the oil cooler and two transfer flex-hoses, was examined to determine the source of the oil leak. A visual examination of the oil cooler revealed a weld fracture along the edge of the cooler housing and a deformed mounting flange. The fracture location and appearance were consistent with impact-related damage. The fracture was cleaned, welded closed, and pressure tested while submerged in a water tank. There was no evidence of any leaks during testing, confirming the integrity of the oil cooler. Similarly, the two transfer flex-hoses did not exhibit any leaks when pressure tested.