N3629FSchweizer Aircraft G-164B · 1984

On July 30, 2021, about 1423 Pacific daylight time, a Grumman G-164B airplane, N3629F, was substantially damaged when it was involved in an accident near Tracy, California. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

The pilot reported that he was just beginning his 8th pass over a hay field that was located beneath high-tension power lines. He planned to repeat his previous passes by flying south below the high-tension power lines, which ran diagonally across the field (northeast to southwest) and the flightpath. When the pilot was about midway through the spray pass, he experienced a total loss of engine power accompanied by a vibration; however, the pilot could not recall if the vibration occurred before or after the power loss. The pilot stated that his airspeed was about 130 mph at the time of the power loss but was decaying rapidly. He was unable to enter a climb due to the power lines located overhead. The pilot flew about 10-15 ft above ground level under another set of power lines at the south end of the field to an adjacent field. He then began a shallow right tun to orient the airplane parallel to a series of raised dirt beds. However, during the turn the airplane contacted the ground and several trees. The wings and fuselage were substantially damaged.

In a subsequent statement the pilot reported that he was likely running the engine at 70-80% of full power as he was nearing the end of the load at the time and did not require a high power setting. The pilot could not recall the exact position of the power lever.

Engine Examination

Postaccident examination of the engine revealed that the engine’s rotating group rotated with some resistance but was free to rotate after the planetary gear assembly was removed from the nose cone assembly. Continuity of the direct drive fuel control drive train was confirmed from the fuel control drive shaft to the power shaft.

Additionally, evidence of rotational scoring was observed in several locations including the propeller shaft, first stage compressor impeller, second stage compressor impeller, and to the entire turbine rotating group.

Compressor shroud metal spray deposits were observed in several locations of the first, second, and third stage turbine rotor blades, and on the first, second, and third stage turbine rotor vanes.

Accessory Testing

Testing of the fuel control showed that the unit operated outside the manufacturer’s specifications in several areas including fuel flow, the flight idle setting, and various test points. However, according to the manufacturer, none of the tests that were beyond specifications would have been contributing factors in a loss of power event.

Testing of the fuel pump also did not reveal any anomalies.

The propeller governor was tested, and both the maximum rpm and minimum rpm were beyond their normal limitations. Additionally, the arm travel range was out of limits. According to the manufacturer, the test data were the results of typical adjustments made to the propeller governor in the field and the operational testing showed that the unit was functioning properly at the time of the accident.

Fuel Solenoid Shutoff Valve with Manual Override

The engine was equipped with a fuel solenoid valve to introduce fuel to the engine when the speed switch is actuated during engine start and to cease fuel delivery when the stop switch is actuated. When the valve is open, fuel passes through a ball valve and out to the fuel atomizers. The manual override lever and shaft at the upper end of the valve is connected to the manual stop and feather lever in the cockpit. The valve can be opened and closed electrically but cannot be opened manually.

An examination of the valve revealed that the scribe mark on the end of the override shaft was positioned at the top of the valve, consistent with an OPEN position. Low pressure air was applied to the fuel inlet port of the valve, which did not pass through the valve, consistent with the ball valve being in the CLOSED position.

According to the manufacturer, the CLOSED position of the ball valve would prevent fuel flow to the engine fuel nozzles.

The manufacturer noted that the closed position of the ball valve is inconsistent with the findings of metal spray in the turbine section, which indicates the engine was rotating and fuel was being delivered to the engine to generate a flame at the time of impact. However, according to the manufacturer, these opposing findings can only exist if the fuel shutoff valve closed at impact, either through pilot intervention, an electrical short, or inertial movement during the impact sequence.

Removal of the beta tube required 18 – 18.5 turns, which was consistent with the turns required in another propeller/engine configuration and consistent with normal alignment.

Propeller

According to the propeller manufacturer, no visible discrepancies were noted that would have degraded normal operation prior to impact. The blades remained in their respective clamps and no slippage was noted. All three pilot tubes were intact and minimal blade-to-hub contact marks were observed. Some deformation was observed on one of the drive dowel holes in the direction of rotation.

All three propeller blades exhibited chordwise/rotational scoring on the camber-side near the tip. Blade No. 2 displayed some S-bend characteristics and an approximately 3-inch-long tip fracture, which was not recovered. Blade Nos. 2 and 3 exhibited some bending opposite the direction of rotation and twisting towards the low pitch. Some scuffing and abrasions were present on each of the blade faces with no evidence of rotational scoring.

Maintenance records showed that the propeller’s most recent overhaul took place in 2012. A work order from that overhaul showed that the blade angles were set to 30 inches radius, and the manufacturer noted this angle was consistent with published guidance.

Lock Brackets, Start Lock Plates, and Piston

All three start lock brackets were fractured and the start lock plates displayed signatures consistent with forceful impact with the start lock pin. According to the propeller manufacturer, the counterweight puncture in the spinner dome was consistent with a low blade angle when the spinner impacted terrain and the signatures on the start lock bracket indicated forceful contact in the high pitch direction.

The start lock pins were actuated by a hand tool and when the blade/clamp assemblies were rotated only blade No. 2 would intermittently lock near the start lock angle. Both the blade No. 1 and blade No. 2 start lock plates passed by the displaced pin/bracket without locking.

Marks on the piston/cylinder showed a mark about 1.85 inches from the bottom of the piston consistent with an approximate blade angle of -1° and a start lock angle of about 2.0°. Another mark located about 2.35 inches from the bottom of the piston was consistent with an estimated blade angle of about 15.6°. Finally, a mark about 1.9 inches from the top of the cylinder was consistent with an approximate blade angle of -1° to 0.7° and corresponded with the piston marks and the blade damage and near the start lock angle.