N346MA

On December 19, 2018, about 1420 eastern standard time, a Diamond DA20, N346MA, was substantially damaged when it was involved in an accident in McCormick, South Carolina. The pilot and passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 post-maintenance test flight.

On the day before the accident, another pilot performed a precautionary landing in the airplane at McCormick County Airport (S19), McCormick, South Carolina, due to a partial loss of engine power. That pilot reported that after landing, the engine would not run when the electric fuel pump was turned off. The engine would run “slightly better” if the primer was running (the primer switch, normally used when starting the engine, operates the electric fuel pump on HIGH).

The accident pilot, who also held a mechanic certificate, was dispatched to S19 to diagnose and repair the airplane on the day of the accident. After checking the oil quantity, he started and ran the engine, which was “not running smoothly.” He connected test equipment to the fuel-injected engine and determined that the unmetered fuel pressure was above the manufacturer-specified limits at 1,000 rpm, and the metered fuel pressure was below the specified limits at full power. He then replaced the throttle and metering unit (also referred to as the “fuel servo” or “fuel control unit”), the engine-driven fuel pump, and the fuel manifold valve (also referred to as a “flow divider” or “distribution manifold”). After replacing the fuel manifold valve, he was able to adjust both the metered and unmetered fuel pressures to within the specified limits. The engine ran satisfactorily at idle, during a normal run-up, and at full power. He asked a coworker to check his work, which was found satisfactory with no fuel leaks observed.

Before takeoff, the pilot performed a preflight inspection and sumped each fuel drain three times to ensure there was no debris or contamination and found none. He checked the fuel quantity and noted that the single tank was about 3/4 full. He started the engine and performed a normal run-up before taxiing for departure from runway 36.

The pilot initially remained in the traffic pattern until he was satisfied that the engine was performing normally. He then began a climb and proceeded toward Newnan Coweta County Airport (CCO), Atlanta, Georgia. At an altitude between 3,000 and 3,500 feet, the engine lost partial power. The engine speed reduced from about 2,500 rpm to 1,000 rpm and would not respond to throttle input. The pilot ensured that the mixture was in the rich position, the fuel boost pump was on, and attempted to adjust the throttle, but was unsuccessful in restoring power. He turned the airplane back toward S19 and prepared for an emergency landing, but realized that he would be unable to reach the runway and attempted to land in a clearing about 1 mile southwest of the airport, during which the airplane impacted trees.

Examination of the wreckage by a Federal Aviation Administration inspector revealed that the left wing was completely separated from the fuselage at the root and a large section of roof and canopy was fragmented.

The engine was examined at the manufacturer’s facility under oversight of the NTSB. The engine was installed on a test stand and found to run very rough and would not idle. The engine remained running only when the test stand fuel boost pressure was increased beyond the normal values. The throttle and metering unit was removed from the engine and tested on a production test bench, where the fuel flow parameter was below the acceptable range at all throttle positions. The throttle and metering unit was disassembled and a piece of green glassy solid debris about 1/16” square was found in the metering plug orifice, obstructing about 90% of the orifice area. The debris was too large to pass through the fuel pump inlet screen or the airframe fuel filter bowl (gascolator). The debris was removed, the throttle and metering unit was reassembled and reinstalled on the engine. All subsequent engine test runs appeared normal.

An examination of the fuel manifold valve that was removed from the airplane prior to the accident flight revealed several pieces of debris on the downstream side of the manifold’s filter screen. The debris appeared to be identical in color to the debris found in the throttle and metering unit. The particles were smaller, but too large to have passed though the manifold screen.

The debris found in the throttle and metering unit was sent to the NTSB materials lab for further examination. A Fourier transform infrared spectrometer examination revealed that the composition of the debris was similar to (but not an exact match) an exemplar sample of “torque putty.” Torque putty is a liquid/paste substance that can be used to mark threaded fittings or fasteners after they have been assembled and properly torqued. When used, a line of putty is typically applied across both sides of a fitting, which dries after application. During subsequent visual inspections of the fitting, any cracks in the putty or displacement of its line is an indicator that the fitting may have loosened. During disassembly of a fitting marked with torque putty, the putty is typically cleaned/removed from the components. At the time of the engine examination, none of the fuel system fittings were found marked with torque putty.

AIRCRAFT INFORMATION

A review of the airplane maintenance records over the 13 months (about 1,300 flight hours) preceding the accident revealed that the only other fuel system related entries occurred in August 2018 (see table).

Table - Fuel System Maintenance Events

Date

Maintenance entry

# flight hours prior to accident flight

8/17/18

Set fuel flow injection pressure

549

8/29/18

Removed/Replaced inoperative (electrical/boost) fuel pump,

Set fuel flow injection pressure

426

8/30/18

Set fuel flow injection pressure

422

Fuel System

Fuel flows from a single fuel tank, through one of two outlet finger screens, to a filter bowl and then the electrical fuel pump. It then passes through a shutoff valve before entering the engine driven fuel pump, which is fitted with an inlet finger screen. Fuel then flows to the throttle and metering unit (labeled “Fuel Control Unit” in the figure below) before entering the manifold valve (labeled “Distribution Manifold”) which is fitted with an internal filter screen. Four lines deliver fuel from the manifold valve to the four engine cylinders through the fuel injector nozzles.

Figure. – Fuel system diagram with annotations