N78988Mooney M20C

On August 26, 2019, about 1644 Pacific daylight time, a Mooney M20C airplane, N78988, was substantially damaged when it was involved in an accident near Reno, Nevada. Both passengers were seriously injured, and the pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, before departure, he sumped the fuel tanks, added oil, and subsequently departed on the accident flight with the fuel tanks about half full. He determined the fuel level by computing the total time the engine had been running since it was last refueled. The pilot reported that he did not have a dipstick for the fuel tanks nor was there one manufactured for his airplane. As he approached the destination airport, he descended below traffic pattern altitude, turned on the electric fuel boost pump, and switched to the right fuel tank just before entering the right base leg of the traffic pattern. Immediately after turning off the fuel pump after switching tanks, while turning onto final approach, the engine sputtered and then lost all power. The pilot switched the fuel selector back to the left tank and turned on the fuel boost pump again. He then pitched the airplane for its best glide speed and configured the airplane for a forced landing. The pilot stated that he was focused on the airspeed indicator during the landing and was not looking outside the airplane when the airplane impacted a fence and then the ground, resulting in substantial damage to the left wing.

GPS data from a third-party commercial application used by the passenger during the accident flight showed that the airplane departed and reached its maximum altitude about 20 minutes after departure. The total duration of the flight was about 35 minutes.

A Federal Aviation Administration (FAA) inspector arrived at the accident site about 30 minutes after the accident and did not smell any fuel around the wing fuel tanks, both of which had ruptured during impact.

The airplane was equipped with two 26-gallon integral sealed fuel tanks located at the wing root of each wing. During normal operation, fuel is fed through aluminum fuel lines that carry it to the engine through a fuel selector valve and an electric boost pump. The system description states:

The electric boost pump is turned on for take-off and landing to provide fuel pressure if the engine driven pump malfunctions.

The pilot reported that he departed with 21.6 gallons of fuel onboard; about 9.6 gallons in the left wing, and 12 gallons in the right wing.

Both propeller blades remained attached to the engine at the propeller hub. One blade exhibited blade tip curling and several gouges. The other blade displayed a slight bend.

The engine was mostly intact, with the exception of the airbox and exhaust, which exhibited minor damage. About 1 gallon of 100 low-lead aviation fuel was plumbed directly into the engine-driven fuel pump. The propeller was not replaced before the engine run. The engine started successfully and was advanced to 1,700 rpm before it was further advanced to the full throttle position and 2,700 rpm. Engine operation was smooth and continuous during the engine run.

The pilot reported that he regularly used the fuel quantity gauges on the instrument panel to verify fuel levels. Postaccident examination revealed that both fuel tank gauges indicated erroneous fuel quantities. Tests completed with and without power showed that the right fuel tank indicated a fuel level near empty with the fuel sender at its lowest position. The fuel gauge indicated 6 lbs of fuel without power and 36 lbs of fuel with power for the left tank, with the fuel sender at its lowest position.

The fuel senders and fuel gauge cluster were submitted to their manufacturers for further examination; however, tests were inconclusive due to the unknown condition of the fuel senders at the time of the accident.

The pilot reported that, before the accident, he had flown from Wyoming, where he last refueled the airplane. The pilot stated that he assumed a fuel consumption rate of 10 gallons per hour and switched tanks every 30 minutes. He further remarked that the accident was likely the result of fuel exhaustion, as he calculated that he had 1 total hour of fuel remaining at the time of the accident. Additionally, he reported that he may have become task saturated during the flight from Wyoming due to turbulence and forgot to switch tanks within the prescribed time period. The pilot stated that the fuel indicators had always reported accurate indications consistent with his total run time computations.

A photograph taken by the rear seat passenger of the instrument panel before departure on the accident flight showed that the left fuel tank contained about 9 gallons and the right fuel tank contained about 12 gallons. Fuel computations were completed using the takeoff and climb and cruise data charts from the pilot's operating handbook. Based on these charts, the airplane would have consumed fuel at a rate of 13 gph for the first 9 minutes, 12 gph for 6.5 minutes, 11 gph for 6.5 minutes and finally 10 gph for the remaining 13 minutes of flight. Based on the pilot's statement of fuel load prior to takeoff, the fuel computations indicated that the airplane had approximately 8.7 gallons of fuel in the right fuel tank and 6.2 gallons in the left fuel tank at the time of the power loss. If the pilot left the fuel selector on either tank for 30 minutes longer than planned during his previous flight, the quantity in that tank would have been reduced by about 5-6 gallons.

The FAA Pilots Handbook of Aeronautical Knowledge contained the following guidance regarding cockpit fuel gauges:

Aircraft certification rules require accuracy in fuel gauges only when they read "empty." Any reading other than "empty" should be verified. Do not depend solely on the accuracy of the fuel quantity gauges. Always visually check the fuel level in each tank during the preflight inspection, and then compare it with the corresponding fuel quantity indication.