Accident Overview
On March 12, 2024, at about 1902 eastern standard time, a Beech K35, registration N59MK, was substantially damaged during a forced landing near Front Royal, Virginia. The pilot, who was the sole occupant, was not injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight.
Flight History
The pilot departed from Shannon Airport (EZF) in Fredericksburg, Virginia, at 1301 on an aerial survey mission. During the flight, the pilot attempted to adjust the propeller control but received no response. After slowing the airplane near the survey area, the propeller became responsive, and the pilot decided to continue. While conducting the survey, the pilot noted an abnormally low exhaust gas temperature (EGT) on the No. 3 cylinder. The pilot continued the flight expecting the EGT to normalize, but it did not, leading him to discontinue the survey and head for the destination airport. As the pilot began to climb, he switched fuel tanks from the right main to the left main. Several minutes later, the engine began to stumble. The pilot checked the fuel selector and found it positioned to an auxiliary fuel tank; he switched it to the left main tank and continued climbing to 6,300 ft. About 10 minutes later, the engine lost total power. The pilot turned toward the nearest airport and attempted to restore power without success. Realizing the airplane would not reach the airport, the pilot executed a forced landing to a field. During the landing, the airplane traveled through a tree line, and the outboard portions of both wings separated from the aircraft.
Fuel System Details
The pilot stated that the airplane departed with full fuel capacity (99 gallons) and was equipped with six fuel tanks. The airplane had two bladder-type main fuel tanks (one per wing), each with a capacity of 25 gallons (3 gallons unusable), two bladder-type auxiliary fuel tanks, each with a capacity of 20 gallons (1 gallon unusable), and two fiberglass wingtip fuel tanks.
Post-Accident Examination
During wreckage recovery, personnel from the recovery company removed about 5 gallons of fuel from the right wing tanks and less than 1 gallon from the left wing tanks. The wreckage was transported to an aircraft salvage facility for further examination. The main and auxiliary fuel tanks showed no signs of breach during the accident sequence. The wingtip tanks remained attached but were ruptured. The main fuel tanks were cut adjacent to wing skin cuts made by recovery personnel for transportation. Wing fuel vent plumbing sections were also cut. Vent plumbing for each main and bladder tank was clear of blockages. The right wing siphon-break plumbing was partially obstructed by dirt near the wing port. The wingtip tank vent plumbing was unobstructed.
Supply lines from each auxiliary tank to the fuel selector were cut near recovery cuts. The fuel supply plumbing, including main wing and auxiliary bladder tank plumbing, the fuel selector, and the fuel strainer outlet, were free of obstructions when air was passed through. Wingtip tank plumbing was also clear. The battery was connected, and the auxiliary fuel pump operated normally. Fuel transfer pumps and solenoid valves functioned when connected to a 12-volt battery. The fuel strainer and its bowl were clean.
Engine and Propeller Examination
The propeller blades were cut midspan for an engine test run. Automotive gasoline was supplied from an external source at the fuel pump inlet, with the fuel pump return rerouted to the external source. The engine started and was run up to 2,500 rpm. The throttle, mixture, and propeller controls operated without discrepancies. The propeller lever controlled rpm as expected. The test run lasted 5 minutes.
An oil transfer collar test was performed: 80 psi was applied to the governor oil pressure port, which maintained 42 psi. The propeller governor remained attached to the engine and was undamaged. Control continuity from the governor to the cockpit was verified. The propeller governor was removed before the engine test run; its gasket screen was unobstructed. The governor pumped oil when rotated by hand and was reinstalled for the test run. After the test, the governor was sent for bench testing, where it passed all requirements per the original manufacturer's manual.
Data Analysis
A J.P. Instruments EDM-900 engine monitor was recovered, and data were successfully downloaded after the engine test run. Parameters from the accident flight, the postaccident test run, and a previous flight were evaluated. Review of the accident flight data showed that about 20 minutes before the end of the flight, engine manifold pressure was about 22 inHg, fuel flow consistently indicated 15 gallons per hour (gph), and fuel pressure was 15 psi. At 1847:20, fuel pressure and fuel flow simultaneously dropped to 0. As they dropped, at 1847:30, engine rpm increased from 2,600 to about 3,300 rpm before reducing to 2,000 rpm. Concurrently, all six cylinder head temperatures (CHTs) and EGTs uniformly began to drop to 0. Engine manifold pressure also began increasing as the airplane descended.