Wrong fuel tank cap leads to accident

The pilot reported that during the preflight inspection, he filled the Piper J-4A’s forward (main) fuel tank to about 1 inch below the top and noted that the auxiliary fuel tank contained 4 gallons of fuel. The pilot used a stick to “dip” the two fuel tanks and validate the amount of fuel in them.

The pilot planned on departing from North Omaha Airport (3NO) in Nebraska for flight operations in the airport’s traffic pattern.

Before the takeoff, the fuel selector was positioned for the forward fuel tank. During the takeoff, the airplane traveled about three-quarters of the way down the runway and climbed to about 300 feet above ground level. The engine sputtered once and stopped producing power. The pilot switched the fuel selector to the auxiliary fuel tank and tried to restart the engine unsuccessfully.

He maneuvered the airplane for a forced landing, but the airplane hit the ground and came to rest next to a tree in a field.

The pilot reported that fuel was pouring into the cockpit after the airplane came to rest, and he quickly got out of the airplane.

The airplane sustained substantial damage to both wings and the fuselage, while the pilot sustained minor injuries.

A post-accident examination of the airframe and engine revealed that the fuel tank cap found installed on the filler neck of the forward fuel tank did not appear to be the correct part for the airplane. It had a part number of 2501621 etched on it with no manufacturer name displayed.

The fuel tank cap appeared to be manufactured with a single vent hole. The fuel tank cap was equipped with two worn gaskets stacked under the cap. The lower gasket had multiple visible cracks present on both sides of it. The upper gasket was wrinkled and was displaced toward the vent hole, which blocked the venting capability of the fuel tank. Based on the available evidence, it could not be determined when the vent hole became blocked. No fuel was observed in the forward fuel tank, which had sustained impact damage.

For the Piper J-4 Cub, the sole source of venting for the forward fuel tank is through a vented fuel tank cap. The approved part numbers for the forward fuel tank caps from the original equipment manufacturer (Piper) are A748 (the older style) or 00062-40 (the newer style).

The pilot reported that the forward fuel tank cap was on the airplane when he purchased it several years ago. The pilot and his brother purchased the airplane together in March 2011, and the pilot became the sole owner in October 2017. The pilot did not have any knowledge of the origin or the background of the forward fuel tank cap.

According to the airplane’s maintenance logbook, during the airplane’s most recent annual inspection, the airplane was inspected per Piper Inspection Sheets 230-3000. The accident occurred 39 days after the inspection, and 2.3 airplane flight hours had accumulated since the inspection.

The FAA Airplane Flying Handbook (FAA-H-8083-3B) discusses preflight procedures and states in part:

Checking the fuel tank vent is an important part of a preflight assessment. If outside air is unable to enter the tank as fuel is drawn into the engine, the eventual result is fuel starvation and engine failure. During the preflight assessment, the pilot should look for signs of vent damage and blockage. Some airplanes utilize vented fuel caps, fuel vent tubes, or recessed areas under the wings where vents are located. The pilot should use a flashlight to look at the fuel vent to ensure that it is free from damage and clear of obstructions. If there is a rush of air when the fuel tank cap is cracked, there could be a serious problem with the vent system.

Probable Cause: A total loss of engine power due to a blocked fuel tank cap vent hole, which resulted in fuel starvation. Contributing to the accident was the mechanic’s and the pilot’s inadequate inspections of the fuel tank cap.

NTSB Identification: 103179

To download the final report. Click here. This will trigger a PDF download to your device.

This May 2021 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.