Fuel Starvation Maule-4-Incorrect Fuel Caps
An Aviation Safety Information Letter from the Transportation Safety Board of Canada (TSB)

On September 30, 2004, a Maule-4 aircraft lost power while cruising at 1 200 ft. The pilot changed tanks and turned on the electric fuel pump, but power could not be restored and the aircraft was forced to land. As the field was too short, the aircraft sustained substantial damage when it hit a fence at the end of the landing roll and overturned. When the aircraft was recovered, the pilot owner was somewhat surprised that fuel remained in the right tank and very little was lost from the left tank after the aircraft had been inverted overnight. The type of cap thereby retaining the fuel in the tanks.

Examination of all fuel tubing did not reveal any anomalies or restrictions. It was also outlined that the aircraft had a similar previous engine stoppage two years earlier. At that time, the aircraft was on skis over a snowy field and made a successful forced landing. Shortly after, the engine restarted and ran normally. Due to lack of other tangible factors, it was felt that it may have been caused by a fuel selector malfunction or positioning. The owner also recalls that whenever operating with the fuel selector on "both," the left tank always fed at a slower rate than the right. He further mentioned having heard air rushing into the tank when opening the left fuel cap for refuelling immediately after engine shutdown.

After the most recent occurrence, the owner was prompted to verify the adequacy of the venting system, which is done through the fuel caps (Figure 1). Air passage on the left fuel cap was found to be erratic; sometimes it would let the air through, but sometimes it would not. Information from the manufacturer indicates that this type of cap is only to be installed on aircraft having been modified with auxiliary wing tanks (located plumbing for a different venting system.

Figure 1: Non-probed fuel cap
Figure 1: Non-probed fuel cap

The caps used on the occurrence aircraft, shown in Figure 1, had been ordered by the previous owner to replace the original caps to which a ram air probe is fitted to assure positive pressure within the fuel tanks (Figure 2). The order voucher indicated that non-leaking caps (non-probed caps) were requested. This was desired partly for aesthetic reasons and also because probed caps allowed fuel to leak out if the aircraft when it was parked on uneven ground. The order voucher included the aircraft serial number. The manufacturer forwarded the non-probed fuel caps without challenging whether the aircraft fuel system was original or it had been modified with auxiliary wing tanks. While the probed caps assure a positive pressure inside the fuel tanks, the air passage through the non-probed caps reduces the pressure within the tank below that of the ambient pressure.

Figure 2: Probed fuel cap
Figure 2: Probed fuel cap

Consequently, any blockage within the cap quickly results in stopping the fuel flow to the engine. As the would normally restore the fuel flow, re-establishing power to the engine. Test bench trials on similar systems, operated by a skilled engine technician aware of the intended fuel starvation test, have demonstrated that it requires 30–45 seconds to restore full power following the engine stoppage.

The investigation into this occurrence has raised a concern about the replacement of parts for different aircraft models, which would affect the airworthiness of the aircraft. The use of non-probed caps on an unmodified airframe has shown that venting is possible when the valve within the caps is working properly. However, as demonstrated in this occurrence, there is no alternate means of venting in case of malfunction. Any change to original aircraft status, regardless how small, must first be authorized by the manufacturer, unless it is approved via a supplementary type certificate (STC)-as these changes can and have created airworthiness disturbances.

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