| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 21 November 2018, the crew of a Cessna Aircraft Company 152 departed Bankstown, New South Wales to conduct a training flight with two crew members on board.
Returning from the Bankstown training area, the aircraft was cleared by air traffic control (ATC) to join downwind for runway 29R. Whilst on downwind, at 1,500 ft above ground level, the aircraft’s engine RPM started to reduce uncommanded. The instructor took over from the student and applied full throttle and carburettor heat to attempt to increase engine RPM and performance. The engine’s RPM increased momentarily and reduced again.
The instructor advised the tower of the engine issues and requested a glide approach. ATC cleared the aircraft for a glide approach for runway 29R. During the glide approach, the engine failed and the propeller stopped windmilling.[1] The aircraft landed safely on the runway.
Engineering inspection
Following the incident, the engineering inspection revealed the right fuel tank was empty and the left fuel tank had 15 L of fuel remaining. The remaining fuel failed to feed through the fuel lines, resulting in fuel being starved from the engine. The engineers inspected the fuel lines and vents for blockages but could not find any fault or blockage in the fuel system.
The Cessna 152 has a gravity fed fuel system that does not have a fuel tank selector switch. Asymmetric (uneven) fuel delivery is a well known phenomenon in single engine Cessna aircraft. It is very common for a 10 to 15 L difference to be found between left and right tanks. Once the aircraft reaches a fuel level equal to or below this common difference, and one tank is dry, the likelihood of fuel starvation increases significantly.
A company investigation identified a non-vented fuel cap on the left tank as a possible contributing factor. The right tank had a vented fuel cap. The Cessna 152 has an underwing vent on the left-hand side. The dual vented caps have become standard as a back up to this vent due to complicated pressure forces created within the fuel system and the tendency of the underwing vent to become blocked.
Airworthiness Directives have been released previously regarding the replacement of non-vented caps on Cessna 150 aircraft (predecessor to Cessna 152), however none have been released for the Cessna 152 as all but the very early models (first year of production) were released from the factory with dual vented fuel caps; and Cessna no longer provides the non-vented fuel cap as a replacement part. Although the majority of Cessna 152 were released from the factory with dual vented caps, many fuel system diagrams still show a vented cap on the right-hand tank only.
Figure 1: Example of Cessna unvented fuel cap
Source: Google Images
Figure 2: Example of Cessna vented fuel cap
Source: Google Images
Safety action
As a result of this incident, the operator has advised the ATSB that they are taking the following ongoing safety actions:
- They will be replacing the left fuel cap with a vented fuel cap.
- They will also inspect their entire Cessna fleet to ensure all aircraft have vented fuel caps on both tanks.
- Additional training for staff and students in measuring fuel levels on uneven ground will take place.
- Additional training for staff regarding total fuel and fixed fuel reserves will be implemented.
Safety message
Simulated total loss of power and a subsequent practice forced landing is at the core of a pilot’s emergency training. It is important that pilots remain aware that despite conducting comprehensive pre-flight checks, unanticipated failures can still occur during flight. Following a complete engine failure, a forced landing is inevitable. In this instance, the crew followed standard emergency procedures to ensure a safe outcome was achieved.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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