| 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 2 August 2021 at about 0900 local time, the student and instructor of a Cessna 172N aircraft, planned to conduct a one-hour local flight from Kalgoorlie-Boulder Airport, Western Australia.
The instructor and student held a pre-flight briefing, which included discussing fuel requirements for the flight including fuel reserves. The student calculated the fuel required based on a fuel burn rate of 38 L/hour and 30 minutes fixed reserve, in accordance with the company operations manual.
The student then dipped the aircraft’s fuel tanks using the approved dipstick to ascertain the fuel on board. The student determined there was 50 L of fuel in the tanks, although the recorded fuel remaining from the previous flight in the daily flight log was 55 L. The student assessed that this amount was sufficient for the flight and relayed the fuel required and fuel onboard to the instructor who replied that ‘sounded about right’.
Although the student believed there was enough fuel, they suggested a refuel to the instructor prior to departure, however, as another flight was planned after the occurrence flight, the instructor determined there would not be sufficient time to do so.
The flight was conducted for approximately one hour and the aircraft was landed without incident.
The aircraft’s usable fuel capacity was 195 L. After the flight, the aircraft was refuelled with 182 L, indicating the aircraft completed the flight with approximately 13 L of usable fuel remaining. This was 6 L short of the fixed reserve of 19 L.
Operator investigation findings
The aircraft operator conducted an internal investigation into the occurrence. Its findings included the following.
- The minimum fuel required for the flight was approximately 65 L. This was made up of the 38 L flight fuel for 1 hour plus the 19 L fuel reserve and taxi fuel 5L (calculated on a consumption rate of 38 L/H)
- As the student was nearing licence stage, the instructor felt some degree of confidence that the student had made the correct calculation. The student had conducted fuel calculations many times prior to the occurrence flight. Post-incident discussion revealed the student may have confused the reserve quantity required for the flight, however it could not be ascertained why the student made an error in calculation.
- It was possible that perceived time pressure resulted in not refuelling prior to the flight. However, the student and instructor thought there was sufficient fuel on board.
- The method of ascertaining the fuel on board is by way of approved clear plastic capillary tube. The tube, called a ‘fuel hawk’, is calibrated specifically to the individual aircraft. The student had been trained and was confident in the used of this device. The student stated that at lower fuel levels the device was harder to read. A survey of the company flight instructors revealed that some error was possible when dipping lower fuel levels. If the aircraft fuel state was found to be near the minimum fuel required, it was common practice to add fuel as a buffer.
- Following the occurrence, it was found that the fuel dipstick had a calibration error. This error made any fuel reading below approximately 20 L harder to read and inaccurate.
Safety action
The aircraft operator advised the ATSB of the following safety actions.
Reinforcement of standard operating procedures
A safety alert will be sent out to remind all company instructors that confirming the amount of fuel required is onboard and is the responsibility of the pilot in command in accordance with the company operations manual. The issue will also be raised at the next instructor meeting.
Recalibration of aircraft dipsticks and fuel tanks
Following the occurrence, the aircraft’s fuel dipstick was recalibrated by the chief engineer. This recalibration and rectification of the error should improve the reliability of the dip sticks when reading lower fuel quantities. It will be recommended that the company’s other Cessna 172 aircraft dipsticks be recalibrated.
Safety message
This incident highlights the importance of correct fuel quantity management. It is the responsibility of the pilot in command to ensure adequate fuel is available for each flight. Also, operators are reminded of the importance of checking fuel quality and quantity before each flight and to use correctly calibrated fuel tank quantity measuring devices.
The Civil Aviation Safety Authority advisory publication, CAAP-234-1 Guidelines for aircraft fuel requirements, provides guidance for fuel quantity crosschecking, specifically that the crosscheck should use at least two different verification methods to determine the quantity of fuel on board the aircraft.
The incident also highlights the importance of clear communication between instructors and students.
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.