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.
On 1 May 2022, at about 1558 local time, the pilot of a Quest Kodiak 100 departed Orange Airport, on a private flight, to Mittagong, New South Wales.
At 1620, while cruising at 9,500 ft, the pilot observed the ‘reservoir fuel’ warning light illuminate. They immediately put the fuel pump on, checked both fuel cocks were on, checked the fuel quantity and balance and reduced the power to preserve fuel. Shortly after, the pilot then received a fuel starvation imminent warning followed by low fuel pressure warnings.
At the time of the warnings, the aircraft was overflying a remote sandstone escarpment region that contained deep gorges and large cliff lines, and was approximately 30 NM (56 km) away from any open land. Due to limited options to conduct a precautionary landing, the pilot continued to cycle the fuel pump, further reduced the power and turned towards an area suitable for a forced landing. During this time, the pilot heard multiple loud bangs.
The pilot continued to cycle the fuel pump and after a few minutes the fuel messages stopped. When the pilot was satisfied that the engine had enough fuel, they tracked direct to Mittagong and landed the aircraft safely at about 1651.
Following the incident, the pilot conducted a post-flight inspection and observed the upper skin on the right wing had crumpled and collapsed. To relieve the vacuum in the wing, the pilot attempted to open the fuel cap by forcing a piece of laminate under the cap. After partly opening the cap, a large amount of air was sucked into the tank and the upper skin of the wing popped back out once the pressure was relieved. However, there was still a large depression of about 11 mm (Figure 1).
Figure 1: Fuel tank depression on the right wing
Source: The pilot, annotated by ATSB
An engineering inspection of the right wing revealed the ribs and stringers had failed and the wing was no longer airworthy. Further inspection of the fuel tank vent line inlet recess also revealed mud wasp nests deep inside the vent lines for both wings, which were unable to be inspected or seen visually during the pre-flight inspection (Figure 2 and Figure 3).
Figure 2: Fuel tank vent line NACA inlet recess on the wing
Source: The pilot
Figure 3: Diagram of the fuel tank vent line within the wing
Source: Pilot, annotated by ATSB
Fuel tank vents are used to ensure the pressure inside the tank is maintained when fuel is being used by the engine. If a fuel tank vent becomes blocked while fuel is being pumped out, it will create a vacuum due to the inability of air not being vented in to replace the fuel, which can cause the tank to collapse.
The pilot advised:
- The aircraft was parked in a hangar and had been flown on the Thursday and Friday prior to the occurrence. No abnormalities during those flights were detected.
- They would typically only fill the fuel tanks halfway, but while at Orange they had filled them up to the top. They postulated that the vacuum occurred in this flight because there was less air in the tank compared to when the tanks are half full.
As a result of this occurrence, the pilot advised the ATSB that they made a protective plug for the fuel tank vent lines to prevent wasps from building nests (Figure 4). The plug consists of a clear plastic tube that is long enough to be inserted into the vent line, which can be used to detect an obstruction in the line if resistance is felt. At the end of the tube is a masonry plug with a screw to seal the tube and streamers to ensure the plug is not missed during the pre-flight inspection. In addition to this, the pilot has also informed other operators at the aerodrome of the potential hazard.
Figure 4: Devised plug to protect the vent line inlet recess
Blocked, or even partially blocked, pitot tubes and fuel tank vents can compromise the safety of the flight. Wasps can begin to build a nest rapidly and significantly block a fuel vent line or pitot tube within 30 minutes. Regardless of whether an aircraft has a short turn-around time or is parked overnight, protective covers and screens should be used on both fuel vent lines, fuel caps and pitot tubes. In addition to visually inspecting pitot tubes during pre-flight inspections, pilots should also inspect fuel vent lines. Moreover, operators and aerodrome personnel should monitor and remove any wasp nest sites in the general area of where the aircraft is stored and maintained to further reduce the risk.
For further information on wasp nest infestations please refer to the CASA Airworthiness Bulletin AWB-02-052.
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.