Fuel Contamination Suspected After Refuelling
A turbine-powered aircraft was refuelled from a bowser at a small airfield. During the pre-flight sump check, the engineer finds a small amount of clear fluid with a slight sheen in the fuel sample from the left tank drain, plus a few dark specks. The right tank sample is clean. The aircraft has 1,800 litres on board and is scheduled for a 2-hour flight with 6 passengers. The bowser's last filter change was recorded 14 months ago, and the airfield has had heavy rain for three days. Outside air temperature is 8 °C and the fuel is Jet A-1.
1. Assess what the sample findings could indicate and rank the possible sources of contamination.
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The sample shows two separate findings: a clear fluid with a sheen, and dark specks. Each has different implications.
Clear fluid with a sheen: the most likely explanation is water with a slight fuel film, which is normal in small quantities at a sump drain. However, a sheen can also indicate a small amount of lubricating oil or hydraulic fluid, which would point to a cross-contamination or a leaking component. Because the sample was taken from a drain, water is the most probable and least alarming explanation, provided the sample clears after draining.
Dark specks: these suggest particulate contamination, possibly filter media, rust, or dirt. With a bowser filter 14 months old, a degraded or bypassing filter is plausible, and heavy rain increases the chance of water ingress into the bowser and of microbial growth at the fuel/water interface.
Ranked sources:
- Water from the bowser after heavy rain, drawn into the left tank.
- Particulate from an overdue bowser filter.
- Microbial contamination promoted by the water.
- Cross-contamination from a shared line or a leaking component - least likely but most serious.
The right tank being clean suggests the contamination entered through the left tank fill, which supports the bowser as the source.
2. Decide whether the flight may proceed and describe the fuel testing and corrective actions required.
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Decision: the flight does not proceed until the fuel is proven clean. A sheen plus particulate in a turbine aircraft with 6 passengers is a stop-and-verify situation. Water in Jet A-1 can freeze at altitude and block filters, and particulate can damage fuel controls and the engine.
Testing and corrective actions:
- Drain the left tank sump repeatedly until the sample is clear, bright, and free of water and particles, using a clear container and a water-detecting test if available.
- Take samples from all drain points, not just the left tank, and from the engine fuel filter drain.
- Check the fuel filter for water and debris, and replace it if contaminated.
- Quarantine the bowser and notify the fuel supplier; a filter 14 months old needs replacement and the bowser needs a water check.
- If microbial contamination is suspected, arrange a fuel sample test for bacteria and fungi before further flight.
- Record all findings and actions.
Departure criteria: all samples clear and bright, filter clean, no microbial evidence. If any water or particulate persists after draining, the aircraft stays grounded and the fuel is defuelled or polished as required.
3. Explain the regulatory and safety responsibilities of the engineer and operator in this situation.
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The engineer who takes the sample has a duty to act on what the sample shows. Finding contamination and releasing the aircraft anyway would be a maintenance error with serious consequences, and the record of the sample and the action taken is part of that duty. The engineer should not be pressured by the passenger load or schedule into accepting a marginal sample.
The operator has a responsibility to ensure fuel is supplied to an acceptable standard and that fuelling equipment is maintained. The bowser filter being 14 months old is an operator and supplier issue, and it should be reported so the equipment is corrected before it serves another aircraft.
Key points:
- The sample result and the corrective action must be recorded in the aircraft records.
- The bowser should be quarantined and the fuel supplier informed; this protects other aircraft too.
- If the flight is delayed or cancelled, the reason should be documented honestly as fuel contamination, not as a vague technical issue.
- Any suspicion of microbial contamination should be reported because it affects the whole fuel supply chain at that field.
The safety culture point is that a small sheen and a few specks are easy to dismiss. Treating them as significant and following through is what prevents a fuel-related engine failure in flight.