Engine Roughness After Fuel Uplift
You have just taken off from a rural strip after refuelling from a drum at the club shed. The drum was labelled as containing avgas but the label was faded and the drum had been standing outside for some months. Five minutes after take-off, at 2,500 ft, the engine begins to run rough. The roughness is intermittent, the cylinder head temperature on one side is rising faster than the others, and the fuel pressure gauge is fluctuating slightly. You have a passenger on board, the strip is behind you, and there is a cleared paddock about 1.5 nm ahead. The engine is still producing partial power and has not stopped.
1. What is your immediate priority, and how do the symptoms shape it?
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The priority is to keep flying the aircraft and to put it in a position where a forced landing is survivable if the engine stops. Intermittent roughness with one CHT climbing and fluctuating fuel pressure suggests either fuel contamination (water or incorrect fuel) or a partial fuel starvation or ignition problem. Because the engine is still producing power, do not make abrupt power changes that could provoke a complete stoppage. Turn towards the cleared paddock, set a safe altitude and speed for the conditions, and run the engine roughness and fuel checks from the aircraft checklist: select the other tank if fitted, apply carburettor heat if carb ice is plausible, check the primer is locked, check mixture is rich, and check magnetos briefly. Prepare the passenger and brief the forced landing. Declare the situation on the area frequency if able.
2. The engine smooths out when you select the other tank. What does that tell you and what do you do next?
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Smoothing on tank change points strongly to contamination or water in the fuel you were feeding from, most likely from the drum. Water settles at the bottom of a tank, so the engine may run normally once the pick-up is clear of it, but the problem is not solved. Land as soon as practicable at the nearest suitable strip and have the fuel drained and sampled from both tanks and the gascolator. Do not continue the planned flight. Record what happened, preserve a fuel sample if possible, and report the occurrence through the approved reporting channel because a contaminated fuel supply is an airworthiness and safety matter that may affect other club members using the same drum.
3. What pre-flight decisions should have prevented this situation?
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The drum should have been positively identified by batch documentation and fuel type, not by a faded label, and it should not have been used if there was any doubt. Fuel should have been sampled from each tank drain and the gascolator before flight and checked for water, sediment and correct colour and smell. The aircraft should have been checked for correct grade of fuel and any signs of misfuelling. If the drum had been standing outside, condensation and water ingress were foreseeable, so an extra sample after uplift and a short ground run before departure would have been prudent. The lesson is that a fuel-related engine problem in flight is nearly always a ground-handling failure that can be prevented by disciplined pre-flight fuel checks.