Engine Roughness and Partial Power Loss Over Hostile Terrain
Cruising at 8,500 ft over rising, sparsely populated terrain, you notice the engine running rough, with a slight RPM drop and a vibration that appeared over about two minutes. Oil pressure is steady in the green, oil temperature is rising slowly toward the top of the green arc, and the EGT on one cylinder is noticeably lower than the others. Fuel selector is on the fuller tank, mixture is leaned for cruise, and carburettor heat is cold. The nearest paved airport is 22 NM behind you; a grass strip appears on the chart 9 NM ahead.
1. What is the likely cause and what is the immediate troubleshooting sequence?
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A single low EGT with roughness and rising oil temperature suggests a cylinder problem - fouled plug, failing ignition lead, stuck valve, or incipient detonation/pre-ignition. Sequence: apply carburettor heat (check for carb ice, which can produce roughness), enrich the mixture, check magnetos briefly on each (expect a larger-than-normal drop on one), verify fuel selector and quantity, check primer locked, and reduce power to lower cylinder temperatures. If roughness persists, treat it as a partial power loss and plan to land.
2. Compare the options: 22 NM back to a paved airport versus 9 NM ahead to an unfamiliar grass strip.
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The grass strip is closer, which matters if power degrades further, but it is unfamiliar and its condition is unknown. The paved airport is known, has services, and is only 22 NM - roughly 12–15 min at reduced power. Decision logic: if the engine is stable at reduced power, the paved airport is preferable because a known, long, hard surface reduces landing risk after a partial power loss. If roughness worsens or oil temperature continues to climb, divert to the grass strip immediately and overfly it once to assess surface and obstacles before landing. Do not delay the decision while troubleshooting.
3. What should you communicate and how do you prepare the cabin and approach?
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Declare the situation on the appropriate frequency (or 121.5), stating position, altitude, nature of the problem, intentions, and persons on board. Squawk the emergency code if able. Prepare for a possible forced landing: brief passengers, secure loose items, review the forced-landing checklist, and select a suitable field as a contingency. Fly a stabilized approach with extra speed margin and no steep turns near the ground, since power response is unreliable. Plan to land long and roll out rather than aiming for a short-field touchdown.