Rough Engine in IMC: Which Airport?
You are single-pilot in a single-engine airplane, IMC at 8,000 ft, 40 NM from destination with 45 minutes of fuel in the selected tank and 30 minutes in the other. Five minutes ago the engine began running rough; EGT is fluctuating and you have lost about 150 RPM. Carb heat produced no change, and switching tanks produced no change. The destination ahead is 600 ft overcast, 2 SM, ILS 200-1/2 with high terrain on the approach. Fifteen miles behind you is an airport with 4,000 ft ceilings, 10 SM, and a 5,000-ft runway. You are in radar contact; MVA is 7,000. True airspeed is 130 kt.
1. State your priorities and how much troubleshooting is justified right now.
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Aviate, navigate, communicate - in that order - and cap the troubleshooting.
Aviate: attitude, power, trim, wings level. A partial power loss in IMC kills people through distraction, not through the engine itself. Navigate: turn toward the better option while you still can. Communicate: tell ATC now - 'we have a rough running engine, request vectors to [airport behind],' and declare if the trend is bad.
Troubleshooting is limited to quick, reversible items, most of which you have already done: carb heat (no change), tank selection (no change), mixture enriched, mags checked, fuel pump on, gauges scanned. That is the checklist, and it is finished.
Set a hard decision point in time, not in hope: e.g., if the roughness has not resolved within two minutes, or if oil pressure falls toward the red line or oil temperature climbs, you commit to the diversion and stop diagnosing. Do not go head-down in the cockpit trying to identify a cylinder at 8,000 ft in cloud - there is no repair available to you in the air. The only productive question now is where does this flight end.
2. Destination ahead or airport behind? Work the numbers and the risk.
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Behind - and it isn't close.
Numbers:
- 15 NM behind at 130 kt = ~7 minutes; 40 NM ahead = ~18.5 minutes.
- Fuel: 45 min in the selected tank (30 in the other), so ~75 min total. Both airports are well within range; fuel is not the deciding factor, and the diversion leaves roughly 68 minutes after landing.
Risk:
- Destination: an instrument approach to 200-1/2 in high terrain, at night, with an engine that is already making 150 RPM less than it should. If you go missed - and a 600 ft ceiling means you might - you now need climb performance, from a sick engine, over rising terrain. A partial power loss can become a total one with no warning and no checklist that prevents it.
- Airport behind: 4,000 broken, 10 SM, 5,000 ft of runway. You can cancel IFR once visual and land in VMC with multiple runway options and no go-around requirement.
Extra: at 8,000 ft with an MVA of 7,000, the behind option also lets you keep your altitude while turning - no descent into terrain with reduced power. The destination trades a small time saving for the one outcome you cannot recover from.
3. What do you tell ATC, under what authority, and what do you keep in reserve?
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Declare, state intentions, and keep an out.
Transmission: 'Mayday, N12345, single-engine airplane, rough running engine, 150 RPM loss, unable to maintain, request immediate vectors to [airport behind], souls on board one, fuel one hour fifteen.' Use §91.3(b): an in-flight emergency permits deviation from the rules to the extent necessary - including priority handling, a turn off your cleared route, and descent below an MVA if ATC coordinates it.
Keep in reserve:
- Options along the route. Know the two nearest airports on either side of your track before you need them.
- VMC escape. The airport behind is 4,000/10; once you are visual and have the field, cancel IFR and land.
- Equipment on standby. If power degrades further, request ARFF on standby at the diversion airport - an entirely routine request and much easier to make from 3,000 ft than from short final.
- Do not cancel IFR early. Stay on a clearance with ATC until you are certain the field is in sight and you can remain VMC; a rough engine is exactly the wrong time to give up radar and someone watching your altitude.