Rough Running at 6,500 Feet

What carburettor ice normally looks like. Heat is applied, RPM drops as the hot air is less dense and the mixture momentarily richens, then RPM climbs back past the original figure as the ice melts and the venturi recovers. The key feature is recovery.

What this engine did. RPM dropped and stayed down, with roughness unchanged over 40 s. That is consistent with three possibilities:

1. There was no ice. The drop was simply the normal rich, less-dense charge from the heat; nothing to melt.
2. The heat system is not delivering - broken or mis-rigged cable, blocked air box, cracked or disconnected ducting. A partial or leaky heat supply can melt ice into water that is not carried into the cylinders, occasionally making things worse.
3. Heat was applied too late into an ice build-up that is not clearing quickly.

What it has not told you. It does not exclude ignition or induction faults, and it does not prove the heat system works. Do not treat 'no recovery' as 'not ice' - treat it as 'this symptom is not being explained by heat', and keep looking. The dew point spread of 4°C and the moderate power setting are exactly the regime where carburettor ice is a live threat, so ice cannot be dismissed on the strength of one test.

1. Mixture. Briefly enrich: a lean misfire (common at 6,500 ft if the mixture was set aggressively) will smooth almost immediately. Risk: an over-rich setting fouls plugs and costs power - enrich, observe, restore.

2. Carburettor heat. Full heat only, not partial, held long enough to be conclusive (30–60 s). Partial heat is the setting that makes ice worse. Risk: rough running and loss of power while hot air is selected; expect it.

3. Magneto check in flight. Momentarily select L, then R. If the roughness vanishes on one magneto, the fault is in the other - likely a fouled plug or a failing magneto. Risk: running on one magneto misfires more, passes unburned fuel into the exhaust and reduces power; keep each selection to a few seconds and accept a small RPM loss.

4. Power settings. Cycle the throttle and pitch gently. A sticking valve or a failing plug often shows at particular manifold pressures; sudden large throttle movements on a rough engine can backfire or worsen a mechanical fault.

Interpretation rule: roughness present on both magnetos is not a single-plug problem - think fuel delivery, induction leak or mechanical. Roughness present on one magneto localises the fault to the other ignition circuit. If no test produces improvement, stop diagnosing and start diverting.

The situation is a degrading engine over rising ground with a field 18 minutes away. The trend is unknown but the risk is asymmetric: a rough engine that becomes a stopped engine over 5,200 ft terrain has no benign outcome.

Decision: divert now, on the power you still have.

• Turn toward lower terrain and the aerodrome while the engine is still producing usable power; a diversion begun at 2,250 RPM is a diversion, one begun at 1,800 RPM is an emergency.
• Set up a shallow descent and keep the mixture correct for the altitude rather than aggressively leaned; keep the engine warm, not shock-cooled.
• Choose a cruise power setting that produces the least roughness, and avoid large throttle changes.
• Brief a forced-landing field before you need one - over high ground the options shrink quickly.

Declare early. With a rough engine and diminishing terrain clearance, a PAN call to alert ATC costs nothing and buys priority, radar help and fire cover on arrival. Continuing for another hour 'to see if it settles' places the decision in the hands of the engine rather than the pilot; the exam-level point is that the diversion is justified by the trend and the terrain, not by a confirmed diagnosis.