Engine Roughness Over the Altiplano

The roughness improves when the mixture is enriched and worsens when leaned further, which points to a mixture-related cause rather than an ignition fault. At 11,500 ft the air is thin, and leaning is necessary, but over-leaning causes lean misfire and rough running, especially if one cylinder runs leaner than the others. The fact that enriching smooths the engine suggests the mixture was too lean for the power setting. Other possibilities should be excluded: carburettor ice is unlikely at +8 °C with no ice indication, and fuel pressure is steady so a fuel pump or line blockage is less likely. Oil temperature near the top of the green arc with pressure in the lower green arc suggests the engine is working hard and hot, which can also contribute to detonation or pre-ignition if the mixture is too lean at high power. The correct first action is to enrich the mixture to the best-power setting and monitor the engine parameters.

First, enrich the mixture smoothly until the roughness subsides, then set the mixture for best power rather than best economy. Reduce power slightly by pulling the throttle back to reduce manifold pressure and cylinder head temperature. Check the magnetos briefly: a small RPM drop on each is normal, but a large drop or rough running on one magneto indicates an ignition problem. Check the carburettor heat briefly to rule out ice, even though conditions are marginal. Monitor oil temperature and pressure; if oil temperature continues to rise or pressure falls below the green arc, treat it as an engine problem requiring a precautionary landing. Given the nearest airfield is 25 nm away at 9,000 ft, the pilot should plan a descent toward it while keeping the engine within limits, and be prepared to divert to a closer suitable area if parameters deteriorate. The flight should not continue to the original destination if the roughness persists after enriching.

At high altitude the air density is low, so the fuel-air mixture becomes richer for a given throttle setting unless the pilot leans. The engine needs leaning to maintain power and to avoid fouled plugs, but over-leaning causes misfire, roughness and high cylinder head temperatures. The correct technique is to lean slowly in small increments while watching RPM or EGT, stopping at the peak or slightly rich of peak for cruise. Because the air is thin, the engine also cools less effectively, so oil temperature and cylinder head temperature rise more readily. The pilot should reduce power, open cowl flaps if fitted, and enrich the mixture if temperatures approach the upper limits. At 11,500 ft the engine is already operating at a high percentage of its available power, so any further leaning must be done cautiously and the engine parameters monitored continuously.