Engine Roughness After a Hot-Weather Departure
You are flying a Cessna 172 with a Lycoming O-320 engine from Sohar on a hot afternoon. Outside air temperature is 43°C, QNH 1004 hPa, and the aerodrome elevation is 100 ft. After a normal full-rich takeoff, at about 800 ft AGL, the engine begins to run rough. RPM falls from 2,300 to 2,100, the oil temperature is rising through 220°F, fuel pressure remains in the normal green arc, and the fuel selector is on BOTH. The mixture is full rich. You have 3 h of fuel, the aircraft is at 1,050 kg against a maximum of 1,111 kg, and there is no carburettor temperature gauge. The engine had been shut down for 25 minutes before this flight, and the cowling was heat-soaked in the sun.
1. What is the most likely cause of the roughness, and how would you distinguish it from carburettor icing?
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The most likely cause is an over-rich mixture at high density altitude, possibly combined with plug fouling, rather than carburettor icing. At 43°C and a pressure altitude near 400 ft, density altitude is well above 4,000 ft. A full-rich mixture at that density altitude is excessively rich, which can cause rough running, RPM loss, and rising oil temperature. Fuel pressure is normal, so a failing pump is less likely, though vapour lock after heat soak is possible. Carburettor icing is less likely at 43°C, but it cannot be dismissed entirely because venturi cooling and high humidity can produce it. To distinguish: apply carburettor heat briefly and watch for a further RPM drop or rough running, which suggests the mixture is already too rich and carburettor heat is making it richer. If carburettor ice were present, carburettor heat would usually cause initial roughness then improve the engine as ice melts. In this case, lean the mixture to best power and see whether RPM recovers.
2. State your immediate actions and the order in which you would take them.
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First, maintain control of the aircraft and establish a positive climb or level attitude. Then apply carburettor heat briefly only if icing is suspected; if the engine worsens, return the control to cold. Lean the mixture to best power or the manufacturer's recommended setting for the density altitude, and check that the fuel selector is on BOTH. Check magnetos on both, then briefly on left and right to identify a fouled plug; do not leave a magneto selected if the engine runs badly. Reduce power if the engine is rough and high oil temperature persists, and enrich slightly if cylinder head temperature is excessive. Turn back toward the aerodrome or select a suitable forced-landing area while you troubleshoot. If RPM and smoothness do not improve within a short period, treat the engine as unreliable and land.
3. Would you continue the planned 1-hour flight if the engine smooths out after leaning? Give your reasoning.
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Continuing is defensible only if the engine runs smoothly after leaning, temperatures return to normal, and you can maintain safe terrain clearance, but a return to Sohar is the safer choice. The initial roughness shows that the powerplant is sensitive to mixture and heat, and the cause has not been proven. A fouled plug, partially blocked fuel injector, or vapour lock could recur. With 3 h of fuel you have time, but the aircraft is near maximum weight and the day is hot, so climb performance is already degraded. If you continue, climb at a reduced rate, monitor RPM, oil temperature, fuel pressure, and cylinder head temperature, and keep a diversion aerodrome within gliding range. If there is any recurrence, land immediately. For a PPL decision, the conservative answer is to return and have the engine checked rather than accept an unexplained rough-running episode.