Oil pressure falling and temperature rising in cruise

Most probable cause: a loss of oil from the system - a leaking seal, gasket, oil cooler or return line - not an instrument fault.

The reasoning is the pairing. A faulty pressure gauge or transmitter alone explains a low pressure reading, but it cannot make the oil temperature climb. Falling pressure with rising temperature means less oil is available to carry heat away and to maintain the film in the bearings; the engine is running hotter because it is being lubricated less, not merely because it is being indicated wrongly.

Supporting evidence:

• The dark streak on the belly: oil being blown aft along the fuselage in the slipstream. This is the strongest physical confirmation and it predates the flight.
• Six quarts at departure with a two-quart top-up suggests the engine was already consuming or losing oil faster than normal.
• CHT rising is consistent: less oil cooling and more friction.

Alternatives worth excluding: fuel dilution of the oil (would give low pressure but not an external leak); a stuck pressure relief valve (usually sudden, with pressure going high or to zero); cold-weather viscosity effects (would give high pressure after start, not low pressure in cruise at -2 C).

Act now; the engine may be minutes from seizing.

1. Reduce power to the lowest setting that maintains safe flight - cruise descent power. Less power means less heat, less bearing load and a lower oil demand, and it buys time.
2. Enrich the mixture slightly. Excess fuel cools combustion and the cylinder heads; do not lean any further.
3. Open the cowl flaps if fitted, and lower the nose to increase cooling airflow.
4. Avoid abrupt throttle movements. Rapid power changes spike the load on a poorly lubricated engine.
5. Plan to land at the earliest suitable opportunity. The farm strip 12 NM ahead is the right target: 500 m of grass is ample for a C152, the terrain is flat and the visibility is 20 km, so an approach can be judged from a distance. The hard runway with engineering support is preferable in principle, but 45 minutes of further running on low oil pressure is a gamble on an engine that may stop.
6. Set up for an engine failure throughout: altitude, field within gliding range, passenger briefed, no flight over terrain that offers no landing option.
7. Do not shut down in flight while the engine is still producing power - an engine at idle still gives electrical power, cabin heat and, above all, options.

Zero indicated pressure with a running engine means the bearings are running on what film remains; seizure is likely within minutes and the engine should be treated as already failed.

• Land immediately at the nearest safe surface, without further diagnosis. At this point the question is no longer 'which airfield' but 'which field'. The farm strip is the first choice if it can be reached in the glide; otherwise a flat pasture.
• Reduce power further or shut down only when committed to a forced landing - a windmilling or idling propeller gives more range and a better flare than a seized one.
• Confirm the indication is real with one cross-check: if oil temperature is also peaking or the engine begins to rattle, it is real. A single gauge reading in isolation does not justify shutting down a healthy engine.
• Do not climb or accept a reroute. Every extra minute of running converts a precautionary landing into a forced landing.
• After landing, the aircraft does not fly again until the cause is found; the belly streak and oil consumption figures are the evidence the engineer needs. The judgement point is that a smooth-running engine is the most dangerous thing about this scenario - it tempts you to continue.