Oil Pressure Falling in the Climb
You are climbing through 2,500 ft in a Cessna 172S (Lycoming IO-360) after departing Ibadan for Lagos, 40 NM ahead. OAT is +31 °C, and you are climbing at 75 KIAS at full power. At brake release, oil pressure was 78 psi and oil temperature 180 °F; six minutes later, pressure reads 32 psi with a slow ±5 psi wander, and temperature has climbed to 232 °F. The oil-temperature redline is 245 °F; the pressure green arc starts at 25 psi. The engine runs smoothly, RPM is steady, and there is no oil on the windscreen or cowling. You checked 6 qt at the last stop 1.5 hours ago, and the last oil change was 22 hours back. Ibadan is 12 minutes behind; Lagos is 25 minutes ahead, with no paved option in between. Visibility is 10 km in haze, ceiling 6,000 ft, and the aircraft has fixed gear and a fixed-pitch propeller.
1. What does the combination of falling pressure and rising temperature tell you, and how would you order the likely causes?
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The two needles are moving in the wrong direction together, and that is the key. At a fixed power setting oil pressure should hold steady and temperature should stabilise. A 46 psi fall with a 52 °F rise in six minutes is a trend, not gauge wander.
Order of likelihood:
- Loss of oil flow or loss of oil - low quantity, a leak not yet visible from the cockpit, or a partly blocked oil cooler/cooler bypass. Rising temperature with falling pressure is what you see when hot oil thins and circulation is restricted.
- Pressure relief valve stuck open or a worn oil pump - pressure falls, less oil circulates, temperature climbs.
- Gauge or sender fault. The 172S oil-pressure gauge is a direct-reading Bourdon type, so a false low reading is less likely; the temperature gauge is electrical. But a single instrument fault would not normally pull both needles in opposite directions.
- Wrong oil grade or fuel dilution - less likely with 22 hours since the oil change at 31 °C.
The smooth engine and lack of external oil do not rule out an internal problem: a small leak or a restricted cooler can be invisible from the cockpit, and an engine can run smoothly while its oil is being cooked.
2. State your immediate actions and the thresholds you would use to decide between returning to Ibadan and continuing to Lagos.
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Immediate actions, in order:
- Reduce power to 2,200–2,300 RPM and lower the nose to hold 85–90 KIAS; less power means less heat into the oil and a better airflow through the cowl.
- Enrich the mixture fully and open cowl flaps if fitted; a richer mixture reduces combustion temperature.
- Avoid abrupt power changes and keep the engine running. A shutdown in flight is worse than a monitored degradation.
- Turn back toward Ibadan now rather than "seeing how it goes". Twelve minutes behind beats twenty-five minutes ahead with a sick engine.
- Broadcast a PAN and tell the tower you may need a priority landing.
Thresholds I would use:
- Pressure below 25 psi (bottom of the green arc) or falling faster than about 5 psi per minute: treat as an emergency and land at the nearest site, including a suitable strip.
- Temperature at or above 230 °F and still rising, with pressure below 40 psi: land as soon as practicable; do not continue to Lagos.
- Even if pressure stabilises at 50 psi or better with temperature below 220 °F, a monitored return to Ibadan remains the right call: the defect has not gone away, you have only stopped making it worse.
3. Suppose the readings stabilise at 30 psi and 238 °F. Is that acceptable to continue the 25 minutes to Lagos? Justify your answer.
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No. Do not continue to Lagos. Reasoning:
- 30 psi is only 5 psi above the bottom of the green arc, and the trend over the last six minutes has been downward. You have no margin for the next 25 minutes plus the approach, landing and a possible go-around.
- 238 °F is 7 °F below redline. There is no way to cool the oil further in a climb, and the approach will add power and heat.
- You would pass the point where returning to Ibadan is practical while carrying an engine that may already have bearing or cooler damage.
- A stabilised abnormal reading after an excursion is evidence of a fault that has been contained, not fixed. The fact that it stopped getting worse does not mean it is safe.
A 30 psi / 238 °F state buys you enough time to fly a short, planned diversion to Ibadan and land; it does not buy you a 25-minute continuation. I would land at Ibadan, shut down normally, and have the oil system inspected before further flight.