Oil pressure loss at FL350: nearest or best?
Level FL350, twin turbofan, 2 h into a 3 h sector. ENG 2 oil pressure falls from 55 psi to 32 psi over about 4 minutes and is now fluctuating ±4 psi. Oil temperature is 12 °C above its normal cruise value and rising slowly. Oil quantity indication has dropped from 5.0 to 3.5 qt over the same period and now appears stable. There is no chip caution. On ENG 2, N2, N1, EGT and fuel flow are normal and the engine responds normally to thrust lever changes. The nearest suitable aerodrome is 85 nm behind with a 2,400 m runway; the destination is 230 nm ahead with a 3,000 m runway and better weather. The QRH gives only the standard low oil pressure drill.
1. Interpret the indications. Which ones drive the decision, and which are distractors?
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Driving the decision - the combination of falling pressure, rising temperature and falling quantity.
- Pressure down from 55 to 32 psi with fluctuation means the system is no longer delivering steady flow at expected pressure: a leak downstream of the pump, a failing pump or relief valve, or air in the system (aeration shows up as fluctuation).
- Temperature rising at constant power means less oil is carrying the same heat away, or a bearing is being heated because it is no longer being cooled properly.
- Quantity down 1.5 qt over two hours is a real loss. The fact that it has 'stabilised' is weak reassurance: quantity is only meaningful in level flight and many gauges are heavily damped or sampled.
Distractors and weak reassurance:
- normal N1, N2, EGT and fuel flow say the gas path is intact, not that the bearings are;
- no chip caution says only that no ferrous debris has been detected - a white-metal bearing cage or an external line leak will never annunciate.
The red lines matter: if pressure falls below the red line or temperature rises above it, the QRH answer is shutdown, not continued monitoring.
2. 85 nm behind or 230 nm ahead? Show the reasoning.
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Time is the currency here, not distance.
- 85 nm behind at roughly 420 kt ground speed is about 12 minutes.
- 230 nm ahead at the same speed is about 33 minutes.
Low oil pressure with rising temperature is a fault that can progress to bearing seizure, in-flight shutdown and an oil-fed fire. Nearly tripling the exposure to that outcome to save a diversion is a poor trade when the nearer aerodrome has 2,400 m of runway - ample for this aircraft - and no reported weather problem.
Counter-arguments to weigh honestly:
- the destination has a longer runway and better weather, which helps if the engine fails on approach;
- fuel, handling and engineering may be better at the destination;
- a single-engine approach is manageable at either field.
But if the engine fails 12 minutes from now you are on the ground; if it fails 33 minutes from now you are still airborne with a fire risk and a single-engine diversion anyway. Turn back, tell ATC so fire cover is available, and brief the approach as a possible single-engine approach with a defined shutdown trigger.
3. How do you manage the engine for the rest of the flight, and what are you preparing for?
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- Reduce to the minimum practical thrust and make thrust changes slowly; a distressed bearing does not need transient loads.
- Keep it running. Shutdown is for a red-line exceedance, vibration, fire warning or clear loss of thrust. A running engine at low power gives options a windmilling one does not.
- Monitor the trend continuously - pressure, temperature, quantity, vibration if displayed - and set the shutdown trigger in advance, for example pressure below red line, temperature above red line, or any increase in vibration.
- Prepare for a single-engine approach and go-around: brief the missed approach, the asymmetric handling and the performance. Do not leave it to the last two miles.
- On landing, stop clear of the runway if possible, shut down, and have ARFF inspect the engine and tailpipe area for an oil fire. Oil that has left the system often ends up sitting in the hot section.
- Write up the defect with all the numbers and file an occurrence report if the operator's system requires one. The engineer needs the trend, not just 'oil pressure low'.