In this article, we outline what causes low oil pressure in piston-engined propellor aircraft, why oil is essential for the operation of aircraft engines and what to do if your engine loses oil pressure.
Introduction
Whether you like engines, or understand them currently, one of the non-negotiable parts of being a pilot is knowing how ALL of your systems work inside and out, especially your engine(s). In humans, our heart pumps blood around our body. If it doesn’t do this efficiently enough, or if another issue in our circulatory system causes low blood pressure, a whole host of issues and symptoms could occur, including fatigue and in severe cases, our body may shut down.
Consider oil as the lifeblood of our engine, which is pumped around to all the moving parts. With oil pressure that is too low, our engines, too, will face many potential issues, including total engine failure.
In this article, we discuss why oil is so crucial to our engine, and what may cause a low oil pressure reading in the cockpit. After reading, you will have a thorough understanding of what to do if you get a low oil pressure reading in flight, including diagnosing the problem and how to deal with a total loss of oil pressure.
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Why does an engine need oil?
Oil plays a vital role in any engine, to the point where a failure in the oil system of a piston-engined aircraft will result in near-immediate damage. Engine oil has five main functions:
– Cleaning: Carries away sediment and residue, which ends up in the oil filter
– Cooling: Helps to remove heat from the engine by carrying it away to the oil cooler, where it dissipates into the air
– Sealing: Fills the spaces between piston rings and cylinder walls and therefore prevents gasses from leaking out of the cylinders
– Lubrication: Prevents metal-to-metal contact, reducing friction and preventing both energy loss and damage to engine components
– Protection: Provides a barrier between metal engine components and water, oxygen, or other corrosive agents. It also acts as a cushion between surfaces under high loads.
“Arguably the most important gauge on the panel indicates the engine’s oil pressure. The integrity of the oil cooling system is crucial to the powerplant’s health. Oil is used both as a lubricant and a cooling medium in air-cooled engines.”
aopa.org
What is oil pressure?
Put simply, pressure is the force exerted by a fluid against an object it is in contact with. It is measured via oil pressure gauges. Oil enters an engine through an oil pump, which creates a positive oil flow through the engine and, therefore, oil pressure. It makes sense, then, that oil pressure will be higher when an engine is cold, as the oil will be more viscous (thick), exerting more force on the galleries through which it flows. Conversely, a low viscosity oil in a hotter engine with wider galleries would result in lower oil pressure.
What is low oil pressure?
Low oil pressure, when referring to aircraft engines, is when oil pressure is reading below the normal oil pressure range, or the “green range” on the oil pressure gauge, in the cockpit of light aircraft. This is often an incorrect indication due to a faulty gauge, but when aviating it is always best practice to err on the side of caution, and so you should land as soon as possible to get the issue checked, regardless of the suspected cause.
A low oil pressure reading becomes an even more urgent problem when it is coupled with an abnormally high oil temperature reading and/or rough running of the engine, as this could indicate low oil quantity.
What can cause a loss of oil pressure?
There is usually only one reason for a genuine loss of oil pressure: a loss of OIL! This excessively low oil level could occur due to a leak throughout the engine, or perhaps the dipstick was not replaced or tightened during the preflight inspection before a long flight. Sometimes, though, oil pumps become defective, as do the bearings throughout the oil system. Some other causes for a loss of oil pressure could be a faulty pressure relief valve or a clogged oil line. Nevertheless, it is usually a gauge problem because oil pumps rarely fail.
What happens when you lose oil pressure?
If a loss of oil pressure is indicated, don’t panic and immediately assume the gauge is right. If oil pressure is genuinely lost, the oil temperature should be indicating high as well, unless oil was lost very rapidly. One of the best indications of oil pressure loss is how the engine sounds. If it’s a single-engine aircraft with a fixed pitch prop, the engine will slowly lose power and may sound noisier in operation, with no oil between wearing surfaces. If a single-engine aircraft has a constant-speed propeller unit (CSU), the prop will go into full fine pitch (high RPM), and only throttling back will control RPM.
If the aircraft is a twin-engine with CSUs, the prop on the engine that has lost oil pressure will go into feather/coarse pitch (lower RPM), which will make it run rough as the feathered blades slap the air until the throttle is reduced to idle. If none of those things happen simultaneously, it’s probably a gauge problem – either a wire has come loose, or the sensor has failed. If you do experience a genuine loss of oil pressure, prepare for engine failure shortly afterward.
What should you do when you lose oil pressure?
In a twin, reduce power or shut down the affected engine only as necessary, feather the prop, and land as soon as practicable. If it’s a single, reduce power and look for somewhere safe to land while power is still available. When you’ve found a spot, reduce power to idle and conduct a power-off approach. An idling engine shouldn’t seize after the loss of oil pressure because it shouldn’t be creating enough heat to cause the parts to expand enough to seize.
“Low oil pressure can be a sign of low oil supply, burned out bearings, a broken pressure relief valve spring or hot oil. High oil pressure may be a sign of a sticking relief valve, an improper pressure relief valve setting or cold oil. If the rise takes place in flight, there may be a possible internal plugging. Fluctuating oil pressure may be an indication of low oil supply or possible oil foaming.”
aviationsafetymagazine.com/features/gauging-safety/
Knowing how aircraft systems work is far more effective than being taught how to follow a recommended recipe for each disaster. Without a doubt, pilots have caused far more accidents than engine failures ever did, and even if the engine did fail, then pilots (more often than not) should have been able to foresee it, prevent it, or at least minimize damage by managing the problem rationally, based on a sound foundation of knowledge. Ignorance is not a valid excuse.
Conclusion
While knowing your aircraft systems back to front, including your oil system, can help you to diagnose oil pressure problems such as low oil pressure as they occur; prevention is always better than a cure so be sure to conduct a thorough pre-flight inspection that includes checking oil quality and quantity and making yourself aware of the maintenance schedule and history for the aircraft which you will be flying. However, no preventative measure is bulletproof, and we hope this guide will help you to safely rectify the issue of low oil pressure, should it ever happen to you.
