Airplanes, specially the large complex ones have low oil pressure ranges where things get critical. Usually when the oil pressure gauges start to show a depletion, as pilots we keep an eye on it to see further drop in pressure levels. And if possible and, if the airline has a frequency for pilot-engineer communication it maybe a good time to talk to them as well. The cool part is, if the value drops to a certain level you get a cockpit warning to alert you on the low oil pressure. Most of the times, loss of oil pressure calls for an engine shut down procedure.
Why it is important to shut down the engine is because without oil, the engine is not being cooled and lubricated. This causes friction in engine bearings and in worst case scenario it can lead to an engine fire. So, shutting down the affected engine and landing at the closest, most appropriate aerodrome is the best way to go.
In propeller airplanes, loss of oil pressure does not completely cause a loss of propeller pitch control. Some props have counterweights to control the coarsening of blades while the oil only controls fine pitch. Thus, air loads it self can increase the propeller blade angle to a safer level for the flight. If the propeller is double acting with oil controlling both the coarsening and fining of blades, then you have other means of reducing blade angle in loss of oil pressure. In these propellers, the loss of oil pressure will cause the blades to move to a lower angle due to aerodynamic loads. To prevent this from happening, a pitch lock mechanism prevents the blades from increasing, from the point of pressure loss. Large turboprops also have what we call an alternate feather system which has an electric feathering pump and its very own oil reservoir to admit oil to feather the propeller in case of main engine oil pressure loss.
Author – Anas Maaz (Airline Pilot)
