Imagine you are pre-flighting your King Air, checking the oil and the oil is off the stick on one side. After uttering the obligatory expletive, what do you do? If your first instinct is to dump a couple quarts in that engine and check the stick again, chances are you will be wiping oil off the flaps and gear doors at the end of your trip because you over-serviced the oil.
Oil is the lifeblood of any engine, so when it is off the stick, it gets your attention, and rightly so. But before you add more, try motoring that engine for about 30 seconds and check the stick again. If you find the oil back at the level you typically expect for that engine, then you are good to go. You have a minor oil migration problem that can be easily remedied at the next phase inspection. As long as you are checking the oil on a cold engine, you’ll want to motor that engine briefly whenever checking that side until the migration problem can be properly addressed.
Checking Cool or Checking Hot?
Pratt & Whitney says you should check the oil level on each engine within 20 minutes after shutdown; but in reality, who does that? First of all, the engines are still piping hot after just 20 minutes. And even though you may have shut everything down and buttoned everything up, there’s still baggage and passengers, instructions for the line guy, possibly a rental car, or maybe just a long drive home. Let’s face it, at the end of a trip, checking the oil on a hot engine is not everyone’s favorite thing to do.
Far more important in my book is just checking the oil, whether the engine is hot or cold. (Those of you out there who never check your engine oil … ever … might want to pay attention.) Whichever way you choose to check the oil, whether it’s post-shutdown or before the first flight of the day, do it that way consistently.
In addition to simply verifying oil is present, you are discovering and then maintaining the oil level at which your engines like to be. I’ve found many PT6 engines like to run at between two and three quarts down. To be clear, on a cold engine, that’s between the second and third hash marks below Max Cold on the stick. Some engines stake their claim at three or even three-and-a-half quarts low. And many times there will be a disparity between your engines – the left side is happy at two down, but the right side prefers two-and-a-half down.
More on Migration
As an engine cools after running, the oil collects in the oil tank located above the oil filter housing. But if the oil is able to get around the housing and travel down to the accessory gear box and the bearing areas below, it will go there. Then you come along, checking the oil before your next trip and (surprise!) the oil is off the stick. It looks like you suddenly lost all of the oil on that side. But it has to go somewhere, so if you don’t have obvious evidence (such as a puddle on the floor and a mess inside the cowl), try motoring that engine for 30 seconds and check the oil again.
Here’s what happens: During every phase inspection, the oil filters and chip detectors are pulled, inspected and reinstalled. Some oil filters are difficult to get out and get back in. The process of removing and reinstalling a stubborn oil filter might jostle the oil filter housing ever so slightly; and if that happens, the O-rings between the filter housing and the engine case could lose a tad bit of their seal. If you have an oil migration problem, these O-rings are the likely culprits. That 30 seconds of motoring gets the scavenge pump to move the oil back into the oil tank.
Funny thing about Pratt’s recommendation, if you only check your oil levels within 20 minutes after shutdown while the engines are hot, you would never know whether or not you had an oil migration problem. It could go undetected for years.
Mass Migration
Oil migration is an oil leak inside the engine. The good news is that the engine is still full of oil and will operate properly.
If you opened your engine cowl and saw oil dripping out of the compressor inlet, you should have reason for more concern. This indicates a larger amount of oil is leaking out of the tank and accumulating inside the engine where the bearings are. When the oil reaches the level of the shaft, it leaks out. You want to get that addressed.
If you read this magazine regularly and think I’m repeating myself, you are partially correct. I wrote about engine oil levels in a 2011 issue, but I barely touched on oil migration. Over the past 40 years of working on King Airs and working with their pilots and owners, I find the topic of engine oil levels, although basic, comes up repeatedly.
About the Author: Dean Benedict is a certified A&P, AI, and has 40 years of experience in King Air maintenance. He is president of Honest Air, Inc., which specializes in Beechcraft King Air maintenance and repair.
If there is a particular main-tenance issue you would like Dean to address in a future issue, please email Editor Kim Blonigen at kblonigen@cox.net.
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