The rate of oil consumption on a PT6 engine is a vital statistic for proper maintenance and accurate troubleshooting. Pratt & Whitney’s guideline regarding oil consumption for PT6s is 1 quart for every 10 hours of operation. This rate of consumption is the maximum allowable and it applies to every type of PT6 engine found on King Airs.
If the engine is being over-serviced, the excess oil is blown overboard, giving the impression the engine is using more oil than it really is. To accurately track the rate of oil consumption, you must ensure each engine is getting the amount of oil it needs and no more.
Whenever I hear an owner or pilot complain of oil mess all over the struts, flaps, gear doors or beaver tails, I suspect they are putting in too much oil. My first question is “Where are you keeping your oil levels?” and my second question is “When are you checking it?”
Finding the level
Every PT6 is slightly different. Like fraternal twins, they look alike but have different personalities, quirks and mannerisms. Preferred oil level is a trait that varies from engine to engine. One engine likes to run at 3 quarts down and another stays at 1.5 quarts down. I’ve found many PT6s running at between
2 and 3 quarts down. (To be clear, that’s between the 2nd and 3rd hashmarks below “Max Cold” on the stick on a cold engine.) Some engines stake their claim at 3 or even 3.5 quarts low.
Many times there will be a disparity between your engines – the left side is happy at 2-down, but the right side prefers 2.5-down. There is nothing wrong with this. I know it’s painful for those who want both of their engines to be perfectly matched along every parameter, but they are not identical twins. As long as each engine makes book figures, you are good to go.
If your King Air is new to you, you need to fly the airplane for a bit to see where your engines want to be oil-wise. If you just had an engine overhauled, you need to fly it for a while to see if the preferred level has shifted. If it goes from 1-down to 2-down fairly rapidly, don’t worry right away. Maybe it will go to 2.5-down and stay there. Your engine will tell you its preferred level. Once that level is ascertained, don’t add oil until the engine has gone down a quart below that level.
Finding the preferred oil level for each of your engines is crucial to monitoring engine oil consumption. All oil added in excess of that level is just blown out the breather, making a mess and wasting money.
When to check
Pratt & Whitney recommends the oil level on a PT6 be checked within 20 minutes after shutdown. I know this flies in the face of everything drilled into your pilot psyche from when you first learned to fly. Checking the oil is a vital part of the all-important preflight routine. So why would the engine manufacturer recommend doing it after a flight?
Here’s what happens in a PT6: After running, the oil has collected in the oil tank located above the oil filter housing; however, 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. It’s called oil migration, and it’s common in PT6s. Therefore, Pratt says to get an accurate read on the amount of oil in the engine, you need to check it before the oil starts to migrate, and that is within 20 minutes after shutting down.
Checking cool or checking hot?
In reality, the engines are still pretty hot after 20 minutes. You must exercise caution to check the oil without burning yourself. Further, at the end of a flight, there’s plenty of chaos with line service instructions, passengers, baggage and ground transportation. Let’s face it, at the end of a trip checking the oil on a hot engine isn’t everyone’s favorite thing to do.
Far more important in my book, is just checking the oil regularly. Whichever way you choose to do it, whether it’s post-shutdown or before the first flight of the day, just do it that way consistently.
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.
More on migration
Imagine you are preflighting your King Air and checking the oil, which is off the stick on one side. That’s a red flag! But if your first instinct is to dump a couple quarts in and check the stick again. you could be over-servicing that engine and wiping up the mess at the end of your trip. Why? Migration. The oil is still in the engine, it’s just hiding below and appears to be off the stick.
Instead of adding oil, try motoring that engine for about 30 seconds and check the stick again. Chances are, the oil will be back on the stick at the level you were expecting and you are good to go. You have a minor oil migration problem that can be easily remedied at the next phase inspection. Until you get the problem addressed, keep motoring that engine each time you check the oil on that side. Motoring gets the scavenge pump to move the oil back into the oil tank where it shows up on the stick.
Oil migration is an oil leak inside the engine. A common culprit is the o-rings between the oil filter housing and the engine case. During every phase inspection the oil filters and chip detectors are pulled, inspected and reinstalled. Maintenance technicians make every effort to be careful, but some oil filters are a pain to get out and get back in. The process of removing and reinstalling a stubborn oil filter can jostle the filter housing just enough that the o-rings lose some of their seal.
Keeping it clean
When it comes to lubrication in general, I would say you can’t have too much of it. The one exception seems to be the oil in a PT6. If you continually have oil on your gear doors, struts or flaps, then most likely you are over-servicing your engines. Be aware that oil migration makes an engine appear to have lost oil when it has not. Above all, ascertain the preferred oil level for each engine and stick with it. This gives you an accurate picture of the oil consumption rate. As an added bonus, you will have a cleaner aircraft.