Maybe you’ve heard about it, maybe not, but there is an issue with TBM 700/850 engines which some are referring to as “dark oil”. As a aircraft broker, I’ve encountered several TBM 700 models and even a few 850s that have dark oil notations in the logbooks. One buyer I worked with chose not to buy a certain plane based on the issue, but did buy another airplane which also had the issue. The following is a synopsis of the the explanations I’ve gotten in working on two different acquisitions that had to deal with “dark oil”. It’s not scientific and should not be the basis for you to decide to buy or not buy a TBM but its informational only.
A little history. The PT6 engine was originally developed 55 years ago. According to Pratt and Whitney’s website over 55,000 PT6s have been built and used in over 140 applications in several variants. PT6s are used in many aircraft including King Airs, Pilatus, Cessna Caravans, Piper Meridians, Air Tractors and Piaggios to name a few.
“My car’s oil gets dark after its been in the engine isn’t it normal for my plane’s oil get dark?” No. The oil in turbine/turboprop engines normally stays much cleaner than in internal combustion engines due to the extremely close tolerances and the highly efficient combustion process so the oil normally doesn’t turn as dark as in a car’s engine but it is a known issue in some turboprop engines such as the PT6A-64 engines in the TMB 700s. The oil gets burned or “coked” onto the side of the bearings which eventually flakes off and is pulverized into tiny carbon particles in the oil which darkens the oil.
What causes the dark oil? According to Pratt and Whitney the problem is too much heat on the bearings inside the engine. The oil gets burned or “coked” onto the side of the bearings which eventually flakes off and is pulverized into tiny carbon particles in the oil which darkens the oil.
Is this a safety of flight issue? Generally speaking, no. The coking does not reduce the lubricating properties of the oil. However, carbon particles are caught in the oil filter and secondary screen and can accumulate and cause reduced oil pressure and induce the oil system to bypass the oil filter which could allow carbon and other particles in the oil to go through the engine unfiltered and potentially cause damage. Following the scheduled maintenance procedures seems to prevent buildup of particles to the point of being dangerous.
How many airplanes have the issue? No one is sure, it seems to begin around 800 hours so more 700s have incurred it than 850s just based on their accumulated engine time. One Maintenance Director I spoke to about this said, “It’s not if but when” a TBM will experience this issue.
Is there a fix? Pratt and Whitney and Dahr/Socata have been studying the issue are working to come up with a solution. The primary focus has been on a different oil. P&W has been testing a different oil which has higher resistance to “coking” than the current oil. There also may be suggest changes to the cooling systems both inside the engine and on the airframe. Stay tuned.
Until then the accepted procedure to manage the condition is to flush the engine oil (it may take multiple flushes) to remove the carbon from the entire engine, change the filter and the secondary screen and then to follow an early oil change procedure and to inspect the filter and the screen for particles and then gradually increase the hours between oil changes back to normal.