Checking in with the dean of aviation oils
Dennis Boggs could be considered something of an elder statesman of aviation oil. When he retired from Phillips Petroleum last summer, Boggs had perhaps as much experience with aviation fuels and lubricants as anyone in the business. Moreover, Boggs had the opportunity — or the cross to bear, depending upon your point of view — to present the ins and outs of oil to pilots and mechanics during countless group and one-on-one sessions.
We sat down with Boggs shortly after his retirement to get his views on aviation lubricants.
Is there a difference in lubricating qualities between straight mineral oils and ashless-dispersant (AD) oils?
With regard to the lubrication itself, in the actual prevention of wear, there is no difference between the two. One will lubricate as well as the other. The main difference is cleanliness. Ultimately, the AD oil's ability to suspend wear particles gives you better wear characteristics because you don't build up a lot of hard carbonation. Particles in the oil, sludges, will cause wear, promote wear. Aside from the ashless additives, the oils are the same.
So the concept that a non-AD oil is less effective at sealing the ring/cylinder-wall gap, and allows the rings to run hotter to promote break-in, is a fallacy?
Yes, that's a fallacy. An AD oil will break in the engine just like a non-AD oil, as long as the difference is just the ashless dispersants. If you put other things in the AD oil, like anti-wear additives, you run into break-in problems. For example, Phillips made an anti-wear version of XC oil that would not have been a good oil for break-in. Ultimately, the cylinders and rings will break in, but the anti-wear ingredients will inhibit the break-in process.
That's why synthetic oils are not ideal for break-in; they would inhibit the process. If you take two straight-weight mineral oils — one with the AD additives and the other without — they will break in the same way.
So there's no practical reason you can't run an engine on AD oil from the start — there's no detriment to break-in?
Nothing except for tradition. It's hard to convince mechanics and owners that it's OK to do so. Much of what the average pilot knows about oil is very old information. And the oil has changed a great deal.
Up until World War II, there were no additives and we didn't even know what kind of base oils were best. People have tried everything — castor bean oil, corn oil, mineral oil; there were even some records of people trying lard in engines. There were all sorts of concoctions. Castor bean was popular because of its lubricity. Back in the days of exposed valves and tappets, the castor bean oil wouldn't slop off as fast. It wasn't a very long-life product because castor oil oxidized very quickly, forcing short oil-change intervals.
World War II came along and [the military was] trying to find the ultimate oil to use in the war effort. They finally wrote a specification for aviation lubricants; and because the only oil around was a non-additive mineral oil, that's all the spec covered.
For about 20 years after the spec was written — into the early 1960s — the basic oil was the same. Save for adding an antifoam agent and allowing for a few different viscosities, those oils were identical to the World War II oils.
By the early 1960s, Shell developed a dispersant. Dispersants are used with automotive oils in conjunction with detergents. Shell — and the mil spec — excluded detergents because it was the theory back in the 1940s that if you put anything metallic in the engine, it might cause preignition. That was a big scare put on at that time so that no additives would go in aviation oils.
Do you think that prohibition on detergents is still valid today?
Never was valid, in my opinion. But it's still around today, and we have to deal with it because the spec has not been changed to allow for detergents.
Will this spec ever change?
I think eventually it will change. But it will change only if and when the engine manufacturers are willing to look at it.
One of the things that people tell you is to never, ever run automotive oil in your aircraft engine because the metallic additives could cause problems. Now, you say the detergents are probably not a problem?
Well, you wouldn't want to run automotive oil like we run in the cars because you have more additives there than an aircraft engine needs or could use. Still, this is all conjecture because I don't know of anyone who has tested auto oil in a conventional aviation engine. So that's the biggest reason I say don't use it.
So the issue is really one of testing and there is a possibility that the additive package for the automotive oil might very well be compatible with the aircraft engine — we just don't know if it is or not?
Well, we know the chemistry would be compatible with the engine; the thing we don't know is how much of that chemistry we need. That's where the testing is needed. We took some automotive-type material and added it to XC at one point in time, highly modified from auto oils. However, it was the same technology. It worked great. It's not marketable, but technically it's solid, it's sound. It's just not marketable because of this taboo against anything that has ash. Now, the people care about the ash, but the engine really is not all that excited by a little ash.
The only real testing we have in the field so far is with the Porsche-powered Mooney PFM and the Rotax 912s in the Diamond Katana, both of which use off-the-shelf automotive oil.
But they had a lot of time; they knew about those engines on the ground before they put them in the aircraft.
In your view, should we be testing these detergent packages in aircraft engines?
We have technology that can be taken advantage of today. But it's too time-consuming and too expensive for anyone to really pursue it under today's rules. And so that's why nothing is happening. There's no motivation for it. We have gone to both [engine] companies and said, "We'd like to do some kind of a joint effort here to look at ways to make aviation better." But they don't have a program for doing that under the current circumstances.
Are we in need of a new generation of oils?
Nothing happens quickly with oil. You can take the least desirable mineral oil available for the engine and run it versus the very best oil you can put together under the mil spec rules and nothing would happen quickly. It would take 1,000 hours to really see the difference, to see what has happened using one versus the other. As long as the viscosities are equal, you're going to come out the same. I mean one may let it run dirtier than another, and you may get more sludge in the prop dome. But more variable than the oil is the maintenance. And the way the airplane is flown, where it is flown, the fuel used in it…I mean, there are so many variables that two engines running the same oil will get very different results.
I've had people call and say, "I changed from this color can to that color can and ran it 27 seconds and now oil is coming out of all the bolt holes." Those are impossible things that you know can't happen. So when somebody says "I switched from this oil to that oil and all these things are happening," it doesn't matter whose oil you had before or whose oil you went to. One oil change doesn't make any difference.
So the problem after the change of oil brand is a coincidence, not a result of the change?
That's right. We see that quite a bit, in automotive and aircraft [applications] and the heavy-duty equipment. People don't understand oil well enough to be rational about it. It has too much mystery about it. It is an emotional purchase.
Many pilots are convinced that they need a more expensive and sophisticated oil for a highly stressed turbocharged powerplant than for low-power engines.
They don't. The oil doesn't care. The oil is good enough to take care of the hardest-working engine. It's tested in worst possible engine tests and it's run under horrendous conditions that the oil never sees again.
The cost of the oil has nothing to do with how it's going to perform. You take the same oil in Texas, and in California it may be a dollar-a-quart difference in price for the same oil. You can't go by the price of oil [or] its performance level — these oils all meet the required performance level. They're all good or they wouldn't be out there in the marketplace.
I'd say that multigrades give you an advantage every time. If you have an option of three different oils that are multigrades, all made for the same purpose, yes, you can use any one of them.
A lot of pilots think that a synthetic or semisynthetic oil is better in turbocharged airplanes because it protects the turbocharger bearing.
People are always talking about synthetic versus mineral oils as one's being better than the other. Synthetic oils are different; they are not better or worse than mineral oils — they're different. If you have an application that requires a synthetic oil, then there is an advantage. If your engine doesn't need a synthetic, then you don't receive the benefits that you paid for. I've been asked why Phillips doesn't produce a synthetic aviation oil, and my answer is always that the engines don't need the synthetic. The engine gets no benefit from it. I get a warm fuzzy glow from paying more, but the engine doesn't get any benefit in the performance.
There's nothing about the way a turbocharged engine operates that a standard mineral oil won't handle, as long as you follow procedures. If you land and shut it off and coast into the hangar to save fuel…then I don't care what you've got in it, you're going to have a problem with that turbocharged airplane. The synthetic may last a little bit longer before the bearing cokes up, but it will eventually.
It's been said that oil quality has been responsible in part for the generally increasing time between overhauls for modern engines. True?
Well, the big break, so to speak, had to be the introduction of AD oil. When we began to put the dispersants in the oil, we were able to run the engine cleaner. If you run it cleaner — by suspending the contaminants rather than having them kicking around inside the engine — you will increase the life of the engine.
You are known as a big proponent of multi-viscosity oils. Why?
Oil in any engine has to seal, cool, lubricate, and clean. To get the sealing done, you have to have enough viscosity, enough body to the oil to create the seal between the piston ring and the cylinder wall. To get that in a straight grade oil, you use a blending component called bright stock. These are the heavy weights of oils, the real viscous stock. However, those heavy molecules contain greatest amounts of lacquers and varnishes that cause some of the problems with deposits.
So with a multigrade, we are able to use lighter molecules; we don't use very much of the bright stock anymore. And without that you build up fewer contaminants. Being able to change the base stocks, you save some in cleanliness; and by running the dispersants, you get a cleaner engine, so you don't stick the rings and clog up the oil passages.
What is your view on TCP or other extreme-pressure additives?
Anything that has a phosphorous component is technically an anti-wear. It doesn't necessarily make a good anti-wear substance for that particular purpose, but technically it's an anti-wear. Anti-wear additives are like detergents or dispersants; you have to have them in sufficient quantity and effectiveness to solve a problem or they don't need to be there. Any time you design an oil for an engine, you have to take into consideration that engine's requirements. What is the problem or potential problem that you're trying to solve here?
For example, the H series [Lycoming] engines had a camshaft problem. Put the camshaft up out of the oil and it takes awhile for the oil to get there. Put straight-grade oil in it and start it at 40 degrees or less and obviously you're going to have a problem. Now you can solve some of the cold-start wear with multigrade, because it will flow faster. But once the engine is started and the oil is flowing, the anti-wear additive doesn't do a whole lot for you.
Lycoming has an additive, and it's there to help the camshaft. We don't think that we should give all of our customers the anti-wear additive even if their engines don't need it. We have looked at some anti-wear additives and think there are some installations where they will do some good, but we'd like to have a more effective anti-wear than TCP.
