Sentencing comes down for Mobil's synthetic aviation oil
Sometimes what works beautifully in the laboratory fails to live up to expectations in the imprecise and unpredictable real world. So it seems to be with Mobil AV-1. And while no one seriously doubts that synthetics — AV-1 included — are excellent lubricants, that's not all there is to being a good oil. In addition to cushioning the engine's internals, the oil must carry away heat and absorb contaminants. It's this last category in which AV-1 seems to have failed the extra- laboratory trials.
In October, Mobil Oil Corporation reached a settlement with plaintiffs in a class-action suit instigated by Malvern J. Gross and Paul Jimison; on November 28, 1995, that settlement was approved by the California courts. Under terms of the agreement, all claims against Mobil regarding AV-1 must be filed no later than three months after the November 28 approval date.
Gross and Jimison claimed that Mobil's AV-1 caused unusual and potentially dangerous build-ups of contaminants. Mobil introduced the oil in 1987 and pulled it from the market in June 1994. At the time, Mobil claimed that the small volume of AV-1 sold made it unprofitable and dismissed concerns of lead-sludge buildup as isolated cases. Mobil believes that about 500 engines are affected, while plaintiffs' attorneys say that the number is between 2,000 and 3,000.
In the settlement, Mobil has agreed to pay for inspection and repair of certain-model engines that have been run on its AV-1 synthetic oil for more than 150 hours. The court has stipulated that this is a mandatory class; owners of affected airplanes cannot opt out or seek separate legal recourse against Mobil. A team of so- called Special Masters — maintenance facilities yet to be named — will act as mediators and have the power to expand on many provisions of the settlement on a case-by-case basis.
The terms of the settlement are reasonably straightforward, calling for inspection of all midsize-and-larger Continental engines (the 360, 470, 520, and 550 series. Originally, the suit covered only the 520- and 550-series Continentals, but during discovery it was determined that some of the smaller engines could be having the same troubles, so they were added). Both turbocharged and normally aspirated engines are included in the settlement.
The settlement outlines the inspection and repair protocols in significant detail. Let's look at each of the inspection areas in brief, since they paint a picture of the trouble AV-1 may have caused in some engines. (It's worth noting now that not all engines run on AV- 1 have experienced problems.)
Sluggish propeller. According to the settlement, any aircraft experiencing sluggish constant-speed prop performance is entitled to removal and inspection of the prop on Mobil's nickel. If it's determined that there is sludge buildup, the mechanic is authorized to de-sludge the parts and reinstall the prop for an operational check. If any of the oil-transfer holes remain clogged or if the prop exhibits sluggish performance after the de-sludge, the settlement authorizes tests of the oil-transfer collar or bearing. If the engine fails these tests — indicating that lead sludge has blocked or damaged the collar or bearing — a bottom-end transfer is called for.
Crankshaft axial runout. According to the settlement, "If the propeller has been removed pursuant to the Propeller Operation Protocol described...or if the cylinders have been removed pursuant to the Top End Overhaul Protocol...the Axial Crankshaft Test shall be performed...." Strict interpretation suggests that if you didn't have the prop off for sluggish operation, you aren't entitled to have this test performed. Basically, the settlement calls for inspection of the crank runout, and if greater than TCM's service limits, authorizes a bottom-end overhaul. Concern here is for damage to any of the main bearing surfaces that may have been starved of lubrication from sludge contamination.
Cylinder inspection. Among the instigators of the AV-1 saga were reports of high oil consumption in low-time engines. As such, the settlement calls for a review of the engine's service history to determine whether there's been an "oil consumption problem." Specifically, the engine must have had a "dramatic increase" in oil consumption while using AV-1, defined in the settlement as a "sustained trend line with a doubling of oil consumption over 150 hours." If the engine meets these criteria, Mobil will spring for a top-end overhaul.
Even if the engine does not meet the definition of increased oil usage, top-end distress will be determined in one of two ways. First, the engine may undergo a differential compression test to assess top-end health. Under the agreement, any cylinder that measures below the allowable limit as spelled out in TCM's Service Bulletin M84-15 may be replaced. Second, if an historical review of the compression scores, obtained by using the same compression- test equipment, shows a large (15 percent) and sudden decrease over 150 or fewer hours, the suspect cylinders may be replaced. If three or more cylinders display low compression, all six may be replaced.
Each engine is supposed to undergo a crankcase pressure test — used to determine whether combustion gases are leaking past the rings and pressurizing the crankcase. According to the settlement, if all cylinders show good compression and have a history of good compression, and the engine passes the crankcase pressure test, the cylinders would be deemed airworthy.
Let's say you've got six jugs with sky-high oil consumption and all the compression of a piece of cheesecloth — you still don't necessarily get a half dozen new cylinders. If the engine or top-end total time is between zero and mid-TBO and the cylinders measure out beyond new limits, you get new jugs. (But this isn't the time to trade chrome for steel; the settlement states that you get new versions of what you already have installed.) For engines midway to TBO or beyond, the critical standards become the service limits. (Chances are they'll need some sort of rework to remedy the high oil consumption and/or low compression that got them pulled in the first place.) Incidental and normally replaced parts used for top-end inspections will be renewed under the settlement, even if the cylinders are found to be within the appropriate limits.
Bottom-end work. If any top-end work is performed, the settlement stipulates that a connecting-rod bearing inspection take place. Generally, if any of the clearances are outside normal service limits, the failed parts will be replaced.
If the engine meets the teardown requirements set forth in the previous inspection protocols — that is, there's sufficient wear in any of these bottom-end components to call for replacement — Mobil will pay for a bottom-end overhaul. All normally replaced parts are covered, and all airmelt cranks will be replaced with vacuum air remelt (VAR) cranks at Mobil's cost (up to a maximum allowance of $2,200). Generally, unless it can be proven that other crankcase parts — like the crank itself, camshaft, and lifters — have been damaged by AV-1, they will not be replaced under this settlement; nor will accessories be covered.
So, what does all this mean? Where did Mobil and AV-1 go wrong? After all, synthetic oils have been a blessing for many other types of internal-combustion and turbine engines for a long time now. Your average automobile performs wonderfully on pure synthetics, with no appreciable sludge buildups and no apparent downside, save the initial cost of the product.
But there are significant differences between auto and aircraft engines. Primary among them is the type of fuel. Cars run on unleaded gas. Synthetics in cars simply don't have to deal with a substance — lead — that aircraft engines ingest in great quantity. Moreover, auto engines have much tighter tolerances and tend to send much less combustion byproduct into the crankcase. In fact, because of emissions regulations, the maximum possible amount of crankcase effluvia is returned to the combustion chamber for re- burning. Finally, auto engines run with much lower combustion pressures than do aircraft engines, especially highly turbocharged models like the TSIO-520 and TSIO-550.
All of these elements — loose top-end tolerances, lead- saturated fuel, and high combustion pressures — aligned to make synthetic oil's life tough. Mainly, it gets back to the synthetic's not being able to suspend and dissolve certain deposits. Apparently, the lead precipitated out of the oil and collected in areas of low flow. Parts of the engines that make ideal centrifuges — the prop hub and the undersides of the pistons, in particular — became halfway houses for these lead deposits flung from the oil.
It's also interesting to note that while all models of mid- and large-displacement Continentals are included in the Mobil settlement, by far the largest group actually having trouble is comprised of the turbocharged 520 and 550 Continentals. Again, the synthetic oil must work against high combustion pressures — some of these engines use 40 inches of manifold pressure or more for takeoff — and the possibility of greater blowby. Over the years, big Continentals have had problems with oil consumption resulting from ring wear and cylinder stepping, or an increase in ring-to-cylinder clearances at top dead center. And when the AV-1 apparently fails to hold lead deposits in suspension, they end up on the undersides of the pistons, blocking the oil-transfer holes that are intended to provide some oil flow to the top end. That, in turn, promotes even more rapid wear of the rings and cylinder walls.
Why, then, is Lycoming not represented here if AV-1 did indeed present problems in aircraft engines? This is probably the result of Continental's much greater penetration of the factory-turbo market- -a rough check shows about a three-to-one margin in turbo installations between Continental and Lycoming. It also may be that Lycomings as a rule don't pass as much combustion gas to the crankcase, and so aren't contaminating the oil as much in the first place.
Are synthetic oils just a bad idea for aircraft engines? Mobil isn't admitting such; and Shell, which makes a semisynthetic, naturally defends its Aeroshell 15W-50. According to Shell engineers, the company looked into a pure synthetic but found that the solvent qualities weren't sufficient. It settled instead on a mix of synthetic and petroleum products with precisely the intent of retaining the good suspensive properties of conventional oil.
Is this the end of a purely synthetic oil for piston aircraft? Probably. It's hard to imagine that any company would, after Mobil's experience, tread these waters again. At least not until sometime down the road, when all these engines have been transitioned to an unleaded fuel — which would remove the main ingredient that felled synthetic oils.
