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Checking the Dipstick

Checking the dipstick

There’s a lot more to checking the dipstick than just noting the oil level. The appearance of the oil is at least as important as its quantity.

We’ve been doing it since our earliest days as student pilots. Now that we’re aircraft owners, we still do it as part of our standard preflight ritual. But are we doing it right?

It turns out that there’s a lot more to checking the engine’s oil dipstick properly than just making sure that the oil level is above the minimum-for-flight level listed in the POH. If we really pay attention, we can learn a lot about the condition of our oil and of our engine.

How much oil is needed?

The engines on my Cessna 310 have 12-quart sumps—13 quarts if you include the quart in the spin-on oil filter. When I first acquired the airplane, my mechanic would fill the sump to its maximum capacity at each oil change. It didn’t take me long to discover that the engines didn’t like that, and promptly tossed several quarts out the engine breathers.

My POH states that the “minimum for flight” oil level is 9 quarts. So I asked my mechanic to service the sump to 10 quarts (instead of 12), and I’d add a quart of make-up oil when the level got down to 9 quarts. That worked better, but I was still seeing a fair amount of oil on the underside of the engine nacelles and the outer gear doors.

After I became a mechanic myself and learned about such things, I checked the Type Certificate Data Sheet (TCDS) for my Continental TSIO-520-BB engines, and found that an oil level of 6 quarts was sufficient to make good oil pressure in all flight attitudes from 23° nose-up to 17° nose-down. Armed with this information, I decided to experiment with lower oil levels.

What I discovered was that oil consumption (and the oily mess on the airframe) was drastically reduced if I maintained the oil level at around 8 quarts on the dipstick. Since then, I’ve avoided filling the sump to more than 9 quarts, and I normally do not add make-up oil until the dipstick reads about 7½ quarts. (This still gives me a 1½-quart “cushion” above what the engine needs to operate reliably in all flight attitudes.)

You might wonder why Continental put a 12-quart sump on an engine that requires only 6 quarts. The answer is that FAA certification requirements demand that the engine be designed to hold twice as much oil as it actually needs:

FAR §33.39 Lubrication system.

(a) The lubrication system of the engine must be designed and constructed so that it will function properly in all flight attitudes and atmospheric conditions in which the airplane is expected to operate. In wet sump engines, this requirement must be met when only one-half of the maximum lubricant supply is in the engine.

The TCDS for my TSIO-520-BB engines states that maximum acceptable oil consumption is about one quart per hour. If my engines actually used that much oil, then I’d need to fill the sumps nearly to their maximum capacity to ensure that I had enough oil to make a 5-hour flight without risking oil starvation. But since I know from long experience that my engines use more like 0.1 quart per hour, there’s no reason for me to carry anywhere near that much oil.

Every aircraft engine installation has an optimum oil level at which oil consumption is minimized and the engine is happiest. I would encourage you to experiment to determine what oil level works best for your airplane. Your engine will operate properly at 50% of its maximum oil capacity—guaranteed. As long as you keep the oil level a quart or two above the 50% point, your engine will be happy.

The best time to get an accurate dipstick reading is just prior to the first flight of the day. If you check the oil level shortly after the engine has been run for awhile, the dipstick reading will be noticeably lower because a significant quantity of oil remains adhered to various engine components. Another reading taken 24 hours later will often show an oil level that is ½ to 1 quart higher.

Oil consumption?

Having assured yourself that there’s enough oil in the engine, your next task is to make note of how much oil your engine is using. Keeping track of oil consumption—particularly any significant increase in oil consumption rate—is an important tool for monitoring engine condition.

The most common method of measuring oil consumption is to record how many quarts of make-up oil are added between oil changes, and to divide the total by the number of hours in the oil-change interval. (For example, if the oil is changed after 50 hours and 6 quarts of make-up oil were added during that time, the average oil consumption rate is 50/6 or 8.3 hours per quart.)

Oil consumption graph

Oil consumption isn’t linear—it accelerates as the oil deteriorates over time. The rate of consumption during the first 10 hours after an oil change is a good indication of engine condition.

However, this approach obscures the fact that oil consumption is not linear over the oil change interval. If you keep track of when you add each quart of make-up oil, you’ll find that less oil is consumed at first, and progressively more oil is consumed as the oil’s time-in-service increases.

The reason for this accelerating oil consumption is that the viscosity of the oil decreases as the oil deteriorates. Mineral oils lose viscosity due to a phenomenon called “polymer shearing” in which the long organic molecules are actually broken apart by mechanical action of the engine’s moving parts. Multigrade oils also lose viscosity because their viscosity-index improvers oxidize when exposed to high temperatures.

The increased rate of oil consumption provides tangible evidence that your engine oil is getting “long in the tooth” and ought to be changed soon.

What does your oil look like?

Whenever you check the dipstick, it’s also important to make note of the oil’s appearance—particularly its color and clarity. The oil’s appearance offers valuable clues to its condition and that of your engine.

Oil color

Color and transparency are important indicators of engine condition. When oil becomes dark and opaque, it should be changed. If this happens rapidly, it suggests that the engine has too much blow-by past the rings, or that oil temperature is too hot.

Fresh engine oil has a light amber color and is so transparent that it’s sometimes hard to read the dipstick level. As the oil remains in service, it gradually darkens in color and becomes progressively more opaque.

The darkening of engine oil is caused by contamination and oxidation. Contaminants include carbon (soot), lead salts and sulfur from combustion byproducts that get past the compression rings and into the crankcase (“blow-by”), as well as any dust or dirt that gets past the induction air filter. Oxidation of the oil occurs when it is exposed to high localized temperatures at it circulates through the engine, and results in the formation of coke. Various oil additives are also vulnerable to oxidation, particularly the viscosity-index improvers used in multiweight oils.

Dispersant additives are blended in the oil to help keep these so-called “insolubles” in suspension in order to keep the engine clean and minimize sludge deposits. As the quantity of insolubles in suspension increases, the oil darkens and becomes opaque.

It is important to note how quickly this darkening occurs. If your oil remains relatively light-colored and translucent after 25 hours in service, you can be reasonably confident that your cylinders and rings are in fine condition and that your oil can prudently remain in service for 40 or 50 hours. On the other hand, if your oil gets dark and opaque after 10 or 15 hours, you’d be wise to change your oil more often—perhaps at 25 hours—and you may want to investigate the possibility that one or more cylinders are excessively worn.

Such rapid discoloration is often a good indicator that the oil is distressed. In one study, 90% of oil that appeared abnormally dark on the dipstick was subsequently found by laboratory analysis to be non-compliant with required specifications. Oil that is dark and opaque from blow-by past the rings is very likely to be rich in acids and other corrosive compounds that can attack your cam and lifters, and that’s probably the #1 cause of engines failing to make TBO. Any time your oil appears dark or opaque, you would be wise to drain it and replace it with fresh oil and a new oil filter, regardless of the oil’s time-in-service.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from Amazon.com in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from Amazon.com in paperback and Kindle versions. (508 pages).

11 Comments

  1. Mike, did the engine temps remain OK with your use of a lower oil amounts?

  2. Mike,
    Isn’t the reg from the CAA manual 107, on oil sump capacity still in affect ?

    “Civil Aeronautics Administration Manual No. 107 (January 1949), titled Aircraft Power
    Plant Handbook, adds oil-tank capacity guidelines when it says, “The customary ratio is
    one gallon of oil for every 25 gallons of fuel, but not less than one gallon for every 75
    maximum continuous horsepower of the engine involved for non-Transport category
    airplanes.”

    • I doubt it. That sounds descriptive, not prescriptive.

      • How old do you think the engine designs are ?
        Most we designed and developed in the ’40s

        The GO-300 in my C175 has a10 qt capacity, and the POH states 4 qts minumim. I run 7-8 qts and the oil temps are ok at 160-180 degF.

    • Is that meant for radial engines?

    • Maximum continuous HP for a Piper Cherokee 140 is 150. Max fuel is 50 Gallons, maximum oil is 2 gallons. That seem to make some sense.

      However a 160 or 180 HP Cherokee does not meet the above guideline.

      And 200 HP 1977 arrow with 72 gallons of fuel has the same 2 gallon maximum oil.
      Looks like a guideline only, to me.

  3. If the POH says 9 quarts minimum for flight, how is taking off with less than that different from taking off with a higher weight than the max specified in the POH? Are those two things subject to different requirements in the FARs for how specifically you have to follow them?

  4. The engines on your 310 are both canted toward the fuselage. Because of the tilt (not level), the right engine oil reads higher than actual and the left engine reads lower than actual. This is because the dip stick is the same part number and the engines are not level.

  5. Reminds me of a Mooney I used to have. It would drop 1/2 to 1 quart after the first few hours, then would stay at that level until oil change at 25 hours.

  6. A guy I worked for (A&P/I.A.) as well as my military training taught me something else to check for when doing the dipstick inspection – smell. Overused, or burnt oil has a distinctive smell, along with that distinctive black coloration, much different than good clean oil.

  7. Mike, while not an A&P mechanic myself (but 30+ years of automotive experience), I, too, got tired of the BO from our Skyways IO520 in our 182. I noticed that once it blew out enough to level off at the 8 qt. mark, it would stay there pretty much forever. I now make it a point to change it with 9 total, one of which stays in the filter, and our oil consumption is now zero for a 50 hr. interval. Oil reports are superb, and oil temp stays exactly where it used to.

    You neglected to point out that oil levels so high that the crank thrashes in it cause a loss of horsepower (I’ve witnessed this from my days at a dyno shop) and it also froths the oil. Air in oil does nothing good for its lubricating qualities…

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