Sunday, November 28, 2010

Oil Change

Yesterday was the day to change N631S's engine oil. It's a long weekend and the time was available. The oil had 46 hours on it - it was about "used up." And finally, cold weather is upon us. The oil that had been in service for the last three months is Aeroshell 100W, which has an SAE 50 viscosity rating. For the winter months I like to switch to less viscous Aeroshell 80W, an SAE 40 oil.

Of course, changing the oil is one of the items of Preventive Maintenance authorized for accomplishment by the holder of a Private Pilot's certificate under 14 CFR 43 Appendix A Section (c).

To change the oil in a Cessna 182 the following items are essential:

  • 10 quarts of oil;
  • An oil filter;
  • An oil analysis kit;
  • Safety wire (the 0.032" flavor);
  • Safety wire pliers;
  • A couple of small nylon cable ties ("Ty-raps");
  • A torque wrench with a 1" socket;
  • A 5 gallon pail;
  • The usual array of basic tools;
  • A helper (to wrangle the lower cowl).
My first step on arriving at N631S's hangar is to remove the top cowling and hook up the Tanis engine pre-heater. The oil has to be warm to drain properly and there are two ways to make that happen - go fly the airplane, or apply pre-heat to the engine. It's easiest for me to use the electric pre-heat system for an hour, since pattern operations are not permitted at KVKX ("National Security" and all that).

To drain the oil on a 182 you have to remove the lower cowl. To do that, you first have to separate the halves of the landing light electrical connector. It lives just inside the access door for the oil dip-stick and is normally secured with small nylon Ty-raps (to prevent the connector from separating due to vibration). Cut the Ty-raps and split the connector.

Next, disconnect the cowl flaps from their operating cables. A 3/8" wrench and 3/8" socket on a small ratchet work out nicely. The nuts are elastic stop nuts with nylon inserts, with a flat washer under each. For each cowl flap, remove the nut and washer, pull the bolt out of the clevis, let the cowl flap drop and put the bolt back in the clevis. Secure the bolts with the washers and nuts so that the hardware bits can't go adrift and get lost.

The third item is up there in the darkness beyond the cowl flap. The air inlet duct from the air filter is secured to the carburetor inlet air box on each side by a 1/4-turn fastener. If you are fortunate they will be "wing-nut" sorts of an animals and you can reach way up there and separate them manually. If you are not so fortunate you may have to engage them with an appropriate screwdriver, which is more difficult and requires more colorful language.

Once all three connections are disconnected you can unfasten the 1/4-turn Cleco fasteners that secure the lower cowl in place (taking care not to forget the two that are way down at the bottom just to the left and right of the nose gear strut) and carefully lower the cowl clear of the exhaust pipe and carry it away. (This is where your helper earns his keep.)

Now you'll be wanting to get the oil started draining. It's really good if one of these "quick drain" fittings (at left) is installed. You can push and turn the knurled collar and the fitting will lock in the open position to allow the oil to drain. On N631S, a piece of 1" Tygon tubing about two feet long fits nicely over the quick drain and allows the oil to flow into the pail in a neat and orderly way. Once the oil is draining you can turn your attention to the filter.

The oil filter is located so as to maximize awkwardness. Access to the filter is obstructed by various components and parts of the engine mount. To further complicate matters, the filter is full of oil that will want to get out and get all over everything as soon as you try to remove it. Everyone has their own set of home-brewed procedures to minimize the mess - here's what I do. First, I reach in with a pair of cutting pliers and cut the safety wire. Then, I use a 1/2" drive ratchet with a 1" socket to loosen the filter until I can just turn it by hand. I then slide a one gallon zip lock plastic bag up over the body of the filter.

Then, the special tool shown at left (an old oil bottle with one side cut out of it) gets slid under the bag and filter to (I hope) collect oil that may escape.

I unscrew the filter as quickly as possible, keeping a roll of paper towel handy.

Once the filter has been detached and extracted and the area has been cleaned up, untwist and remove any left-over safety wire that may still be hanging from the filter adapter. Then, take a good look at the filter adapter. There is an Airworthiness Directive, AD 96-12-22, that applies to Cessna 182 aircraft. It requires inspection of the oil filter adapter at every oil change and in particular focuses on the integrity of the torque putty applied to the joint at the base of the adapter. (If the putty is broken it indicates that the filter adapter is not secure.) This is one of the few inspections that can be accomplished by a Private Pilot, and it has to be logged in the airplane's maintenance records.

The oil has been draining for a few minutes by this time so it's a good opportunity to capture a small sample of the used oil to send off for spectrometric analysis. I use the kits provided by Aviation Laboratories of Houston, Texas. They provide very nice on-line reports of their analytical results.

As the oil continues to drain you can thread the new filter into place, having coated its rubber gasket with Dow-Corning DC-4 silicone grease (or, if you are fresh out of DC-4, a film of clean engine oil). Use your calibrated torque wrench (as shown above) to tighten the filter to a torque of 16-18 ft. lbs.

Cut a longish piece of safety wire (a couple of feet at least) and thread it through the drilled "ear" on the filter adapter. Pull the wire through until you have two strands of equal length. Mark a spot on the double strand corresponding to the distance to one of the holes on the filter - pick one that, if restrained, will prevent the filter from "unscrewing." Add about 20% more length to account for the fact that when you twist the wire it will shorten up, and clamp the safety wire pliers onto the two strands at that point. Use the pliers' twisting mechanism to twist the paired wire, aiming for about 6 to 8 turns per inch.

When the length of the twisted wire is about right to reach the tie-off point on the filter, thread an end through that hole and then use the pliers to twist the free ends together thus securing the filter. Cut off the excess wire, leaving about a 1/2" "pig-tail" and use pliers to turn the pig-tail under, so that the next person to stick their hand near there doesn't get "bitten."
Now the filter is securely installed, and the oil is probably about done draining from the crankcase. Close the quick-drain fitting and begin pouring quarts of fresh oil into the engine. The crankcase capacity of the O-470 engine in the 182 is 12 quarts but I've learned that the engine will blow off two quarts very quickly and will settle down at the 10 quart level. So I put in 10 quarts to begin with.

With the oil in the engine, pull the airplane out of the hangar and start it up. Watch the oil pressure closely, and if the gage doesn't respond in a few seconds, shut down and find out why not. Don't run the engine too long, as it's un-cowled and therefore isn't being cooled properly. After a couple of minutes shut down and inspect the engine for any evidence of leaks.

If everything looks tight and dry you can re-cowl the airplane (usually the hardest part of the entire job). You'll need your helper again to get the lower cowl in place. Remember to re-secure the two fasteners for the air-box duct, the two cowl flap clevises and the landing light connector - which will need a couple of new Ty-raps.

And that's it! All that's left is to make the appropriate entry in the engine's maintenance log, send off the oil sample to the lab, and cut open the filter to inspect the media for stray metal (which process will be the subject of a future post.)


Roger D. Parish said...

Are you supposed to reuse those nylon locking nuts on the cowl flaps clevises?

Frank Van Haste said...

Hi, Roger! Thanks for reading...

In a word, yes. I know that re-use of elastic stop nuts is not regarded as "best practice", but these are not under load and as long as the Nylok(tm) element retains enough resiliency to resist loosening through vibration, you're good to go. I think you'll know, when you thread them into place, whether it's time to replace.



Anonymous said...

The other action I always like to add to an oil change is to take about 20 minutes with a flashlight and inspection mirror to look for anything else amiss: exhaust leaks, muffler cracks, cooling baffle problems (they're common), loose or damaged wiring, throttle/mixture/prop/carb heat cabling, missing lockwire or cotter pins, worn mounts, fuel stains, or leaks of any kind.

I also like to do a mag timing check. As an owner-pilot, you can't make the adjustment if it's off, but you'll at least know if it's within limits, and if it isn't, it's quick for the shop to fix.

After a couple of oil changes, one should have a pretty good idea of what things should look like normally, and will then recognize when something is wrong.

Frank Van Haste said...

Good advice, airphoria! And, don' t neglect the interior of the cowling. Baffles, lights, wiring, air ducting. The stuff in there tends to take a beating.

Thanks for reading and commenting,


Anonymous said...

Frank, I have found it easier to feed the safety wire through the top eyelet prior to screwing the new filter in place.

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