Recommendations for Changing Oil of Aircraft Engine

Draining Oil

Oil, in service, is constantly exposed to many harmful substances that reduce its ability to protect moving parts.

The main contaminants are: 
  • Gasoline 
  • Moisture 
  • Acids 
  • Dirt 
  • Carbon 
  • Metallic particles 

Because of the accumulation of these harmful substances, common practice is to drain the entire lubrication system at regular intervals and refill with new oil. The time between oil changes varies with each make and model aircraft and engine combination.

In engines that have been operating on straight mineral oil for several hundred hours, a change to ashless dispersant oil should be made with a degree of caution as the cleaning action of some ashless dispersant oils tends to loosen sludge deposits and cause plugged oil passages. When an engine has been operating on straight mineral oil, and is known to be in excessively dirty condition, the switch to ashless dispersant oil should be deferred until after the engine is overhauled.

When changing from straight mineral oil to ashless dispersant oil, the following precautionary steps should be taken: 
  1. Do not add ashless dispersant oil to straight mineral oil. Drain the straight mineral oil from the engine and fill with ashless dispersant oil. 
  2. Do not operate the engine longer than 5 hours before the first oil change. 
  3. Check all oil filters and screens for evidence of sludge or plugging. Change oil every 10 hours if sludge conditions are evident. Repeat 10-hour checks until clean screen is noted, then change oil at recommended time intervals. 
  4. All turbocharged engines must be broken in and operated with ashless dispersant oil. 

Oil and Filter Change and Screen Cleaning

One manufacturer recommends that for new, remanufactured; or newly overhauled engines and for engines with any newly installed cylinders, the oil should be changed after the first replacement/screen cleaning at 25 hours. The oil should be changed, filter replaced or pressure screen cleaned, and oil sump suction screen cleaned and inspected. A typical interval for oil change is 25 hours, along with a pressure screen cleaning and oil sump suction screen check for all engines employing a pressure screen system. Typical 50-hour interval oil changes generally include the oil filter replacement and suction screen check for all engines using full-flow filtration systems. A time maximum of 4 months between servicing is also recommended for oil system service.

Oil Filter Removal Canister Type Housing

Remove the filter housing from the engine by removing the safety wire and loosening the hex head screw and housing by turning counterclockwise and removing the filter from the engine. [Figure 1] Remove the nylon nut that holds the cover plate on the engine side of the filter. Remove the cover plate, hex head screw from the housing. To remove the spin-on type of filter, cut the safety wire and use the wrench pad on the rear of the filter to turn the filter counterclockwise, and remove filter. Inspect the filter element as described in the following paragraph. Discard old gaskets and replace with new replacement kit gaskets.

aircraft engine oil system
Figure 1. Housing filter element type oil filter

Oil Filter/Screen Content Inspection

Check for premature or excessive engine component wear that is indicated by the presence of metal particles, shavings, or flakes in the oil filter element or screens. The oil filter can be inspected by opening the filter paper element. Check the condition of the oil from the filter for signs of metal contamination. Then, remove the paper element from the filter and carefully unfold the paper element; examine the material trapped in the filter. If the engine employs a pressure screen system, check the screen for metal particles. After draining the oil, remove the suction screen from the oil sump and check for metal particles.

aircraft engine oil system
Figure 2. Oil sump screen

[Figure 2] If examination of the used oil filter or pressure screen and the oil sump suction screen indicates abnormal metal content, additional service may be required to determine the source and possible need for corrective maintenance. To inspect the spin on filter the can must be cut open to remove the filter element for inspection. Using the special filter cutting tool, slightly tighten the cutter blade against filter and rotate 360ยบ until the mounting plate separates from the can. [Figure 3] Using a clean plastic bucket containing varsol, move the filter to remove contaminants. Use a clean magnet and check for any ferrous metal particles in the filter or varsol solution. Then, take the remaining varsol and pour it through a clean filter or shop towel. Using a bright light, inspect for any nonferrous metals.

aircraft engine oil system
Figure 3. Cutting open a spin-on type oil filter using a special filter cutter

Assembly of and Installation of Oil Filters

After cleaning the parts, installation of the canister or filter element type filter is accomplished by lightly oiling the new rubber gaskets and installing a new copper gasket on the hex head screw. Assemble the hex head screw into the filter case using the new copper gasket. Install the filter element and place the cover over the case, then manually thread on the nylon nut by hand. Install the housing on the engine by turning it clockwise, then torque and safety it. Spin-on filters generally have installation instructions on the filter. Place a coating of engine oil on the rubber gasket, install the filter, torque and safety it. Always follow the manufacturer’s current instructions to perform any maintenance.

Troubleshooting Oil Systems

The outline of malfunctions and their remedies listed in Figure 4 can expedite troubleshooting of the lubrication system. The purpose of this section is to present typical troubles. It is not intended to imply that any of the troubles are exactly as they may be in a particular airplane.

Trouble
Isolation Procedure
Remedy
1  Excessive oil consumption
Oil line leakage
Check external lines for evidence of oil leakage
Replace or repair defective lines.
Accessory seal leakage
Check for leak at accessories immediately after engine operation.
Replace accessory and/or defective accessory oil seal.
Low grade of oil

Fill tank with proper grade oil.
Failing or failed bearing
Check sump and oil pressure pump screen for material particles.
Replace engine if metal particles are found.
2  High or low indicated oil pressure
Defective pressure gauge
Check indicator.
Replace indicator if defective.
Improper operation of oil pressure
Erratic pressure indications either excessively high or low.
Remove, clean, and inspect relief valve accessory oil seal.
Inadequate oil supply
Check oil quantity.
Fill oil tank.
Diluted or contaminated oil

Drain engine and tank; refill tank.
Clogged oil screen

Remove and clean oil screen.
Oil viscosity incorrect
Make sure correct oil is being used.
Drain engine and tank; refill tank.
Oil pump pressure relief valve adjustment incorrect
Check pressure relief valve adjustment.
Make correct adjustment on oil pump pressure relief valve.
3  High or low indicated oil temperature
Defective temperature gauge
Check indicator.
Replace indicator if defective
Inadequate oil supply
Check oil quantity.
Fill oil tank.
Diluted or contaminated oil

Drain engine and tank; refill tank.
Obstruction in oil tank
Check tank.
Drain oil and remove obstruction
Clogged oil screen

Remove and clean oil screens
Obstruction in oil cooler passages
Check cooler for blocked or deformed passages
Replace oil cooler if defective.
4  Oil foaming
Diluted or contaminated oil

Drain engine and tank; refill tank
Oil level in tank too high
Check oil quantity.
Drain excess oil from tank.
Figure 4. Oil system troubleshooting procedures

aircraft engine oil system
Image (figure 4)