Dealing With A Dirty Belly

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

Sure, you can't see it when you approach the plane, but a dirty belly can be a sign of problems in the engine compartment. AVweb has suggestions to fix the problems and clean up the mess.


It's an unfortunate fact of life that aircraft engines are often not the cleanest things in the world, especially when compared to a typical auto of today. Loose engine-fit tolerances, the use of leaded fuels, substantially higher constant power levels and designs optimized for power rather than cleanliness tend to make oil staining and leaks a common problem for many aircraft operators. This is certainly compounded by the decades-old average airplane in today's fleet.

This article will deal with the build up of oil and combustion byproducts on an aircraft belly as well as what you can do to minimize the problem.

Obviously, an engine that has significant blow-by issues is one in which controlling the mess on the belly can be of diminishing returns. Therefore, a sick engine must have repairs first and foremost if there is any hope of controlling the problem. And make no mistake: Blow-by is not an issue that can be let go indefinitely. It can lead eventually to engine failure if not addressed.

One easy item is to establish an oil level at which your engine operates with minimal throw-off losses. Most engines are far less profligate with oil after the first quart or so is lost in the first hour or two. Therefore, experiment a bit with operating a quart or two below maximum. This is a perfectly safe and well-accepted practice, and may significantly reduce further oil loss.

Order of Battle

Spray Wand: This can be powered by shop air-pressure, a gasoline-powered compressor or a compressed-air tank dialed to 90 psi. The black hose connects to the air supply and a plastic hose connects to the wand and a separate supply of cleaner in a bucket. Just pull the trigger for the powered spray.

The first effort should be to find the source of leaks in the engine compartment. This is accomplished by a cleaning of the engine compartment with a cleaner such as Extreme Simple Green. Be sure to cover vulnerable components such as the alternator/generator, magnetos and vacuum pump with plastic bags before cleaning the engine compartment.

Then proceed to use a spray wand with a plastic drain hose dipped into cleaner and connected to shop air for the atomization of the cleaner. Spray wands of this type are readily available and inexpensive to acquire. An alternative is to use a pressure sprayer such as used to spray garden chemicals, but it is nowhere as effective as the shop air/sprayer method. Both the cleaners and spray wand tools are available from Aircraft Spruce & Specialty.

Pushrod Tubes: On Continental engines the pushrod tube seals are the most common sources of oil leaks. New rubber seals are the only answer to stem the oil flow.

Once the engine is clean, fly the plane around the pattern to see where the leaks are coming from. With Continental engines the most common source of oil leaks is the pushrod tube seals, which tend to dry out over time if the engine is not used almost daily. Since there are two seals for each cylinder, the potential oil mess is significant.

Changing these seals is not easy, but it can be done without cylinder removal. A special tool is available to compress the seal springs. An A&P must do this for certificated aircraft if you want to stay legal.

If you have case-parting surface leaks, it is generally impractical to do much until overhaul time, since there are no easy sealant or epoxy patches for stopping leaks short of opening the case and resealing from scratch.

Lastly, if a case crack is found as the source of the leak this often means overhaul. That said, there are some cracks that are acceptable and may be patched by an epoxy patch. Continental has an extensive Service Bulletin dealing with locations and procedures.

Blow-By Solutions

Compressed Air Sprayer: If you have no way to get an external compressed-air source, then a simple bug sprayer (or larger capacity version) will serve to spray a gentle spritz of cleaner solution on either the engine or the fuselage.

Blow-by is when combustion gasses pass from the combustion chamber into the crankcase, pressurizing the crank case. Oil residue is blown out the breather onto the belly. Blow-by generally is caused by worn or clogged rings, so it is a sign of more critical issues. Additionally, if the crankshaft front seal is worn the airstream can leak past the seal. This also can pressurize the crank case.

The proper solution is first to assure that your rings are in decent condition with a compression test as well as a borescope exam. (Clogged rings can sometimes make for a decent compression test, so if you are using a quart of oil an hour and it is not primarily a leak, then your rings are shot regardless of a compression test. Don't fool yourself.)

The next step is to be sure that oil breather lines are clear and routed properly, since many times these breather lines are never cleaned in decades and are often misrouted in the course of other maintenance. Proper routing is critical to proper operation of the breather.

Given that the rings are OK, the breather lines are routed properly and the front seal is intact, using an air/oil separator is a controversial solution. If your engine is in good condition there should be no reason to need one.

Be that as it may, the air/oil separator has a measure of popularity because it is an "easy" if not always successful fix to the symptom rather than a fix to the problem. Some users swear by them; others swear at them. It usually depends on the degree of blow-by you are attempting to control. It is also very installation-sensitive. If you decide to go this route, Aircraft Spruce can provide a selection of products here as well.

Mixture Setting

Homemade Compressed-Air Tank: This will work about the same as the smaller sprayer. The only advantage is the spray tip is able to get into tighter spots and the air supply lasts much longer. It is still nowhere as effective as the spray wand powered by shop air.

One other source of belly staining of a drier appearance is caused by the pilot operating the engine too rich in all modes. The other cause may be an incorrectly adjusted fuel system or fouled plugs or spark plugs with the incorrect heat range being used for the situation. All these elements need to be checked as a systematic approach to solving the problem.

If you are constantly fouling the bottom plugs from oil getting past the rings, a switch to fine wire plugs for the bottom plugs in the cylinders will buy you some time (but not solve worn rings) because they will fire in oil.

While beyond the scope of this article, it is sometimes possible to free up oil-clogged rings without an overhaul by the use of solvents such as Marvel Mystery Oil being used directly in the cylinder and then rinsed out. (Talk to an experienced A&P about this sometimes successful technique for oil-caked rings.)

Belly Cleaning

We have approached the topic from a preventative perspective to this point. Now let's look at getting the gunk off. There are a number of products that are dedicated to this purpose. If the degree of staining is not too extreme, you will probably meet with success with two or so applications of full strength Extreme Simple Green applied preferably via the shop-air/spray-wand approach.

We recommend against a high-pressure power washer unless you are well-experienced with appropriate cautions of using the right chemicals as well as reduced pressures and a less-focused spray tip. You can easily dent the metal or remove paint with indiscriminate use of high-powered washers. You can even induce corrosion with the wrong chemicals driven under skin lap joints if the paint is breached.

Dress for Success

Creeper: An inexpensive mechanicís creeper works well to easily slide around under the aircraft on hard surfaces. Never go under the plane without some form of eye protection -- either a shield or goggles.

Since you will be in the thick of it, so to speak, be sure to dress appropriately such as in coveralls or a Tyvec disposable suit, as well as eye protection and latex gloves. While the Extreme Simple Green is nontoxic, the oil residues are not particularly good for your skin, especially with prolonged or repeated exposures. Think safe; think long-term exposure effects.

If you need to really hand-work hard residue or reach far down the belly, then a mechanic's creeper provides a far more mobile solution to maneuvering under the aircraft -- at least on a hard, even surface.

Don't resort to any sort of abrasive pads, no matter how fine, unless you are planning on painting, since this type of mechanical abrasive will scratch paint. Sponges are fine. What you need to do is change the chemical if the residue doesn't want to come off.

A couple of cleaners that have had good reports are Gear-Up Exhaust and Oil Remover or Belly Wash Degreaser or Crud Remover. All these products are available from Aircraft Spruce.

If all else fails, pure mineral spirits will generally remove oil-based stains when the hardest stains are hand-rubbed. If you go this route, however, be sure to thoroughly wash the area with an aircraft safe cleaner to get any residue off. Actually the same holds for any cleaner you use: Water is the universal solvent.

Keeping It Off

Once it's clean, there are two things to do beyond fixing the sources of oil leaks or oil burning. The first is to use a cleaner/polisher product on belly-stain-prone areas to help keep the pained surfaces more resistant to allowing future stains to accumulate -- the same way you use leading-edge polish to reduce bug residues from building up as rapidly or sticking as bad. Here, Rejex Soil Barrier and Stain Protector will do the job. The second method is to not wait until the stain buildup is so bad as to require heavy-duty actions to remove it.

All that said, nothing beats a mechanically healthy engine for keeping things cleaner longer.

For more about maintaining general aviation aircraft, check out AVweb's Maintenance section.