Exhaust System Care
In Star Trek movies, warp factor means nothing but speed. In our airplanes it means nothing but trouble, as Mike Berry reported in Light Plane Maintenance.
This article originally appeared in the June 2002 issue of Light Plane Maintenance, and is reprinted here by permission.
One of the biggest recurring headaches for the light aircraft owner and mechanic is that of exhaust manifold leaks. With the majority of aircraft in the general aviation fleet exceeding 30 years old, problems with exhaust systems seem to be happening on a regular basis.
While the repair in many cases is not difficult, time spent analyzing the problem is the proper way to prevent further difficulties from cropping up. A long-term solution should be the goal rather than stopgap measures, and this will definitely make your flying safer as well as save money by keeping major repairs to a minimum.
Not all aircraft exhaust systems are created equal, and that makes financial sense. The demands on an O-200 are far lower than on a tightly cowled, turbocharged IO-540. Exhaust metals vary from basic carbon steel to the exotic forms of stainless and even Inconel.
All exhaust systems are characterized by very thin wall thickness in our never-ending efforts to stay light in weight. While we achieve that goal, the price is a constant battle with heat, corrosion and differential expansion -- different parts of the system expand at different rates, promoting leaks and warping metal. This differential expansion is another reason why there are so many slip joints and provisions for system movement.
|Light filing of the flange is OK to make for a perfect fit of the gasket. Too much metal removed will promote warping. (Click top picture for larger version.)|
Problems caused by exhaust manifold leaks are concentrated in four areas. First, exhaust leakage into the engine compartment can introduce carbon monoxide into the passenger cabin through leaks in ducting, firewall seals and poorly sealed areas around doors and windows. Second, erosion of exhaust outlet surfaces on individual cylinders is a common occurrence. Third are the fire hazards associated with exhaust leakage onto induction pipes, primer lines or other areas that contain or transfer fuel.
Also troublesome is the exhaust leakage and erosion problem on mating surfaces of cylinders, which, if allowed to continue, could lead to cylinder cracking and eventual replacement of the cylinder.
Galvanized or zinc-plated tools should never be used around or on aircraft exhaust systems. Additionally, aircraft exhaust systems should not be marked with a lead (graphite) pencil but rather chalk or India ink.
A mark with zinc, pencil lead or galvanized tool is absorbed by the metal of the exhaust system when heated, creating a distinct change to its molecular structure. A change such as this softens the metal in the area of the mark, causing cracks and eventual failure.
Exhaust leaks are usually identified as a flat gray, light brown or sooty black streak on the pipes and adjacent areas. Any discoloration from the adjacent "normal" areas is suspect as a possible leaking area or a weakened area on its way to failure.
A leak is often the result of poor alignment of the two mating exhaust system members such as exhaust flanges mating up to the individual cylinder. Additional leaks may occur at welded or clamped connections and slip joints.
Leakage around exhaust gaskets sometimes may not be easy to detect due to the color and type of metal of the surrounding areas. Keep these areas clean and use a strong light and a mirror in some cases to detect a leak in these mating flange areas.
An old trick used by mechanics to check for leaking exhaust gaskets is to try to slide a feeler gauge (.010" or less) between the exhaust flange and the cylinder surface. If it fits, there's a problem.
|Mounting flanges must be machined perfectly flat if you expect to have a long-lasting exhaust pipe mating.|
The key to correcting exhaust leaks is taking the time to determine where all the leaks are coming from and what might be the best course of action to correct the entire problem. Replacing just one leaking gasket might seem to correct the immediate problem, but often the problem is related to misalignment of connecting stacks or warped flanges.
Repositioning clamps and flanges or loosening clamps or nuts and tightening in a different sequence to align mismatched parts can often correct alignment. Other times a different style or even a mixture of different styles of exhaust gaskets on different cylinders can be used to eliminate leakage if the mating surfaces are fairly even. In most cases the cylinder surface is made of aluminum and will be eroded quickly as compared to the steel exhaust pipe flange to which it mates.
If the cylinder flange is eroded to the extent that an exhaust gasket will not seal properly, the corrective action is to grind the cylinder surface flat again. A special machining tool such as the Pneuport grinder available from Aircraft Spruce can be used while the cylinder is attached to the engine to grind the cylinder flange flat again.
This grinding action will reduce the exhaust flange mating surface to eventually cause another problem -- that of mismatched heights of the cylinder surface as compared to adjacent cylinders. This height-matching problem can be dealt with by using different styles of exhaust gaskets to take up the space left by grinding.
When doing a top overhaul with reconditioned cylinders, try to match adjacent cylinders with exhaust flange heights nearly the same to prevent a mismatched condition.
Many times mechanics elect to remove a severely eroded cylinder for a more thorough inspection for cracks or related repair problems such as broken exhaust studs.
Severe erosion and uneven heating leads to cracks in the exhaust port and cooling fin areas adjacent not easily detected without proper cleaning such as glass bead blasting.
Slightly warped exhaust flanges on the stack side can be corrected with the use of some blue dye to identify the "high" areas and then filing them to an even height for a proper seal. This filing should be limited to a reasonably small amount of material since a large reduction in thickness of the exhaust stack/flange material would weaken the flange and cause warping.
If a stack is warped enough to require a large amount of grinding or filing, the stack or manifold is in need of a professional aircraft exhaust specialist. They can replace the flange and use a jig to set the correct angles. Removal of large amounts of material will cause warping and is counterproductive.
Exhaust Stud Removal
|Not much examination is needed to see this hole -- unless it's out of normal visual range. These things can happen without warning, so check frequently. (Click photo for larger version.)|
While removal of broken exhaust studs has been covered before, this is a problem that is always lurking out there to present itself when working with exhaust systems. Many times broken exhaust studs can be prevented by using penetrating fluid on the attaching nut and a slow easy pull on the wrench as opposed to a quick jerking on the nut to "break it loose." Working the nut back and forth slowly along with penetrating fluid may help to loosen up the nut allowing it to be removed without breaking the stud. Studs that are eroded but not broken are much easier to remove and replace than studs broken inside the cylinder.
Removing broken exhaust studs with an "easy-out" can be very difficult as the metal used in the stud is of hardened material to prevent breakage. The most important factor in removing a broken exhaust stud is to drill into the broken stud as close to a parallel angle with the correct size drill for the "easy-out" to be used. Cobalt drills are suited for hardened steels. A properly drilled stud will allow for successful use of the "easy-out" and prevent damage from drilling into the cylinder itself.
Special drill fixtures, such as the Exhaust Stud Replacement Kit for TCM O-470 and O-520 engines, are available at Aircraft Tool Supply (800) 248-0638. Aircraft Spruce and Specialty (800) 824-1930 sells a two-step stud extractor that has proven to be very useful when some stud piece is still protruding.
The use of heat on the exhaust area near the stud is very helpful in expanding the aluminum, allowing the stud to "free" up and back right out. Use of a propane torch prevents overheating aluminum to approximately 450 to 500 degrees Fahrenheit.
Temperatures higher than this will cause permanent damage to the aluminum. When broken or damaged exhaust studs are replaced, check for the proper replacement sizes, as often these require an oversize such as plus .003 to .012.
The use of the approved aircraft parts (nuts, studs and washers) along with the proper installation torque (40 inch-pounds minimum for Lycoming exhaust nuts) will help prevent studs from breaking in the future. Some aircraft supply houses sell stainless steel exhaust studs as an after market approved part that has worked well.
|Over time, slip joints deteriorate, especially the inner portion. Keep your eyes out for telltale signs of leakage. (Click photo for larger version.)|
There are generally three types of exhaust gaskets in common use today on aircraft engines: the no-blow or spiral wound type, the soft, thin metal type such as copper, and the fiber/copper (formerly asbestos/copper) sandwich type.
While the no-blow types are most popular and do work well to prevent the gasket from "blowing out," these work best on clean, flat, uneroded surfaces because the seal material is somewhat harder than the copper alternatives.
The thin metal copper type has a small beaded area that is used to form the positive seal against the stack flange. These types are assembled interlocked in pairs with the bead facing the exhaust stack flange surface. The soft metal such as copper has more flexibility, allowing the soft material to fill in and seal minor surface defects, as in the case of slightly eroded cylinders.
The fiber/copper sandwich type is thicker, allowing the gasket to fill in where gaps would occur between cylinder flange and exhaust stack flange due to misalignment. The soft metal (copper or similar) also will fill in minor surface imperfections just as the plain copper style would.
The problem with the copper/fiber and the plain copper type of gaskets is that they do eventually blow out, as they are soft material and do not seem to stand up to the continued heat and pressure.
An additional "crutch" used by mechanics has been to use stove-pipe cement to help seal uneven sealing surfaces such as the cylinder surface, and even around exhaust clamps, typically used in Cessnas. While using the cement works to seal up leaks in many cases, you need to be honest with yourself and properly repair the parts if the leaking areas are in poor condition.
|The first step is to check for ADs, Service Instructions and Service Bulletins for exhaust work. This example SB used by permission of Lycoming. (Click photo for larger version.)|
Repairs to exhaust components by local non-aircraft welding shops are frequently not effective in the long term because of the special requirements of aircraft exhaust systems. The composition of the base metal used in aircraft exhaust systems changes to a degree during the service life of the part. This change, together with the task of determining the exact material the original part was manufactured from, makes it very difficult to properly repair some exhaust parts.
In addition to identification, it is very difficult to properly clean the part prior to welding; any impurities that are trapped in the weld will reduce the strength of the weld. Even with good cleaning, proper repair materials that match up with the base metal may be impossible to duplicate, causing the welded areas of different metals to fail again in a short period.
While somewhat costly, most welded repairs should be accomplished by an aircraft exhaust shop that has the proper equipment (jigs) and fixtures and repair material to do the job properly. Some repairs can be accomplished locally; welds that are not done to aircraft quality and fail are not a bargain but a liability. Welds that penetrate through the typically thin wall of the exhaust tubing produce hot spots that eventually weaken the structure.
Failures of exhaust system components are a very serious problem in turbocharged aircraft systems with higher exhaust pressures and temperature; repairs must be properly accomplished the first time by an aircraft exhaust repair facility. These turbocharged exhaust systems require inspections that are more frequent, and any leaks should be corrected before the aircraft is operated again.
Aircraft exhaust system repairs can be expensive, as most repairs by aircraft exhaust shops are actually major replacements rather that patches welded onto existing components. To produce or repair the exact replacement part without distortion and have the part fit correctly means that special fixtures and welding procedures are used for each part. Stick with the pro shops that do this every day and you will be far more pleased with the long-term results and your peace of mind.