Maintenance and Certification

Maintenance and certification

Exhaust Systems: Inspection and Maintenance Tips

by Joe Escobar, Editor, Aircraft Maintenance Technology (AMT) on-line magazine (www.amtonline.com). This article originally appeared in the July 2005 issue of AMT Magazine and is reprinted with permission.

An aircraft's exhaust system is critical to flight safety. Defective exhaust systems can lead to carbon monoxide poisoning, fire, or loss of engine performance. There are some tips that can ensure you are properly inspecting and maintaining these systems. I talked to Tom Heid, President of Aerospace Welding Minneapolis Inc. (AWI) to learn some of these tips. Heid is an A&P who is very familiar with exhaust system inspection and repair. Here are some pointers he shared during our conversation.

General inspection tips
Before inspecting the exhaust system, be sure to remove all shrouds and shields from the muffler and stacks to permit full inspection. Some mechanics get in a hurry and instead of removing the shroud, they will open it up, split it open manually, and just kind of look around in there, and then close it back up. Heid has several examples of cracks and deformities in mufflers that wouldn't have been caught if the shroud were only partially removed.

During inspection, you want to look for signs of leaks. Inspect the surface areas of components next to the exhaust system for signs of exhaust soot. Also look for signs of leaks on the exhaust system itself. Leaks will appear as a yellowish or orangish powdery residue. Any time you have that kind of discoloration in an area of an exhaust part is a good telltale sign that you have a leak. You want to pay particular attention around welds, clamps, and flanges.

Another way to find leaks is by performing a pressure test. Refer to your maintenance manual for detailed procedures of a pressure test. In general, to do a pressure test, you insert an air source such as a shop vac (in reverse mode) or regulated shop air in the tail pipe and pressurize the exhaust system to about 3 to 5 psi. Be careful not to overpressurize the system as exhaust system and/or engine damage can occur. You can then spray a soap and water solution on all the joints and the system in general to make sure there are no cracks, pinholes, or any excessive leaks at the clamp or slip joints.

Exhaust system
Proper exhaust system inspection is critical to safe operation.

You also want to inspect all surfaces for metal fatigue. This will be indicated by bulges, distortions, or cracks. Examine bends in pipes for pitting and thinning of material. You can use an awl to probe material in suspected weak spots.

Use a flashlight to shine into pipes for inspection. You can also use a borescope to examine internal components.

Inspect for damaged or missing heat studs, fins, or other heat-sink material. These defects can cause uneven heating of the muffler surface and lead to holes in the muffler can.

Look to see if the muffler has internal baffles or tubes. If the baffles are damaged or missing, repair or replace the muffler. Broken baffles may become dislodged and restrict the outlet and cause power loss.

Inspect internal areas where possible for wear, pitting, cracks, and broken baffles. Corrosion may be occurring on a component that looks good externally.

Installation tips
AWI offers the following installation tips for exhaust systems.

  • Don't force fit any parts: cracking will occur and shorten component service life.

  • Do not reuse gaskets.

  • Make sure that all parts are properly aligned. First, loosely mount on aircraft, then tighten all connectors to OEM specifications; retighten after a hot run.

  • Use an anti-seize compound rated to at least 1,400 F such as Bostik Never-Seez® or Loctite C5-A on all slip joints.

  • Inspect all hardware and clamps for wear, pitting, or heat stress. Replace as necessary.

Turbo 182
On a Turbo 182, unlike other exhaust systems that have a turbo system installed, there is no support bracket for the turbo. All of the weight of the turbo rests on the exhaust header (or Y assembly as some people refer to them). This puts a lot of stress on that header. There have been several of these exhaust headers that crack and break. This can cause an in-flight fire, and it is an area that needs to be inspected carefully. It is an extreme safety factor.

Repairs
Since many mechanics don't have the tungsten inert gas (TIG) welding equipment, expertise, or comfort level to do an exhaust system repair, sending it out for repair is a common practice. There are repair stations like AWI that specialize in exhaust system repairs. As an alternative, some mechanics choose to take the exhaust part to their local welding shop to have it repaired. If you are having a local shop do the repair or if you are tackling the repair yourself, there are several things you need to know to help ensure you get a good repair.

Alignment. Proper alignment is important when repairing an exhaust system component. Most repairs need to be done in a jig in order for the component to fit properly during re-installation. Not using a jig can cause improper alignment, setting up stress after installation that can damage the part.

Lack of experience. Many welders don't have experience working with aircraft exhaust systems. Exhaust systems are comprised mostly of 321 stainless or 601 or 625 inconel. There aren't many other things that are made out of these alloys, and most general welders don't have the experience of working with them. Even if the welder has welded stainless before, it is not the same as 321 stainless, which requires a specific rod and specific techniques. Using the wrong rods coupled with wrong procedures will result in a weak joint.

Proper cleaning. Thorough cleaning of the part is critical. The outside of an exhaust system component is typically dirty with oil and other deposits on it. But just cleaning the exterior of the part is inadequate. The inside of the part is full of carbon deposits left behind from burnt fuel and fuel additives. As soon as you start welding, the crack opens up from the heat, and that contamination from inside the part is pulled right through into the weld puddle creating a weak weld. So the part needs to be thoroughly cleaned inside and out before welding.

Purging. A final tip for welding is to ensure the part is purged when welding. Purging is the process of providing a separate source of argon to the inside of the part. This pushes all of the air out of the part and creates a pure argon atmosphere inside the part. This pure argon atmosphere helps pull the weld puddle through the crack during the welding process, and the resulting weld is as clean on the inside as it is on the outside. Not purging will cause oxidation of the weld puddle on the inside, creating a rough, jagged bead. Not only is this a weaker weld joint, but the jagged edges will disrupt the gas flow creating hot spots that will set up spots for future failure.

Stainless vs. inconel
Another thing that mechanics need to be aware of is that exhaust system components can be manufactured out of either stainless or inconel. These two materials are similar in appearance and can be difficult to differentiate without chemical tests or destructive (grinding) analysis. It is important to realize that these two materials have different characteristics that affect the visual indications of a pending failure.

Over time, stainless steel tends to deteriorate. The molecules of the metal start to break down, and the metal starts to stretch, bulge, and deform. This is a good visual indication that the part is close to failure and requires repair or replacement.

Because of this susceptibility of stainless to weakening and bulging over time, some engineers decided to go with a stronger metal that could withstand the heat and prevent these bulging failures. They chose inconel, a metal in the stainless-steel family that has more nickel and chrome in it, allowing it to withstand higher heat. Typically, inconel does not bulge and deform like stainless does. But what it does do is pit out from the inside. The metal properties of inconel aren't very compatible with the mineral deposits that are left behind in AvGas. This causes severe pitting, almost like a cancer. Heid notes that 95 to 98 percent of the inconel parts that come through their door are severely pitted out.

The bottom line is that as a mechanic you may not know what material you are inspecting. Just because there are no cracks or bulges on the system, doesn't mean that there aren't any defects. You could be looking at an inconel part that is pitting from the inside.

Proper exhaust system inspection is critical to safe operation. By knowing proper inspection procedures and ensuring your parts are being properly repaired, you can do your part to ensure the aircraft doesn't experience an exhaust system failure.

Inspection tips
AWI has produced a free catalogue titled Aircraft Exhaust & Engine Mounts. It shows all the applicable part numbers for major components and extras like gaskets and clamps. The catalogue also points out specific areas where special attention should be paid during inspection. To request your copy of the catalogue, you can call (800) 597-4315 or you can download it at
www.awi-ami.com.

Beware of backfires
Engine backfires are extremely hard on exhaust systems, especially mufflered systems. Backfires stress the entire exhaust system in a very abrupt, severe manner. They can damage baffles, possibly breaking them loose. Backfires can also bulge or crack the can. Any backfire warrants a thorough inspection of the entire exhaust system.

Additional resource
Aerospace Welding Minneapolis Inc.
(800) 597-4315
www.awi-ami.com

Fuel Starvation Due to Fuel Selector Condition

The following is an Aviation Safety Information Letter from the Transportation Safety Board of Canada (TSB)

On September 12, 2006, a privately operated Piper Cherokee PA-28-180 was in the circuit pattern for Runway 06R at the Montréal/St-Hubert, Que., airport. On base leg, at about 700 ft above ground level (AGL) and prior to landing, the pilot changed fuel tanks and selected the electric fuel pump to “ON” in accordance with the aircraft approach checklist. After a few seconds, the engine lost power. The pilot elected to turn off all electrical switches and execute a forced landing into a busy intersection. On final approach for the forced landing, the aircraft vertical stabilizer struck a telephone pole retaining wire, the right wing struck a vehicle, and the aircraft flipped over, striking three more vehicles before coming to rest. The vehicle occupants, pilot, and aircraft passenger sustained minor injuries. The TSB classified this accident as a Class 5 occurrence (A06Q0160).

When the aircraft was recovered, the fuel selector was found positioned midway between the right and left fuel-tank selection positions. Testing of the fuel system with the engine in the accident aircraft running revealed that, by positioning the fuel selector in the midway position, the fuel flow to the engine was reduced to a fuel starvation state, followed by engine stoppage. Examination of the fuel system revealed no anomalies, with the exception of the fuel selector. The fuel selector was very difficult to move, and the detent positions were barely perceptible. The condition of the fuel selector valve assembly was consistent with a component that was not maintained in accordance with the manufacturer’s maintenance recommendations.

The fuel selector valve installed in the accident aircraft is a three-position type valve. An integral part of the valve is a tapered plug cock. This tapered plug cock, unless properly lubricated, is subject to binding or “freezing” caused by fuel coming in contact with the plug cock and gradually dissolving the film of lubricant, by the presence of foreign material, or by hardened or congealed lubricants, usually of the wrong type.

Fuel selector, as found
Fuel selector, as found

On June 5, 1972, Piper Aircraft Corporation issued Service Bulletin (SB) No. 355, titled Fuel Selector Valve Lubrication. SB No. 355 is applicable to Piper aircraft model PA-28-180 (serial numbers 28-1 to 28-7105179 inclusive), as well as to other small Piper aircraft. The primary objective of this SB is to ensure that the fuel selector valve is periodically and properly inspected and lubricated. The compliance time was within 10 hr of operation of the effective date indicated on the SB. The inspection/maintenance provisions of this SB were to be repeated at: each 100-hr interval, until the aircraft reached 400 hr of operation; then every additional 400 hr of operation or annually, whichever occurred first; or whenever the fuel selector valve was difficult to operate. This SB had not been completed on the occurrence aircraft, nor was the owner aware of its existence.

In Canada, owners of small, non-commercially operated aircraft may use the maintenance schedule provided by Transport Canada (TC) in Canadian Aviation Regulation (CAR) 625, Appendix B, Part 1 and Appendix C. Alternatively, owners could use the aircraft manufacturer’s maintenance checklist, if available, provided that this checklist includes at least all the applicable items listed in CAR 625. The tasks listed in the TC maintenance schedule are described only in general terms; whereas, the maintenance checklist produced by the manufacturer is detailed and includes references to the applicable service letters and SB produced by the manufacturer. The occurrence aircraft was maintained in accordance with the CAR 625 maintenance schedule.

The CARs clearly state that the maintenance of an aircraft is the responsibility of the owner. Therefore, should owners choose to use the less-detailed maintenance schedule in CAR 625 to maintain their aircraft, they are still responsible for developing an appropriate checklist for use with the maintenance schedule, and for being aware of any additional maintenance items such as out-of-phase items, service letters, SB or Airworthiness Directives (AD) that may apply to their aircraft.

SAR Flash

by Major James Pierotti, Officer in Charge, Joint Rescue Co-ordination Centre (JRCC) Victoria

Recently, a Cessna 140 with two people on board was transiting northern British Columbia. The weather was beautiful and the aircraft was in great shape, so what could go wrong? In a particularly remote, heavily forested area, a flock of birds rose up, right in front of the aircraft. Despite manoeuvring, one bird struck the air intake and shut down the engine. The pilot did an excellent job of controlling the forced landing, and settled into the trees with only minor injuries to both occupants.

At this point, the full scope of their troubles became evident: they had not filed a flight plan; they did not have an emergency locator transmitter (ELT); and they had very limited survival gear. Fortunately, a concerned aviator at one of their last stops had noticed their lack of emergency beacon and had loaned them a portable distress beacon in the hopes that, if anything really bad happened, they would have some method of alerting the search and rescue (SAR) system.

After the crash, they made hourly calls on 121.5 MHz on their still functioning radio. Unfortunately, there was no one to hear their calls on 121.5 MHz. After some difficulty with an unfamiliar device, they were able to activate the distress feature on the portable beacon, and the emergency message was heard and relayed to JRCC Victoria. Had they been injured more extensively and unable to do so, these two aviators would likely have died out there because no one knew to look for them.

You are probably saying to yourself that this could never happen to you because you always file a flight plan and have a functioning 121.5 MHz ELT. Remember that the 121.5 MHz ELT requires high flyers to hear the signal, so it can take a long time in a remote area; in the case above, the hourly calls on 121.5 MHz did not produce any help.

Our plea to you is to always make sure you file a flight plan or itinerary, and to strongly recommend that you fly with a properly registered 406 MHz ELT, so when everything works against you, you have a tool that automatically sends the cry for help automatically for you.

 

Transport Canada’s Safety Management Systems (SMS) Information Session

Montréal, Quebec, Fall 2010

www.tc.gc.ca/eng/civilaviation/standards/sms-info-menu-638.htm

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