Inside the exhaust system it's tubular hells
Exhaust systems ought not to be ignored. Recent crashes involving turbocharged twin Cessnas have once again shone the spotlight on the perils of downplaying exhaust-system maintenance. To illustrate the point, consider a letter sent in April by the Cessna Pilots Association to owners of turbocharged 300- and 400-series twins that points out a chilling statistic. "In the last 29 months, at least 29 people have died in a dozen fiery crashes … in which the failure of the exhaust system is known or suspected," the letter said.
In one fatal accident in 1995, a Cessna 310R suffered an exhaust-system failure that resulted in an engine fire. According to the National Transportation Safety Board report, "the last 8 to 12 inches of the right overboard tube had separated circumferentially from the exhaust system." Also, "the exhaust stack revealed evidence of fatigue … [and] numerous hot gas corrosion cracks … and there was evidence that a breach of the right exhaust stack allowed hot, high-velocity gases to escape and initiate a fire." Other, even more grisly accidents have seen this under-cowling conflagration spread to the wing and fuel systems before the hapless pilot could maneuver the airplane to a landing site.
These accidents have set in motion an interesting chain of events. It's well understood that the turbo twin Cessna exhaust system is prone to failure — to be fair, much like many turbo setups. A 1975 airworthiness directive addresses some of the problems, calling for 50-hour repetitive inspections on the stainless steel systems and placing life limits on certain components. AOPA records show that 11 years later the NTSB issued a inspections or scheduled replacement of some of the components; at the time, the FAA felt that the existing AD was sufficient. A decade later, the NTSB called again, this time asking for repetitive inspections every 100 hours that would entail disassembly of the exhaust system. The FAA agreed that the existing inspection protocols weren't doing the job and so informed turbo twin Cessna owners that it was considering a new AD to supersede AD 75-23-08.
The notice of proposed rulemaking on this topic is expected soon. Among the aspects of the AD the FAA is considering for big changes are these: Making the current 50-hour inspection a 100-hour event, but with a pressure check (the current requirement is strictly visual); restricting weld repairs of the exhaust system to designated facilities; and calling for removal and inspection of the parts aft of the slip joints (components that are difficult to inspect in situ and that are believed to be responsible for many of the in-flight troubles) after 2,000 or 3,000 hours and service every 500 hours thereafter.
AOPA is actively supporting CPA's recommendations. Both associations endorse the 100-hour pressure check, do not oppose the limitations on who may weld-repair the exhausts, but they oppose the repetitive disassembly portion on the grounds that complying with this potential AD would create more problems than it would cure.
Some mechanics point with criticism to the very configuration of the Cessna system, in which the turbocharger and its associated plumbing is not directly bolted to the engine. In fact, these airplanes locate the turbo on the airframe, necessitating complex and highly stressed slip joints and ball joints to accommodate relative movement of the engine in the airframe. But much evidence points to inadequate maintenance as a major culprit here. Corporate operators who follow rigorous inspection procedures reportedly get excellent service from their turbocharged twin Cessnas.
Admittedly, this burn-and-crash scenario is a dramatic and uncommon conclusion to exhaust-system neglect; airplanes are hardly falling out of the sky by the case lot because the exhausts have been ignored. But as the fleet ages and owners try to assess ways to keep the rising cost of periodic maintenance from looking like the DNC budget, understand this: The exhaust system is not the place to scrimp.
It's no wonder the exhaust system gives problems in the first place. In most airplanes, the materials used are the lightest possible, designed to fit in tight spaces, cost as little as possible (for the original manufacturer), and are exposed to extremes of temperatures as well as punishing vibration. In fact, the exhaust system plays several roles. Besides ducting the hot exhaust gases to the atmosphere (or turbo system) in the most efficient way possible — and this measure may be biased toward packaging efficiency more so than for better breathing of the engine — the exhaust systems must protect under-cowl components from the heat. When the system fails, blowtorch-like gases can bear on delicate components, oil or fuel lines, and structural members, producing serious consequences.
Low-power airplanes use stainless steel exhaust systems, while some higher-power, turbocharged models employ pricey (but much more temperature-tolerant) Inconel steel. The physical properties of the most common stainless steel systems are compromised at temperatures above 1,600 degrees Fahrenheit; the reduction in strength with additional temperature is quite rapid. You might not know this, but, for the most part, exhaust systems in normally aspirated airplanes are built by the airframe maker, while most of the turbo installations are constructed by the engine manufacturer — there are some notable exceptions, however.
And yet it seems to be the attitude of many owners that the exhaust system should last the life of the airframe with little more than a tap on the muffler and a tightening of clamps now and again. Moreover, most modern airplanes' cowlings are designed so that a detailed look at the plumbing during preflight is next to impossible. The best you can hope for is to tug on the tailpipe and take a cursory look for gross leaks.
You might say it's fortunate that the common exhaust maladies are well known. For the simple singles — non-turbocharged, modest-power installations in particular — the critical element is the integrity of the exhaust pipe under and around the heater muff. The vast majority of light aircraft receive hot air for cabin heating and defrosting from a shroud wrapped around a section of exhaust pipe. To promote transfer of heat from the pipe to the air, these muffs employ welded-on swirls or pegs whose job it is to increase the surface area of that portion of the pipe. Over time, cracks can form under the muff, or the welds holding the swirls or pegs in place can crack — tearing away part of the exhaust pipe as well. (The weakest part of any weld is the material along the bead's border.)
Unfortunately, this part of the heater system is hidden from view, even during a cowling-off preflight or service call. And the dangers from this heater system's leaking exhaust gas — rich in carbon monoxide (CO) — are well known. A Piper Dakota that crashed last year was found to have a breach in the exhaust system, which contaminated the cockpit with CO. All aboard presumably passed out and perished when the airplane crashed.
Two items will help to prevent such accidents. First is the presence of a carbon monoxide monitor in the cabin. Many pilot shops sell cards with CO-sensitive dots that will change color in the presence of elevated CO levels. These are effective and inexpensive devices for catching CO intoxication before it knocks you out; but you should note that they have limited useful lifespans and should be replaced when the due date comes up. There are also electronic CO sensors available.
The second part of the equation is consistent inspections. For some aircraft, the heater muff (and entire exhaust system for that matter) must be inspected and tested periodically to comply with the pertinent AD. (For some aircraft with original exhaust components, this could be as often as every 50 flight hours.) But even if your installation is AD-free, you should ensure that the entire exhaust system gets a thorough examination at the annual or 100-hour inspection.
John Frank, technical director of the CPA, says, "A lot of attention has been focused on the twin Cessna exhaust systems, and for good reason, but many of the other models deserve a closer look than they may be getting." While it's true that most A&Ps work hard to inspect the exhaust system at the annual, says Frank, "We're seeing evidence that this diligence is not universal." Rather than additional regulatory measures, Frank and company would like to see shops voluntarily pressure-check systems at regular inspections. "It's an easy test to perform," says Frank. "And it will tell you a lot about the integrity of the exhaust system." Pressure testing is usually performed by attaching the outlet side of a shop vacuum to the exhaust pipe — the point is to develop a slight pressure differential — and spraying a solution of soapy water on the exhaust components. Bubbles will appear at the source of leaks.
The Wall Colmonoy Corporation manufactures and overhauls exhaust components, and was a pioneer in high-temperature alloys in the late 1930s. (The root name of the company — Colmonoy — stems from the names of inventors Normal Cole and Walter Edumonds, linked to the term alloy.) According to Sue Seckel, the company's sales manager, the most common problems noted on incoming parts include blown baffling in mufflers, general cracking and bulging, and poor repairs. "We see some weld repairs that have obviously been left in place too long," Seckel says. The idea of performing a weld repair on a crack is to get you home, she says. "It is not a permanent repair." As the fleet ages, Seckel says her company sees more and more components that have been repaired too often. "At some point, you must simply replace components, because the deterioration is too great."
And while the thought of having to ante up for a new set of pipes may give most owners the night sweats, at least the service is available. "We can overhaul systems for about 95 percent of the fleet," says Seckel. "For 75 percent of the airplanes, we can produce a good number of new parts. More importantly, we have the jigs and dies that allow us to bring the exhaust system back to original specifications." That's an important concern for the remaining life of the parts; an exhaust system that fits poorly or that has been knocked out of shape through repairs will quickly self-destruct when returned to service.
Bearing in mind that an extensive examination and a pressure test are a potent pair in determining the health of your exhaust system, here are the common maladies you'll be looking for.
Flange frailties. Where the exhaust system meets the cylinders is a violent place. The aluminum head grows and shrinks at a different rate than do the steel exhaust pipes, and so the flange that mates the two sees plenty of movement. The poor gasket trying to keep the glowing exhaust gases directed down the pipe faces a difficult life. Blown gaskets are, unfortunately, pretty common. Be looking for exhaust residue on the adjacent surfaces — usually powdery, it can be gray, red, or black in color. Sometimes the exhaust comes out with such force that there's not much residue right on the leak, but plenty is on the cylinder fins a few inches away. A few installations — like the turbocharged Lycomings with the two-bolt flanges — are more prone than others to blowing out gaskets. Take your bright light and mirror to look all the way around the port. Remember, too, that you cannot reuse the flat or beaded gaskets, but you can return the "blow proof" spiral-wound models to service if there's no evidence of a leak.
Paying attention to flange leaks now will save money later, because a blowout gone unfixed will eventually erode the softer port face. Only a certain amount of refacing is allowed — Lycoming says no more than 20-thousandths of an inch — so a longtime leak could cost you a cylinder. Most often, flange leaks are the result of improper torque on the hold-down nuts. Too much and the flange warps; too little and the seal will loosen through the normal expansion and contraction cycles.
Slip joints. Because the engine "grows" slightly as it warms, most exhaust systems are built with slip joints. These connectors use no clamps per se but are designed to seal themselves when the exhaust system reaches operating temperature. That's why it's normal to see some exhaust residue around the slip joints — they will leak slightly when cold — but there should not be an excessive amount. A great degree of leakage will lead to fouling of the joint with exhaust byproducts, eventually rendering the joint solid. At this point, the slip joint will start experiencing localized cracking at the highest stress points.
Loose connections. In the same vein as the flange inspection, look at all clamped connections for evidence of leakage. A broken connector will certainly not heal itself and may well transfer loads to another part of the exhaust system, leading to more cracking and deterioration.
Internal erosion. Most of the pipe decay takes place from within, so it's worth borrowing a borescope to examine any parts of the exhaust system you can gain access to.
Muffler mayhem. It's common for internal baffle and flame tubes to come dislodged in the muffler. They can create additional troubles by banging around in the muffler and causing cracks. In the worst case, the offending piece will lodge itself in the muffler outlet, dramatically increasing back pressure and stopping the engine cold. A sharp rap on the muffler with your palm will often disclose loose internals, but a visual inspection is also a good follow-up.
Fatigue failures. Exhaust-system metal works hard. So you should be looking for cracks — around any parts of the system that are welded as a matter of course, and particularly on welded repairs. Look for bulges and distortions; often these problems are related to localized overheating, but they may also be an artifact of simple old age.
In many ways, age is the issue. Just how long should an exhaust system last, anyway? Once again, it depends upon the application. The pipes on a Piper Warrior ought to go a couple of engine TBOs before needing significant work, but something on a Cessna 414 should, according to several sources, be sent off to an approved exhaust-system repair facility at each engine overhaul.
Regardless of what you fly, it's clear from the accident reports and feedback from the maintenance world that pilots and owners should be paying more attention to the exhaust system. Like wing-spar bolts and connecting rods, the exhaust is not a component to take lightly where preventive maintenance is concerned.
E-mail the author at [email protected].
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