February 1, 1998
By Bruce Landsberg
The maintenance safety record of old airplanes is generally good. The technicians who keep old aircraft aloft generally do an outstanding job, and the statistics prove it. Over the past 15 years, the National Transportation Safety Board has identified maintenance as a probable cause in only about 19 percent of all accidents. However, there are occasional maintenance-related accidents, and they serve as a painful reminder as to why shortcuts cannot be tolerated in aircraft maintenance.
This is a delicate subject because pilots and owners understandably want to control costs and the use of substitute parts and practices is sometimes rationalized. After all, a hose clamp is a hose clamp, right? That, of course, depends on where the clamp is used. Fuel and exhaust systems must be handled with extreme care.
In-flight fires, fortunately, are relatively rare. In the last 15 years there have been 346 in-flight fires in all kinds of aircraft, which works out to an average of 23 per year. This presumably includes everything from cabin fires where a loose cigarette started things off to engine-compartment and fuel-tank conflagrations. The result was 68 fatal accidents, so it's not something to be taken lightly.
Last year a Piper Cherokee was returning from a spring fly-in with a flight instructor, two student pilots, and another passenger on board. The weather was good VFR, and a VFR flight plan was activated. An aeronautically qualified witness noted the aircraft flying very low to the ground — just above the treetops about three-quarters of a mile away. Other witnesses near the accident site heard the engine tone change pitch. The aircraft was observed at very low altitude "banking from side to side, with white smoke trailing from it." The white smoke turned to black, and one witness saw "orange flame coming from the left side of the engine compartment." The aircraft rolled to the left and struck the ground in a near-vertical dive. There were no survivors.
The pilot in command was a 23-year-old flight instructor who had accumulated 400 hours total time and had been flying for about two years. A student pilot in the left seat had received her student certificate 25 days before the accident and had logged about 11 hours of dual instruction. The toxicological tests were negative and carbon monoxide analysis was not performed because of a lack of specimens.
The airplane was found inverted in an open field with the nose approximately 70 degrees below the horizon. The engine appeared to have been running at the time of impact and was buried in the ground up to the accessory case, which is located on the rear of the engine. A post-impact fire destroyed the airplane.
Investigators noted that the lower left portion of the engine cowling, where it attaches to the fire wall, was located 10 feet from the engine and on the edge of the post-crash burn area. An 8- by 18-inch hole was burned through the cowling and adjacent to the hole was an area that showed no soot deposits. This is indicative of an extremely hot flame — greater than 700 degrees Farenheit. Bordering the hole in the cowling was another burn hole in the engine fire wall that measured 2 by 4 inches.
The lower left rubber engine mount was consumed by fire, while the other three mounts sustained minimal damage. Likewise, the left magneto had significantly more fire damage than the right. The handheld fire extinguisher in the cabin was severely burned and damaged. However, the plastic safety tie and seal were found broken and unburned — an indication that the extinguisher may have been used.
The fuel selector and mixture control positions could not be determined because of fire damage, so it is unknown whether the pilot attempted to shut off the fuel.
Of particular interest to the investigators were the exhaust clamps used to attach the muffler to the exhaust stacks. According to the Piper parts list, a special clamp with a pin on the inside is used to hold the muffler in place. The pin penetrates a hole in the muffler and in the exhaust stack to prevent the pipes from separating. The accident aircraft had the appropriate alignment holes in the pipes and the muffler, but automotive-type clamps without pins had been installed.
A 1972 FAA advisory circular states, "The exhaust system often operates at red-hot temperatures of 1,000 degrees or more; therefore, parts such as ignition leads, hoses, fuel lines, and flexible air ducts should be protected from radiation and convection heating by heat shields or adequate clearance." Fuel lines, which run in areas around the muffler and exhaust system, should have protective fire sleeves. According to the manufacturer, "The purpose of the fire sleeve is not to increase the temperature of a hose line but to protect the hose from direct fire long enough to allow appropriate action to be taken." The upper temperature limit of the fuel line increases from 300 degrees to 500 degrees with the use of the sleeve.
A fuel supply line that had a fire sleeve attached showed little discoloration or deformation, but a line leading to the cockpit fuel pressure gauge had no sleeve. According to the NTSB it showed "considerable deformation and exhibited a melting pattern consistent with high-velocity airflow." The implication is that the line was subjected to very high heat that occurred in flight when airflow, and presumably exhaust pressures, were high. Post-crash fire damage and melting would not normally show high-velocity airflow. This also implies that a properly attached fire sleeve might have delayed or prevented the fuel line failure.
The NTSB cited the probable cause as the improper installation of incorrect exhaust clamps by unknown persons, which led to an exhaust system disconnect and a subsequent in-flight engine compartment fire.
The Cherokee was built in 1975 and had flown approximately 7,050 hours. The aircraft tachometer showed only 10 hours of flight time since a routine annual inspection that had been conducted three days before the accident. The maintenance history was normal for a flight school airplane with the required 100-hour and annual inspections.
A year prior to the accident, during the previous annual inspection, the front muffler had been replaced along with other exhaust-system parts. It is possible that the wrong clamps were installed at that time. The exhaust system had to be disassembled and reassembled to complete that work. Several months later a top overhaul was performed that could have required the disassembly of the muffler system, so the clamps could also have been replaced then. After the 1995 annual, several 100-hour inspections were conducted by different mechanics, but no irregularities were noted in the log regarding the exhaust system.
It appears that one of the technicians, either out of ignorance or deliberately, substituted improper clamps, left off the fire sleeve, and returned the aircraft to service. This could have occurred because of parts unavailability or cost. According to another Cherokee owner, the approved clamps are about $30 apiece and can be very hard to find. In his case, he had to have the pin welded onto a clamp and approved. It was expensive and time-consuming. His inspection-authorized mechanic also confirmed that the proper clamps were essential.
The exhaust system is one of the most critical portions of the aircraft. Mufflers, pipes, flanges, gaskets, clamps, fire sleeves, and heat shields are designed to keep the system intact and the 1,000-degree Farenheit gases inside. The inspection-authorized mechanic who performed the final annual inspection just before the accident used a checklist to guide his work. This indexes 150 items to be approved, and each must be initialed as airworthy. He signed off on two critical elements in the engine group: "inspect exhaust stacks, gaskets, etc.," and "inspect mufflers and shrouds." This implies that the offending clamps and the missing fire sleeve were overlooked. FAA records showed that the IA technician was highly experienced and had no prior infractions.
According to one of the accident investigators, the approved clamps and the SAE "warm-type" clamps actually used do not look similar, and it seems likely that they should have been spotted. The automotive-type clamp is used in some warm-air aircraft applications, such as heater and fresh-air ducting, but never high-temperature applications like the exhaust system. It is also troubling that the clamps and missing fire sleeve were not identified during the several inspections prior to the last annual, assuming that the improper work was done the previous year. Missing flight-critical items once is bad enough, but to continually overlook such a potentially dangerous area implies an extensive breakdown in the maintenance management of this aircraft.
According to the FAA rules, the last mechanic to work on an aircraft is the one who takes the responsibility — even if he or she did not install the offending part. This explains why many technicians are particularly fussy about checking things — their certificates are literally on the line. A typical sanction is a 90-day suspension for the first offense.
There is a natural desire to save money. To some, a hose clamp is a hose clamp. As we've seen, it ain't necessarily so. To maintain an increasingly elderly fleet, everyone from pilot to technician should clearly understand that there are no safe maintenance shortcuts in critical systems. Compare the great risk to the relatively small savings.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.
Aircraft Power and Fuel,
FAA Information and Services,
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