July 1, 2000
By Bruce Landsberg
Both aircraft and people require more maintenance as they grow old. As our general aviation fleet ages, pilots often ask when the useful life of an aircraft is over. The easy answer is when the economics of maintenance exceed the value of the airplane. But many of us don't fly based on economics alone and will continue to maintain an airplane well beyond fiscal feasibility. All that matters is that the airworthiness be maintained. Occasionally, however, there are owners who attempt to take more out of a machine than is safe without the requisite maintenance investment, and that can lead to accidents.
Airliners and business jets are built with a certain number of cycles in mind. A cycle is usually defined as a pressurization and depressurization of the cabin. There is no doubt that pumping up the cabin puts more stress on an airframe than does unpressurized flying. It was the catastrophic failure of the cabin roof of an elderly Boeing 737 that alerted the airline maintenance community to this problem. But what about unpressurized aircraft? The DC-3 has flown for more than 60 years, and many light aircraft soldier on after age 40.
Corrosion is a constant problem for aluminum aircraft. If they have not been internally corrosion-proofed and are operated in a wet or salty environment, they will rot much faster than in a dry climate. A business colleague flew a high-time (more than 10,000 hours) Piper Arrow for nearly a decade after it had been a trainer on the Florida coast for a number of years. My friend's steady diet of liver and onions also may have contributed to a corrosive atmosphere inside the cabin that led to an earlier demise of the aircraft. At the last annual inspection the maintenance technician could no longer sign off the old bird without extensive and expensive repairs, so the Arrow became an organ donor with her serviceable parts going to more viable machines. She was deemed unairworthy before becoming unsafe.
I've spent much of my flight time in mature aircraft and they can be perfectly serviceable, but only when the owner is willing to invest substantial sums. The best philosophy is to replace certain items on a regular basis whether they need it or not. Batteries, for example, are good for about three years. Vacuum pumps will generally last for about 500 hours, and fuel tank bladders or cells will generally last 10 to 15 years, provided they are maintained properly. There are always exceptions to average life spans — shorter and longer. Beech, in the pilot's operating handbook for the newest Baron model, listed the life span of the vacuum pumps at 600 hours. All time-limited components ought to be listed where the pilot can easily find the information.
Engines have a defined life, and there are several variables that can affect their longevity. Frequent use is beneficial, since there can be a tendency for the cylinders to rust. Moisture in the air corrodes the interior cylinder wall, which then abrades the piston rings, resulting in high oil consumption. This by itself will not lead to engine failure, but eventually the engine will lose power. There are special cylinder treatments to reduce this, but aircraft are built to fly — so why not enjoy it and help your engine out at the same time? Time between overhauls, or TBO, is recommended by the manufacturer but is not considered mandatory for Part 91 operations. The engine is the part of an aircraft most prone to failure because it is the most complex and under the most stress. In every safety review that ASF has done on a specific model of light aircraft, the engine is at least twice as likely to be the cause of or a factor in an accident than other aircraft-related causes.
An old aircraft can be partially rejuvenated by simply dropping in a replacement engine. It gets challenging when parts are no longer obtainable to overhaul the engine. A good machine shop and mechanic are invaluable then, and the cost will assuredly increase. The engine, of course, is not the only item that wears out.
Do accidents increase as the fleet ages? As a general rule, no. If we look at accidents that were caused by real mechanical failure and not where the pilot mismanaged the fuel or forgot to add oil, the trend appears to be slightly downward. According to the AOPA Air Safety Foundation's database, there were 366 maintenance-related accidents in 1983 and 235 in 1998. Recognizing that the estimate of hours flown is just an estimate, the rate through the years hovers around 1.07 accidents per 100,000 hours flown. This works out to about 14 percent of the annual accident total, with just the slightest downward trend. Fatal accidents due to mechanical engine failure have averaged about 2.5 per month in the past 16 years.
Is this a statistically significant change? Probably not, but we do not have good data on the age of the particular engines that failed. The NTSB does not collect that. The manufacturers do not release data on the mean time between failures. While we are always concerned about mechanical problems, it remains the smallest part of the accident picture. Pilots are still by far the most likely component to fail.
Parking an aircraft outside instead of in a hangar increases the aging process. Safety problem? Not necessarily, but it will increase expenses and it puts the burden on the owner to invest appropriately. Pay hangar rent or pay for more maintenance — this seems to be one of the axioms of ownership. Paint fails more quickly when subjected to the sun, and when that happens the aircraft skin begins to corrode. Water invariably finds its way into some inaccessible crevices, and again corrosion potential increases.
Windows age in the sunlight and start to take on a permanent haze, which makes it harder to see and avoid other aircraft. Has it been documented in a midair collision that an old windshield was a contributing factor by obscuring the pilot's view? Not that we know of, but it seems obvious that a clear view is a safety item.
How an aircraft is used will have some bearing on how well it must be maintained to be safe. Day VFR is the most forgiving environment. If an alternator or a vacuum pump fails, it is an inconvenience. At night or in the clouds the consequences are far more serious. An aerobatic airplane that routinely pulls 4 to 5 Gs needs to be in better shape than the cross-country flier that spends most of its life at one G. Pipeline patrol aircraft routinely take a beating from low-level turbulence. In 1987 a Piper Archer with more than 7,000 hours suffered an abrupt wing separation. The investigation showed fatigue at the wing root near one of the carry-through attachment bolt holes. An airworthiness directive was issued to remove the wings to inspect affected PA-28s and PA-32s with more than 5,000 hours. It was later rescinded when reports from the field showed that to be an overreaction.
Prolonging the life of an aircraft is similar to extending human longevity. Take care of yourself, and chances of living longer are improved. However, with aircraft the role of heredity has not been established — although some of the type clubs will have some strong opinions on that topic. Heredity does seem to play a part in ADs that are issued against a particular aircraft. These frequently grow out of design decisions that may have been made decades ago. As a result of accident investigations and inspections, the FAA may issue an AD against a part that has shown to be a recurring problem. The age of the part frequently plays a role. Stress cracks develop, seals fail, or corrosion begins. The list is endless and frequently is as much a symptom of old age, and possibly inadequate maintenance, as of poor design.
Periodic inspection and replacement of worn or time-limited parts keep small problems from becoming critical safety issues. An aircraft has not been built that doesn't require ongoing maintenance — typically the more complex the machine, the more is needed to keep it airworthy. One of the big maintenance factors is retractable landing gear. It's pretty reliable when properly maintained, but valves, hydraulic lines, pistons, cylinders, uplocks, electric motors, and microswitches, as well as wiring, all wear much more faster than a solid piece of spring steel or an oleo strut. In many aircraft, depending on the design, equipping it with retractable landing gear will add between 12 and 20 knots and about another four to six hours of technician time to the annual inspection.
Belts, hoses, seals, gaskets — just about anything that rubs against something else — will need periodic attention. One gasket or seal that should be routinely replaced is on the fuel tank caps. This is a major source of fuel contamination as rainwater finds its way into the tank — sometimes in significant quantities. Every year tens of thousands of dollars are paid out in insurance claims when a two-dollar gasket would have prevented the problem entirely.
One area that does not seem to have much discussion is wiring. It has come up recently in some airliner crashes. When insulation fails, a fire can result. It's a bigger problem in bigger aircraft with miles more wire and higher current loads, but it's important on any aircraft with an electrical system. Aging plastic insulation, chafe, and flex can all lead to a short circuit or just failure of the affected electric equipment.
Flight instruments wear, and those with the most movement wear the most. Gyros are usually the first to go. Altimeters and airspeed indicators, although two of our most important gauges, seem to last a long time. Engine instruments and fuel gauges also seem to wear well, although their accuracy can degrade significantly. That can be a safety-of-flight item. Is fuel consumption really increasing, or is the tachometer just reading too low?
Sadly, there are some old birds that the owners really can't afford. They are headed for the boneyard, and a few will wind up in accidents. It's hard to know when something is merely tired-looking or on the verge of failure. This brings up annual inspections. You can generate a healthy discussion on whether we need to tear into a perfectly functioning aircraft every 12 months regardless of flight time, just because the calendar says so. The flip side is that many items are caught early and fixed, precisely because the aircraft is getting a good once-over. AOPA has spent considerable time discussing this with the FAA and is participating in a GA aging-aircraft initiative. In the meantime, make sure that both you and your aircraft reach a ripe healthy old age by taking care.
See also the index of "Safety Pilot" articles, organized by subject. Bruce Landsberg is executive director of the AOPA Air Safety Foundation.
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