Some parts must be periodically replaced, regardless of the number of times t

When good parts go bad

It's understandable that no equivalent to your local auto-parts store exists for business jets. Aircraft maintenance records must be documented scrupulously, and the pedigree of every nut, bolt and washer has to be recorded for possible FAA review. So, when it comes time to replace a fuel pump or engine mount-or even a tire-it makes a difference where the part comes from. The people who maintain jets put their careers on the line every time they sign off on a repaired or replaced part.

In the first place, every original part or assembly used on an airplane must be tested and certified to perform its assigned duty under all predictable scenarios. Airplane parts do fail, but it is extremely rare. You can rest assured that the design and materials, right down to the smallest rivets, have been tested to several times the load limits they are expected to endure. And stress tests can predict how long they can be expected to last. Some parts have life limits based on hours of operation. Others are limited by "cycles"-for example, the number of takeoffs and landings, the number of landing-gear retractions and extensions or the number of times a cabin is pressurized and depressurized. Each cycle contributes measurable stress and wear, so certain parts must be replaced well in advance of their expected "mean time before failure" or MTBF.

In some cases, parts must be replaced when a certain amount of time has passed since installation, regardless of the number of hours the aircraft has flown or the number of cycles it has experienced. That's because some parts simply deteriorate over time. Various rubber hoses, gaskets, seals and washers are among those items that must be periodically replaced, regardless of the number of times the airplane has flown. In fact, some parts are known to deteriorate more quickly if the airplane sits idle.

All aircraft have published maintenance procedures that must be adhered to. In some cases, the procedures are specific to the operator-such as an airline that flies in a particularly demanding environment. When I visited the flight department at Corning Glass years ago, it was in the process of establishing its own maintenance procedures for its then-new Dornier Do 328 turboprop twins. The Do 328 was a popular regional airliner, and the manufacturer had established its maintenance procedures based largely on the assumption that the airplanes would be flying multiple flights per day, seven days a week. Working closely with Dornier, Corning rewrote many of the maintenance protocols based on its much lower utilization. The replacement schedule for some parts that had been based on flight hours in airline service had to be recalculated using calendar-based limits.

There is a major source of frustration-even outright anger-between some aircraft manufacturers and operators. It involves parts for older, out-of-production jets. In many cases, a third-party vendor supplies parts to the manufacturer and replacement parts come from the same vendor. But over time, some of these vendors get sold-often more than once. The new parent company might have little interest or motivation in retaining its cordial relationship with the aircraft manufacturer or its customers. Depending on the original contract, the vendor might hold legal rights to the design of the part and the aircraft manufacturer may have little recourse but to pay the asking price. Often, that manufacturer will absorb the added cost to maintain good will with the operator-but not always.