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Ask Marilyn: More Information on Pre-Flight Testing
John Hazlet of Pasadena, California, writes:
Marilyn: A few comments on your column with a question from a commercial pilot regarding repetitive preflight testing of aircraft equipment and systems. (November 11, 2012)
1) Some time ago, airlines began to recognize part of the basis of the question—that in some cases, you actually contribute to wearing out components by repeatedly cycling them during preflight tests, resulting in more risk from wear-induced failure than from an undetected malfunction. To address this, airlines adopted tests of certain systems at longer intervals, such as once per trip, once a week, or by maintenance personnel (rather than flight crew members) at a calculated frequency that confirms the system's proper operation but balances malfunction risk against wear-generated failure.
2) A major curse word in airline operations is "unscheduled maintenance." Due to airlines' desire to avoid unsafe conditions, costly delays, and irritated customers, analysis of the service lives of aircraft components and systems has become something of an art. Data analysis can often predict with reasonable accuracy how long a valve, relay, widget (or the switch that actuates it), will last. The components are then assigned a service life figure and replaced with new or reconditioned units before they reach the point that probability of failure increases. The service lives are also subject to ongoing study: If it is determined that component X never fails with a service life of 1,000 hours, the airline may submit data to the FAA requesting that the interval be raised. Likewise, if they encounter premature failures at 1,000 hours, they'll request that the interval be reduced. Then this interval is analyzed to ensure that the subsequent failure or malfunction rate is within acceptable limits. So, in a perfect world, that theoretical push button in the question would get changed every 7000 cycles and never fail. Perfection is hard to achieve, of course, and the button may still fail very occasionally, but the failure rate will have been significantly reduced. And they will keep trying to reduce the rate further—maybe even designing a better push button.
3) Passengers don't like to hear this, but ask any experienced pilot, "When was the last time you flew a "perfect" aircraft, nothing whatsoever wrong with it?" If they're honest and were paying close attention to their preflight inspections and in-flight operation, the answer will be "never." There's always something wrong, even if it's as minor as one tiny light bulb that illuminates an instrument (when there are two other ways to ensure that instrument is adequately lighted). For this reason, your comment, "...you can't fly the craft again until you... fix it," isn't quite correct. Airlines can use FAA-approved Minimum Equipment Lists, Nonessential Furnishings Programs, Carryover Items Programs, and manufacturer's Configuration Deviation Lists, etc., which allow the airplane to continue in service under controlled conditions and time or flight cycle limitations, with certain items inoperative or removed, provided procedures are followed, ensuring an equivalent level of safety.
So, when your reader pushes the button to test component X during his preflight inspection, and it doesn't test properly, he'll call the company's line maintenance department, and they may: (a) fix it, if it can be done without causing a delay; (b) delay the flight until they can fix it; (c) defer it and keep the plane in service if their maintenance programs and policies allow it—and if the captain will accept the aircraft in that condition; (d) tow the craft off the gate and bring in another one (if they have one); or (e) cancel the flight.
Whew. Sorry to wax so prolix, but having recently retired after nearly 50 years in the aviation business, accumulating more than 20,000 flight hours in large and small airplanes, serving as director of operations for three companies and a stint as vice president of maintenance for one, and working as a captain, training and check captain, and an FAA-designated airline transport pilot examiner for decades, I felt the urge to weigh in.
Thank you, John. I'm happy you just couldn't resist!