“There are known knowns. These are things that we know that we know. There are known unknowns. That is to say, there are things that we know we don’t know. But there are also unknown unknowns. There are things that we don’t know we don’t know.” Former Defense Secretary Donald Rumsfeld disclosed this intelligence on Feb. 12, 2002. Rumsfeld’s focus was Iraq. But, as far as aviation maintenance managers are concerned, it might as well have been airplanes.

“Any way you look at them, non-routines are an unknown,” says Yossi Zohar, deputy manager of Bedek Aviation Group’s aircraft division. “In some cases…they become the major portion of the maintenance event.”

Predicting the Unpredictable

Ironically, as aircraft become more digital, the ability to predict when a component will go down becomes more of a problem. “Mechanical equipment will tend to wear out, and failure will occur on a more predictable basis,” says Marc Wilson, director of safety, quality and asset management for Morten Beyer & Agnew consultants. “Failure probability with an electronic piece of equipment is essentially random."

Certainly ACARS, the Aircraft Communications Addressing and Reporting System, has mitigated the problem since the 1990s, especially out on the line. Digital datalinks “allow the maintenance staff at a destination to plan an unscheduled event and take the required actions to potentially avoid an AOG,” says Frank Martin, a principal with Seabury Aviation and Aerospace. ACARS was a quantum leap in easing the impact of “unknown unknowns,” but that doesn’t mean they have gone away.

Bedek models heavy checks and assigns non-routine to routine ratios, pegging its predictions to aircraft type, age and utilization. It allocates a Boeing 737NG 0.25 non-routine man-hours to each man-hour of routine work. By contrast, a Boeing 727-200 might consume two hours of non-routine work for every hour of routine maintenance.

Oliver Wyman associate partner Brian Prentice cites experience that mirrors Bedek’s. “Fully half of the work done within a letter check is responding to non-routines,” he says. He has worked with some operators flying aged aircraft where as much as 70% of the ultimate work package is non-routine. He also has worked with carriers fielding young fleets with 10-15%. “But, on average, 40-50% of all work done in a hangar is non-routine.”

Bedek assigns an approximate non-routine/routine formula to its customer’s aircraft for a couple of reasons: to help better apportion resources, and to ease friction with customers. “I plan the workload and the hangar slot, assuming that those non-routines…will apply,” says Zohar.

Then there are relations with operators. He says non-routine ratios render them less contentious. Whenever it can, the MRO includes “a certain portion of non-routines…into our fixed price,” says Zohar. Bedek puts 50%, perhaps more, of the non-routines into that column. He says the idea is to “move the work ahead, rather than bargaining for each hour of non-routine.” The idea of negotiating a full hour to determine the price tag of an hour of labor is “absolutely illogical and worthless,” maintains Zohar.

What is worthwhile is the experience gleaned from going over scores of aircraft possessing similar characteristics. That’s how Bedek, and others, have traditionally planned for non-routines. “We…accumulate more and more events and learn [from] those events what will be the behavior of the next event,” Zohar says.