PdM Begins with Condition-Based Maintenance

The origins of predictive maintenance (PdM) begin with condition-based maintenance, which many attribute to CH Waddington, who along with two Nobel laureates, four Fellows of the Royal Society and a Fellow of the National Academy of Sciences Australia had no particular expertise in any kind of maintenance and how to improve equipment availability. In addition to not being mechanical engineers,

“None of us felt committed to any special expertise in Queuing Theory, or Games Theory or Decision Theory or what have you, so we were ready to stick our noses into what everyone told us did not concern us and to follow wherever that led.”  — CH Waddington. author of OR in World War 2 (1973).

And thankfully they stuck their noses into the “intolerably bread-and-butter affair” of organizing maintenance of Royal Air Force Coastal Command 502 Squadron and after a five-month trial applying their recommendations, aircraft availability in

“..the squadron average exceeded the previous maximum by 61% and exceeded the best average of any squadron over a similar period by 79%’.

Before the recommendations were implemented, aircraft were inspected based on planned maintenance schedules as determined by the manufacturer. Some steps required dis-assembling parts of the aircraft so they could be inspected. One of the most unexpected findings was

“The rate of failure or repair is highest just after an inspection and thereafter falls, becoming constant after about 40-50 flying hours.”

And they concluded,

“But the fact is that the inspection tends to increase breakdowns, and this can only be because it is doing positive harm by disturbing a reasonably satisfactory state of affairs.

Predictive Maintenance Thwarts Waddington Effect

The planned preventative maintenance whose purpose was to prevent unplanned failures, was actually creating unplanned failures. The behavior was termed, “The Waddington effect.” It was as if the old parts were jealous of the new parts and chose to ruin it for everyone.

Their advice was to change the maintenance process to be in-tune with the actual condition of the equipment and its actual usage patterns. It was the beginning of conditioned-based maintenance. It was also the beginnings of using economic and probabilistic information to determine inspection cycle strategies which becomes the foundation for predictive maintenance.

It took a scientist, not an engineer, using observational data and a scientific approach to really improve equipment availability by changing how maintenance was planned. Marshal of the Royal Air Force Sir John C Slessor GCB, DSO commented,

“It never would have occurred to me what the RAF soon came to call a ‘Boffin,’ a gentleman in grey flannel bags whose occupation in life had previously been something markedly unmilitary such as Biology or Physiology, would be able to teach us a great deal about our business. Yet so it was.”

So, if you find yourself fixing the same things on the same equipment over and over again, you may be stuck in The Waddington Effect and need to move to a maintenance approach that is more condition-based and predictive. Furthermore, you may need to enlist the help of a modern-day “Boffin” who can bring modeling and probabilistic maintenance methods to your organization.


Peter Darragh | EVP Product Engineering

Peter defines and executes the product roadmap for Mariner.