unk-unk applied to Aircraft Inspections

"There are known knowns; there are things we know we know. We also know there are known unknowns; that is to say, we know there are some things we do not know. But there are also unknown unknowns – the ones we don’t know we don’t know." Donald Rumsfield, Secetary of Defense

Inspecting an airplane using best industry practices, applied by consensious and trained mechanics, with help and guidance by our regulator authorities, one is still left with rumsfeldian unknowns; Something still might be wrong.

When something does go wrong, often the mechanic is blamed because "he should have caught it". Before this claim can be made one has to determine what the mechanic knew and did not know and why. It could be:

1. He might have known but ignored the condition (known-known).
2. He might have known that a condition could or did exist (corrosion for example) but its severity or  signifigance was unknown. (known unknowns)
3. He might simply have not known about the condition and therefore it never occured to him that there could be a problem. Example, might be an undisclosed manufacturing defect.  ;This is the unk-unk (unknown-unknown). "I never heard of that problem."

The proper response depends on which of these three catagories the problem belongs to. If the problem falls into the unk-unk then the problem is not a mechanic issue.

unk-unk is a slang engineering term thought to have originated at Lockheed. The problem of "we don't know what we don't know" is a trap anyone doing maintenance can fall into, not grasping the significance (or danger) of an action or inaction. The best defence against unk-unk problems is to follow well established procedures and standards even though "It's always more fun to go off rapidly on your own and invent your very own personal mistakes rather than look up and actually study somebody else's stuffy reference book."

"if it flew in it will fly out" -- Feynman's Criteria

If one is asked to sign-off on safety, based on "not having failed yet," in the presence of deviations in performance, then I suggest you read  Feynman's Appendix to the Rogers Commission Report on the Space Shuttle Challenger Accident . Feynman^1. outlines the classic tension between managers and technicians (engineers or mechanics).

This is what Feynman had to say in regards to the success of flights in the presence of deviations:

 "The acceptance and success of these flights is taken as evidence of safety. But erosion and blow-by are not what the design expected. They are warnings that something is wrong. The equipment is not operating as expected, and therefore there is a danger that it can operate with even wider deviations in this unexpected and not thoroughly understood way. The fact that this danger did not lead to a catastrophe before is no guarantee that it will not the next time, unless it is completely understood. When playing Russian roulette the fact that the first shot got off safely is little comfort for the next."

Metal particles from oil filter

Metal particles in the oil filter is evidence that the equipment is not operating as expected. Given the finding of metal shavings in this aircraft engine oil filter, using Feynman's criteria, should this aircraft be removed from service until a more through understanding of the problem is identified?



1. Richard Phillips Feynman was an American theoretical physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics (he proposed the parton model). For his contributions to the development of quantum electrodynamics. http://en.wikipedia.org/wiki/Richard_Feynman

 "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." Richard Feynman's famous conclusion to his report on the shuttle Challenger accident: