Sunday, January 01, 2012

B-nut Torque and Loosening

 "Failure of maintenance personnel to properly tighten the fuel supply hose at the engine-driven fuel pump." 
Injuries: 3 Fatal.
NTSB Identification: ERA09FA068

It's long been recognized in engineering and among the common man that properly tightened threaded fasteners can become loose. There is an entire industry devoted to making devices that prevent properly tightened threaded fasteners from working loose; lock nuts, lock washers, adhesives of many types, special thread forms.Yet, none of these devices are used on aircraft B-nut connections. (A few aircraft did use lockwire drilled B-nuts, but these are seldom seen).

A survey of aircraft accidents where the B-nut was found loose reveals a bias among accident investigators. Investigators are not investigating why B-nuts may be loose because their bias tells them they already know why; the mechanic didn't tighten it properly. This sloppy and unprofessional work degrades the entire process of accident investigation. Any non-retained threaded fastener can work loose.  It is not acceptable to assume that any loose threaded fastener was caused by " improper torque" by the installer without doing some additional inspection work. There are many reasons why a B-nut can work itself loose: thermal expansion and contraction of the joint, vibration, malformed seating surfaces, etc.

A pilot or mechanic or engineer who fails to learn and repeats a mistake can be the probable cause of a future accident; an accident investigator can also be the probable cause of an accident as he had it in his power to prevent it but did not. 

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