NTSB Safety Alert SA-028 - Proper Use of Fiber or Nylon Self-Locking Nuts

Proper Use of Fiber or Nylon Self-Locking Nuts

Inspect ing nuts for age and condition and properly

installing nuts is critical to ensuring tight connections

The problem

 Some pilots and maintenance personnel are not adequately inspecting or installing fiber or

nylon self-locking nuts , which have fiber or nylon inserts incorporated into their c onstruction to

provide tight connections that will not loose n under vibration .

 Fiber or nylon self-locking nuts ’ ability to provide tight connections degrade s with each use.

They may not meet the minimum prevailing torque value , which can lead to loose conne ctions

or, eventually, liberate hardware and lead to accidents and incidents .

A new nylon insert. An old, degraded nylon insert .

Related accidents

Sadly, the circumstances of each new accident are often remark ably similar to those of previous

accidents. This suggests that some pilots and mechanics are not taking advantage of the lessons

learned from such tragedies that could help them avoid making the same mistakes. The following

accident summaries illustrate some common —and preventable —maintenance -related accident

scenarios:

 A P-51 Mustang airplane collided wit h the airport ramp in the spectator box seating area

following a loss of control during the Na tional Championship Air Races . The investigation

found that the trim tab attach ment screws could be rotated through the nuts by hand , and the

fiber inserts were visibly worn and displayed the full thread form . The degraded nuts allowed

the screws to repeatedly loosen at race speeds, and, during the accident flight, led to

aerodynamic fl utter of the elevator trim tab and the subsequent loss of airplane control . The

commercial pi lot and 10 people died and about 64 people were in jured . (NTSB accident

number WPR11MA454 and report number AAB-12-01.)

SA-028 December 2013 , revised December 2015

 During landing, the nosewheel separated from the nose landing gear of an Ercoupe 415 -C

airplane . The nuts on the nosewheel bolts could be rotated by hand , and only slight

resistance could be felt as the bolt entered the locking portion of the nut . The fiber inserts

were visibly worn and displayed the full thread form. (ERA13LA007 )

 An experimental , amateur -built SeaRey airplane crashed during takeoff . The pilot stated that ,

all of a sudden, the airplane was upside down . The right horizontal stabilizer lock nu t at the

forward attachment point was missing , and the bolt that secure d the right horizontal stabilizer

was backed out. (MIA08CA088 )

 An experimental , amateur -built Air Command Elite gyrocopter collided with terrain following

an in -flight separ ation of a control system rod . Postaccident examinations revealed that the

bolt and lock nut used to attach the control rod in the cyclic system were missing.

(LAX06LA236 )

 During a forced landing of a WSK PZL Mielec airplane , the throttle would not retard all the

way, and the airplane struck a pole. T he bolt that normally connect ed the throttle linkage

clevis to the throttle arm was missing , which allowed the throttle cable to disconnect from the

throttle control arm. A sister airplane was examined, reveal ing that a nut with a nylon insert

had been installed on the clevis bolt and that it was loose enough to be removed by hand.

(SEA04TA147 )

What can pilots and maintenance technicians do?

 Comply with instructions in your aircraft’s applicable maintenance manual or Federal Aviation

Administration Advisory Circular 43.13 -1B, chapter 7 , section 4 , if there is no guidance on nut

reuse in the manual.

 Confirm that the specified hardware is installed correctly .

 Ensure that adequate torque is required to spin the nut on the bolt. Be aware that binding in

the assembly may produce a false indication of nut effectiveness.

 Throw out a nut that can be rotated on the thread by hand .

 Ensure that sufficient thread protrusion exists .

 Do not run a tap through the fiber or nylon insert because doing so remove s the locking

capability of the nut.

REMEMBER: WHEN IN DOUBT, THROW IT OUT !

Interested in more information?

The reports for the accidents referenced in this safety alert are accessible by NTSB accident

number from the NTSB’s Aviation Accident Database & Synopses web page at

www.ntsb.gov/aviationquery/index.aspx. Each accident’s public docket is accessible from the

NTSB’s Docket Management System web page at http://dms.ntsb.gov/ pubdms/ .

AC 43.13 -1B chapter 7 , section 4, paragraph 7 -64 provides general information about nuts and

specifically states when not to use nuts with fiber or nylon inserts.

GA Maintenance Alert: Safety and Security of Components . This GA maintenance alert and others

can be accessed from the FAA Safety Team website at www.faasafety.gov.

This NTSB safety alert and others can be accessed from the NTSB’s Safety Alerts web page at

http://www.ntsb.gov/safety/safety -alerts/Pages/default.aspx .