Thursday, August 02, 2012

Reusing Locknuts

lock nuts
MS21044 locknut comes in two styles
MS21044 Spring Beam Nut (left):
This type of nut has thin slots cut down through the top few threads with the resulting fingers bent slightly inward. At installation, the bolt springs the fingers out and the fingers grip the bolt with a prevailing torque.
MS21044 Distorted Thread Nut (right):
This type of nut has been slightly crushed at the top. (notice the marks on the side of the nut). When the round bolt reaches the oval portion of the nut it springs the nut back round. This spring action grips the bolt and adds friction that prevents loosening.

A review of research papers tends to conclude that of the two; the elastic lock nut is more re-usable than the all-metal lock nut.

Two problems with the all-metal design:
  1. The all-metal locking mechanism rubs the threads and removes the protective plating and damages the threads. This makes the bolt more prone to corrosion and galling damage. Replacement with a new nut does not restore the damaged bolt threads. 
  2.  The wearing away of metal tends to loosen the grip of the all-metal lock nut.
These problems are best illustrated by the loss of 17 lives in a Sikorsky S-92A where all-metal self-locking nuts were used in areas that required frequent removal. In this accident a titanium stud was used with a silver plated locknut. Titanium is prone to galling and the silver plating on the nut was used to prevent galling. However, as described in the accident report:

"Examination of a new stud and nut showed that galling damage developed after the first installation and that the damage became progressively more severe with repeated installation/removal cycles. Testing of the occurrence and exemplar studs and nuts showed that after 13 to 17 assembly cycles, the nut self-locking feature was significantly damaged and fragments were separating from the crests of the threads" Report A09A0016

The elastic lock nut is more friendly to the threads, locks out moisture and prevents corrosion; and the nylon deforms rather than gouges. Except in areas of high-temperature, the elastic lock nut is generally preferred.

Recommendations for use per FAA AC43.13-1B Acceptable Methods and Practices:

  • Elastic lock nuts are not to be installed in areas exceeding 250 degrees F.
  • Do not reuse elastic lock nuts if the nut cannot meet the minimum prevailing torque values shown in the chart
  • Do not use self-locking nuts on parts subject to rotation
  • Do not use self-locking nuts where the loose nut, bolt, or washer may fall or be drawn into the engine air intake scoop.
  • Do not use self-locking nuts to attach access panels, doors, or any parts that are routinely disassembled before or after each flight.
Several aircraft accidents (Canadian TSB Report A97O0055)  and aircraft control problems (C-130 aileron) and the loss of Bell-206-L-1 helicopter, N2138Y NYC01LA088, and Sundance Helicopter caused by lock nuts coming loose have challenged the idea that any style lock nut can be reused.

The idea of inspecting a lock-nut for minimum prevailing torque sounds good on paper, but as illustrated in the Canadian accident, it may not be a good practice to use in the field. In this accident the engine manufacturer's maintenance manual (ROTAX), stated: "self-locking nuts must be replaced with new items after removal in the event  the friction torque has diminished."

When the Transportation Safety Board did their own tests on the M8 lock nut PN 942-035 used on the exhaust,  they found that friction torque diminishes each time the M8 locknut is installed and removed.  In this accident the loose lock nuts created a situation that led to an accident.

The usual method a mechanic would use to check for "diminished friction torque" is to see if the nut would unscrew by hand. If it does, then it is no good and is replaced. If it cannot be unscrewed by hand then it is OK. THIS METHOD DOESN'T WORK as they found out in the loss of Bell 206 N2138Y.

The "finger test" of the re usability of a locknut is not valid. Informal tests done by the author show that it takes 4 to 5 inch-pounds of torque to prevent a mechanic from screwing a 3/8-16 AN nut down by hand. For a 3/8-16 elastic stop nut, the minimum breakaway torque is 12 inch-pounds and a new nut produces 80 inch-pounds according to the Acceptable Minimum Limits for self-locking nuts per MIL-N-25027 (in actual tests 102 inch-pounds were required to free run the nut).

Notice that a new nut in our example produces 80 inch pounds of prevailing (friction) torque when new but can degrade down to 12 inch pounds before rejection! Could it be that in some high-vibration applications (read helicopter) that 12 inch pounds is not sufficient to hold the nut in place?

Maintenance Flow Issues in re-using and inspecting locknuts:
In the Sundance Eurocopter AS350 Accident, the loss of one locknut caused the loss of control of the helicopter. Instead of using a new nut, both the FAA, NTSB believe that the nut should only be replaced if it fails inspection. Why inspect a 50 cent nut if the helicopter and 5 people's life depends on it? Especially if these "nut inspections" have resulted in previous accidents (Bell 206 N2138Y).

Nut inspection  introduces not only an extra inspection requirement but a bottleneck to workflow. Most likely a proper inspection is more expensive than the nut itself. If the nut fails inspection then you have the cost and time of the inspection and still have to procure a locknut. The tendency in human nature is to get the job done and the nut is probably not all that bad. However, if it was in the mechanic's mind that critical locknuts are not re-used then a new locknut is made available as a standard item for replacement. Given the critical nature and the saving of an inspection step -- why would anyone suggest that the mechanic should inspect the darn thing?

The NTSB's "Safety issues" regarding Sundance Eurocopter AS350 with 5 lost lives is:
  • Reuse of degraded self-locking nuts 
  • Maintenance personnel fatigue
  • Lack of work cards with delineated steps
  • Lack of human factors training for maintenance personnel
I have a better safety suggestion; "Don't reuse critical locknuts" Simple and to-the-point.

Air Force T.O. 1-1A-8 now states: "New self-locking nuts shall be used each time components are installed in critical areas throughout the aerospace vehicle."  This seems a more practical policy given the low cost of a lock-nut.  And from the UK:  Fasteners with a fibre or nylon friction element should only be used once, and must not be used in  locations where all-metal stiffnuts are specified.  All-metal stiffnuts should not be re-used in locations  vital to aircraft safety (e.g. control runs) but may be re-used in other locations providing the locking quality remains satisfactory. CAP 562

 If the NTSB and FAA had made this a requirement after the loss of N2138Y, then the Sundance AS350 accident need not have happened. Shall we add another Safety Issue:
  • The failure of the NTSB and FAA to correct known industry safety issues. 

NAS679 Nut
NAS 679 Low Height Lock Nut

In 1969, Cessna issued Service Letter SE69-28 after finding that NAS679 nuts, size 10-32 were cracking due to "heat treatment embrittlement". SE69-28 indicated that Cessna had discontinued their use and called for their replacement in certain critical applications.

NAS679 nuts, other than size 10-32 were used extensively by Cessna until 1980 when they were replaced by MS21042 nuts. While the company amended its parts catalog for the affected aircraft to reflect the change, Cessna did not specifically indicate that the NAS679 nuts had been superceded and there was no service information requiring replacement of NAS679 nuts, other than those called out in SE69-28.

There has been other reports of these nuts cracking in the Cessna Wing Carry Through Spar Assemblies. For this reason, were allowed and applicable, MS21042 may be a better choice than NAS679
Service Difficulty Reports (SDR) published stated that the 16 nuts were MS21042L6, although when investigated by the Transportation Safety Board of Canada it was found that the cracked nuts were actually NAS679. Reference AAC6-54 dated 2/95

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