Sikorsky S-92: From Design to Operations

[riff_raff]

Originally Posted by [Concentric

From the same report, however:

1.12.5 “…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… “

Curiously, although Cougar CH191 and worldwide fleet maintenance records were averaging around 200 – 220 hours between filter replacements, the CHC VH-LOH had averaged only 72.6 hours over 17 replacements, so the self locking effectiveness may have been lost anyway.

Sikorsky did not recommend renewing the nuts at every filter removal/replacement until September or November 2008, several months after the CHC failure.

So, at least as far as nut (and preload) retention is concerned, it may have been little worse than refitting a worn self locking nut. Neither is desirable but only the improvised repair looks obviously suspect. The studs of both failed, at much the same time, the fatigue damage possibly having been largely done under the original attachment conditions, as later with CH191[/I][/FONT].

Concentric-

Looking at at photo of the failed filter housing attachment it appears the type of locknut used is an MS21043. This is an A286 corrosion resistant steel all-metal locknut with silver plating finish that is widely used in the aerospace industry, and it uses a deformed thread locking feature located at the upper end of the nut. These nuts are qualified to MIL-DTL-25027, which includes testing to verify the locking feature's prevailing (locking) torque remains within specified limits for a minimum number of installation/removal cycles.

I'd estimate the stud in the filter housing photo is maybe 1/4" diameter. One common issue with all-metal deformed thread locknuts at smaller sizes like this is difficulty in accurate control of prevailing torque during manufacture, and maintaining consistent prevailing torque over repeated installation/removal cycles. For this reason, every aerospace manufacturer I have worked with has fastener installation procedures that require checking the prevailing torque of every locknut at each installation to verify it is within specified limits. These procedures are normally carried over to the maintenance instructions since it is a safety/reliability compliance issue with the threaded fastener dual locking function requirement contained in FAR 29 & FAR 27.

The prevailing torque problem becomes worse as the thread size gets smaller. These #4-40 size locknuts are often used to secure electrical connectors, and I have seen situations where maybe 10% or more of brand new ones had out-of-tolerance prevailing torque and had to be scrapped. I think one reason for this is the sampling rate used for acceptance, which is only 5 samples per 10,000 parts.

One last thing I would point out about the locknuts shown in the photo of the filter housing linked above is that they appear to be silver plated, which would be standard for an MS21043 locknut. It was reported that the mating studs were titanium alloy. It is well known that silver coatings should not be used in contact with titanium, since it can cause embrittlement of the titanium. Here's what MMPDS has to say about it: "Under certain conditions, titanium, when in contact with cadmium, silver, mercury, or certain of their compounds, may become embrittled. Refer to MIL-HDBK-1568 for restrictions concerning applications with titanium in contact with these metals or their compounds."