magnetic permeability of fasteners around magnetic flux valve 1

[brin]

Does anyone know of a reliable guideline for selecting fasteners around a magnetic flux valve based on magnetic permeability? The original fasteners that attach an access cover were bronze per AN509 but these are no longer available. MS24694 CRES screws have a magnetic permeability of 2.0, but that exceeds the permeability of the original bronze screws.

This is for a Sperry C-14 Gyrosyn compass system. This is now part of the Honeywell empire and I haven't been able to locate a contact for OEM support.

 

[der8110]

Would brass instrument screws work? MS24693BB or AN507B. Next, Austenitic stainless steels (302, 304) are what you want, permeability is in the single digits (other alloys being orders of magnitude more). Another trick is to have a "balanced" pattern around the flux gate so no single fastener throws it off. Saw that done once with steel stem blind rivets. Also don't forget the nutplates; easy to get fixated on what the screws are made of and totally miss what the nutplates could do.

 

[Sparweb]

There are a lot of other brass screw spec's other than AN509. MS35215 comes to mind.
MS20160 anchor nuts are stainless steel, and if you trace through the material spec's you can probably find an allowable permeability.
There are a number of anchor nuts, clip-nuts, and screws that are designed specifically for use in instrument panels, and accordingly they have magnetic permeability limitations in their material spec's.
Don't forget that equipment ground return wires (AND the metal structures they are attached to) carry electric current. Current = magentic field.
Flux gates are often found in aircraft wings. If this is the case for you, consider the proximity of your compass to control cables, rods, counter-balance weights, etc...



STF

 

[brin]

Thanks DER8110 and SparWeb. We will probably do a bench test to better evaluate the options.

 

[edmeister]

Additional considerations should include a process whereby an unwary mechanic will not be able to mistakenly replace any of the critical fasteners with the incorrect ones. There are several historical examples of aircraft deviating from route due to this error. Either critical fastener location is CSK compared with adjacent non-critical protruding locations / or label installed such as "install Non-magnetic fasteners this location / or locations marked with a triangle symbol .." etc.
Note how some mechanics may complete a wing inspection / all panels may be removed at the same time & fasteners grouped in a common container until reinstalled. A painted stainless screw will look identical to a steel screw.

 

[brin]

This has been observed on one location where a steel screw was found installed instead of CRES. As mentioned, this can be very difficult to prevent because the CRES and alloy steel screws look alike after paint has been applied, etc.