Fin Dorsal IPC clarificATION

[MoonlightVFR]

I am trying to learn about Spherical Washers.

Some time ago I overheard discussion about 170 tail fin attachment

Story was that there is a concave or spherical washer used in tail fin dorsal. If the installer made mistake of installing washer upside down it could compromise the safety of aircraft.

I have looked and looked at Cessna IPC fig 13 and page 25 and this washer story just does not JUMP off the page for me.

Why would It make any difference ?

[Bruce Fenstermacher]

I think what your are thinking about is not on the dorsal fin at all but the vertical stabilizer attachment bracket bolt.

Vert Stab attachment bracket.png

The bolt is item 7, a NAS 145-18. This is an internal wrench head bolt which does not have a flat bottom to its head. See the red arrows on the drawing below.

NAS 145-18.png

You can not use a standard AN bolt here or put a standard washer under the NAS bolt head. If a washer was to be used with this bolt it would have to be relieved to fit the head. If you had such a washer and it was installed upside down, the bolt head would not seat correctly. The IPC does not call for a washer here so the bracket must be relieved to fit the head or the correct washer is not depicted in the IPC.

To be honest I've never seen a bracket without a bolt in it and I can not recall if mine and others has a washer under it but I tend to think not.

[Bruce Fenstermacher]

Update:
OK as with most things 170, we've already discussed this and the answers to NAS 145 bolts and washers and pictures can be found here: http://cessna170.com/forums/viewtopic.p ... 708#p68257

I guess there are washers under them bolts. I'll never forget again.

[minton]

Bruce,

First; When trying to fit up a bolt and washer I've had issues with enough room laterally for the washer (interference with the machined radius of the bracket). That has tipped me off that there is no requirement for such washer.

Second; I have not heard differently in my 45+ years in the business sitting through countless "AI" meetings.

There are other places these "NAS" bolts are used in Cessna's fleet IE, Main wheel axels on later models. In those applications, NO washer . The reason? The hole edge has a "Radious" or rounded hole edge and in the engineers opinion enough "Bearing surface for the bolt". If you look closely the hole in the vertical stab. bracket is also, thus, NO washer. If it is not for some reason a special reamer is required to do the work as the hole is not just "Chamfered" (a straight edged 45 degree bevel). I have used a wood router cutter in the past as finding such a reamer is almost impossible as well as access to the area is limited with larger tools. As the material is relatively soft you can spin the tool by hand and get a good result. Be sure to put the cutter guide for appropriately sized hole in place to assure a 90 degree alignment and even cut. Getting a seating of the bolt is required without contact of the radius on the bolt to the hole. (So don't over do it and lessen the bearing surface available to the seated bolt). That radius on the bolt is to prevent / relieve a stress building up as the bolt is in "Tension" rather than "shear". Also don't chatter the hole. All of this attention to detail is of course to prevent any additional stressed areas within the hole and bracket. Now once the bolt is in place take the time to torque it properly with your recently calibrated torque wrench. (Standard torque values for the bolt size apply) and don't forget to take into account the lock nut friction in that calculation.

Now you will all chime in and say, "Well mine isn't that way" Well, they should be! My thinking for the lack of a radius is? The folks doing the finish work did not comply with the specifications. Think it through before raising the "BS" flag. Fastener engineers didn't go to all of that trouble for nothing when designing the bolts. nor Cessna when designing the bracket.

Hope this is of help.