The jack struts are designed to transfer braking loads up to the fuselage and triangulate the load away from the main gear bushings for longer life.  There are three designs and four part numbers.

The early style two piece Gear (with upper and lower main legs), utilizes a LH and RH jack strut constructed of 4130, thin wall steel tubing which is NOT heat treated.  It may be repaired by welding then normalized for stress relief.  

The lower mounting is fitted with a threaded bushing normally of 5/16" NF construction, into which the strut fork (not really a fork, but PN S-9 or 08311-4), is screwed, and affixed after adjustment with a plain jam nut.   This fitting is a structural weak link, designed to break in overload.   There is also an alternate S-9A fitting which has a 3/8" NF thread pursuant to an engineering order/repair authorization. It is jam nutted with a 3/8" plain nut.  Jack strut(s) must be drilled and rethreaded to accommodate these oversize forks.   S-9A forks are VERY rare and not generally recommended- better to install a new threaded plug and use the readily available 5/16 NF fittings.

There is a cross member fitting required on the lower gear leg of the 08311 gear to stabilize and triangulate the structure.

The cross member fitting which screws into, or through the main gear legs, is also made of thin wall tubing with a 5/16" threaded plug in the end.  A specially threaded bolt with extended threads (5/16" NF, PN08311-2), is used with two jam nuts to secure the cross member to the main gear leg at the through bolt hole in the gear legs.  This bolt is also safetied at the head with safety wire between the bolts, or to the gear leg itself.  Pursuant to a service letter on gear rigging, this bolt may be upsized to 3/8" if necessary.

NOTE:  The LH cross member is straight, and absent any bends or kinks.   The RH cross member weldment has one or two bends in it to accommodate the landing gear shackle movement in the RH gear, while maintaining a perpendicular installation in the arc.   IT IS NOT BENT!

The upper Jack strut mounting is accomplished with a standard AN5 (5/16" NF bolt) and castellated nut, because it is subject to rotation in service.  Under the seat pan this bolt passes on the front side through a mild steel fitting riveted into an aluminum extrusion riveted to the floor pan (it is located under a 1.5" X3" steel fitting visible on top of the floor pan). 

The aluminum U-channel extrusion is a common failure point which frequently splits lengthwise and creates looseness in the attachment.   There is an upgraded U-channel part engineered from 4130 steel that can be used to replace the aluminum extrusion. Remember that the application of any steel by riveting it to aluminum should have a barrier installed such a thin vellum or Teflon to prevent dis-similar corrosion at the contact area.

The bolt then passes through the jack strut itself, which is all steel, and not equipped with a replaceable bushing. (NO Need do to large bearing area).  This area should be lubricated with grease and or oil to keep the bolt from seizing in the jack strut itself and wearing at the front and rear bearing surfaces. (see below).

Lastly the 5/16" bolt passes through the thin aluminum #2.5 bulkhead which is fitted with a steel plate reinforcement.  The total bearing area at the fuselage hard points is less than 3/16" v/s about a 1" bearing area in the jack strut steel bushing area.   Guess which parts wear first? 

To properly inspect this area it is necessary to remove the rear mounting bolt first. Inspect it for wear or grooving from concentrated loads at the head and threaded ends.  If minor wear is noted, any looseness may be attributed to that wear, and the bolt replaced, returning the aircraft to service (1/16" play is about the max allowed).   The good news is that often a bolt replacement will remove most of the play noted by the pilot.

If no wear is noted in the bolt, then a removal of the Jack strut is indicated for a detailed inspection by mirror and lights at the front and rear upper jack strut fittings.   If the repair of either fitting (Left or Right, or front and rear) is indicated, do all of them.  The easiest repair is to remove the seat pan and #2.5 bulkhead as a unit so that the seat pan unit and bulkhead may be removed to a bench for replacement of all the various fittings as needed. Drill, ream and test fit-up a bolt before re-installing the floor pan to insure proper alignment.

Many if not all of the hardware, parts, and fittings described above are available from Classic Aero Support, 480-650-0883.

The Silflex jack struts are very similar with a few notable exceptions.

It utilizes a LH and RH jack strut constructed of 4130, thin wall steel tubing which is NOT heat treated.  It may be repaired by welding then normalized for stress relief. 

The lower mounting is fitted with a threaded bushing into which the strut fitting weldment (adjustable) (PN 51346-95), is screwed, and affixed after adjustment with a plain jam nut.

The cross member fitting is not required on the Silflex leg to stabilize and triangulate the lower gear leg, as on the 08311 gear, because the thicker and heavier silflex installation need no reinforcement.

The upper Jack strut mounting is very similar / nearly identical to the standard gear (minor fitting variations- same principles).  The above standard gear description of wear points therefore applies:

Mounting is accomplished with a standard AN5 (5/16" NF bolt) and castellated nut, because it is subject to rotation in service.  Under the seat pan this bolt passes on the front side through a mild steel fitting riveted into an aluminum extrusion riveted to the floor pan (it is located under a 1.5" X3" steel fitting visible on top of the floor pan). 

The aluminum U-channel extrusion is a common failure point which frequently splits lengthwise and creates looseness in the attachment.   There is an upgraded U-channel part engineered from 4130 steel that can be used to replace the aluminum extrusion. Remember that the application of any steel by riveting it to aluminum should have a barrier installed such a thin vellum or Teflon to prevent dis-similar corrosion at the contact area.

The bolt then passes through the silflex jack strut itself which is steel, BUT equipped with a replaceable bronze bushing which rarely wears. (NO Need do to large bearing area).

 Lastly the 5/16" bolt passes through the thin aluminum #2.5 bulkhead which is fitted with a steel plate reinforcement.  The total bearing area at the fuselage hard points is less than 3/16" v/s about a 1" bearing area in the jack strut steel bushing area.   Guess which parts wear first? 

To properly inspect this area it is necessary to remove the rear mounting bolt first. Inspect it for wear or grooving from concentrated loads at the head and threaded ends.  If minor wear is noted, any looseness may be attributed to that wear, and the bolt replaced, returning the aircraft to service (1/16" play is about the max allowed).   The good news is that often a bolt replacement will remove most of the play noted by the pilot.

If no wear is noted in the bolt, then a removal of the Jack strut is indicated for a detailed inspection by mirror and lights at the front and rear upper jack strut fittings.   If the repair of either fitting (Left or Right, or front and rear) is indicated, do all of them.  The easiest repair is to remove the seat pan and #2.5 bulkhead as a unit so that the seat pan unit and bulkhead may be removed to a bench for replacement of all the various fittings as needed. Drill, ream and test fit-up a bolt before re-installing the floor pan to insure proper alignment.

Many if not all of the hardware, parts, and fittings described above are available from Classic Aero Support, 480-650-0883.

Copyright © 2004, Doug Combs,