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CTSW Undercarriage Leg replacement details

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Hi Guys,

I've just had to replace one of my CTSW undercarriage legs and thought that other owners may be interested in how to do this and some detail in getting the holes drilled accurately!

 

Here in the UK, we have a mandatory 300 hour inspection of the undercarriage legs (Service Bulletin CT145) and my 2006, 890 hour CTSW with over 1,000 landings had its third inspection done this week. 

This entailed raising the main gear a few inches off the deck using a homemade raising gadget!  Then the spats and brake lines were removed, brake fluid drained and axle/wheel assemblies removed.  The leg fairings were then slid off and then the legs were unbolted from the 'plane.  Non-destructive testing was done by employing special chemical penetrant dyes and UV light.  The only areas tested were in & around the main centre hole and around the 'waist' at the top of the leg just below the thicker part that goes up inside the top bracket.

Unfortunately, the port leg on my 'plane had a very tiny crack just inside the aft edge of the centre hole which was only visible with a x100 jeweller's loup.  The leg was rolled on a propeller balancing rig and was found to be dead straight but a crack is a crack.  The crack was absolutely minuscule but my engineering inspector said that cracks only get bigger and are always a possibility on a metal item subject to repeated bending loads.  Anyway, he had a new replacement leg in stock and proceeded to fit it. 

The new leg only comes with the centre hole already pre-drilled.  Another modification we have in the UK is to peen the edges of the main attachment hole thus leaving residual compression stress at the hole edges intended to suppress fatigue crack initiation (Mod 309).  As this wasn't done on my factory fitted original legs, the 'old' starboard leg and new port leg had this engineering process applied.

First, the new leg was fitted and the (new) M6 centre bolt tightened up together with the cover plate and its four (new) bolts.  Then a custom made centre-punch tool was fitted into the white bracket's top and bottom mounting holes and hit with a hammer to make an indentation on the new leg for drilling.  This custom made tool is a short steel dowel with a very small hardened point in its centre.  The tool sits snugly in the white bracket's hole(s) so that the tiny raised point is exactly in the centre of the hole.  The leg was then removed and drilled from both sides just slightly undersize.  It was then re-fitted and the top hole reamed out and a new nut, bolt and washers fitted.

 

The correct drilling of the bottom hole is critical so that the aircraft not only sits level but the wheel tracks correctly.  First, ensure the tyres are exactly the same pressure and the same amount of fuel is in each wing.  Then slide the complete axle and wheel assembly onto the new leg and just nip up the pinch bolt. Lower the 'plane onto its wheels and roll it backwards and forwards a few yards to settle it.  Then accurately measure the height from the floor to the base of the last aileron bracket on both wings.  You must have a flat workshop floor for this!!  You may have to raise the 'plane, loosen the pinch bolt and 'wiggle' the wheel assembly up/down the leg a few times and repeat the above before you get exactly the same floor-to-height measurements on both wings' brackets.  Once you have the same heights, scribe a faint line along the top of the white bracket onto the leg as a reference.

Now we needed to adjust the tracking.  We assumed that because the tyre wear on the starboard leg was very even, we would use that side as a datum to set the port wheel.  With the 'plane on the ground and the undercarriage legs fully assembled and tightened (but with the the new leg's wheel assembly just pinched up), we first checked that the stabilator was exactly level using a digital level and measuring from the rear corners to the ground.  It was!  Then we put some masking tape along the leading edges of the stabilator.  Using a laser level held against two of the brake disc mounting bolts on the starboard side wheel, we rotated the wheel until the laser beam 'hit' the masking tape and we then marked this spot with a felt pen.  We rotated the wheel and used all the disc mounting bolts to check that it hit the same spot.  It did!  My Marco wheel must have been a good one!!!!    

Next, we measured the distance of the felt pen mark from the fuselage on the starboard side and made a similar mark on the masking tape on the port side of the stabilator.  We then held the laser level against the port side brake disc mounting bolts and rotated the wheel to see where the laser dot hit the masking tape.  It was off to one side of the mark so we loosened the pinch bolt and carefully twisted the wheel assembly on the leg until we got the laser dot to hit the mark - checking that we still had our scribed line in the right place and hadn't accidentally moved the wheel assembly up or down the leg!

Now, at last, we were ready to drill the bottom hole in the new leg using the same technique as we used for the top hole.

The rest is just a matter of reassembly and brake bleeding.  For this, we used a big large diameter hypodermic syringe bought off eBay to pump the brake fluid in through the bleed nipples and up into the brake fluid reservoir to which we fitted a piece of tubing going into an empty container.    

 

I'm sorry I haven't got any photos but I hope the explanation above might help someone in the future who has to do this job.

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The whole job took about 4 hours but in order to get a perfectly level aircraft and correct wheel tracking, it needs to be done properly!  The hardest part was wiggling the hub assembly up and down the leg to get the level correct.  It's a tight fit on the leg and we found that we had to clean out the inside of the tube with some fine emery paper wrapped around a piece of tubing in order to get it to move up and down the leg.  A smear of ACF50 on the end of the leg helps too.

The trouble is that FD don't seem to have a uniform way of mounting the hubs to the legs.  My engineer has seen some undercarriage legs that have been inserted down inside the hub tube as far as they'll go (until they touch the horizontal hub tube which can be seen inside the slanting hub tube of the hub assembly) whilst others have only been inserted about 3/4 before the bottom hole was drilled. 

So this means that the new leg which has to be drilled, needs to be wiggled up/down the leg accordingly to get the aircraft level set correctly.  Even if the leg on the other side of the aircraft was fully inserted into the hub tube before its hole was drilled, this doesn't automatically mean that the aircraft will be level if the new leg is also fully inserted.  I'm not sure why this is but my engineer tells me that of the many legs he's done, it just doesn't work that way.  Similarly, he says that measuring the distance of the bottom hole on the 'bent/cracked' leg whilst it is on the workbench and transferring this measurement across to the 'new' leg doesn't work either! 

You only get one chance to drill this hole in order to get the aircraft sitting level and the tyre tracking correct and unfortunately, there ain't no short cuts!

 

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I forgot to add that having seen a number of CT's out there with slightly uneven wing levels (and pilots on this forum who have admitted that their aircraft don't sit exactly level), there's either a lot of folk having very heavy landings (!) or the undercarriage legs weren't fastidiously drilled correctly in the first place!

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When I bought my CT I noticed one wing slightly low so after some investigation I found the right leg bent and the left knuckle bent. I replaced the gear but it takes a lot of thinking and work to get it right, one mistake and it will cost someone a lot of money. Anyone who is able to replace the gear properly has my respect. I don't believe today's average A&P will get it right.

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I've replaced a few gear legs, and it is not that difficult. I have also fixed a toe in issue without replacing the legs.

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9 minutes ago, Madhatter said:

You must be a lot better at it than I am.

I did have one that took the better part of a day. I had issues getting it to come out of the top socket. The airplane had been through a flood, and I think that was the issue with getting it to come loose.

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22 hours ago, Tom Baker said:

I've replaced a few gear legs, and it is not that difficult. I have also fixed a toe in issue without replacing the legs.

Any tips on doing that work, Tom?

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