Turbine AgCat tube repair

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Turbine AgCat tube repairEvery aircraft--worldwide, has to have an annual inspection by a licensed A&P, etc.This involves looking and testing, basically everything on the aircraft. The a/c cannot fly until it is signed off. Annual's can get pricey in a hurry.It's 'that time' for the AgCat. In a month, it will be very busy, seeding hundreds of tons of rice.The steerable, rear tail wheel, was shimmying badly. (the above shows the tube repair area in red. The blue outline is the spring and removed tail wheel.)The kingpin and bushings are being repaired/replaced. This, plus thumpin' down on rough dirt strips, has cracked the rearmost longeron tubes that brace the spring mount for the tailwheel. The tubes had been previously welded, now new fatigue cracking has appeared outside the old weld. A&P (who also has Inspection Authorization) calls for reinforcement, via doublers on both tubes, right and left.(This rear area is inside paneling, that cannot be removed until the rudder is removed, which is why this wasn't noticed, before. TCA removes all accessible panels, frequently during the year, to look at things, before they break. ) (If mfr. had extended the central gusset plate all the way dow, then run nice doubler tubes up 6 inches, instead of those slapped-on triangle plates, this failure most likely would not have occurred. Cracking occurs at the weakest point of the assembly---exactly where this did exhibit cracking. Additional, hidden cracking under one edge of the triangle plate was discovered during this repair. Tubes are real chome-molly---4340, 1.0 x .050 wall.4130, 4140 ain't chrome-molly, never was, despite folks now calling it that. It's cheaper, so that makes it okay, I guess.)The old repair welds were sanded down flush to allow fitting the doublers, the new crack was carbide burr grooved, stop drilled at both ends The crack and stop drilled holes are welded, then sanded flush.Doubler of 1.25 x .125 wall CRS tubing was made, splitting the tube into two halves on vertical band saw, then templating, and lots of fiddling to trim to fit tight. Doubler was welded fully around, to seal---against all the chemicals that wet down this tail.(When burning off more paint with propane torch, the place smelled like a sulfur mill!) Multiple clamping is needed to squeeze the doublers tight to the tube, since they spring some--while being split, sliced and diced to fit.Ditto for the right side tube.Post welding stress relieve, the un-plated tube areas, directly adjacent to the doubler tops. Performed with rose bud, looking for just a hint of dull red. Yes, I have temple-sticks. Welding was done with MIG--not TIG. Could have tigged it. This is a highly stressed structure. The less heat input from welding, the less stress induced. The additional strength is from the doublers, not the welding, which only serves to mate the doublers to the frame tubes. Nice full heat and melt penetration in those .050 wall tubes, will crack out the doublers in a heart beat. On tube repairs for race cars and aircraft, I've had much better durability with MIG repair welds than TIG--especially in 4340.Blackbird
Reply:nice repair ChuckASME Pressure Vessel welder
Reply:An A&P can not sign off an annual but you did mention that the A&P holds an IA so that's ok. Unless the FAA has changed it's mind in the last few years there is no blanket approval for MIG welding. Any MIG welded repairs would have to be specified the manufacture of the aircraft. Was there a form 337 done. Even though most sprayplanes fly in restricted catagory I wouldn't be cought MIG welding an aircraft structure unless the current issue of 43.13 (acceptable methods and practices) says something that mine doesn't.Last edited by tresi; 03-26-2008 at 10:02 PM.
Reply:9/27/01AC 43.13-1B CHG 14-74. GENERAL. This section covers weldrepairs to aircraft and component parts only.Observe the following procedures when usingsuch equipment as gas tungsten arc welding(GTAW), gas metal arc welding (GMAW),plasma arc welding, and oxyacetylene gaswelding. When repairs of any of theseflight-critical parts are required, it is extremelyimportant to make the weld repairs equal to theoriginal weld. Identifying the kind of metal tobe welded, identifying the kind of welding processused in building the part originally, anddetermining the best way to make welded repairsare of utmost importance.Blackbird