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#61
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Depression after Washing
On Jul 5, 3:58 pm, More_Flaps wrote:
Why did your push-pull test not detect it? I prefer to give a shake and feel the nature of surface response. That's why I have a push-pull test now. Didn't do it until after I flew that airplane and took it apart after I got it here. Found a lot of other stuff, too: lower strut attach bolts with no nuts, and backing out of their holes; cracks in lots of places; scary stuff galore. The owner trucked it away. In pieces. Dan |
#62
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Depression after Washing
On Jul 5, 4:26 pm, Peter Clark
wrote: On Sat, 5 Jul 2008 09:45:11 -0700 (PDT), wrote: evasive action. We run several 172s and have seen cracks, up to four of them, in a spar; we now forbid students to push down on the things. No more cracks. Just out of curiosity, is this before or after Cessna put the reinforced spars (R and S models?) in at the factory? I don't believe the newer models are prone to this kind of issue, but wouldn't do it in practice anyway - using the towbar is always better for the aircraft, and done by hand I don't think it's possible to damage a 172/182 nosewheel. There's a Service Bulletin dealing with this, and it's dated to before the R/S models went into production, so it's safe to figure that Cessna fixed the problem in these models. The SB calls for stopdrilling and monitoring every 100 hours, and fixing it by installing a doubler either right away or when the cracks go too far. The problem with waiting is that the centre section needs to be replaced if the cracks reach the flanges. I'll know more later. We're getting a new or newer one, maybe two, soon. Interesting to see what they've improved, and what they haven't but should have. Which reminds me: there are other places that crack, not readily visible. The rudder hinge brackets (on the rudder itself, not the fin) will break in the bend radii. The top hinge is the worst, as it has the loads of the lead balance weight wobbling around in the turbulence to deal with. Got to use a tiny mirror and lots of light to see the break. We spin these airplanes all the time, and in a spin the tail wiggles around a lot, so maybe the non-spinning pilot won't have a problem with these. Spins are also hard on gyros, whacking the internal gyro cases against their stops and Brinelling the bearings. But that's all part of flight training, and we charge enough to cover things like that. We go through more propellers, too, operating off rougher strips and picking up small stones. These rudder brackets also wear against the fin lugs, since the spacing at the factory was often screwed up so that the bottom hinge's top bracket doesn't ride on its bearing flange like it's supposed to, letting the middle and/or upper hinges take the thrust loads so they wear thin. No thrust flanges on those bearings; just aluminum against aluminum, and sitting outside in the wind those hinges get full of grit and the wind wiggles the rudder and the brackets eat themselves. I've asked Cessna to issue some teflon washers to shim the bottom bearing and get the load off the others, but they pay no attention to a hick from rural Canada. The law here says we have to use only the stuff from the parts manuals, and they don't list any such washers. The rudder bar springs put tension on the rudder cables. Those cables aren't pulling perpendicular to the hinge line becease the rudder is tilted back, so the rudder is pulled down, increasing the load on the hinge thrust faces. Dan |
#63
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Depression after Washing
wrote in message
... On Jul 4, 4:56 pm, "Mike" wrote: Obviously, you haven't seen some of the things I have seen. One of the more tender, and more invisible spots on some airframes is where the horizontal stabilizer connects to the fuze. Many designs allow a tremendous moment arm for any non-balanced load on the stabilizer to stress the attachment points. This shows up as cracked spars on Cessnas, and I have seen stressed and cracked fittings from another airframe. How can this happen? Well on Cessnas it happens from folks using improper procedures to back the plane into a parking spot. It can also happen from innocent (but ignorant) bystanders, mowers, animal activity, or any of thousands of other posibilities. I always give each side a good heave up and down for this very reason, so such can easily be checked on the preflight for impending failure. A good heave up and down on the end of the stab of a 172 flexes the center of the forward spar, eventually cracking it. A gentle bit of push-pull is all that's needed, to see if there's unusual tip travel. I bet your spar is cracked now. Many are. Cessna calls for stopdrilling the crack unless it has reached the spar flange, in which case it has to be repaired. I once flew a 172 that I subsequently found had a broken spar, busted clear through both flanges so that the skin was all that was holding it. The thing could have killed me if I gotten into turbulence or had to take violent evasive action. We run several 172s and have seen cracks, up to four of them, in a spar; we now forbid students to push down on the things. No more cracks. 172s suffer cracking at the bottom of the aft doorposts. Some models crack inside the forward doorposts. Do I need to point out that these doorposts are what the wing pulls on to lift the airplane, along with the struts? No preflight will find those. The wing spar attach lugs are known to crack at the bolt holes. In older 172s the forward elevator bellcrack bracket would break loose, reducing elevator control. In newer 172s (rod-style gear; 1973 or so and on) the landing gear retaining bolt sometimes shears and totals the airplane on landing. As the years go by, these older airplanes will become the subjects of ADs addressing age-related airframe failure, probably after a couple come apart in flight. Sooner or later. 1. By "good heave" I don't mean raising the nosewheel off the ground. I just mean applying enough pressure both directions to listen for any irregularities. 2. I don't have a 172. |
#64
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Depression after Washing
On 2008-06-30, John Smith wrote:
It really isn't as bad as you make it out to be. You polish on rainy spring days, before the real flying season begins and let it go the rest of the year. Maybe touch it up in August on days when it is too hot to fly. Coming in to this thread very late... Actually, it is as bad as that unless you live in the desert. I owned a polished plane (Cessna 140). It was only half polished, too - just the lower half of the fuselage, the tailplane, and flaps. The rest was painted. To keep it looking nice required a whole day of polishing, a minimum period of once every two months. By whole day I mean at least 12 hours. (I rarely did it all in a day, usually I spent a weekend doing it so I could do other stuff too). This was for an itty bitty plane and only half polished with the more difficult to polish bits (things like upper wing surfaces) painted. A completely polished Cherokee, for example, would be at least three days work every two months to keep looking nice. The plane absolutely must be hangared too. We also used Nuvite, the final polish grade of that stuff also leaves a bit of a coating that keeps it shiny for much longer. The result with our C140 was of course stunning, especially when the plane was parked on a rural turf airfield, with the polished underside and tail reflecting the green grass, trees and blue sky. But it was a BIG commitment to keep it that way. On the plus side you really get to know the skin of the plane well and each session of polishing is a rather thorough inspection. -- From the sunny Isle of Man. Yes, the Reply-To email address is valid. |
#65
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Depression after Washing
On Fri, 11 Jul 2008 12:00:56 +0000 (UTC), Dylan Smith
wrote: On 2008-06-30, John Smith wrote: It really isn't as bad as you make it out to be. You polish on rainy spring days, before the real flying season begins and let it go the rest of the year. Maybe touch it up in August on days when it is too hot to fly. Coming in to this thread very late... Actually, it is as bad as that unless you live in the desert. I owned a polished plane (Cessna 140). It was only half polished, too - just the lower half of the fuselage, the tailplane, and flaps. The rest was painted. To keep it looking nice required a whole day of polishing, a minimum period of once every two months. By whole day I mean at least 12 hours. (I rarely did it all in a day, usually I spent a weekend doing it so I could do other stuff too). This was for an itty bitty plane and only half polished with the more difficult to polish bits (things like upper wing surfaces) painted. A completely polished Cherokee, for example, would be at least three days work every two months to keep looking nice. The plane absolutely must be hangared too. We also used Nuvite, the final polish grade of that stuff also leaves a bit of a coating that keeps it shiny for much longer. The result with our C140 was of course stunning, especially when the plane was parked on a rural turf airfield, with the polished underside and tail reflecting the green grass, trees and blue sky. But it was a BIG commitment to keep it that way. On the plus side you really get to know the skin of the plane well and each session of polishing is a rather thorough inspection. I once had the care of a polished all aluminium cessna 150A.(lovely thing) polishing it took a weekend. what caused it to be painted eventually was the realisation that the night time condensation appeared to penetrate the wax polish. this continued the activity in surface pitting that had developed all over the aircraft. the eventual cure to the pitting was the wash and phosphoric acid prep done prior to alodining the aircraft and painting it. since painting, not a problem since. in hindsight if I wanted an all aluminium finish aircraft I'd paint it that colour. Stealth pilot |
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