The International Cessna^® 170 Association

Can anyone tell me the thickness of part number 0513000-14, skin-Fuselage front section left? Item number 17, figure 20 of your IPC.

I did not find this info listed in the IPC. Where can one find this info for our airplanes?

thanks.

I do not have the IPC in front of me but I believe that would be .032 a standard alum. sheet thickness. Funny that you mention that skin as I have had to repair a series of cracks that were located about the lower L/H engine mount. The cracks originated from the cowling attach holes and had been there for awhile. For some reason they began to grow recently and I was going to replace the panel rather than patch but I settled on the later to limit down time.

It is .032" 2024T3 material. Cracks propogating from the cowling attach holes are common. Whether you replace the entire skin panel or repair by stop drilling the cracks and installing a doubler, I would remove the tinnerman gang strap if it's still installed. Then install a .040" or .050" strip doubler in place of the tinnerman gang strap. Then, instead of re-installing a gang channel, install individual tinnermans for the cowl attach.
The original design places too much load and tranfers too much vibration to a flange of .032" skin. The gang channel does not allow the cowling to get comfortable, whereas the individual slip-on tinnermans (with a slightly larger hole in the skin) will allow the cowling to find the place it wants to be as you tighten the screws from bottom to top.

Del,
Is there any particular reason you say tighten from bottom to top? I always put on my bottom cowl with losely fitted screws, then the top cowl with losely fitted screws, make any adjustments, then start with the middle screws at the top and work my way down. I don't want to sound argumentative or anal, I was just curious if going the other way keeps the metal from cracking. Thank you, Jon

Jon,
The way your doing it is fine. I like to start all the screws before tightening any also. And I also start at the middle of the top cowl, working outboard and down. But I like to start at the bottom and work up on the bottom cowl. Then I like to see how the top and bottom align.

This is just my theory/opinion. The common cracks typically start at the 2 lower bottom cowl screw holes; if you start at the top of the bottom cowl, and there is any slack, it'll pooch out when you get to the bottom. If you tighten up those screws to suck up the pooch (like that terminogy?), it'll pre-load the attachment structure which is only a .032" skin. Also, there is less opportunity to preload when fastening in a relatively flat area (the sides), than in a curved area (where those lower screws are).

If your cowling fits well, and you don't have any cracks yet, I would keep on truckin the way your're doing it. Inspect the tinnerman gang strap because it will crack before the skin.

Del, Thank you for your response. The bottom hole on the bottom cowl was already cracked, (it was that way when I bought the plane). I have not had any other cracks, thank goodness. I am going to stop drill and patch both corners like they were before, but with a thicker patch. I am also going to explore the idea of welding the crack, prior to adding the patch. I don't know if the metal will become brittle in that area or not, that is the only thing that concerns me. I have an 0-360 and constant speed Hartzell, and the shut down shakes more than the 0-300 used to, but in flight there is hardly any noticable difference, (except for the power ). The 80" prop has a dampener and does away with any RPM restrictions, but like I said the vibrations are very slight. I have been toying with the idea of making a thin rubber stand off between the cowl and the fuselage, (bottom hole only) to see if this has any effect. I have noticed several other 170s with the same problem at the same location. Jon

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Here are two things we can do to cut down on the cracking that the cowling on our 170's seem prone to. First have your prop balanced and keep it that way and number two use the new baffle seal material sold by McFarlane. I have have flown about 5 hours sense replacing my orange silicone baffle material with McFarlane's new stuff and I am a beliver. The only question I have is how it will age and it's performance over long term.

I suspect it is metal fatique cracking occuring and due to the age of the materials it is to be expected.

I am looking for the SB which installed doublers at the lowest point of the 170B ('53 and later) cowls to deal with this situation. When I find it, I'll post it. Alzheimers.

Thanks for the replies, I hope I didn't hijack this thread! Seems this is a common problem for alot of our aircraft. Looks like I won't be welding. George, I didn't realize there was an SB on this problem, good find. Jon

wingnut wrote:Jon,
The way your doing it is fine. I like to start all the screws before tightening any also. And I also start at the middle of the top cowl, working outboard and down. But I like to start at the bottom and work up on the bottom cowl. Then I like to see how the top and bottom align.

This is just my theory/opinion. The common cracks typically start at the 2 lower bottom cowl screw holes; if you start at the top of the bottom cowl, and there is any slack, it'll pooch out when you get to the bottom. If you tighten up those screws to suck up the pooch (like that terminogy?), it'll pre-load the attachment structure which is only a .032" skin. Also, there is less opportunity to preload when fastening in a relatively flat area (the sides), than in a curved area (where those lower screws are).

If your cowling fits well, and you don't have any cracks yet, I would keep on truckin the way your're doing it. Inspect the tinnerman gang strap because it will crack before the skin.

Thanks, Del. This is what happened to me. The nut strip cracked first and then the skin cracked back through on of the rivet holes. It is down by the lower left engine mount. I like the idea of replacing the nut strip with .040. I have some and will do that.

The main info I needed was the thickness of the skin. Is there a document that lists the stock thicknesses of the skin on our planes or do we just have to measure them?

Bruce

I have located the Service Letter which applies to cowling cracks at the lower attach points. It is dated 4-22-59 and is S.L. 170/172-15, found on page 20 of the Cessna Service Information Summary I handed out to all attendees at the Kelowna convention MX Seminar. Here is the text, and following is the Service Kit SK172-11 Cowling Reinforcement Instructions. (Although targeting the early C-172 airplanes, this SK is retroactively applicable to 1953 and later 170-B models with the pressure-cowl.)

" Date: 4-22-59, S.L. 170/172-15,
Subject: Cowling Reinforcement

There have been a few cases reported of small fatigue cracks appearing on either side of the lower engine cowling of the Model 172 (and 170B) , where it attaches to the firewall. To correct this condition, we are now installing a doubler in this area, which makes it more rigid and less susceptible to engine vibrations.

This change can be installed on the earlier airplanes very easily, and the installation details are covered in Service Kit, SK172-11, see Fig. 2-3. Kits are available from the Cessna parts Department, priced $3.80. (Ha!)

Just looking at the lower cowling corners, it looks like they should have added one more fastner close to the bottom, but it probably would crack anyway. It would seem some of the newer cowls that mount on rubber washers (?) would be better because the rubber would seem to isolate some of the vibration. Del can probably tell us what he sees on those kind of cowlings regarding their cracking or lack there of. Maybe some of the carbon fiber or fiberglass cowling Selkirk makes are less prone to cracking. I don't think Cessna, or any other aircraft mfg's, planned on us flying these planes for 50+ years.

There's cracks in the cowl, but the cracks in the fuselage are more troublesome to fix as once the crack starts it will head straight to the nearest attachment point (rivet). Stop drilling really isn't an option as you have to contend with the firewall attachment angle. I cut out the damage and fabricated doublers but the fix to make it look pretty will be a new skin. I'll resched that to the next "D" check due to workload conflict.