Lower Forward fuselage cracking

[piclr60]

I have read posts about some cracking of the forward lower fuselage near the lower engine mounts. I had a crack and it is starting to expand. Has anyone replaced the skin or just put on a doubler?

Thoughts or prayers.

Thanks

Brad

[voorheesh]

Can you post a picture?

[c170b53]

I'd stop drill the crack and watch it. I've had two cracks at those areas. The first one I didn't get to in time and made a doubler picking up the original holes on the stringer of the lower mount and the firewall angles. The second crack on the other side I caught in time, stop drilled and it's been good ( no further crack progression).

[mike roe]

One area to check are the rubber isolators at the engine mount to firewall. These are 2 piece and can be hard over time and not working. Paragraph in the early 100 series service manual on installation torque of these.
Mike Roe

[hilltop170]

If you have an air box cowling, what I think is going on is when you are in flight, the air loads push down on the top of the cowl. You will most likely see wear on the top front of the air box where the two top cowl hinge support angles are bearing down on the front of the air box. This down force also translates into the lower cowl, then into the lower corner of the cowl where the down force is finally reacted by the fuselage skin at the lower corners of the lower cowl. Over time, the two lower corners crack out. My 1951 A Model had similar cracks that were repaired and patched when the plane was restored.

Before the failure process was understood, the upper cowl hinge support angles were cut out where they contacted the air box thinking that would correct the problem. However, it made the problem worse because the top cowl could then deflect further downward putting more stress on the lower cowl corners causing them to crack out.

What I did was to make some teflon bumpers that attach to the top front of the air box center section support with Monadnock fasteners that fit just inside the top cowl hinge support angles. The top cowl angles rest on the teflon blocks which keeps them from deflecting down during flight. That allows the down-force from the top cowl to be reacted into the engine then engine mount instead of the fuselage skins. The pictures show where the Monadnock fasteners and teflon blocks are installed. In 300 hours since restoration, no more cracks have formed.

Also, checking the rubber isolators at the engine mount to fuselage attach points is a good idea but many rubber isolators have been changed to aluminum bushings which eliminate the rubber isolators. If the rubber isolators have collapsed and the bolts are bearing into the firewall, that could allow the engine to sag and which in turn would allow the down force on the top of the cowl to force the top cowl down farther which will apply more stress to the lower cowl lower corners causing the cracks to continue.

Monadnock fastener before teflon block is installed

Teflon block installed and safetied with .040 safety wire

Teflon block with safety wire that wraps around the Monadnock fastener

Top side of teflon block that contacts the top cowl hinge support angle

[piclr60]

Thanks. I will post a picture next week. Unfortunately I flew home a couple hours and the crack that I stop drilled worked back towards the front and a piece the size of a nickel is now gone.

Thanks for the info. I will take this and determine how to attack it. Unfortunately I don't have much patch experience.

Brad

[johneeb]

...........................I don't have much patch experience so I will be looking for somebody trustworthy in the Chicago area to help fix.
Brad

Brad,
You might try another Brad. Brad Brady is a member here and is located in Dwight, look him up in the directory.

[minton]

There are many old posts referring to cracking in that area as well as cracking engine mount stringers. I'd do a search for those posts.

[edbooth]

I am confused are you talking about cracking on the lower cowling, or cracking on the lower fuselage near the engine mount?

[voorheesh]

Fuselage skin behind the lower engine mount. Apparently a known problem. I have two small cracks that propagated from the same location on the right side of my 170A(approx 1/2" each). Stop drill holes kept them from progressing for a few years. I am concerned about corrosion in that area because it is in the exhaust path. Have others installed a doubler on the fuselage skin leaving room for the cowl attach screw? Will be interested to hear more info on this. I searched as Minton suggested but didn't come up with much on this site. I am probably not using the right word(s).

[c170b53]

With a doubler, the cowl will be spread further away from the skin which as you have correctly surmised poses further problems. Stop drilling is a short term solution (although I've been looking at the same hole for awhile now), skin replacement will solve the problem.
In my case, I had a fair amount of surface corrosion that I had removed from the ica in side of the fuselage skins. The corrosion I believe was due to stringer that houses the engine mount had provided a horizontal surface (to the skin) which allowed debris and contaminates from the cabin to accumulate on.

[piclr60]

Thank You for the good advice. If I change the skin, how does that solve the problem? Won't the same thing happen again?

Brad

[mike roe]

My thoughts are the cracks either start, or are not helped by old, hardened, and collapsed rubber isolators in the firewall. These are out of sight out of mind. See if you can look in the log books to see when, if ever they were replaced. Cessna did away with these on the redesigned early 172 firewall. If I had to do it over I would eliminate the isolators and install the seaplane metal bushings in their place. This is only my thoughts and I am sure some will agree or disagree. Replacing the skin, if its the one I think it is, is not a easy task. It is also the glare shield, which means the windshield will have to come out. And also the engine removed. Which come to think of it, will be a good time to replace the isolators. I would go with a internal doubler and hang the engine and move it forward enough to replace the isolators. You might get by with using a Martin head countersunk solid rivet to attach the doubler for a cleaner look on the outside. These have a shallower head than the AN426 countersunk.
Mike Roe

[n2582d]

... You might get by with using a Martin head countersunk solid rivet to attach the doubler for a cleaner look on the outside. These have a shallower head than the AN426 countersunk.
Mike Roe

Hi Mike,
Where do you find Martin head countersunk rivets? Never heard of them. I used NAS1097 reduced head countersunk rivets to install nutplates on the glare shield for the windshield retainer. They are not to be used in structural applications though.

[c170b53]

Makes sense to reason why the cracking took place. In my case the individual nut plates were attached to the skin whereas I believe there should be a nut plate strip at the forward skin lip on both sides below the top mount down to the lower end of the skin. If anyone with a B wants to confirm that or dispute it please feel free. My cracks eminated from an individual nut plate hole.
If any loads other than skin loads are imposed on the skin likely it will crack. So if your cowls fit poorly like Richard suggests then it's likely you'll get cracks somewhere at the attach points. Fatigue in the skin is also a possibility if your vibration devices have deteriorated.
As for skin replacement, depends on which skin and which model in terms of difficulty.