Firewall Damage Suddenly Appears

Firewall Damage Suddenly Appears

I replaced my engine about 57 hours ago.  It was a great install and I am very happy with it.

At 40 hours, I had some small things taken care of which required removal of both the upper and lower cowling.  Today, the same A&P was in my engine fixing a busted Lamar starter (a whole ‘nother topic) and he called me urgently to say there was a 10 cm crease on my firewall which he had not see 17 hours ago! 

To me – with an untrained eye - the damage looks superficial.  Basically, the skin appears to be slightly buckled in a diagonal line extending down from the pilot’s side of the firewall and extending out across the firewall surface at a 45 degree angle.  It measures approximately 6 to 10 cm long.  The buckle is worst at the top where it’s maybe 5 to 10 mm in height and then it recedes back to the skin surface as it spreads out.   

As I was racking my brain to pin point when I may have landed hard on the nose, which I honestly, I never do, I then remembered that I have hi res photos of my plane with the engine removed showing the bare firewall I took during the engine install.  I looked, and low and behold, I can see the beginnings of the same buckle or crease, but only not so long. But clearly this is the spot I am seeing now only now it's much longer. 

This is a 1973 M model with only 1760 TT airframe.  I know things deteriorate over time, but I am suspicious if maybe something happened when removing the engine? 

Or, was my strut too pumped up or maybe too low? 

Were the new engine mounts too tight on the firewall?

Is this the accumulation of ordinary wear and tear? 

How is this sort of thing fixed? 

How many shop hours does it usually take to repair this sort of thing?

Is my plane grounded now or can I still fly it with precautions taken?

I’m freakin’ out here, so what can you tell me about this situation?

Thanks for your help.

SkyPilot594
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Re: Firewall Damage Suddenly Appears

This year my A&P performed an inspection of the firewall brackets for cracks in the engine mounting points on my 1978 C-172N.  Evidently there is a service bulletin issued by Cessna for this inspection.  I would think this is something to check on your airplane and I wouldn't recommend flying it until this has been checked.

jplaxton00
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Re: Firewall Damage Suddenly Appears

Does that service bulletin apply to your '78?  I don't have it in front of me but I recall that it did not apply to my '77 172.

Gary

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Re: Firewall Damage Suddenly Appears

Hi Gary:

I would have to check with my A&P.  As far as I know, the only significant difference between the '77 and '78 is the 28 volt electrical system.  SkyPilot has a '73, but it seems to me they are all Lycoming powered and would have a very similar firewall mounting arrangement.  I will check with my A&P and see if I can get a copy of the SB.

Jon

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Re: Firewall Damage Suddenly Appears

Well, even if it applies, it's too late now.

What will it take to fix this? Should I ground the plane? Any one know?

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Re: Firewall Damage Suddenly Appears

SkyPilot,
Can you give a better description of where the crease is at?
If there was a hard landing, look at the lower firewall doubler in the area where the lower nose strut trunnion mount attaches; look on each side of it for wrinkles. Also look at the lower firewall, just above the doubler for wrinkles.
The Service bulletin to inspect the engine mount attach brackets does apply to your aircraft. We are replacing a firewall in a 172XP right now for hard landing damage and complying with the SB# SEB07-2 at the same time.
Maybe email a picture?

Del Lehmann
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Re: Firewall Damage Suddenly Appears

Del,

I hope I can describe it... Let me try: 

As the upper portion of the nose strut runs upward along the firewall, there is a raised "shelf" on either side of the nose strut.  These shelves come about a 1/2 inch out from the firewall and cover the V-shaped stringer which is under the surface and is not visible. Make sense? 

I have a crumple or a crease that begins on the left hand side (pilot's side) of this firewall shelf, above the rivet which holds the corner of the shelf together.  At that point, it's about 1 cm high and runs away from that corner at a 45 degree angle along the face of the panel gradually getting shorter and shorter until it's no longer visible. Lower down on the panel, the crease does run between two rivets that are into the V-shaped stringer. The crease is longer that I first believed, now between 10 and about 12 cm long.

UPDATE: Tonight, we removed the panel and insulation under the panel.  No visible bowing or crumpling was observed on the cabin-side of the firewall and no loose or busted rivets were seen. There are no tears or rips in the metal on either side.  We rocked the plane back and forth at the wing tip as the mechanic looked for unusual movement of the firewall, and he saw no buckling or shifting in the area of the defect. 

We are coming to the opinion that someone (perhaps without knowing it) dropped something against the firewall at the top of the shelf, causing the crease to run down the panel.  Our plan is to not service the area but inspect it again in 20 hours or so.  I am also considering having an external skin added as a doubler over the impacted area. 

Does anyone have any additional input to offer?  Remember, I just had a new engine installed and in fact, the crease was present but was ruled out as a big problem.  That was 57 hours ago and for some reason it's grown.  I would hate to have to remove the engine, but I certainly don't want a collapse.

Give me your e-mail and I can send you photos if you like.

Jeff

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Re: Firewall Damage Suddenly Appears

Jeff,
I would like to see some pics.

This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Thanks,
Del

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