Cessna 172, 182 etc Costly AD.

[scifi]

It has been reported that the FAA have introduced a Cessna AD that checks for cracks in the doorframes of many aircraft....

https://www.avweb.com/aviation-news/ad-affects-thousands-of-cessna-singles/

The repair will likely cost £8-10k and prevents failure of the wing-strut mounting point.
This could be un-economic in some cases.
.

[Pilot DAR]

£8-10k may be a conservative price, I have done a trial of this repair on a 182Q airframe, and it was more work than the service letter suggests. But, when I began the installation of the kit, I found the crack in the underfloor bulkhead which is the concern of the service letter and AD. And, this airframe had only 3300 hours TTSN. I have spoken with the FAA Engineer who drafted that AD (who knows her stuff, as she used to work as an engineer at Cessna), and she reported that there is a suspicion that a factor in this strain on the fuselage is simply the airplane being parked in the wind for decades. A 182 in Wyoming was reported as having the cracks, even though its airframe time was low. But, it had sat tied down, rocking in the wind its whole life.

The strain which causes this cracking is the relative motion of the wings (cabin top) to the landing gear (floor). So in reality, flying the plane gently is about the least straining thing to do to the airframe. It's noteworthy that any floatplane version of the affected types will have crossbracing in the windshield, which is there to resist this straining during rough water operations (no suspension). But even a landplane which had been operated on rough ground a lot, or even sitting parked in the wind, can cause this strain.

The AD does not affect 170's, 180's, 185's and early 172's and 182's as they have a different structure in that area. Similarly, 150's and 152's are not affected, ans their wingstruts mount differently. But, out of an abundance of caution, I did remove and inspect the wingstruts and mounting areas on my 150M during the spring annual inspection. It's not affected by the AD, and it's comparatively low time, but it's still an aging aircraft, so deserving of the additional inspection. But, for the affected airplanes, it is a very valid concern.

[NutLoose]

I did remove and inspect the wing struts and mounting areas on my 150M during the spring annual inspection.

If you pulled the top strut bolts make sure they have gone back together correctly, the packer that goes on the aft side of the spar has a chamfer on it to face the spar and allow for the radius of the spar cap.
Unfortunately only one side of the spacer has the chamfer so if it goes in the other way round ( and it can ) the sharp edge can impinge on the spar radius and crack the spar. It, if memory serves me correctly it does not mention this in the manuals, but i seem to remember the picture shows it the right way round... we had a new 172R with them fitted incorrectly from the factory!


..

[The Ancient Geek]

The required regular inspection is not expensive. The cracking is rare but dangerous, very few aircraft will need expensive work.

[Pilot DAR]

Here's what the bulkhead looks like cracked:

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1600x1200/crack_032f2871bc9036c007596d5fb19d144925a0bb76.jpg
I removed this part of the bulkhead from the airframe I was doing the installation evaluation on

The crack will difficult to see, but more so, difficult to be certain is not there. The radiused cutout (above my "crack" bracket) is in a spot where the intersection of the floor and doorpost is open, and gunk can drop down, so is usually not clean. And, a fuel line passes across in front of the bulkhead, and doesn't help the view. A endoscope camera is a good way to get a look in there, though you have to be sure that the area being examined is clean, and the camera has good enough resolution to see a crack, if it were there. Though it would be very difficult to see by looking down between the floor and doorpost, the edge is exposed just below there. Cleaning the area to be inspected with a pipe cleaner soaked in solvent would certainly make it easier for the inspector. If the edge of the bulkhead were well cleaned, and you had a good magnifier, you might see the crack in the top edge of the radiused cut out by looking down from above, but that would not be a conclusive inspection by itself. It'll be necessary to look upward, outboard and toward the aft, through an inspection hole in the floor under the pilot's/copilot's heels - it's just awkward.

If the repair kit is to be installed, it's necessary to chisel off the red swaged collars over the HiLock pins holding the strut attach forging. It's really difficult to work up under there, even after cutting away the floor as per the Cessna instructions. And, it's vital that while chiseling off the collars, the chisel is not allowed to slip and scar the forging itself. The view under the floor in doing the inspection (on a happily really clean plane!) is:


https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/640x480/182_1977_lh_to_aft_d5bc68b82a5087f75eca8c8fd1aa3325a15836db. jpg

[scifi]

Is there one each side, that both need replacing..?

[Pilot DAR]

The arrangement is left/right symmetrical in the fueslage. Only fasteners are replaced, otherwise, if there is a crack, a doubler kit is installed over the cracked bulkhead. Replacing the bulkhead itself would be a massive job. It is possible that only a left or right side were cracked, and I suppose you'd only have to install the repair kit over a cracked side, though the kit comes both sides, so you may as well install both while you have the plane apart. A 182RG I bought on behalf of a friend 7 years ago had the kit install preventatively at some time before the purchase, though it had been a US government plane, so I suppose the cost of the kit and labour were not factors!

[Droop Snoot]

Is there any consideration of modifying the relief to have one large radius instead of 2 smaller radii? This would considerably reduce the stress concentration factor. The surface should also be deburred and polished. The doubler would make this unnecessary, but re-configuring could prevent future cracks.

Is NDI required to detect the crack?

[Pilot DAR]

It is not possible to rework the bulkhead, nor debur and polish, as that bulkhead is up against a doubler angle on the back, removal of which is not practical. Any attempt at rework would jeopardize the angle behind, which is critical structure. A larger radii would not hurt, if formed before assembly, but the structural characteristics of the bulkhead and angle are very different, so they stretch differently with airframe loads. The angle is more carrying the tensile load between the struts, and the bulkhead is more trying to maintain the stiffness of the doorpost portion of the fuselage (doorposts displacing as a parallelogram under load). The bulkhead is not up to the task in some cases.

The service letter does not mention NDI inspection processes, so I infer it's a visual inspection. NDI would be pretty hard to do effectively down in there.

For pilots, an indicator that the fuselage has a bit too much "give" in the axis of concern is that when the airplane is taxied over uneven ground, the base of the windshield moves left/right relative to the glaresheild at the base of the windshield. For my experience, a 1/8" lateral movement is common for an airworthy fuselage while taxiing slowly over undulating ground, movement of 1/2" laterally would have me inspecting the plane before further flight. But, that's my subjective criteria, by no means authoritative, nor "quantified", as undulating ground is not quantified.

[Droop Snoot]

It is not possible to rework the bulkhead, nor debur and polish.

Pilot DAR....

Thanks, I got a bit confused because I saw the exemplar cracked part had been removed from the aircraft.

Just to develop the point about NDI... how severe is the failure mode? If it is a catastrophic mode, there has to be some protection against an inspection that misses a crack. Which then opens up the discussion of how small of a crack needs to be detected. Is there a repetitive inspection done in this area?

Does the doubler restore the original service life of that structure?

This is all good stuff, but perhaps the most interesting part for me was the hypothesis that the cracking could be due to wind loads while parked. I believe Navy H-3 airframes were evaluated for fatigue damage due to quasi static sea state loads while parked on an aircraft carrier. Definitely a good observation and great piece of forensic load development and stress analysis.

DS