CL-650 Aft Pressure Bulkhead, Design wants to attach lavatory cabinets to it.

[bigbertha912]

I was asked to Analyze an aft Lavatory compartment that is being attached to the Aft Pressure bulkhead for a CL-650. This is my first time seeing it today and basically the lateral brackets (see attached photo) use the existing holes of the support structure to attach.
To me this is a major RED FLAG because i would imagine that F&DT is going to be a real bitch (I am not a DT expert). I also believe that in the 1G sitting, the upper lavatory compartments you can see in the photo will be pre-torquing the structure.

Any advice, or suggestions as to why this is okay or not okay? Analytically i can make it work, but that doesn't mean its a good idea, in my opinion.
I also attached the SRM which shows the angles/webs and stiffeners on the aft side of the pressure bulkhead.

http://files.engineering.com/getfil...4a6cb1&file=Lavatory_Pressure_Blkd_Mounts.JPG

http://files.engineering.com/getfil...ccc-a49d3d664990&file=SRM53-31-03-00.full.pdf

 

[SWComposites]

Sounds like a bad idea.

 

[FastMouse]

I fully agree with SWComposites.

I'm not totally familiar with accepted Part 23 completion practices or Part 23 bulkhead design philosophy, but amongst other things I am a DT guy. I have always treated bulkheads with caution that verges on terror. One might say that the bulkhead its no more sensitive to F&DT issues than the skin, and the skin is routinely modded, so what's the big deal? The big deal is:
(1) Inspectability of the bulkhead is typically much worse than that of the skin.
(2) Corrosion of the bulkhead may be more likely than for the skin (very design-specific, but I can think of examples), and corrosion + poor F&DT detail = severe, severe, severe concern.
(3) (A bit hand-wavy here) if a crack becomes critical in the skin, current design practice is such that will tend to arrest. Not a good day for anyone and never to be encountered if everything is correct, but if it is encountered, the situation can be recoverable. However, if a crack goes critical in a pressure bulkhead, all hell breaks loose. I don't know if Part 23 now precludes subsequent damage to the empennage due to bulkhead failure, such as happened to Japan Airlines Flight 123 where the bulkhead failed (granted, because of an improperly executed repair), and the resulting damage to the vertical stabilizer rendered the aircraft uncontrollable. I recall that Part 25 was amended to prevent empennage damage resulting from bulkhead failure, but it can easily be that a mod is performed on a pre-amendment airframe.
(4) Etc (Time is short, and the key points are above.)

The mod in the sketch is far more than simply adding a couple of unloaded rivet holes remote from any pre-existing stress concentration, and the comment about introduction of additional load to the bulkhead is valid. I guess that the design is well-advanced, and nobody wants to hear about a configuration change. The only thing that I can suggest is that without an utterly compelling reason, perhaps an impossibly compelling reason, do not touch the pressure bulkhead. If the ultimate decision is to proceed with this configuration (and I have never been in a situation where the organisation accepted a bulkhead mod beyond the unloaded rivet example above), make very sure that you have over-conservative and achievable post-mod ICA, and, in particular, have addressed all changes of loading, inspectability and environmental damage (I am thinking about condensation between the bulkhead and the compartment panels, combined with the possibility, however remote, of a leaking hose).

I suggest that you consult with the CVE or DER prior to further activity.

FastMouse

 

[Andries]

I fully agree with the comments above. Remember the old saying "if it looks right it is right". This mod sure does not look right to me.

Andries

 

[rb1957]

CL650 should be part 25.

In one sense the design is good, as it looks to be hanging off the bulkhead, rather than tying the bulkhead to the floor (at least that's how I see it, could easily be wrong). Obviously this adds extra load to the bulkhead, but nothing that can't be analyzed. Maybe improve the bulkhead by adding a second skin, to the top of the stiffeners, so the bulkhead works more load a plate. Maybe back-up the lav attmts with bathtub fttgs (on the aft side of the bulkhead web) tying into the stiffeners.

That said, it would seem to be way more natural to have the lav mounted onto the floor (seat tracks); the way the "intelligent designer" intended things to be.

another day in paradise, or is paradise one day closer ?

 

[bigbertha912]

FastMouse
Thank you for taking the time to write your comments. I am not a DT expert by any means, but in my limited knowledge I questioned the designer how inspections would take place since there would now be several layers of metal and places where all possible cracks would not be visible by the naked eye and require "sepcial" inspection.
I also realized that they would want to attach water systems directly to the pressure bulkhead and if there were to be any leak what so ever the primary structure would have no barrier, giving it direct exposure to corrosion.

Again, thank you and i will try to push that they consult a F&DT DER immediately.
Also, i was looking into going to a DT training somewhere, do you have one that you recommend?

 

[WKTaylor]

BB912..

The aircraft I work on experienced a catastrophic failure of the aft pressure bulkhead during a '3 stooges ground pressurization test' many years ago. It was determined that the bulkhead failed above ultimate design pressure load; however failure would have been delayed to an even higher pressure load, except there were several unauthorized class 2 holes with surrounding nutplate rivet holes drilled in stiffener flanges on the interior side of the pressure-dome. These holes/nutplates were added to mount locally-developed equipment/tools on the bulkhead, out of everybody's way.

Several of the bulkhead stiffeners collapsed [kinked/buckled] suddenly in compression... centered on these holes... which precipitated the inward collapse of the bulkhead/dome. The resultant damage caused the pressure vessel skins to tear-out around the bulkhead in sudden decompression. The aircraft aft fuselage was destroyed causing it to fall onto the concrete ramp. I repeat... it was fortunate this happened during a ground engine run

One lesson learned from military structural mishaps, like this, is that NO secondary screw/bolt holes shall ever be allowed on primary flight critical structure. It is acceptable to permanently fasten secondary attachment structure to primary structure... which may then have the secondary components mounted to it with non-permanent fasteners[crews/nutplates, etc]. This allows for cracking on the secondary structure that is isolated-from [will not transfer the cracking-to] the primary structure.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[FastMouse]

rb1957: Oops, yes, of course. Part 25. I have given myself a rap on the head.

Bigbertha912:

http://www.sigmak.ltd.uk

...but I think that when becoming more experienced in F&DT, nothing beats working alongside a good mentor.

 

[Sparweb]

No way.
Even if the floor doesn't offer enough attachment strength alone, I would use a semi-rigid attachment at the ceiling structure first, rather than touch the aft pressure bulkhead.
There are many kinds of lav's in these Bombardier aircraft but most of the tall ones install the same way: 3 or 4 attachments through the floor and a redundant tie-rod at the ceiling.
Your designer should look at some of these lav's to figure out what to do with his.

You should get your hands on the Bombardier Completion Center Handbook for the CL-600's.

STF

 

[bigbertha912]

I want to thank all of you for your in-put into this matterm, its been two months of explanation and still my manager and director are refusing to listen to me and have basically lied (talked the position into their favour) to the company working with Transport Canada about how this is really no big deal and that F&DT is not necessary (for empahsis, they are not doing the analysis, they are requiring me to do the analysis). I just concluded a meeting about it and I am having a real issue with this. If anyone out there has a good example of when its okay to attach this much to an aft pressure bulkhead and use the aft pressure bulkhead as your primary load path to hold an entire lavatory, please show me, or send me an example. I am really really nervous about this.

 

[rb1957]

hey, you're just a cog in the machine. If your company mgmt think its QED (Quite Easily Done) then it is ... until it isn't (then they'll look for scapegoats, and I think you're first in line).

The design is approved by TC, maybe via your company's DADs, maybe by "mother B"s, maybe by TC.

If TC accept your cert plan, then that's pretty much that. You could "whistle-blow" to TC, but they'll probably ask "who the heck are you ?"

you've had your say in the design, so have other people, particularly the guy who signs the cheque. In my opinion, if you use the language you did to describe your mgmt, look for another job ... either they'll give you the boot, or the situation will be too stressful for you to put up with for too long.

another day in paradise, or is paradise one day closer ?