Fatigue issue on floor beams 6

[Kakalip]

Since we are working on DTA, thought may be some one of you who may know about this issue.
Due to the new 14CFR Part 26 we are required to do some DT analysis on some of the old Challenger 850 aircrafts STC. On these aircrafts, we are using floor plinths or pallets to instal the Business Jet Seats and divans. For installing these pallets, we are making some minor changes to the floor beams. Note that the modifications have been substantiated structurally. The floor beams are never considered as fatigue critical previously. Now because the floor beams are included as part of the fatigue critical baseline structure (FCBS) by Bombardier (new part 26 issue) we are required to do a DT analysis of the floor beams.
Since the floor beam is not typically fatigue critical, the inspection intervals are not fatigue driven. So I wanted to know how the inspection intervals are derived by the OEM’s (Boeing) for the floor beams. Let me know if any one of you can provide some information on this.

 

[bf109g]

Kakalip,

I believe that any DTA assumes an initial flaw in the structure, grows that flaw to a critical size at which the structure can no longer sustain limit load, and then divide the growth time by a factor of 3 or 4 to establish inspection intervals in that area. In the case of a floor beam, you're looking at tension due to pressurization and bending due to floor loading. I wouldn't think your load spectra would change too much from original, unless you've strenghened the floor structure to increase area load capability.

Since your STC has modified the baseline structure, you will have to determine how your modifications affect the inspection intervals provided by Bombardier. Without knowing more about your installation, I'd first try to compare your installation detail to somehting previously analyzed by Bombardier like a floor beam repair. Are you using similar fasteners and installation methods? Are you creating a hard point somewhere on the floor beam that will attract load?

There are several people in this forum who have more DTA experience than I. Perhaps they can shed some light on the subject.

 

[rb1957]

which floor beam are we talking about longitudinal or lateral (ie part of the fuselage frame).

if longitudinal, then yes, the fatigue loading from flight is very low, and the crack life of any flaw would be very long. i don't know if the FAA does this, but TC limits inspection intervals (at least on the a/c i'm used to) to 1/2 the a/c life. consider too the effects of corrosion, probably not too significant on an u/floor beam but something i consider whenever i'm below the floor (condensation, spillage, ...)

if the lateral floor beam, then there is a pressure load, model a typical frame, wack on 1 delta p.

either way i don't expect too much to happen, and neither should anyone else. a conservative analysis producing long intervals, a conservative inspection program (visual at 1/2 life) should be more than enough to satisfy "them".

good luck

 

[der8110]

OEM's include it in their full-scale fatigue test article, then use it to calibrate analytical methods. Modifiers do it by analysis. Only difficult part is the spectrum, how many cycles bue to bending loads and how many due to pressurization. Long inspection intervals would be expected. Corrosion is a very common problem on seat tracks and it sometimes involves supports below; but that is not looked at in the fatigue crack growth (but can use material props for wet environments).

Some confusion on which aircraft you are working on - Challenger business jets would not be subject to Part 26, but if it is one of the RJ conversions then yes. In any event, look in the SRM for a concise briefing on DT and Bombardier's view of what factors to divide by to get inspection intervals.

Many aspects of Part 26 are a shake down, be sure to keep up on your payments of street tax.

 

[SAITAETGrad]

The Challenger 850 is a rebranded CRJ200.

http://en.wikipedia.org/wiki/Bombardier_Challenger_850

 

[FastMouse]

Kakalip,

You wrote "Since the floor beam is not typically fatigue critical, the inspection intervals are not fatigue driven. So I wanted to know how the inspection intervals are derived by the OEM's (Boeing) for the floor beams."

If the inspection intervals are not defined by crack propagation then they will be corrosion-driven, either as part of the MRB Structures Program or the integrated CPCP. (I've no idea if your aircraft is MSG-3 or not, but I'd guess so.) The method for deriving corrosion-driven inspection requirements should be detailed in the PPH (Policy and Procedures Handbook). This publication does not make up part of the aircraft's formal documentation, but there are probably a few copies floating around aircraft users' larger organisations since operators are supposed to be part of the MSG-3 process. If you are lucky and can lay your hands on a copy, the PPH should explain how corrosion-driven inspection requirements are derived.

FastMouse

 

[Kakalip]

Thanks all for further inputs and helps!Part 26 affects only the Challenger 850 aircrafts, which were previously called RJ’s. Most of these aircrafts are approved under Canadian LSTC. Transport Canada does not have this part 26 rule yet. Now the reason we are doing part 26 is becoz, TCDS (A21EA) for these aircraft allows for a passenger capacity of up to 50 and now some of the customers want to get their FAA STC using Canadian LSTC (familiarization). Previously when we did the DTA for these aircrafts (25.571) we never considered the floor beams as fatigue critical & no analysis was made to address these modifications.

We do not have access to the Bombardier load spectra & I don’t think they would like to share any of that data with us. Also we do not know what the baseline configuration for these aircrafts based on which the inspection intervals are derived. For a 50 Pax seat capacity the aircraft is equipped with double seats with one leg of the seat installed on the side rails & the other leg installed on the floor beam BL 9.0.

Our STC installs only 6 single seats, 2 double seats & 2 three place side facing divan all installed on the floor plinths. In addition, the seat plinths distribute the interface loads from the seats into all three-floor beams BL 9, 27 & 42, which make the OEM aircraft worst case. Seat plinths are installed using existing holes in the floor beams on BL 9 & 27. However, some modifications are made to the BL42.0 by adding channels & angles to support the plinth. We have to justify that, the inspection intervals provided by OEM are not affected by these modification & the fatigue life of any attachment that is newly created is better than an existing details on that F/B.

The only data that we have from Bombardier is completion center handbook, which gives the floor beam allowable. Since this is, the first time we are doing this, we are not sure how we can proceed to resolve this issue & this problem is not something similar to a pressure vessel penetration problem.

 

[rb1957]

your structure is pretty much only loaded (in fatigue) by pressure. you could model a frame (since it is the lateral u/floor beam your modifying), you could guess (10ksi due to cabin pressure, hopelessly conservative). the idea is to define, as simply as possible, a once-per-flight (GAG) stress. then make a conservative crack model, visual detectable about 1".

the main difficulty you'll have is justifying your hand-waving assumptions, you want to be conservative. your plane is a bit of a "hangar queen" (not a commercial jet used 8-10hrs every day) so additional inspections shouldn't be too much of a problem. of course if you have to have a whole bunch of stuff out of the cabin to look under the floor you don't want to do that often, maybe 1 every 4 years ?? (every 1,000 cycles, sounds very conservative). when would the operator be looking under the floor for corrosion damage ??

 

[Kakalip]

Thanks for your response on the floor beam issue. I am working on the report for this but not sure, if only the pressure loads are sufficient to do the analysis. What about the interface loads that go into the floor beam from the seats & divans?

Please advise.

 

[GBor]

The pressure loads will likely drive, but I'm not so sure that 10,000psi is conservative. I've been performing the DTA on floor beams for 717's and 737's. We've always done DTA on our STC's, so Part 26 compliance was pretty much just a matter of sending in our 8110-3's showing compliance with 25.571, but we tied a rack to the floor beams...it's been a mess.

What ACO are you dealing with? They are not all handling Part 26 the same way.

Calculate the tensile load in the floor beam due to pressurization of the fuselage.

Calculate the required residual strength for a gust load which will induce tension and bending in the beam.

We usually assume a 0.05" rogue crack

Usually assume the crack is starting at a fastener hole.

Since you are looking at pressurization, your crack will tend to run vertically.

See if there is some other feature in the floor beam, or if you will be able to run the crack all the way to the floor.

Grow the crack...should result in a large number of cycles.

Now try the crack the other way trying to come up with some reasonable means of loading the beam so that a crack would grow from fastener to fastener...this can be tricky.

Once you determine the cycles to grow from fastener to fastner, you can then assume a crack continues to "unzip" the fastener row and using a 0.005" crack, see how much more you get.

Initial inspection cycles are determined as the least of the following:

Ndet = number of cycles until the crack is detectable
1/2 Ncrit = number of cycles to failure
1/4 Ndsg = number of cycles in the design service goal
1/3 Nfat = number of cycles to fatigue failure (not exactly sure if this last one is right...would have to double check my notes)

Subsequent cycles are:

(Ncrit - Ndet) / 3 This is to ensure that you inspect a minimum of 2 times between the time you can detect the crack and the time it fails.

So, initial may be 10000 with a follow-on of 1500 meaning that you would inspect first once you hit the 10000 GAG cycles and then every 1500 GAG cycles afterward.

If you are in Seattle, you have the FAA fatigue King, so make sure you do it right. There is a lot more to cover than just what all of us have put into this thread. In the end, you have to have a DER to sign an 8110-3 unless, I guess, you are an OEM, a DAS, or an ODA. Lots to think about.

GBor

Garland E. Borowski, PE
Engineering Manager
Star Aviation