Airplanes when they crash are like hot M&Ms: crunchy on the outside, soft & gooey on the inside.

5.26.2006

Wet Paper Sack

Did you know that many airplanes use paper for strength?

Imagine a PB&J sandwich. Now imagine that the 'bread' is made of fiberglass, and the Jam is made of a thick slice of perforated paper. If you do like I do when I eat a PB&J, then you smash it really thin before eating it. This is, in essence what composite honeycomb is. Have a look:






Image courtesy of DIGIRAY: http://www.digiray.com/

Now, having a thick core like this is actually a 'good thing' (best Martha Voice). It keeps the structure light and gives it strength. These are also 'good thing's for airplanes.

But there's a problem. When paper gets wet, what does it do? It falls apart, right? that means that if water somehow got into your composite then your core can fail, the composite colapses and *poof*, no more structural member. Now, normally, well made composites are sealed from the atmosphere. H2O can't get in, and everything is fine.

Just for fun, though, let's imagine that there's a small crack in the composite. It's tiny, hardly even visible; in fact, you can't see it but it's there. Let's say this composite is part of an airplane that does lots of takeoffs in a hot tropical climate. At 30,000 ft the air temperature is -50. What might happen to the warm, moist air that gets into the composite? It condenses, right? Right. As Julia Roberts once said 'Big mistake. Huge!' Now do this, say 500 more times, and now the water starts to pool in the composite. Check this out:




Image courtesy of DIGIRAY:
http://www.digiray.com/

The green sections represent water in the composite. If this were a paper core, then it wouldn't be long before that core fails, and quickly thereafter, the rest of the composite. And failure or delamination, as it's called, is caused by not just water, but almost any type of fluid that can saturate the paper. Here's a report from the TSB of Canada detailing the delamination due to hydraulic fluid:

http://www.tsb.gc.ca/en/media/fact_sheets/A05F0047/recs_a0606_goudou.asp

In order to find this kind of problem, you pretty much have to x-ray or scan the structure. Have YOU seen any x-ray or scanning equipment at the airport? (other than at security, smart***) How does the airline know if this is happening? Short answer: in my opinion, they don't. At least not until a thorough inspection happens. The fix is problematic, too. Heating up the composite can blow up the sealed water, destroying the structure, and trying to suck it out is no good either. Best just to not let it happen. The FAA is starting to take a closer look at maintenance procedures, lately, and trying to determine if more and closer inspection needs to be done to prevent water ingress.

In the meantime, we keep flying.

Btw, what happens to freezing water as it cools? it EXPANDS, right? Right....Happy thoughts until next time.

5.25.2006

Heya!

Lacking any great skill in writing, and only wanting to share a few war stories, I've elected to start my blog here. My name is Bling! and I work for the Industrial-Defense-Maritime-Aerospace-Federal-Banking complex. And 'complex' is the word for it, too. Complex' because it's crazy, confusing, & senseless. Com'plex because anyone working in it needs treatment.

So "tinfiles & aerobabble?" you say. Well, you're asking, so I'm telling: a tinfile, as my friends define it, is an aviation related story or factoid that I think they would find interesting. Sometimes morbidly so. Generally these are the types of things that pop up in their mind, unbidden, as their flight leaves the ground and the fuselage starts to pop and swell from the change in altitude pressure. So if you're a queasy flier, um, best not read any further.

Aerobabble is just that: babble from, related to, or aimed at Aero people. Most of it is crap and completely untrue. However, some of it is eye opening and some I've seen happen myself. I'll let you decide.

A word of caution: if you're one of those "sound bite" readers, that is, unable to finish a paragraph in US News, then you should probably bow out. I'm gonna just assume from this point forward that you're intelligent, think clearly, and have a wry sense of humor. You're gonna need it.

Airplanes are not all that they seem.