What would cause a 738 to creak like a submarine at depth?

[slip and turn]

Flown down the back on a couple of 738s recently with sound effects which surprised me. As they were from the same fleet there is a faint possibility it was the same airframe.

On depressurisation during taxy, and in smooth air during climbout there was a distinct irregular rapping type creak from the upper rear fuselage, possibly behind the luggage bins. Seemed to be from about Row 22 and back a few rows. It was coming from at least two points, one on each side. On both flights it was distinct enough for more than my pair of eyes to be scanning the ceiling/bins in puzzlement.

It was loud enough to still be noticeable during the take off roll over the din of the engines right outside the window. On the second flight I noticed a ceiling panel had been incorrectly re-seated above seat 20C. I also noticed that three or four internal cladding side panels had been refitted with variable spacing between, rearward of the starboard wing.

The creak or "rapping" sound if you like might be the same kind of noise you would get if you rapped your knuckles hard enough to hurt on a hard surface like a table surface, or possibly a luggage bin cover.

The frequency of the "creaks" / raps tailed off as we climbed in smooth air and only reappeared in the first phase of flight when we hit a little light chop near top of climb.

No sound was heard during cruise. No sound was heard again until about 5 minutes into descent. Then it returned intermittently for about 5 minutes, then was barely noticeable until the landing roll when it was at it's loudest and most frequent as we shook under reverse thrust. My teenage FSX pilot son fancied some of it was noticeable whilst rudder was being used in the final approach, but I didn't associate it with that.

Now then, I am pretty certain that this noise could not be explained by loose items in the bins, and I would be very surprised to learn it was the normal sound of the outlet valves or other parts of the aircon / pressurisation system, and having experienced it twice now without being reduced to a dribbling wreck, I have an open mind about it.

Any takers?

[OzExpat]

Buggered if I know. I've done my fair share of travel in 738s, albeit mostly within Oz, but I've never noticed the symptoms that you refer to. However, it's possible that you've answered the question yourself because of the effects of pressurisation changes.

Maybe someone else has a better idea? I suspect there's been enough problems over the years that all decent operators take all the now standard precautions with airframes that have logged a lot of pressurisation cycles.

[FCS Explorer]

1. "variable panel spacing" is simply an US-engineering feature.:}
2. depress is not during taxi in but starts during descend, valve full open either at preset landing field alt or at touchdown, whatever comes first.
3. no offense, but quite frankly posting about "i heard a noise in a boeing" is a waste of sunday. especially near the wings/main gear bay could be anything, could be nothing. i had a senior stewardess sitting there on her vacation flight. the sound she reported turned out to be the flap drive.... go play.

[Jetdoc]

I would take a wild guess and say that the noises are coming from the riser ducts that take the conditioned air from the manifold to the main distribution duct in the ceiling. Most Boeings have atleast 2 ducts per side. I never worked on a later series 737 but I believe that they would still be located just forward of the wings.

[slip and turn]

FCS - depress to go up. repress to go down, even on a Sunday.

Where was that you said you had a senior stewardess again? ... :hmm:

Thanks for the input doc ... maybe those bits you speak of are further back in the NG? (like 5 or 6 rows behind the wing?)

[FCS Explorer]

FCS - depress to go up. repress to go down, even on a Sunday.


the dive tank in a sub marine or what?
and even if the cabin alt during cruise is at about 6000ft higher than airport elev we still call it "pressurized"

[mustafagander]

I wonder if the sounds could be the movement of the locating thingos for overhead bins etc. as the fuselage expands/contracts during a pressurisation cycle? The B747 classic flight deck used to be nearly symphonic sometimes as the FE's panel moved around. The Boeing fix was to grease the locating dowels. It worked!!