Creaking / cracking during ascent phase - symptom of metal fatigue?

[scroggins]

I was on a flight from Muscat to Salalah (Oman) today and noticed a rather disturbing phenomenon. During the ascent, and only during the ascent, the hull (particularly over our heads and towards the left side of the plane) was making quite a loud creaking / cracking sound, like one would expect from a submarine. I've been flying for years and have never encountered a sound like this. Even my wife, who is by no means a nervous flier (which is more than I can say for myself), was noticeably shaken by the sound.

Because the sound only occurred during the ascent phase, and gradually tapered out as we reached cruise altitude, I suspected it may have been related to pressurization. The sound occurred during segments of the flight where there was not even the slightest hint of turbulence.

As a frequent flier I've read a reasonable amount on the subject, and understand the concept of metal fatigue and its relationship to the pressure changes associated with take off / landing cycles. This particular plane (Boeing 737-800) seemed to be at least 10 years old, and as it is being used on a short flight (70 minutes), I'd guess that it has experienced a higher number of cycles than average.

Could this creaking have been a symptom of metal fatigue? Should it be reported to the airline as a possibly urgent maintenance issue?

Thank you in advance for your time!

[PAXboy]

My first (uneducated) guess is that it relates to the internal fittings of the cabin. As you know, all aspects of the aircraft are designed to flex - like a skyscraper building does. Ther is a heck of a lot of materials in the cabin and the oft discussed topic of overloading of lockers may have a bearing.

IF this was related to the actual hull of the aircraft, I cannot imagine that you would hear such sounds over the noise of the engines during climb.

I do not doubt that you heard something new and expect that professionals can help. I have beeb a pax for 45 years and heard many sounds - but we do not all hear sounds the same way.

[scroggins]

Thanks Paxboy, I appreciate the response. I initially thought it was something to do with the overhead cabins as well, until I noticed that the sound was occurring during perfectly calm stretches of the ascent. It also ceased entirely (apart from a few odd groans here and there) once we reached cruise altitude, and was not noticeable during the descent in spite of moderate turbulence.

I've also read that metal fatigue has been a serious concern on Boeing 737s (admittedly the older models), and less stringent safety / maintenance protocols here in Oman might lead to such cracks slipping through the proverbial one.

[Capot]

Those of us who were lucky enough to travel extensively in the back of the L1011 Tristar would worry quite a lot if creaks, rattles and assorted unidentifiable squeaks and groans did not start when the brakes came off, reach a crescendo on the taxiway, continue during the climb, cruise and descent, reach another crescendo on touchdown as the galley came apart, and carry on until the gate was reached.

The shenigans of the crew in the downstairs galley on a long flight was one of the noise sources, of course, but the rest was down to the aircraft's design concept as a collection of loosely connected but inter-dependent parts flying together.

[L'aviateur]

"less stringent safety / maintenance protocols here in Oman might lead to such cracks slipping through the proverbial one."

Any basis for that remark?

[easyflyer83]

Try not to worry. As others have said it was most probably the fittings within the cabin. Airbus aircraft also creak like mad on the accent including some, very un-nerving (for the average/first time pax) cranking and creaking around the doors.

[sevenstrokeroll]

is it at all possible that :

1. the overhead compartment contained something unusual like a metal container which expanded during CLIMB (we don't say ascent).

2. that a particularly old flight attendant or pilot was nearby and they were creaking?

10years is pretty young for a plane.

[BUGS/BEARINGS/BOXES]

If you happened to be sat near the overwing exits, it's a common phenomenon. Well actually it's just plastic stretching against plastic. The overwing exit cabin side wall feature doesn't fit vey well with the main plastic sidewall. So as differential pressure increases with climbing, do the 'plug' gets more snug, and rubs and creaks as it does so. A clicking sound can sometimes also be heard.

All quiet common. The odd thing is sometimes you notice it, sometimes you don't. I'm sure there is a logic to it somewhere.

[scroggins]

Thanks for all the replies.

Bugs/Bearings/Boxes: We were indeed sitting in the row behind the overwing exits, so hopefully your assessment is correct. Strange though that I often sit near the overwing exits but have never noticed this sound before.

easyflyer83: Thanks and hopefully that is correct.

L'aviateur: I've flown with this airline many times and have always been impressed. However, we both know that things aren't always done up to Western standards here, regardless of what the official line may be.

[Bigmouth]

16000+ hours and I can´t say I have ever noticed nor been informed of any creaking during climb such as you describe.
Then again I tend to stay away from any large moving object that is owned, operated, or maintained by fatalists.
Btw, the reputation an airline has among its customers is almost exclusively based on inflight service levels. The reputation the same airline has among owners, operators or technicians is based on entirely different factors and is not infrequently diametrically opposed to that of the passengers.

[scroggins]

I've sent a quick email to the airline just to be on the safe side. They can do with my observation what they wish, but at least I've done my part.

[easyflyer83]

during CLIMB (we don't say ascent).

You have to remember which part of the forum you are in here. i.e SLF/Questions. Whilst from a purely aviation industry perspective you are correct, from a purely physics based term the accent is actually just as correct, if not more so. Not withstanding that we all knew preceisely what the OP meant.

[sevenstrokeroll]

it was not an admonition...it was a simple correction. educate the masses.

repunzel, repunzel, let down your hair, so that I may ASCEND the golden stair?

CLIMB!!!!

[easyflyer83]

But he didn't need educating because what he said wasn't incorrect. That was my point.

[ExXB]

But I did, and appreciate the gentle way I gained further knowledge. One of the great things about SLF Is we get the guys from the pointy end from time to time. Please don't scare them away!

[scroggins]

I was not at all offended by sevenstrokeroll's correction, but I did find his dismissive response to my observation to be somewhat uncalled for.

Passengers do have the capability to differentiate between normal and abnormal sounds. I've read elsewhere on this board of incidents where passenger observations helped to avert potential disaster. The eyes / ears of the crew can't be everywhere at all times, so shouldn't passengers report apparent anomalies?

[old,not bold]

However, we both know that things aren't always done up to Western standards here,

Let's just stop that one right in its tracks. This kind of complacent, xenophobic rubbish is what allows many "Western" civil air transport operators AND regulators to get away with some of the worst practices in the world.

I have worked all around the world except Africa, including 15 years in the Gulf, 3 of which were in Oman, and my experience is that generally speaking standards are as good or better in most of the countries you would consider as "non-Western" whatever that stupid generalisation is supposed to mean. I was involved in setting up the Oman register and then working under its jurisdiction for 3 years or so, and its standards were then and are now as good as or better than many "Western" regulators.

In the Gulf, especially, much of the work is done and/or certified by contracted, experienced "Western" engineers.

I don't know about Africa; there is obviously evidence suggesting that some States are unable/unwilling to operate and regulate to ICAO standards.

By contrast I have seen operational and maintenance practices in the UK, Europe and USA which would put any State to shame. And they continue unchecked, in Europe at least, while the regulator (EASA, who else) creates an ever-increasing mountain of paper and ineffectual, process-driven, bureaucratic, fluffy oversight.

In the UK, the CAA is now a self-serving "Agency" obsessed with generating revenue to sustain itself and its employees' generous pay and conditions, which seems to have lost all contact with the realities of aviation safety, as bad (ie unsafe) practices continue in its "customers".

Is that what you mean by "Western standards", I wonder? I could show you a few "Western standards" - within 5 miles of LHR T5 - that would make your hair curl, if you had any idea of what you are talking about.

[scroggins]

The comment was hardly xenophobic; having worked in the region for a couple of years myself, I know that even in spite of the best official intentions, people on the ground often don't adhere to the rules. Of course this happens everywhere in the world, including in the West, but then most Western airlines - and everyone knows what that term means - have a safety record which is far superior to most airlines from other parts of the world.

Airlines in the Gulf region do tend to have excellent safety records (exluding Saudi, of course), but seeing the routine breach of rules and regulations that occurs every day here in front of one's eyes, it is not a stretch to begin to wonder if similar breaches are occurring in other fields as well.

[TomU]

Scroggins, as a licenced engineer and inspector of structures for more decades than I care to mention, I would say this:
There are many creaks and groans in aircraft, especially in plastic trim, as has been mentioned. Much of this is normal and no cause for concern.
But there are also chances that frame to skin and frame to stringer cleats, ties and brackets crack and let go, causing more flex than should be in adjacent structure.
As you are an experienced pasenger, and found these noises to be abnormal. it was correct of you to notify the airline concerned. Reviewing, and possibly following up with a physical inspection, on any number of pax concerns is better than one structural failure in flight.
Happy flying.

[old,not bold]

but then most Western airlines...............have a safety record which is far superior to most airlines from other parts of the world.

Oh really?

Africa excluded, you may find that this assertion is unsubstantiated, especially if my understanding of your use of the word "Western" is right.

It also depends on what statistics you use to assess the "safety record".

[scroggins]

Thanks for the constructive comments, TomU, your opinion is much appreciated.

old,not bold: For a single specific source, you could review the stats at airsafe.com. With a few notable exceptions, airlines in the U.S. / Canada / Western Europe / Australia have fatal event rates well under 1 in 1,000,000 - with most under 1 in 2,000,000. For most other parts of the world, this is unfortunately not the case.

And even though various airlines in the Gulf have thus far amassed exemplary safety statistics, the relatively low number of flights compared to, say, American Airlines or British Airways, makes their safety records not quite as convincing.

[givemewings]

I hear this many times especially on Airbus near upper deck doors, they creak a lot on climb and quiet almost as soon as level off. Always been told its normal when reported. Bearing in mind it was not by any means "metallic" but clearly plastic components on the inside of the door flexing (-slideraft cover usually)

[scroggins]

Thanks givemewings, hopefully that's all it was. The sound didn't strike me as being one that plastic would ordinarily make, but of course it's difficult to say that with any degree of certainty.

I should stress again, though, that the sound was primarily emanating from the upper left and left side of the fuselage - neither me nor my wife noted the sound from the exit doors we were sitting closest to, which were on the right side of the plane.

[old,not bold]

the relatively low number of flights .... makes their safety records not quite as convincing.

Why, exactly? That is a statistical fallacy. As you say, various airlines in the Gulf have thus far amassed exemplary safety statistics and indeed they have. The reason is NOT the "relatively low number of flights"; it IS because they have had few, if any, accidents. Gulf Air has been operating for 50 ++ years; from recollection, their record, apart from terrorist action, includes a DC3 which failed to get off the ground before the end of the Seeb runway from which everyone walked away (1960-s?), a total loss of an Airbus (?) at Bahrain more recently, and perhaps 1 or 2 others. Not bad for 50 years, don't you think?

Oh, and two incidents following uncontained catastrophic disintegration of TPE 331 engines in Skyvans due to a gross design error (1970s). After the second the Omani authorities issued an AD for immediate corrective action to ensure that it could not happen again, as it could without the action. The UK CAA (design authority) and FAA decided, under commercial pressure, that 4 years should be allowed, and they would probably do the same today in similar circumstances except that EASA would be the design authority. So much for integrity, an asset sadly lacking in the UK CAA today, as it always has been.

I suspect that you have an instinctive faith that so long as white people, preferably English-speaking, operate, fly and maintain the aircraft all is OK. I know that that is an ad hominen remark, but this belief shines through your posts. If you had had the exposure to UK and European airlines' methods and practices, and to regulators in UK and Europe, that I have had since 1980 and continue to have now you may not be so sure.

Edit; The DC3 incident was not Seeb, it was a strip somewhere towards Seeb (definitely not Bait al Falaj, where the outcome might have been different). Was that called Azaiba? Memory is fading fast!

[Piltdown Man]

Returning back to the OP's question:

Could this creaking have been a symptom of metal fatigue?

When things make noise, something is getting worn out or broken. But the important question is: Will it break on my flight? The answer is most unlikely. Aircraft are near enough hand made and each and every one is just slightly different from another. So some will make more noise than another. I'll suggest that the root cause of the noise described is the aircraft (slightly) changing shape due to pressurisation. Virtually every airline follows a manufacturer's routine inspection procedure which should identify components which need replacement/repair well before their failure. And even if parts do fail, most aircraft still remain flyable (Aloha, Air SouthWest etc.)

[scroggins]

Thanks for offering your thoughts, Piltdown Man. I did have the Aloha incident in mind, though the fact that it was able to land strikes me as a bit of a fluke. China Airlines Flight 611 also came to mind, which as you know did not have such a fortunate outcome.

Still, the key point in your post is the unlikelihood of something actually breaking on one specific flight, out of the the tens thousands in the life of any given aircraft. Yes, it is always extremely unlikely, but nevertheless the mere suggestion of anything out of the ordinary is quite worrisome.

ONB: Your decision to utilize unfounded personal attacks in our discussion makes continuing the debate rather undesirable. However, I will simply point out that my observation was not a "statistical fallacy." If a person is shooting free throws on a basketball court and makes 10 for 10 it is an impressive feat, but I'd put my money any day on the guy who has made 999 out 1,000.