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Airliner Stress Questions  
User currently offlinefaro From Egypt, joined Aug 2007, 1620 posts, RR: 0
Posted (1 year 3 months 3 days 3 hours ago) and read 4424 times:

For a normal, uneventful flight what, in descending order of severity, are the greatest mechanical stresses imparted to an airliner structural frame?

What is the single greatest stress event allowed for by airworthiness regulations: severe weather encounters, beyond-MLW landings, MTWO rejected take-offs, etc? If a specific airframe accumulates a statistically significant series of such peak stress events, may it have to be retired early? Who makes that decision, the operator or the maker of the ariframe?


Faro


The chalice not my son
11 replies: All unread, jump to last
 
User currently offlinePhilBy From France, joined Aug 2013, 673 posts, RR: 1
Reply 1, posted (1 year 3 months 3 days 1 hour ago) and read 4377 times:

Quoting faro (Thread starter):
For a normal, uneventful flight what, in descending order of severity, are the greatest mechanical stresses imparted to an airliner structural frame?

That will depend on the part of the structure but normally for an uneventful flight

Landing
Take-off
Gust loading

Quoting faro (Thread starter):
What is the single greatest stress event allowed for by airworthiness regulations

Crash (for much of the airframe) followed by hard landings.

Quoting faro (Thread starter):
If a specific airframe accumulates a statistically significant series of such peak stress events, may it have to be retired early?

After certain peak-stress events the a/c will be inspected. If damage is found the a/c may require more frequent fatigue inspections. Normally the routine take-off, flight and landing cycle (or operating from San Francisco) is more punitive to the airframe than infrequent peak stress events.
Retirement of a modern a/c due to fatigue issues is rare. Normally it is for commercial reasons but increased maintenance costs may play a part.


User currently offlinefaro From Egypt, joined Aug 2007, 1620 posts, RR: 0
Reply 2, posted (1 year 3 months 2 days 22 hours ago) and read 4311 times:

Quoting PhilBy (Reply 1):
Quoting faro (Thread starter):
For a normal, uneventful flight what, in descending order of severity, are the greatest mechanical stresses imparted to an airliner structural frame?

That will depend on the part of the structure but normally for an uneventful flight

Landing
Take-off
Gust loading

Are landings more stressful because of thrust reversal? Otherwise it would seem to me that landings are quite pedestrian occurrences stress-wise...

Also, by gust loading I imagine that this is the non-CAT type.

Quoting PhilBy (Reply 1):
Quoting faro (Thread starter):
What is the single greatest stress event allowed for by airworthiness regulations

Crash (for much of the airframe) followed by hard landings.

What is the peak vertical G deceleration which regulators consider as "survivable" for i) passengers and ii) the airframe itself?


Faro



The chalice not my son
User currently offlineroseflyer From United States of America, joined Feb 2004, 9818 posts, RR: 52
Reply 3, posted (1 year 3 months 2 days 22 hours ago) and read 4287 times:

I would assume it is landing because that is the only real impact in a routine flight.

In general I am not sure how to answer the question since stress analysis is compartmentalized and focused on each area. I have not really seen it at an airplane level.



If you have never designed an airplane part before, let the real designers do the work!
User currently offlinefaro From Egypt, joined Aug 2007, 1620 posts, RR: 0
Reply 4, posted (1 year 3 months 2 days 21 hours ago) and read 4276 times:

Quoting roseflyer (Reply 3):
I would assume it is landing because that is the only real impact in a routine flight.

From what I recall, pavement loadings and change in kinetic energy are much greater during takeoffs, mainly because of the high weights as compared with landing weights...don't know really...


Faro



The chalice not my son
User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 5, posted (1 year 3 months 2 days 20 hours ago) and read 4255 times:

Quoting PhilBy (Reply 1):
Normally the routine take-off, flight and landing cycle (or operating from San Francisco) is more punitive to the airframe than infrequent peak stress events.

Is this due to the corrosive salt air, or are you making a joke?


User currently offlinePhilBy From France, joined Aug 2013, 673 posts, RR: 1
Reply 6, posted (1 year 3 months 2 days 19 hours ago) and read 4234 times:

Quoting LH707330 (Reply 5):
Is this due to the corrosive salt air, or are you making a joke?

The runway surface is sufficient that there are FAA mandated (I believe) fatigue loadings specific to this runway i.e it may be the worst runway in regular commercial use. I'm not entirely sure of the specifics.

Quoting faro (Reply 4):
From what I recall, pavement loadings and change in kinetic energy are much greater during takeoffs, mainly because of the high weights as compared with landing weights...don't know really...

Pavement loadings are higher due to the higher MTOW but the take-off is more gentle.
Wheels-up landing can also be a little severe.

Quoting faro (Reply 2):
What is the peak vertical G deceleration which regulators consider as "survivable" for i) passengers and ii) the airframe itself?

Best place to get the exact answer is CS25 (EASA or FAA) . Airframe must be certified for 6g vertical, I'm sure that I saw something aout seats being certified for 16g but that's hearsay.


User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 7, posted (1 year 2 months 3 weeks 4 days 23 hours ago) and read 3791 times:

Quoting PhilBy (Reply 6):
The runway surface is sufficient that there are FAA mandated (I believe) fatigue loadings specific to this runway i.e it may be the worst runway in regular commercial use. I'm not entirely sure of the specifics.

Something to do with the earthquakes and the construction? How would other Californian airports measure up?



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlinebikerthai From United States of America, joined Apr 2010, 2197 posts, RR: 4
Reply 8, posted (1 year 2 months 3 weeks 4 days 23 hours ago) and read 3783 times:

Early in my career I had the opportunity to tour the Boeing Fatigue test facility. So bear with me as my memory is not that great.

They don't fly the fatigue airplane with max load every flight. They do the standard Ground Air Ground (GAG) profile. Then once in a while they would throw in a series of spikes to simulate random turbulence.

Max Rejected take off is tough on the airplane but the load is more fore aft. The fuselage and wing is very strong in that direction.

Hard/heavy landing is one of the max stress condition where they will need to go in to inspect structure components for damage.

I'm not sure whether they inspect the structure after heavy/severe turbulence.

Quoting PhilBy (Reply 6):
Airframe must be certified for 6g vertical, I'm sure that I saw something aout seats being certified for 16g but that's hearsay.

16 g for seat is for forward loading only and is a dynamic load situation where the load profile varies with time. The 6g vertical also applies to the seat but it is purely a static load.

So if an airframe do experience one of these high load situation, then the fatigue life is affected and is reduced. But the reduction is all estimates and calculation to aide in determining the maintenance intervals of the frame. Final determination of whether the frame is still structurally viable is best obtained through visual inception and determine if the crack growth exceed the point where repair and maintenance cost prohibited.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlinePhilby From France, joined Aug 2013, 673 posts, RR: 1
Reply 9, posted (1 year 2 months 3 weeks 2 days 4 hours ago) and read 3465 times:

Quoting PhilBy (Reply 6):
The runway surface is sufficient that there are FAA mandated (I believe) fatigue loadings specific to this runway i.e it may be the worst runway in regular commercial use. I'm not entirely sure of the specifics.

It seems that there was a "severe bump" on the runway prior to resurfacing.

From CS25 AMC25.491
"Some manufacturers used actual runway profile data to calculate loads. The runway profiles of the San Francisco Runway 28R or Anchorage Runway 24, which were known to cause high loads on aeroplanes and were the subject of pilot complaints until resurfaced, have been used in a series of bi-directional constant speed analytical runs to determine loads."

Also:

"The profiles of those runways are described in NASA Reports CR-119 and TN D-5703."

Quoting bikerthai (Reply 8):
So if an airframe do experience one of these high load situation, then the fatigue life is affected and is reduced.

It is also possible for a one-time peak load situation to increase crack initiation life for a structure as plastic strain can leave residual compressive stresses that mitigate the tensile portion of the cyclic fatigue loading. Think of it like pre-stressed concrete.

Crack initiation due to fatigue loading is probabilistic. It will not occur reliably at the same number of cycles for each identical structure under the same loading. For this reason a large scatter factor is applied meaning that, on average a component will have a fatigue life several times longer than the certified life of the a/c. For critical structure analysis is often based on showing that the time taken for a crack to reach a critical length after it is detectable exceed the inspection interval or that loss of a piece of structure will not cause loss of safe flight.

Quoting faro (Thread starter):
Who makes that decision, the operator or the maker of the ariframe?

The manufacturer will define operationally acceptable structural damage. The operator will decide if he wants to pay repair/maintenance costs.


User currently offlinedh121 From United Kingdom, joined Aug 2013, 9 posts, RR: 0
Reply 10, posted (1 year 2 months 3 weeks 23 hours ago) and read 3275 times:

I seem to recall from my aero apprenticeship many years ago that a severe "braked taxi" was one of the highest stress loading cases. Worst, I guess, in a take off condition.

User currently offlineCitationJet From United States of America, joined Mar 2003, 2469 posts, RR: 3
Reply 11, posted (1 year 2 months 2 weeks 5 days 23 hours ago) and read 2999 times:

Design limit loads are determined for each component on the aircraft (fuselage, wing, tail, gear, flaps, etc) for various scenariois (taxi, turbulence, maneuvers, landing, etc). The component that sees close to a limit load on most every flight are the flaps and speedbrakes, as they are usually deployed close to the max flap extension speed for almost every flight. Landing gear loads for any given landing are usually much lower than the maximum sink rate for the design limit load (10 ft/sec?).

[Edited 2013-10-03 06:45:56]


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