b767
Topic Author
Posts: 86
Joined: Mon Feb 18, 2008 9:23 pm

How Strong Is The 767 Structure?

Mon Jul 09, 2012 7:35 pm

A few weeks ago a 767,ANA had a landing accident in Narita with a B 767- 300 series with damage to forward fuselage.There is a video of the accident.I am no pro,but I guess what is happening is that the pilot flying is pushing forward on the stickh to get the nose down,combineted with a cg well in front of the main gear, instead of let the nose gently fly itself on the ground.Thing is that this has happend a lot on the 767,and to my knowledge only on the stretched 300 series.Damaged in the same area.So how strong is the 767 structure.Was the 300 seies beefed up structually to compensate for the increase in length?But on the other hand ,I have never heard of a 767 landing gear or wing break up as a result of a hard main gear touchdown, and it is certicated for a rather high number of cycles compared to other medium to long range jets.Are there any 767 doctors here,please share your wisdom.
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: How Strong Is The 767 Structure?

Mon Jul 09, 2012 11:26 pm

Quoting b767 (Thread starter):
So how strong is the 767 structure.Was the 300 seies beefed up structually to compensate for the increase in length?But on the other hand ,I have never heard of a 767 landing gear or wing break up as a result of a hard main gear touchdown, and it is certicated for a rather high number of cycles compared to other medium to long range jets.

This is a really hard question to answer. The real answer is it is strong enough to take the required loads with the required safety margin. It’s hard to give a better answer because loads are analyzed based on the flight envelope requirements. The safety margin varies depending on what type of structure it is, location on the airplane, corrosion, manufacturing method, etc. TdsCanuck is pretty well versed in damage tolerant design so he likely can shed more light.

To answer the question of was the 300 structure beefed up, and the answer would be yes. Structure does get bigger and stronger and the weights go up.

The 767 is a higher cycle airplane than most widebodies. I think it is a 40,000 cycle airplane (might be wrong on that but it is between 40 and 60). The 747 is only 20,000 cycles whereas the 737 is 75-80,000 cycles.

With a high enough sink rate, any airplane will have its gear fail. Also with a high enough sink rate and high enough loads, the forward section of fuselage can exceed maximum design load.
If you have never designed an airplane part before, let the real designers do the work!
 
User avatar
Starlionblue
Posts: 17117
Joined: Fri Feb 27, 2004 9:54 pm

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 12:23 am

Video here. http://www.youtube.com/watch?v=Jw-aUVa3a0U. Wow, that looks like it would have cracked teeth!

Looks like the right wing dipped just before touchdown and the aircraft bounced on the right main. The next thing to touch down was the nose gear, another bounce. Ouch.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
747400sp
Posts: 3850
Joined: Sat Aug 09, 2003 7:27 pm

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 12:30 am

I really, really believe, that a 767, is nowhere near as strong, as an old L1011 or DC10. Heck, I put an old A300 against a 767, and believe that the A300 would win. Just my   
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 12:32 am

Quoting Starlionblue (Reply 2):
Looks like the right wing dipped just before touchdown and the aircraft bounced on the right main. The next thing to touch down was the nose gear, another bounce. Ouch.

It is best to avoid touching the nosewheel down first in a tricycle-geared aircraft   (even if you bounce a main first). Don't know about jet transports, but the generally accepted method of arresting a bounce (so it doesn't degrade into a pilot-induced series of bounces down the runway) is to "blip" the throttle at the peak of the first bounce. Jets might not spool up quickly enough to use this technique...

Nose-first arrivals in many tricycle geared types result in collapsed nose struts, and in the case of single-engined aircraft, often a bent firewall, too  
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
User avatar
jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 1:29 am

Quoting 747400sp (Reply 3):

I think your sentiments would have more than a grain of truth to them. I'd say that due to advances in manufacturing techniques and computer analysis (FEA & CFD), we can now predict loads and stresses with much less uncertainty compared with 30 or 40 years ago. In addition, we can probably manufacture more advanced structures than we could prior.

Because of the greater levels of uncertainty 30 or 40 years ago, aircraft had to be overdesigned in order to meet a given set of certification requirements. These days, we can optimise the structure to a much greater extent, and hence, the amount of overdesign and excess strength is probably much less.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 1:32 am

Quoting b767 (Thread starter):
I guess what is happening is that the pilot flying is pushing forward on the stickh to get the nose down,combineted with a cg well in front of the main gear, instead of let the nose gently fly itself on the ground.

CG is always well in front of the main gear. The issue here was really skewy winds right at touchdown, causing a severe bounce.

Quoting b767 (Thread starter):
Thing is that this has happend a lot on the 767,and to my knowledge only on the stretched 300 series.

It's happened to other airplanes. This is the standard failure mode for overloading the nose gear.

Quoting b767 (Thread starter):
So how strong is the 767 structure

What Roseflyer said...strong enough to meet the requirements (100% of limit load without permenant deformation, 150% of limit load without structural failure). This fuselage deformed but didn't fail, so it went somewhere between 100 and 150% of limit load...in other words, the airplane experienced more load than it was ever supposed to withstand in its life.

The exact margin depends, a lot, on which load case you're talking about and which specific derivative. I don't offhand know which is more restrictive, the negative g requirement or the landing load requirement (those are the two most likely critical cases for this particular loading). The fuselage is almost always fatigue critical, which also means it may have had a lot more static capability than necessary for FAR static loading. The 767 is more likely to be in this regime due to it's relatively high cycle life.

Quoting b767 (Thread starter):
Was the 300 seies beefed up structually to compensate for the increase in length?

Yes, although it's very hard to say without detailed inside knowledge if the particular structural members in this situation (crown skin and stringers) were upsized or not.

Quoting 747400sp (Reply 3):

I really, really believe, that a 767, is nowhere near as strong, as an old L1011 or DC10

That's almost certainly true; the 767 used a decade better analysis tools. The art of structural design is always to make the thing as light as you can and still meet the requirements. The requirements haven't changed (much) over the years but, as the analysis tools get better, the engineers get better at "toeing the line."

Tom.
 
User avatar
Starlionblue
Posts: 17117
Joined: Fri Feb 27, 2004 9:54 pm

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 1:36 am

AFAIK the only aircraft specifically designed to hit on the nose gear are carrier based aircraft. In case the hook snags the cable before the mains touch down (it happens) the nose may well come down (HARD!) before the mains. Keep your tongue away from your teeth for this one...
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
amccann
Posts: 124
Joined: Sat Mar 08, 2008 9:14 pm

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 1:51 am

Quoting b767 (Thread starter):
So how strong is the 767 structure

Along the lines of what Roseflyer has said, the 767 is strong enough for certification. Structural analysis is performed in terms of margins of safety. The term margin of safety MUST be thought of as a step function, a positive margin of safety is a safe airplane or structural component. There is no degree of "more safe" or "safer" structure. http://en.wikipedia.org/wiki/Factor_of_safety

Quoting Roseflyer (Reply 1):
The 767 is a higher cycle airplane than most widebodies. I think it is a 40,000 cycle airplane (might be wrong on that but it is between 40 and 60).

The number of cycles are dependent upon 767 model/variant. (-200/-300, -300F, -400ER)

Quoting Roseflyer (Reply 1):
damage tolerant design

I believe the damage tolerant design principal you are referring to is more related to fatigue damage and not operational damage.

Quoting b767 (Thread starter):
Thing is that this has happend a lot on the 767,and to my knowledge only on the stretched 300 series.Damaged in the same area.

If you think about the situation it makes simple sense. There are a significant number of 767-300s flying every minute of every day in all corners of the globe. The damage being in the same are also makes sense. Imagine the fuselage between the nose landing gear and the main landing gear to be a pen or pencil (something long and slender), a simply supported beam. Now load the tip of the pen/pencil with a moment and notice the point of maximum deflection. As you will notice on the pen/pencil the point of maximum deflection will be near the midpoint of the beam and is approximately the same location as the previous locations of crippled skin on the 767s.

Edit - Corrected beam "analysis"

[Edited 2012-07-09 19:41:02]
What one person receives without working for, another person must work for without receiving. - Ronald Reagan
 
User avatar
jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 3:03 am

Quoting amccann (Reply 8):
Imagine the fuselage between the nose landing gear and the main landing gear to be a pen or pencil (something long and slender), a simply supported beam. Now load the tip of the pen/pencil with a moment and notice the point of maximum deflection. As you will notice on the pen/pencil the point of maximum deflection will be near the midpoint of the beam and is approximately the same location as the previous locations of crippled skin on the 767s.

I get the feeling that if a cantilevered pencil is equivalent to the fuselage between the nose landing gear and main landing gear, the maximum deflection should be at the unsupported end of the cantilever (nose gear end). The maximum bending moment however, would be at the supported end (main landing gear end).

To my way of thinking, the fuselage as a whole is a pair of cantilevered beams supported by the wing, which would tend to place the maximum bending moments somewhere near the forward and aft spars of the centre wing box. I suspect it is the excess bending moment (and shear force) near the forward spar of the centre wing box that causes the crown buckling seen in these incidents.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
amccann
Posts: 124
Joined: Sat Mar 08, 2008 9:14 pm

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 3:45 am

You are correct for a cantilevered condition. But as I said the section of fuselage between the nose landing gear and main landing gear acts as a simply supported beam, not a cantilevered beam. The aft fuselage can be "somewhat" represented by a cantilevered beam but these are all drastic over simplifications.

This is the only website I can quickly find to graphically depict my previous explanation.

http://learntoengineer.com/beam?f=0,u1|10,u2&d=&m=0,100

Edit - I definitely see the validity of the cantilevered assumption (support near the wing box) and applied load at the end of the beam. I'd love to know if anyone on the forum has worked this type of analysis and could tell us the assumptions they have utilized.

[Edited 2012-07-09 21:06:45]
What one person receives without working for, another person must work for without receiving. - Ronald Reagan
 
User avatar
jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 9:18 am

Quoting amccann (Reply 10):

I can certainly see how the fuselage between the nose and main landing gears can act like a simply supported beam. This would be most likely to occur when the aircraft is sitting on the ground, with the bending moment due the weight of cargo, passengers and interior fittings. However, in the case of a nose gear first touchdown, or a heavy rotation onto the nose gear, I still feel that the cantilever beam analogy would be more appropriate. As you say, this is most certainly a gross simplification of the actual situation.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: How Strong Is The 767 Structure?

Tue Jul 10, 2012 4:38 pm

Quoting 747400sp (Reply 3):
I really, really believe, that a 767, is nowhere near as strong, as an old L1011 or DC10. Heck, I put an old A300 against a 767, and believe that the A300 would win. Just my

This is a really hard statement to quantify. In older designs, less capable analysis tools resulted in some assumed safety factors in design which were on the conservative side. With refined tools, structural analysis has allowed some of the safety factors to be relaxed because the analysis is better and also control of manufacturing is better. The result is overall a less conservative and more efficient design, and in some cases not as strong.

However, improved design & analysis methods have allowed designs to be lighter yet just as strong.

And of course you get into cases where when it was designed it met the requirements, but using current analysis tools it would not. There are many parts that are acceptable because they were before and have not had in service problems. One example is magnesium. It used to regularly be used in the flight deck and in the airplane. However with improved analysis tools, the allowable strength for cast magnesium is only about 11-18 KSI in structural loading parts. That’s far off the typical strength of 32 KSI, but because of lack of proper inspection criteria and defects in casting, a conservative casting factor is used. The result is that magnesium parts no longer look very appealing (in addition to the fact that it burns). However pilots love the feel of magnesium, so the flight deck tiller & yoke are magnesium.

Quoting KELPkid (Reply 4):

It is best to avoid touching the nosewheel down first in a tricycle-geared aircraft (even if you bounce a main first). Don't know about jet transports, but the generally accepted method of arresting a bounce (so it doesn't degrade into a pilot-induced series of bounces down the runway) is to "blip" the throttle at the peak of the first bounce. Jets might not spool up quickly enough to use this technique...

Adding throttle is not going to help with how long it takes to spool an engine. However jet transports also don’t land as close to a stall as small props, so pitch control can be used.
If you have never designed an airplane part before, let the real designers do the work!
 
MD-90
Posts: 7835
Joined: Mon Jan 17, 2000 12:45 pm

RE: How Strong Is The 767 Structure?

Wed Jul 11, 2012 5:41 am

Quoting Roseflyer (Reply 12):
However pilots love the feel of magnesium, so the flight deck tiller & yoke are magnesium.

Now that's interesting. What does magnesium feel like?
 
User avatar
HAWK21M
Posts: 29867
Joined: Fri Jan 05, 2001 10:05 pm

RE: How Strong Is The 767 Structure?

Wed Jul 11, 2012 8:12 am

In this particular case,considering the extend of damage to the fuselage,why did the MLG not shear off on impact.....vide the shear bolts.
I may not win often, but I damn well never lose!!! ;)
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: How Strong Is The 767 Structure?

Wed Jul 11, 2012 12:46 pm

Quoting MD-90 (Reply 13):
What does magnesium feel like?

It feels...weird. Like very hard, cold, plastic. It's so low density that it doesn't have the "heft" we usually associate with metal.

Quoting HAWK21M (Reply 14):
In this particular case,considering the extend of damage to the fuselage,why did the MLG not shear off on impact.....vide the shear bolts.

The MLG fuse pins are designed to protect the wing box; this landing primarily loaded the forward fuselage by introducing a huge load through the nose gear. There's nothing the fuse pins could do to protect the fuselage in a case like this; if they'd sheared the plane would have just settled onto the runway and the damage would be that much more extensive (sheared off landing gear make a big mess on their way out, e.g. BA38).

If they had hit hard enough to tear open the wing box (with the related huge fire risk) then the MLG would have sheared off to protect that (even worse) outcome.

Tom.
 
b767
Topic Author
Posts: 86
Joined: Mon Feb 18, 2008 9:23 pm

RE: How Strong Is The 767 Structure?

Sun Jul 15, 2012 12:51 am

Thanks for the comments all of you.Althaoug the landing looks ugly,I was a little suprised to see the amount of damage.I have been seen a landing like this on a short body 737(300 or 500) many years ago,It was perhaps not quite so bad, but it was a non event.Are widebodies more prone to damage if a landing like this happend? And are skin thikness relative to size,so that a big jet have thikkher skin than a smaller jet?
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: How Strong Is The 767 Structure?

Sun Jul 15, 2012 5:50 am

Quoting b767 (Reply 16):
And are skin thikness relative to size,so that a big jet have thikkher skin than a smaller jet?

Skin thickness is most closely related to maximum certified altitude. The 767 was certified up to 43,000ft which is higher than most narrowbodies.

Quoting b767 (Reply 16):
Are widebodies more prone to damage if a landing like this happend?

Yes and no. Widebodies are heavier, so loads go up, but also the structure is stronger to react those loads.
If you have never designed an airplane part before, let the real designers do the work!
 
amccann
Posts: 124
Joined: Sat Mar 08, 2008 9:14 pm

RE: How Strong Is The 767 Structure?

Sun Jul 15, 2012 6:49 am

Quoting b767 (Reply 16):
And are skin thikness relative to size,so that a big jet have thikkher skin than a smaller jet?
Quoting Roseflyer (Reply 17):
Skin thickness is most closely related to maximum certified altitude. The 767 was certified up to 43,000ft which is higher than most narrowbodies.

Fuselage skin thicknesses are related to both the size of the aircraft and its certified altitude. A fuselage is for the most part a simple pressure vessel (http://en.wikipedia.org/wiki/Pressure_vessel). In sections of the fuselage which body bending does not account for much stress the pressure vessel stress is usually the highest and therefore the design condition.

Because the highest pressure vessel stress is highest in the hoop direction, and is calculated via stress = P * R / thickness, you can see that both size and certified altitude drive skin thickness. Wide body airplanes have a higher radius of curvature (larger fuselage) and therefore a higher R. Certified altitude drives the maximum pressure differential (P). Because it is typical to maintain a maximum cabin altitude of 8000ft if an airplane is certified to a high altitude its pressure differential will also be high to maintain the maximum cabin altitude of 8000ft.

Therefore you can scale the skin thickness, with a set P and R, to obtain a stress level satisfactory for the number of cycles you expect the airplane to see during its lifetime. The final driving factor of skin thickness is the number of certified cycles in a lifetime.
What one person receives without working for, another person must work for without receiving. - Ronald Reagan

Who is online

Users browsing this forum: No registered users and 9 guests