m404
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NW 757 Nose Cone "collapse"

Mon Jul 07, 2008 5:56 am

http://www.cnn.com/2008/US/07/06/nose.cone/index.html

Without closeups I can;t really see a chip or crack pattern indicative of ice but was wondering have any techs here heard or seen of a cone collapsing from former mends or structural failure.
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HAWK21M
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RE: NW 757 Nose Cone "collapse"

Mon Jul 07, 2008 7:55 am



Few months ago,I've seen a Bird strike to the centre of the radome on a B752 with a dia dent of 6-8 inches.Very similiar but smaller dent.And no blood stains visable.

Was this a Bird hit?
Looks very likely.

regds
MEL.

[Edited 2008-07-07 01:18:42]
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EMBQA
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RE: NW 757 Nose Cone "collapse"

Mon Jul 07, 2008 2:00 pm

Maybe even an older bird strike. The composite was cracked or weakened.. water gets in, freezes, causes the composite core to delaminate and over time causes a 'soft' spot
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avioniker
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RE: NW 757 Nose Cone "collapse"

Mon Jul 07, 2008 5:56 pm

If you want a real shock look up the radome soft spot limits for a 747 in the SRM.
In short, without a pressure blow the nosecone should never fail inward unless the operating limits are exceeded by a considerable margin.
It's much the same as a thinwalled beer can. If it isn't dented it'll support your weight just fine . . . until someone touches the side of the can.
That's the principle of monocoque construction and why Boeing planes aren't twice as heavy as they'd otherwise be.
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PhilSquares
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RE: NW 757 Nose Cone "collapse"

Mon Jul 07, 2008 6:44 pm



Quoting M404 (Thread starter):
Without closeups I can;t really see a chip or crack pattern indicative of ice but was wondering have any techs here heard or seen of a cone collapsing from former mends or structural failure.

A couple of years ago, unknown to us, on departure we hit a bird dead centre on the radome on a 744F. Continued on to DXB where we had quite a crowd waiting for us on our arrival. The only indications we had were a WX RDR status for both sets. There was no increased noise, no buffeting, no airspeed abnormalities and the fuel burn was spot on.

Took 5 days to get a radome to DXB from SIN.
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HAWK21M
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RE: NW 757 Nose Cone "collapse"

Mon Jul 07, 2008 7:00 pm



Quoting EMBQA (Reply 2):
Maybe even an older bird strike. The composite was cracked or weakened.. water gets in, freezes, causes the composite core to delaminate and over time causes a 'soft' spot

A weakened radome with water entrapped would likely to give errorous weather radar echoes.Thats a clue.
regds
MEL
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320tech
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RE: NW 757 Nose Cone "collapse"

Fri Jul 11, 2008 3:36 am

We had an A320 bird strike with almost identical results. Impressive damage, but I think we repaired the radome.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
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HAWK21M
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RE: NW 757 Nose Cone "collapse"

Fri Jul 11, 2008 5:27 pm

Whats confusing is that this was not a Bird strike,but the radome collapsed from airloads on a suspected weakened Radome.
The question is what weakened the radome.
regds
MEL
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SlamClick
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RE: NW 757 Nose Cone "collapse"

Fri Jul 11, 2008 6:21 pm



Quoting EMBQA (Reply 2):
Maybe even an older bird strike.

Seems quite likely. Though I've had bird strikes with-and-without blood and feathers remaining. Could also have been struck by a piece of ground equipment, even in a hangar somewhere, then the persons involved decided not to tell anyone.

Quoting PhilSquares (Reply 4):
There was no increased noise, no buffeting, no airspeed abnormalities and the fuel burn was spot on.

Funny that. Instinctively you'd expect buffeting etc. I've had leading edge damage, punctures even, to tail rotor blades without any vibration at all. I've had ice ingestion on a JT8D while looking at the gauges and had no indication at all, despite significant damage. You never know.

But still, this is why we need to pay some attention to the "Flight With Unreliable Airspeed" charts in our manuals. Here is a single-point failure that could cause that, and 'basic pitch and power' would still save the airplane.
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RE: NW 757 Nose Cone "collapse"

Fri Jul 11, 2008 8:26 pm

 
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Starlionblue
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 12:38 am



Quoting SlamClick (Reply 8):
But still, this is why we need to pay some attention to the "Flight With Unreliable Airspeed" charts in our manuals. Here is a single-point failure that could cause that, and 'basic pitch and power' would still save the airplane.

Aeroperu 757.
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wingscrubber
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 4:04 am

Those radomes are usually composite, though I don't know the specific material on the 757. Carbon fibre and glass fibre aerospace composites aren't normally subject to fatigue in the same way as metals, so any 'weakening' must have been caused by a manufacturing structural defect.
The internal volume of a radome isn't pressurised and isn't sealed, so perhaps at altitude, the ground level-pressure air in the radome leaked out; so on descent with high altitude low-pressure air in the radome the external air pressure increased plus the ram affect of the air on the nose created a big enough pressure differential to implode the nose.
It seems like that would have to be one hell of a manufacturing defect...or a big pressure differential. That's my best explanation!
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320tech
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 4:39 am

Really? You guys think air loads did that? I haven't been following the story very closely, how did they determine it wasn't a bird strike?
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
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HAWK21M
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 7:51 am



Quoting 320tech (Reply 12):
how did they determine it wasn't a bird strike?

I would state Bird hit too but.
Since the FAA states so.....Probably due lack of Bird residue & the Altitude at which the collapse occured.
I still feel previous structural weakening of the radome due an impact earlier or inadequate previous repair job.

regds
MEL
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SlamClick
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 4:48 pm



Quoting 320tech (Reply 12):
You guys think air loads did that?

Almost certainly. Of course this would only happen to an already weakened structure. The basic part is engineered to handle the airloads but that assumes no damage to its material. Ding it, and you weaken it, and that is one of the problems with aircraft made from man-made, composite materials. It is a challenge to find simple tests for the integrity of the material. A bag cart bumps a cowl ring and you might have to replace it just because you don't know if it was weakened. Metal dents. Composites may hide their weaknesses.

There was a well known incident where one of the super guppies had its upper fuselage area collapse in flight, under airloads.

Countdown until 2H4 posts a picture of that:

three
two
...
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tdscanuck
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 5:04 pm



Quoting SlamClick (Reply 14):
The basic part is engineered to handle the airloads but that assumes no damage to its material.

Not if it's a damage-tolerant design, which the 757 is. The radome probably isn't considered primary structure so may not have that kind of analysis, but in general you always assume the material is damaged prior to calculating limit loads.

Quoting SlamClick (Reply 14):
Ding it, and you weaken it, and that is one of the problems with aircraft made from man-made, composite materials.

That's a problem with all monocoque and semi-monocoque structures (the shear capability of dinged panel drops precipitously). It's not unique to composites.

Tom.
 
SlamClick
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 5:18 pm

Quoting Tdscanuck (Reply 15):
That's a problem with all monocoque and semi-monocoque structures (the shear capability of dinged panel drops precipitously). It's not unique to composites.

Absolutely correct, but what I was referring to was the tendency of sheet metal to dent, showing us where the weakness has been introduced. Metal castings and forgings and the like also get weaknesses and there are a number of tests for that as well (dye checks, magnaflux and the like) but I have to confess much ignorance in the area of such testing for the various types of "composite" materials. My experience so far is mostly in just seeing it replaced when damage is suspected.

[Edited 2008-07-12 10:21:33]

second edit: First edit did not post.

While the radome is not 'primary' the importance of its integrity should not be diminished. Since on almost all jets, the pitot and AOA probes use the airflow just downstream, aicraft have been lost, as alluded to above, because of the sort of instrument error that can be caused if the radome is severely deformed or missing.

[Edited 2008-07-12 10:26:15]
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2H4
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 10:10 pm



Quoting SlamClick (Reply 14):
There was a well known incident where one of the super guppies had its upper fuselage area collapse in flight, under airloads.

Countdown until 2H4 posts a picture of that:

Here you are, sir:

http://i37.tinypic.com/w1b9xc.jpg

Click for larger version


Would have posted it sooner, but I've been out all day.  Wink

2H4
Intentionally Left Blank
 
SlamClick
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 10:33 pm



Quoting 2H4 (Reply 17):
Would have posted it sooner, but I've been out all day.

A little slow on joinup but you fly good formation.  Smile
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2H4
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RE: NW 757 Nose Cone "collapse"

Sat Jul 12, 2008 10:44 pm

Intentionally Left Blank
 
tdscanuck
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RE: NW 757 Nose Cone "collapse"

Sun Jul 13, 2008 12:41 am



Quoting SlamClick (Reply 16):
Metal castings and forgings and the like also get weaknesses and there are a number of tests for that as well (dye checks, magnaflux and the like) but I have to confess much ignorance in the area of such testing for the various types of "composite" materials. My experience so far is mostly in just seeing it replaced when damage is suspected.

Composite NDT has come a long way. Though still not quite as good as metal, you can pick up an awful lot with ultrasonics. There's also the highly sophisticated "tap test" that's actually quite good for detecting skin/core disbond in honeycomb panels. NDT is good enough that you can make the solid-rocket boosters for a Delta out of CFRP and catch any critical cracks before the case delivers.

Tom.
 
slimshady
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RE: NW 757 Nose Cone "collapse"

Mon Jul 14, 2008 9:43 pm



Quoting Tdscanuck (Reply 20):
Composite NDT has come a long way. Though still not quite as good as metal, you can pick up an awful lot with ultrasonics. There's also the highly sophisticated "tap test" that's actually quite good for detecting skin/core disbond in honeycomb panels. NDT is good enough that you can make the solid-rocket boosters for a Delta out of CFRP and catch any critical cracks before the case delivers

I agree, mostly, but to tag on to your point,

In composite NDT, you are looking for existing damage, delamination, water ingress or the presence of existing moisture. This can be done with a tap tester, (most common), thermography, X-Ray, and maybe some sort of ultrasoinc tester. (in my days of working on composite commercial radomes [6 years worth] we never used ultrasonic)

It is hard, or even impossible to determine if a certain area will be more susceptible to damage on a composite strucrue. (In metalurgy, it is much easier) I mean, how do you physically test for strucural integrity without destroying the sample? You cant, which is why most repair facilities use test coupons which are destroyed in a lab via drum peel or lap shear tests to make sure the test fails within the proper paramaters.

You can bet the test coupon records for our NWA bird are being retreived and evaluated by our friends at the FAA who are only here to help.....  Smile
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smcmac32msn
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RE: NW 757 Nose Cone "collapse"

Tue Jul 15, 2008 4:39 am



Quoting SlamClick (Reply 8):
Seems quite likely. Though I've had bird strikes with-and-without blood and feathers remaining. Could also have been struck by a piece of ground equipment, even in a hangar somewhere, then the persons involved decided not to tell anyone.

You are joking about the hangar rash right??? If somebody damaged that in the hangar, then it got towed to the gate, where there would be 100+ people in the gate area or on the ground that would notice something THAT MAJOR that is wrong with the plane. If this IS what happened, I'd suggest everybody in DTW get fired that worked the outbound flight, and the flight crew for not doing a proper pre-flight and noticing it.
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Starlionblue
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RE: NW 757 Nose Cone "collapse"

Tue Jul 15, 2008 6:39 am

I think what Captain Click meant was that it might have been damaged without any evidence visible from the outside. Minor damage in other words. Obviously if the radome was caved in when it left the hangar people would have noticed. Then in flight the minor damage became major.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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HAWK21M
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RE: NW 757 Nose Cone "collapse"

Tue Jul 15, 2008 7:50 am



Quoting Starlionblue (Reply 23):

I think what Captain Click meant was that it might have been damaged without any evidence visible from the outside.

That what I read from the post too....The damage may have weakened the structure but not visible externally to that extent.
regds
MEL
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