richiemo
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A380 Thrust Reversal

Wed Oct 22, 2008 2:29 pm

Curious as to why only the inboard engines on the 380 have thrust reversers. Are the engines sostrong that only two are needed or does anyone know of any other reason why this would be.
 
manu
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 2:35 pm

Simple... the outer engines overhang on turf or other material (off the runway). Reversing there would stir things up and cause debris to be ingested into the engines. So they don't do it.
 
manu
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 2:40 pm

You can see what I am talking about in this 747SP landing for retirement on a 15m wide runway. Normally it is 60m.

http://www.youtube.com/watch?v=ap_nyRzEOMI
 
qf744fan
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 2:45 pm



Quoting Manu (Reply 1):
Simple... the outer engines overhang on turf or other material (off the runway). Reversing there would stir things up and cause debris to be ingested into the engines. So they don't do it.

Spot on!

Add to this the amazing design of its wing. The "gulling" effect is on full display when this beauty is on final approach, enabling it to come in no faster than a much smaller aircraft
 
WingedMigrator
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 3:31 pm



Quoting QF744FAN (Reply 3):
The "gulling" effect is on full display when this beauty is on final approach, enabling it to come in no faster than a much smaller aircraft

The gulling has nothing to do with approach speed, and everything to do with ground clearance for the engines. The low approach speed is enabled by the very large wing, which carries less weight per unit area than most other airliners.
 
qf744fan
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 3:43 pm



Quoting WingedMigrator (Reply 4):
The gulling has nothing to do with approach speed, and everything to do with ground clearance for the engines. The low approach speed is enabled by the very large wing, which carries less weight per unit area than most other airliners.

That's me not being very clear... my apologies.

Such a large wing surface area, and such a long wing/wide wing span. If they weren't gulled they'd be scraping the ground on full tanks.

Large surface area allows for slower speeds with safety due to greater increases in lift production
 
AA737-823
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 8:43 pm



Quoting Manu (Reply 1):
Simple... the outer engines overhang on turf or other material (off the runway). Reversing there would stir things up and cause debris to be ingested into the engines. So they don't do it.

That is not the reason AT ALL.

In fact, in five replies, only ONE of them even ADDRESSED thrust reversers!

Quoting Richiemo (Thread starter):
Are the engines sostrong that only two are needed or does anyone know of any other reason why this would be.

By that logic, you'd only need 2 to takeoff, as well.

The answer is WEIGHT. Airbus had originally designed the aircraft with four reversers (as anyone in their right mind would expect on the heaviest, hardest-to-stop airliners ever mass produced), but late in the game, when the aircraft was coming up SEVERELY overweight, they decided to yank out the reversers on #1 and #4. Further, it was determined that it would really be a great idea to pull off the brakes on the aft axles of the body gear. Those six wheel trucks? Yep, four brakes only.

It all makes perfect sense to me- an airplane drastically larger than a 747, yet with the same number of wheel brakes, and only half the reversers.

Off of my soapbox, if you'll do a search on this forum, you will either find or not find a VERY LONG, HEATED discussion we had here. Not surprisingly, the opinions were based more on which side of the Atlantic ocean the poster was typing from. The Euro's thought it was a great idea, as reversers can't be used for landing runway length certification anyway, and the Americans thought it was stupid, because there are instances (particularly in Anchorage, Alaska, where the ground is icy 7 months a year, and UPS and FedEx were both planning on sending A380F's.... by the way, did I mention that it's illegal to dispatch an aircraft on known icy taxiway/runway conditions with an INOP thrust reverser? Yeah, how interesting) anyway, there ARE INSTANCES where you want all the reverse you can get.

I hope that answers your question.

It has nothing to do with FOD ingestion- by that standard, the 346 and 744 wouldn't have them, either, as there are more than a few runways where 744 outboard engines are exposed to turf.
 
WingedMigrator
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RE: A380 Thrust Reversal

Wed Oct 22, 2008 11:34 pm



Quoting AA737-823 (Reply 6):
That is not the reason AT ALL.

FOD is very much a reason for not having thrust reversers on the outboard engines. No other airliner has 50 meters between outboard engines and routinely operates on 45 m runways... certainly not the 744, nor the A346.

Refer to Norris's book on the A380 for more details of the story; there is rarely a single reason to do something in engineering. FOD and weight were both important considerations.

Quoting AA737-823 (Reply 6):
Further, it was determined that it would really be a great idea to pull off the brakes on the aft axles of the body gear.

Those brake units were never "pulled off" the A380-800 design, since there is sufficient capacity without them. They were present in the heavier A380-800F baseline. Future A380 models, with MTOW well north of 600 tonnes, may add those brake units. This is only one of many "scars" specifically intended for growth of the airframe.
 
tdscanuck
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 2:00 am



Quoting Richiemo (Thread starter):
Curious as to why only the inboard engines on the 380 have thrust reversers. Are the engines sostrong that only two are needed or does anyone know of any other reason why this would be.

That was what they could get EASA & FAA to agree on...they originally wanted none, because they don't actually need them to meet the regulations.

Quoting QF744FAN (Reply 3):
The "gulling" effect is on full display when this beauty is on final approach, enabling it to come in no faster than a much smaller aircraft

True, it has a good approach speed, but that doesn't help the fact that you have a huge amount of mass to stop.

Quoting AA737-823 (Reply 6):
Quoting Richiemo (Thread starter):
Are the engines sostrong that only two are needed or does anyone know of any other reason why this would be.

By that logic, you'd only need 2 to takeoff, as well.

Not at all...the performance requirements for reverse and takeoff are extremely different. On an A380, you only need 3 to takeoff anyway, but there's no regulation governing engine-out reverse capability.

Quoting AA737-823 (Reply 6):
The answer is WEIGHT. Airbus had originally designed the aircraft with four reversers (as anyone in their right mind would expect on the heaviest, hardest-to-stop airliners ever mass produced)

Actually, I heard that they wanted none from the start (weight, cost, & complexity). Unfortunately, I can't find any data to prove which version is true.

Quoting AA737-823 (Reply 6):
It all makes perfect sense to me- an airplane drastically larger than a 747, yet with the same number of wheel brakes, and only half the reversers.

Number of brakes doesn't mean squat...energy capacity of the brakes is what matters. If Airbus could design a brake with ten times the energy capacity of the 747 brakes, they could probably do just fine with four.

Likewise, number of reversers doesn't mean anything by itself. You need to look at the total reverse acceleration (total reverse thrust / mass).

Tom.
 
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HAWK21M
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 9:25 am

Wasn't exceeding planned Weight limits,the real issue for A380 opting out of the Outboard T/Rs?
regds
MEL.
I may not win often, but I damn well never lose!!! ;)
 
Finkenwerder
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 9:31 am

All above partially correct....the wing is not designed to sustain 4 engines in TR torsion would be excessive on the outer pylons.
 
AA737-823
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 9:50 am



Quoting Finkenwerder (Reply 10):
All above partially correct....the wing is not designed to sustain 4 engines in TR torsion would be excessive on the outer pylons.

Well that's the easiest explanation to believe so far, considering that the ONLY A380 wing ever tested to failure actually broke BEFORE THE LEGAL LIMIT.

Quoting Tdscanuck (Reply 8):
Likewise, number of reversers doesn't mean anything by itself. You need to look at the total reverse acceleration (total reverse thrust / mass).

True, but I think we can agree that FOUR reversers will provide more acceleration than TWO, all else being equal, which in this case, it is.

Quoting Tdscanuck (Reply 8):
because they don't actually need them to meet the regulations.

No airliner does, yet with the noted exception of the BAe-146, all of them managed to get them... and they're heavy on all the other ones, too.

Quoting Tdscanuck (Reply 8):
Actually, I heard that they wanted none from the start (weight, cost, & complexity). Unfortunately, I can't find any data to prove which version is true.

That certainly may be the case, I don't know. But I am under the strong impression that there were four reversers until VERY late in the game, well after the first aircraft had been assembled.

Quoting Tdscanuck (Reply 8):
If Airbus could design a brake with ten times the energy capacity of the 747 brakes, they could probably do just fine with four.

Yes, you're right. But the 747-400 has very good brakes- good enough that you can dispatch with an ENTIRE gear's worth of brakes (4 wheels, I mean) INOP. With the heavier A380 weights, do you (or anybody here) know if they've got that capability.
 
Finkenwerder
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 9:53 am

Bear in mind for performance purposes reverser availability gains no credit for stopping distance. It's all about the brakes...
 
PhilSquares
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 9:57 am



Quoting AA737-823 (Reply 6):
by the way, did I mention that it's illegal to dispatch an aircraft on known icy taxiway/runway conditions with an INOP thrust reverser?

Not according to our MEL!
Fly fast, live slow
 
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Starlionblue
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 12:26 pm



Quoting AA737-823 (Reply 6):
The answer is WEIGHT. Airbus had originally designed the aircraft with four reversers (as anyone in their right mind would expect on the heaviest, hardest-to-stop airliners ever mass produced), but late in the game, when the aircraft was coming up SEVERELY overweight, they decided to yank out the reversers on #1 and #4.

It's the other way around. It was originally designed with no reversers. Comparing brakes and reversers, the brakes are hugely more effective. It's all about the brakes. Reversers are a nice to have in the vast majority of cases.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Tristarsteve
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RE: A380 Thrust Reversal

Thu Oct 23, 2008 3:45 pm



Quoting PhilSquares (Reply 13):
Quoting AA737-823 (Reply 6):
by the way, did I mention that it's illegal to dispatch an aircraft on known icy taxiway/runway conditions with an INOP thrust reverser?

Not according to our MEL!

We have the following
B767 and B757 One reverser may be inop, Operation on Slippery runways not allowed.
A320 Both reversers may be inop. It is inadvisable to operate on slippery runways as the required distance may be more than available.
B777 One may be inop. no restrictions
B744 Two may be inop, must be symmetrical, and most of the brakes must be working.
 
tdscanuck
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RE: A380 Thrust Reversal

Fri Oct 24, 2008 3:56 am



Quoting AA737-823 (Reply 11):
Well that's the easiest explanation to believe so far, considering that the ONLY A380 wing ever tested to failure actually broke BEFORE THE LEGAL LIMIT.

That's true, but it doesn't mean they violated regulations. I know you didn't say that, but you seem to be implying there was something improper...the FAR's (and whatever EASA is calling their regulations now) just require that the OEM show that the wing withstand ultimate load without breaking...they don't say how you have to show that. A test to almost the limit, plus analysis of design changes, may be sufficient. In the case of the A380, it was.

Quoting AA737-823 (Reply 11):
Quoting Tdscanuck (Reply 8):
Likewise, number of reversers doesn't mean anything by itself. You need to look at the total reverse acceleration (total reverse thrust / mass).

True, but I think we can agree that FOUR reversers will provide more acceleration than TWO, all else being equal, which in this case, it is.

Well, yes, but eight engines will provide more thrust (and reverse) than four, all else being equal...that doesn't mean they should have eight engines.

Quoting AA737-823 (Reply 11):
Quoting Tdscanuck (Reply 8):
because they don't actually need them to meet the regulations.

No airliner does, yet with the noted exception of the BAe-146, all of them managed to get them... and they're heavy on all the other ones, too.

It's not necessarily about regulations, it's also about performance. Despite the rampant myth on airliners.net, current certifications *do* allow you to take reverser credit on wet runways, which is important for some performance situations.

Quoting Finkenwerder (Reply 12):
Bear in mind for performance purposes reverser availability gains no credit for stopping distance.

Only on dry runways...on wet, you can take credit. This was a fairly recent change (last ~12 years or so)...all the prior stuff never had certified limits for wet runways.

Tom.
 
Finkenwerder
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RE: A380 Thrust Reversal

Fri Oct 24, 2008 5:29 am



Quoting Tdscanuck (Reply 16):
Only on dry runways...on wet, you can take credit. This was a fairly recent change (last ~12 years or so)...all the prior stuff never had certified limits for wet runways.

Tom.

Your quite right, the obvious exception....  banghead 
 
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HAWK21M
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RE: A380 Thrust Reversal

Fri Oct 24, 2008 10:45 am



Quoting PhilSquares (Reply 13):
Not according to our MEL!

Which types are you reffereingtoo.
Out here its no-go under wet/rainy conditions with Inop T/R for
A300/310/320/330/340/B737/747/757/767/777.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
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Jetlagged
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 3:29 pm



Quoting AA737-823 (Reply 6):
Off of my soapbox, if you'll do a search on this forum, you will either find or not find a VERY LONG, HEATED discussion we had here. Not surprisingly, the opinions were based more on which side of the Atlantic ocean the poster was typing from. The Euro's thought it was a great idea, as reversers can't be used for landing runway length certification anyway, and the Americans thought it was stupid, because there are instances (particularly in Anchorage, Alaska, where the ground is icy 7 months a year, and UPS and FedEx were both planning on sending A380F's.... by the way, did I mention that it's illegal to dispatch an aircraft on known icy taxiway/runway conditions with an INOP thrust reverser? Yeah, how interesting) anyway, there ARE INSTANCES where you want all the reverse you can get.

Please don't generalise so. I'm a "Euro" and I thought reversers were probably a good idea on the A380. I'm certain there are "Yanks" who think the A380 is marvellous. Reducing the A v B debate to nationalism is crazy.

My argument was they should either have four reversers or none. Not two with and two without. If one of the inboard engines is out, the other inboard reverse thrust would create a lot of asymmetry, so effectively you have no reverse.

The usual answer to the icy runway/no reversers argument is "divert". However with a mammoth like the A380 your choices are more limited. Not to mention 500 pax to transport to their proper destination.  Wink

Quoting Tdscanuck (Reply 8):
True, it has a good approach speed, but that doesn't help the fact that you have a huge amount of mass to stop.

It's also about energy, which increases with the square of speed. So a few knots off approach speed makes a big difference to stopping distance.

Quoting AA737-823 (Reply 11):
No airliner does, yet with the noted exception of the BAe-146, all of them managed to get them... and they're heavy on all the other ones, too.

Not just the 146. The Fokker F.28 had no reversers either. Actually the 146 has a low ground idle setting which is surprisingly effective in increasing deceleration, not to mention liftdumpers AND a speedbrake (like the F.28).

The FOD argument is a red herring in my view. The A380 outboard engines aren't much further outboard than those of a 747. As long as reverse is cancelled before speed gets too low, foreign objects won't get into the engines.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
PhilSquares
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 4:04 pm



Quoting Jetlagged (Reply 19):
If one of the inboard engines is out, the other inboard reverse thrust would create a lot of asymmetry, so effectively you have no reverse

To be honest, there is very little asymmetry from a reverser inop. Generally, the EEC/FADEC will limit reverse thrust to 70% N1 or so. I have worked for airlines where SOP is to use all reverse in an engine out landing. Don't worry about asymmetry as normal steering through the rudders will take care of that.
Fly fast, live slow
 
PGNCS
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 6:31 pm



Quoting AA737-823 (Reply 6):
Airbus had originally designed the aircraft with four reversers (as anyone in their right mind would expect on the heaviest, hardest-to-stop airliners ever mass produced),

Hardest to stop? It has slow approach speeds and excellent brakes. We all know you hate Airbus, but give it a rest.

Quoting Tdscanuck (Reply 8):
True, it has a good approach speed, but that doesn't help the fact that you have a huge amount of mass to stop.

But having appropriate brakes for the mass of the aircraft does. In any case a landing at normal weights is no real challenge to the brakes; the RTO requirements are much more taxing.

Quoting AA737-823 (Reply 11):
considering that the ONLY A380 wing ever tested to failure actually broke BEFORE THE LEGAL LIMIT.

So you're a certification expert now, too? The wing failed so close to the design limit that the issue was addressed by analysis and changes that the authorities allowed without requiring the very expensive retest of the wing.

Quoting Tdscanuck (Reply 16):
Quoting AA737-823 (Reply 11):
Well that's the easiest explanation to believe so far, considering that the ONLY A380 wing ever tested to failure actually broke BEFORE THE LEGAL LIMIT.

That's true, but it doesn't mean they violated regulations. I know you didn't say that, but you seem to be implying there was something improper...the FAR's (and whatever EASA is calling their regulations now) just require that the OEM show that the wing withstand ultimate load without breaking...they don't say how you have to show that. A test to almost the limit, plus analysis of design changes, may be sufficient. In the case of the A380, it was.

Well put, Tdscanuck. Thank you.

Quoting Jetlagged (Reply 19):
Please don't generalise so. I'm a "Euro" and I thought reversers were probably a good idea on the A380. I'm certain there are "Yanks" who think the A380 is marvellous.

I am an American and think the A-380 is a brilliant piece of engineering. I wish my operator would buy some. Jetlagged, while I have had disagreements with you before, I always find you to be rational and well-informed. Just because some here rail against Airbus, doesn't mean that all of us are closed-minded and self-righteous. Having flown both Airbus and Boeing, I find the Airbus to be a superior product overall, so count me as a "Yank" who thinks the A-380 (and other FBW Airbus models) are marvelous.

Quoting Jetlagged (Reply 19):
My argument was they should either have four reversers or none. Not two with and two without. If one of the inboard engines is out, the other inboard reverse thrust would create a lot of asymmetry, so effectively you have no reverse.

I respectfully don't buy this given my prior experience. The carrier I flew the 744 for used all reversers on an engine out landing and directional control was never difficult in any way. I don't know the internal deliberations that occurred at Airbus on this design, but I DO think the engineers there are more than competent and I trust them to have made the most appropriate design trade. Certainly Airbus has knowledge on this subject far exceeding our collective insight, and the design caused no difficulties in certification.

Quoting PhilSquares (Reply 20):


Quoting Jetlagged (Reply 19):
If one of the inboard engines is out, the other inboard reverse thrust would create a lot of asymmetry, so effectively you have no reverse

To be honest, there is very little asymmetry from a reverser inop. Generally, the EEC/FADEC will limit reverse thrust to 70% N1 or so. I have worked for airlines where SOP is to use all reverse in an engine out landing. Don't worry about asymmetry as normal steering through the rudders will take care of that.

Agreed.
 
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Jetlagged
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 8:18 pm



Quoting PhilSquares (Reply 20):
To be honest, there is very little asymmetry from a reverser inop. Generally, the EEC/FADEC will limit reverse thrust to 70% N1 or so. I have worked for airlines where SOP is to use all reverse in an engine out landing. Don't worry about asymmetry as normal steering through the rudders will take care of that.

Fair enough, I well understand that reverse asymmetry is a fraction of the asymmetry you might have on takeoff, especially if it is inboard. However I was thinking particularly of low friction contaminated runways, where reverse thrust asymmetry might become more important since wheel braking and lateral tyre friction are minimal. Rudder effectiveness reduces rapidly with forward speed, and reverse thrust reduces it even further.

Still these are rare circumstances and the design should not be compromised by one special case.

Quoting PGNCS (Reply 21):
Jetlagged, while I have had disagreements with you before

Really? Nothing serious I hope.  Smile

Quoting PGNCS (Reply 21):
Certainly Airbus has knowledge on this subject far exceeding our collective insight, and the design caused no difficulties in certification.

100% agree.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
PhilSquares
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 8:46 pm



Quoting Jetlagged (Reply 22):
Fair enough, I well understand that reverse asymmetry is a fraction of the asymmetry you might have on takeoff, especially if it is inboard. However I was thinking particularly of low friction contaminated runways, where reverse thrust asymmetry might become more important since wheel braking and lateral tyre friction are minimal. Rudder effectiveness reduces rapidly with forward speed, and reverse thrust reduces it even further.

It's called judgement. I would argue you wouldn't want to go into a airport as you describe unless there is no other alternative. With autobrakes, even landing on a contaminated runway, isn't that big of a deal. Remember, stopping distances don't take into account reverse thrust, so even though you can use asymmetrical reverse there is no mandate you have to.

I don't understand your comments about reverse thrust decreasing rudder effectiveness. On every aircraft I have flown, that has had reverse thrust, there is no decrease of rudder effectiveness with the use of reverse. It does decrease as airspeed decreases. On a contaminated runway, judicious use of reverse on the asymmetrical engine minimises any problem with directional control.

That's what we get paid for!!!!.
Fly fast, live slow
 
AA737-823
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 9:20 pm



Quoting Jetlagged (Reply 19):
Please don't generalise so. I'm a "Euro" and I thought reversers were probably a good idea on the A380. I'm certain there are "Yanks" who think the A380 is marvellous. Reducing the A v B debate to nationalism is crazy.

Haha, fair enough. I think that reducing it to nationalism is crazy as well, and yet that's what nearly always happens. I am not a fan of the A380, but it's not because I'm an American. Boeing doesn't really build anything to compete with the aircraft, so I have nothing to gain by senseless nationalism.
By the way- you're not a Euro, you're from the UK. Big difference!

Quoting PhilSquares (Reply 13):
Not according to our MEL!

The MEL for the airline I USED to be associated with stated that ANY inop reverser was a no-go item if the taxiways and runways were icy, which in Anchorage, Alaska is the case much of the year. That COULD be tied to an... incident... that carrier had here many years ago, but for what it's worth, they were no-go for us 6 months out of the year.

Quoting PGNCS (Reply 21):
Hardest to stop? It has slow approach speeds and excellent brakes. We all know you hate Airbus, but give it a rest.

Really? Odd, as I choose to fly in their aircraft across Ye Olde Atlantique on 66% of my flights.
Further, I have absolutely RAILED on Randy Tinseths' blog over at Boeing, regarding the noise issue in the 787 cabin.
Before you make inflammatory statements, you should make sure the person you're accusing is actually as childish as you think.
I'm not quite there..... unless you push me REEEEEEAL hard.

Quoting PGNCS (Reply 21):
So you're a certification expert now, too?

Haha, any idiot with a baguette and a glass of wine can say "oh crap" when a wing breaks before it's supposed to. And I don't even have a baguette or a glass of wine.
Doesn't take an expert, buddy.... doesn't take an expert.
 
WingedMigrator
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RE: A380 Thrust Reversal

Sat Oct 25, 2008 11:52 pm



Quoting Jetlagged (Reply 19):
I'm certain there are "Yanks" who think the A380 is marvellous.

 checkmark  I'm one.

Quoting Jetlagged (Reply 19):
The A380 outboard engines aren't much further outboard than those of a 747.

They are, by 23%. The A380 outboard engines are 51.4 m apart (center-to-center); the 747's are 41.7 m apart. When you operate on a 45 m (150 ft) runway, that is quite a difference: with the 747 you've got 3.3 m to spare, and with the A380 you're 6.4 meters over the edge.
 
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Jetlagged
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 12:30 am



Quoting PhilSquares (Reply 23):
I don't understand your comments about reverse thrust decreasing rudder effectiveness. On every aircraft I have flown, that has had reverse thrust, there is no decrease of rudder effectiveness with the use of reverse. It does decrease as airspeed decreases. On a contaminated runway, judicious use of reverse on the asymmetrical engine minimises any problem with directional control.

The effect is most noticeable with tail mounted engines for obvious reasons as the engines are close to the fin so airflow over the rudder is directly affected. With wing mounted engines the reversed fan airflow disrupts the aerodynamics of the wing. The effect is significant enough for the FAA to make it mandatory to be part of any simulator aerodynamic data package. In the Boeing 747 aero model, with symmetric reverse thrust there is no effect on directional control, but with asymmetric reverse thrust there is an effect in roll, yaw and sideforce. As a pilot you will be feeling the direct effect of asymmetric thrust moment, so the secondary aerodynamic effect may not be obvious. Of course your experiences vis a vis the simulator versus the aircraft may say different.  Smile
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
tdscanuck
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 3:43 am



Quoting Jetlagged (Reply 19):
My argument was they should either have four reversers or none. Not two with and two without. If one of the inboard engines is out, the other inboard reverse thrust would create a lot of asymmetry, so effectively you have no reverse.

The yaw control requirements for an engine out at takeoff will vastly overshadow any adverse yaw you'd get from reverse asymmetry. Since the aircraft has to prove it can meet the former requirement, it's going to meet the latter by default.

Quoting PhilSquares (Reply 23):
Remember, stopping distances don't take into account reverse thrust

For aircraft certified since about 1996, that's only true on dry runways. On wet, the certified stopping distance does include use of reversers.

Tom.
 
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Jetlagged
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 5:30 am



Quoting WingedMigrator (Reply 25):
They are, by 23%. The A380 outboard engines are 51.4 m apart (center-to-center); the 747's are 41.7 m apart. When you operate on a 45 m (150 ft) runway, that is quite a difference: with the 747 you've got 3.3 m to spare, and with the A380 you're 6.4 meters over the edge.

You are assuming there is no concrete beyond the marked runway edges. The paved width could well be twice the runway width. It will certainly be a lot more than 45m. The chances of an A380 engines overhanging the grass on takeoff are low.

Quoting Tdscanuck (Reply 27):
The yaw control requirements for an engine out at takeoff will vastly overshadow any adverse yaw you'd get from reverse asymmetry. Since the aircraft has to prove it can meet the former requirement, it's going to meet the latter by default.

I concur with your first sentence though "vastly" is a bit of an exaggeration. However, can an aircraft maintain yaw control with takeoff thrust asymmetry below its VMCG? No, but in reverse thrust yaw control must be maintained down to 60 knots or so. The two things are not comparable, so you can't say meeting one requirement meets the other by default.
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astuteman
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 10:00 am



Quoting PGNCS (Reply 21):
But having appropriate brakes for the mass of the aircraft does. In any case a landing at normal weights is no real challenge to the brakes; the RTO requirements are much more taxing.

Which the A380-800 passed with ease, BTW....  Smile

Quoting Jetlagged (Reply 28):
Quoting WingedMigrator (Reply 25):
They are, by 23%. The A380 outboard engines are 51.4 m apart (center-to-center); the 747's are 41.7 m apart. When you operate on a 45 m (150 ft) runway, that is quite a difference: with the 747 you've got 3.3 m to spare, and with the A380 you're 6.4 meters over the edge.

You are assuming there is no concrete beyond the marked runway edges. The paved width could well be twice the runway width. It will certainly be a lot more than 45m. The chances of an A380 engines overhanging the grass on takeoff are low

Maybe so, but the chances of FOD ingestion have to be a lot greater when the outboard engines are on the margins of the paved area.

Rgds
 
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Jetlagged
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 3:01 pm



Quoting Astuteman (Reply 29):
Maybe so, but the chances of FOD ingestion have to be a lot greater when the outboard engines are on the margins of the paved area.

The distance from the outer side of one outboard engine pylon to the other is approx 55m on an A380. On pavement 90m wide, there is therefore 17.5m on either side from the outside of each pylon to the grass. Foreign objects are not going to be kicked up and ingested back at this distance, especially at speeds above 60 knots. The chances of ingested damage are slightly more perhaps, not a lot more.

If you look at the video posted above of a 747SP landing on a narrow runway in RSA, there is a lot of dust thrown up, but how much, if any, goes anywhere near the intakes?

Strictly speaking, FOD is the damage, not the stuff that causes it.  Smile
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Francoflier
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 4:21 pm



Quoting AA737-823 (Reply 11):
Yes, you're right. But the 747-400 has very good brakes- good enough that you can dispatch with an ENTIRE gear's worth of brakes (4 wheels, I mean) INOP.

The 744 has 'good brakes' alright, but you are being a bit generic and vague with your arguments (you'd make a good political campaign adviser  Wink ), and since this is Tech/Ops...:

An aircraft such as the A340-200/300 has 8 wheel brake units, which it uses to stop 260T, that's 32.5T / brake unit.
The 744 has 16 brake units for a gross weight of 396T, and that's only 24.7T / brake unit...

The 744 doen't have extraordinary brakes, it just has more...

Oh, and the 744 MEL I have at hand (granted, they vary amongst different operators) only allows dispatch with 2 brake units deactivated. At MOTW, with 14 brake units working, that's still only 28.3T / brake unit.

Figures for the A380: 560T, 16 brake units: 35T / brake units.
Seeing how it passed its RTO test with 'flying' colors, I'd be inclined to say that these are darn good brakes!
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astuteman
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RE: A380 Thrust Reversal

Sun Oct 26, 2008 8:08 pm



Quoting Jetlagged (Reply 30):
The distance from the outer side of one outboard engine pylon to the other is approx 55m on an A380. On pavement 90m wide, there is therefore 17.5m on either side from the outside of each pylon to the grass

Can't argue with that - if the pavement is 90m wide...  Smile

Rgds
 
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Starlionblue
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RE: A380 Thrust Reversal

Mon Oct 27, 2008 12:03 am



Quoting Francoflier (Reply 31):
An aircraft such as the A340-200/300 has 8 wheel brake units, which it uses to stop 260T, that's 32.5T / brake unit.
The 744 has 16 brake units for a gross weight of 396T, and that's only 24.7T / brake unit...

The 744 doen't have extraordinary brakes, it just has more...

Oh, and the 744 MEL I have at hand (granted, they vary amongst different operators) only allows dispatch with 2 brake units deactivated. At MOTW, with 14 brake units working, that's still only 28.3T / brake unit.

Figures for the A380: 560T, 16 brake units: 35T / brake units.
Seeing how it passed its RTO test with 'flying' colors, I'd be inclined to say that these are darn good brakes!

20 years later than 744, 15 years later than 340. It's amazing how technology progresses.
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Viscount724
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RE: A380 Thrust Reversal

Mon Oct 27, 2008 2:11 am



Quoting Jetlagged (Reply 19):
Quoting AA737-823 (Reply 11):
No airliner does, yet with the noted exception of the BAe-146, all of them managed to get them... and they're heavy on all the other ones, too.

Not just the 146. The Fokker F.28 had no reversers either. Actually the 146 has a low ground idle setting which is surprisingly effective in increasing deceleration, not to mention liftdumpers AND a speedbrake (like the F.28).

The F-28 and BAe146 were used without problems in northern Canada for years where snow-covered runways are common 8 months of the year.
 
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Jetlagged
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RE: A380 Thrust Reversal

Tue Oct 28, 2008 2:24 pm



Quoting Astuteman (Reply 32):
Can't argue with that - if the pavement is 90m wide...

OK, there won't be many runways with 90m pavement, but those that an A380 will use will likely be a lot wider than 45m. 60m would be more than enough to keep the outer engines inside the paved width, albeit outside the markings.  Smile
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Finkenwerder
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RE: A380 Thrust Reversal

Tue Oct 28, 2008 7:43 pm

Incidently the A380 MMEL permits dispatch with 1 or more reversers inop. As long as they are locked out, with appropriate ECAM indications.
 
Northwest727
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RE: A380 Thrust Reversal

Thu Oct 30, 2008 1:45 am

If I am not mistaken, another reason the outboards on the A380 don't have reversers is to reduce weight and costs.
 
Viscount724
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RE: A380 Thrust Reversal

Thu Oct 30, 2008 3:00 am



Quoting Northwest727 (Reply 37):
If I am not mistaken, another reason the outboards on the A380 don't have reversers is to reduce weight and costs.

The standard VC-10 had reversers on the outboard engines only. The prototype had reversers on all 4 engines but not the production aircraft. Weight was probably a factor. The Super VC-10 originally had reversers on all 4 engines but at some point after they'd been service for several years the inboard reversers were removed. The similar Ilyushin-62 also has reversers on the outboard engines only.

The Tu-154 and H.S. Trident lacks reversers on the center engine, unlike the 727.
 
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Starlionblue
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RE: A380 Thrust Reversal

Thu Oct 30, 2008 5:12 am



Quoting Northwest727 (Reply 37):
If I am not mistaken, another reason the outboards on the A380 don't have reversers is to reduce weight and costs.

As mentioned in Reply 6...
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
planewasted
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RE: A380 Thrust Reversal

Thu Oct 30, 2008 12:52 pm



Quoting AA737-823 (Reply 6):
as anyone in their right mind would expect on the heaviest, hardest-to-stop airliners ever mass produced

Wonder if this is true? The hardest to stop part..
With the higher landing speed of the 747 it might actually have more kinetic energy to brake away.
Anyone know the landing speed and max weight of the 747 and A380?
 
Nicoeddf
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RE: A380 Thrust Reversal

Thu Oct 30, 2008 2:58 pm



Quoting Jetlagged (Reply 35):
but those that an A380 will use will likely be a lot wider than 45m. 60m would be more than enough to keep the outer engines inside the paved width, albeit outside the markings

I doubt the 380 will always get priority for 25R/7L in FRA as opposed to 27L/7R.
The former has 60m, the latter 45m. So I think it will be more than common the 380 will touch down on runways narrower 60m.
 
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Jetlagged
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RE: A380 Thrust Reversal

Fri Oct 31, 2008 3:14 am



Quoting NicoEDDF (Reply 41):
I doubt the 380 will always get priority for 25R/7L in FRA as opposed to 27L/7R.
The former has 60m, the latter 45m. So I think it will be more than common the 380 will touch down on runways narrower 60m.

The published width of 45m refers to the marked runway. A quick look at Google Earth shows there is a few meters of tarmac outside this. The prepared surface looks to be around 60m wide.

There are some runways where there is little width outside the marked width. KSFO 10R/28L is one such runway. But I would suggest this is rare for major international airports. Even so, I doubt the A380 would be at serious risk of FOD landing on that runway if it did have outboard reversers.
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WingedMigrator
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RE: A380 Thrust Reversal

Fri Oct 31, 2008 4:57 am



Quoting Jetlagged (Reply 42):
Even so, I doubt the A380 would be at serious risk of FOD landing on that runway if it did have outboard reversers.

I dunno. For reference, here's a take-off on a 45 m runway:


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Photo © Stewart Andrew



Maybe that picture tells you everything you need to know Big grin
 
astuteman
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RE: A380 Thrust Reversal

Fri Oct 31, 2008 7:11 am



Quoting NicoEDDF (Reply 41):
The former has 60m, the latter 45m. So I think it will be more than common the 380 will touch down on runways narrower 60m.

I think the issue here is not the frequency of landing on 45m wide runways, but the fact that it undoubtedly will at some time, and has been designed to do so.
The A380 was designed to land on the same runways as a 747 - ergo, it was designed to land on 45m runways (even if this doesn't occur at every landing)
Ergo, thrust reversers have been omitted on the outboard engines, which clearly, and completely, overhang a 45m runway

In which case this

Quoting Jetlagged (Reply 35):
OK, there won't be many runways with 90m pavement, but those that an A380 will use will likely be a lot wider than 45m

doesn't matter....  Smile

Rgds
 
tdscanuck
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RE: A380 Thrust Reversal

Sat Nov 01, 2008 6:22 pm



Quoting Astuteman (Reply 44):
Ergo, thrust reversers have been omitted on the outboard engines, which clearly, and completely, overhang a 45m runway

Reversers don't inhale FOD until the speed is relatively low (below 80 knots or so) and you normally kill reversers by then anyway. Plus the A380 outboard engines are a looong way off the ground...why the enourmous FOD concerns?

Tom.

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