747400sp
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Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 02, 2006 9:26 pm

I been told that the Airbus A380 has only two trust reverser, I also just saw a photos of an A380 with it's trust reversers deployed an see that it just has two trust reversers. So is Airbus crazy!!! this plane at landing, has the weight of a DC-10-30 also what if an A380 has to abort take off, how or the pilots going to stop a 1,200,000 lb plane on a takeoff roll with only two trust reversers. 707, DC-8, 747 and A340 has trust reversers on all four of there engines, an A380 is much heavier than these aircraft, so I would think it would be a safe ideal to have the out board engines fitted with trust reverser also. Do not get me wrong an A380 looks cool at landing but safe out rules cool.

Just my Two Cents

PS: Please do not trun this into an A vs B topic I like all four engines airliners.
 
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zeke
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Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 02, 2006 9:42 pm

Quoting 747400sp (Thread starter):
I been told that the Airbus A380 has only two trust reverser, I also just saw a photos of an A380 with it's trust reversers deployed an see that it just has two trust reversers.

A couple of reasons, firstly its does not need them, a 146 does not have reverse thrust either. It has a lot of brakes.

Second reason is FOD, on most runways the outer engines will pick up FOD if reverse is deployed.

Reverse does not decrease stopping distance on landing with any of the newer airbus aircraft, they decelerate at pre-programmed rates. If you were to use reverse, less brake pressure would be applied to achieve the same deceleration rate without the use of reverse thrust.

Quoting 747400sp (Thread starter):
how or the pilots going to stop a 1,200,000 lb plane on a takeoff roll with only two trust reversers.

Aircraft are certified for normal stopping with NO REVERSE. Reverse is only considered operable for some specific takeoff scenarios, like a wet (not damp) runway.
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fr8mech
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Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 02, 2006 10:07 pm

Weight savings would be my guess.
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Matt72033
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Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 02, 2006 11:20 pm

OH MY GOD!!!!! a huge design flaw by the Airbus engineers there, they'd better go back and re-design the whole aircraft!!!!!  Yeah sure

i mean seriously......do you not think they thought about this when designing the aircraft? do you seriously think they would let it fly if it was going to be a problem?

whats a trust reverser anyway? some kind of unit to reverse somebody's trust?  Wink
 
bohica
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Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 03, 2006 11:03 am

Quoting Matt72033 (Reply 5):
OH MY GOD!!!!! a huge design flaw by the Airbus engineers there, they'd better go back and re-design the whole aircraft!!!!!

There's another production delay.  duck   duck   duck   biggrin   biggrin   biggrin 
 
n8076u
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Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 03, 2006 4:06 pm

Apparently, Airbus has faith in the brakes the A380 is equipped with, and feels that two reversers are all it needs, besides any FOD/weight issues.

Chris
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NZ8800
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Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 03, 2006 4:25 pm

80% or more of an airliner's stopping power - including the A380 - is in the brakes. If the pilots jam them on full, they could put the passengers through the seat in front of them  Wink
They can land completely without the thrust reversers. As for safety - it will have been checked, and re-checked, the prototype has flown, and they are now into the production run (however delayed that is!) - if it were not safe, it would not have been certificated.
Also technology has improved - we may see the 747-800 with only two thrust reversers as well, if that is all that is required.
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tepidhalibut
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Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 03, 2006 8:22 pm

I believe that I read that the A380 was designed without any TRUs at all, but the FAA "recommended" that Airbus reconsider that plan. (Can't find a reference at the moment.)

I'm sure some folk are wondering why an airline would prefer to use brakes rather than TRU. My understanding is :

# Stopping with Brakes - Does result in wear of brake pads, which will eventually need replacing.

# Stopping with TRUs - Doesn't involve wear of the brake-pads, but unfortunately, an engine at idle doesn't produce much thrust (forward or reverse.) So the pilots have to apply RevThrust and accelerate the engine. This obviously used more fuel, and contributes to the Cyclic Life useage in the engines.

The decision on whether brakes or R/T is used comes down to economics. As brake technology and life improves, that options becomes more attractive.
 
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Starlionblue
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Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 03, 2006 8:58 pm

Quoting TepidHalibut (Reply 10):
I believe that I read that the A380 was designed without any TRUs at all, but the FAA "recommended" that Airbus reconsider that plan. (Can't find a reference at the moment.)

Also heard this rumor.

Quoting 747400sp (Thread starter):
I been told that the Airbus A380 has only two trust reverser, I also just saw a photos of an A380 with it's trust reversers deployed an see that it just has two trust reversers. So is Airbus crazy!!! this plane at landing, has the weight of a DC-10-30 also what if an A380 has to abort take off, how or the pilots going to stop a 1,200,000 lb plane on a takeoff roll with only two trust reversers. 707, DC-8, 747 and A340 has trust reversers on all four of there engines, an A380 is much heavier than these aircraft, so I would think it would be a safe ideal to have the out board engines fitted with trust reverser also. Do not get me wrong an A380 looks cool at landing but safe out rules cool.

To summarize:
- Weight savings.
- FOD problems on the outboards if they hang too close to the runway edges. Not so much a problem in forward thrust since small FOD would simply fly out the back. But in reverse gravel and other debris could potentially fly out of the reversers and hit the wing, the fuse, the other engines.
- Thrust reversers are a bonus. If the pilots use max braking, the pax would hang by the seatbelts. Landing calculations are made without thrust reversers and they have a 50% pad factor. Planes can come to a stop very very fast.

Quoting 747400sp (Thread starter):
707, DC-8, 747 and A340 has trust reversers on all four of there engines, an A380 is much heavier than these aircraft, so I would think it would be a safe ideal to have the out board engines fitted with trust reverser also.

You're making two logic errors here.
- You assume that the 747 and 340 could not be built with two reversers. They could, and inop thrust reversers do not preclude dispatch. It's a matter of economics (presumably, if those aircraft could be operated more efficiently with two thrust reversers, that's how they would have been build. Also, modern carbon brakes require less cooling time compared to older brakes. So the need for thrust reversers has decreased further.
- You assume that the enormous weight of the 380 is a problem in itself. The plane is large, but all other things (control surfaces, engines, etc...) are scaled up to match, including brakes.
"There are no stupid questions, but there are a lot of inquisitive idiots."
 
speedracer1407
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Why The Airbus A380 Has Only Two Thrust Reverser?

Tue Jul 04, 2006 4:56 pm

Quoting TepidHalibut (Reply 10):
The decision on whether brakes or R/T is used comes down to economics. As brake technology and life improves, that options becomes more attractive.

Your post suggests that engineers and airlines are faced with an "either-or" propostion; whether to use brakes, RT, or both. But as has been posted here already, and elsewhere, the decision to use RT or bakes is not a matter of economics. Brakes, in all airliners, provide the vast majority, and often, the entirety of braking. TR is a bonus, usually deployed as a matter of SOP to account for unexpected failures. It is never relied upon to provide the majority of deceleration unless there is some sort of incredible brake failure.
O
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Starlionblue
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Why The Airbus A380 Has Only Two Thrust Reverser?

Wed Jul 05, 2006 12:12 am

Trust me on this one...  stirthepot 
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HAWK21M
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Why The Airbus A380 Has Only Two Thrust Reverser?

Wed Jul 05, 2006 12:20 am

Quoting Speedracer1407 (Reply 12):
It is never relied upon to provide the majority of deceleration unless there is some sort of incredible brake failure.

Considering The Value of Engine Life.Brake Use seems a cheaper option.  Smile
Trust me its Thrust Reversers.
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Jetlagged
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Why The Airbus A380 Has Only Two Thrust Reverser?

Wed Jul 05, 2006 10:21 pm

Thrust reversers are installed particularly for operation on contaminated runways, where brakes might be useless. Usually they are only used in symmetric pairs on four engined aircraft. Certainly the BAe 146 and the Fokker F.28 have no reverse thrust, but they have very effective airbrakes.

Consider three things.

1. Either of the two inboard engines might be shutdown
2. A large engine running at idle produces significant forward thrust
3. Even at max reverse, the thrust produced is very much less than that produced in forward thrust.

So I wouldn't want to be onboard the first A380 which lands on an icy runway with an inboard engine out. No effective brakes, two engines still in forward thrust and only one asymmetrically in reverse.

Two outboard reversers would provide significant additional redundancy for a modest weight increase, and better spares commonality (operators will have to spare inboard and outboard engines separately). The FOD argument is spurious. 747 outboard engines are already very close to the runway edge. Why would there be more debris from the off tarmac areas anyway. I'd argue less, and then mainly dust and grass.

By the way, FOD stands for Foreign Object Damage, i.e. the damage done, not the stuff that does the damage. Sloppy usage has led it to come to mean the runway debris itself. I know the Boeing website talks about Foreign Object Debris, but why not just Debris, which by definition will be a foreign object for the engine.
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HAWK21M
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Why The Airbus A380 Has Only Two Thrust Reverser?

Thu Jul 06, 2006 5:10 am

Airbus Must have Def considered Alternatives,Prior to Deciding on No Outboard T/Rs.
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Starlionblue
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Why The Airbus A380 Has Only Two Thrust Reverser?

Thu Jul 06, 2006 6:07 am

Quoting Jetlagged (Reply 16):
Certainly the BAe 146 and the Fokker F.28 have no reverse thrust, but they have very effective airbrakes.

And they are an option on the ERJ. Some operators don't have them.

Quoting Jetlagged (Reply 16):

So I wouldn't want to be onboard the first A380 which lands on an icy runway with an inboard engine out. No effective brakes, two engines still in forward thrust and only one asymmetrically in reverse.

If there was a problem with this scenario, don't you think the FAA, the JAA and the operators would have insisted on 4 reversers? As Mel says, I'm sure Airbus has done the math.

On an icy runway, stopping distance is just calculated as longer. This calculation is not affected by the presence of reversers.

Also, two engines would certainly not "still be in forward thrust". They would be idle.

Quoting Jetlagged (Reply 16):
The FOD argument is spurious. 747 outboard engines are already very close to the runway edge. Why would there be more debris from the off tarmac areas anyway. I'd argue less, and then mainly dust and grass.

Maybe, maybe not. 747 outboards are several meters further inboard. Consider that the wingspan of the 380 is 8 meters more than the wingspan of the 747.

Quoting Jetlagged (Reply 16):

Two outboard reversers would provide significant additional redundancy for a modest weight increase,

Too much weight increase and not a lot of redundancy increase. And with current landing distance calculation methods, reversers are redundancy for brakes (which are redundant in themselves with multiple circuits and axles), not for each other. In essence, you could leave them off on most planes. They are "reaching desired turn-off" mechanisms unless the brakes are quite broken AND your runway is very short. Just popping the spoilers will brake the plane quite well in the speeds where reversers are effective.

Quoting Jetlagged (Reply 16):
and better spares commonality (operators will have to spare inboard and outboard engines separately).

On most large engines, the reversers are not part of the engine package in the way you describe. They are part of the nacelle. The engines themselves are all sans reversers.
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David L
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Why The Airbus A380 Has Only Two Thrust Reverser?

Thu Jul 06, 2006 6:35 am

Quoting Jetlagged (Reply 16):
So I wouldn't want to be onboard the first A380 which lands on an icy runway with an inboard engine out

Do 747s often land on icy runways where reverse thrust can mean the difference between staying on the runway and over shooting? Not a loaded question, just curious.
 
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Starlionblue
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Why The Airbus A380 Has Only Two Thrust Reverser?

Thu Jul 06, 2006 9:21 am

Quoting David L (Reply 19):
Quoting Jetlagged (Reply 16):
So I wouldn't want to be onboard the first A380 which lands on an icy runway with an inboard engine out

Do 747s often land on icy runways where reverse thrust can mean the difference between staying on the runway and over shooting? Not a loaded question, just curious.

I can hear SlamClick in my ear... "Why would you want to land an airliner with a failed thrust reverser on an icy runway when you can divert to somewhere with better traction?"

But in any case, I agree with what I think Mr. L is saying. If thrust reverse is needed to stop within the required distance on that icy runway, you won't be allowed to land there anyway, regardless of whether your reversers are in working order or not.
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Jetlagged
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Why The Airbus A380 Has Only Two Thrust Reverser?

Thu Jul 06, 2006 11:41 pm

Quoting Starlionblue (Reply 18):
If there was a problem with this scenario, don't you think the FAA, the JAA and the operators would have insisted on 4 reversers? As Mel says, I'm sure Airbus has done the math.

I'd love to share your confidence. History shows that sometimes "the math" is not done properly, and unforseen accident scenarios occur from time to time. It may not have been considered with an inboard engine failure wiping out half the reverse thrust and introducing asymmetry. I'm only speculating. I don't suppose the rules envisage a situation with only half the engines having reversers, normally it is either all or none.

Quoting Starlionblue (Reply 18):
On an icy runway, stopping distance is just calculated as longer. This calculation is not affected by the presence of reversers.

Much, much longer stopping distance for wheel brakes only on ice. On icy runways, the only really effective braking you have is aerodynamic and reverse thrust. Friction for wheel braking is minimal.

Quoting Starlionblue (Reply 18):
Also, two engines would certainly not "still be in forward thrust". They would be idle.

They certainly will be in forward thrust, at idle, which is a significant component of forward thrust nevertheless.

Quoting Starlionblue (Reply 18):
Maybe, maybe not. 747 outboards are several meters further inboard. Consider that the wingspan of the 380 is 8 meters more than the wingspan of the 747.

747 outboards are about 140 feet apart, centerline to centerline, which means the outer side of the nacelle will be roughly over the edge of a 150 foot wide runway. Apparently the FOD argument didn't also apply to the 747, which is why I think it is spurious.

Quoting Starlionblue (Reply 18):
Too much weight increase and not a lot of redundancy increase.

How much is too much? Do you know the reverser weight? Redundancy would be doubled by adding outboard reversers. What about the RTO scenario after an engine 2 or 3 failure for a very heavy A380? You will certainly want max reverse then, and I'd rather the outboards were not still producing forward thrust, even at idle.

Quoting Starlionblue (Reply 18):
On most large engines, the reversers are not part of the engine package in the way you describe. They are part of the nacelle. The engines themselves are all sans reversers.

Rolls Royce for one have always supplied complete engine/nacelle assemblies on their turbofans, including reversers. I confess to not knowing whether this is true for the A380 Trent engines, or the Engine Alliance equivalents.
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David L
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 1:06 am

Quoting Jetlagged (Reply 21):
It may not have been considered with an inboard engine failure wiping out half the reverse thrust and introducing asymmetry

I just can't see something as obvious as that being overlooked. Isn't it the same situation as you have with twins like the 737, 777, 320 and 330? If asymmetry is a problem, can't you just refrain from applying reverse on the other inboard?

Quoting Jetlagged (Reply 21):
On icy runways, the only really effective braking you have is aerodynamic and reverse thrust. Friction for wheel braking is minimal.

But then we get back to the question of landing on such a runway in any type.
 
2H4
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 1:29 am




Quoting Jetlagged (Reply 21):
Quoting Starlionblue (Reply 18):
Too much weight increase and not a lot of redundancy increase.

How much is too much?

Well, to quote Bill Lear....

"I'd sell my grandmother to save a pound of weight"


 Wink



2H4


Intentionally Left Blank
 
buckfifty
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 5:36 am

If one engine has failed on either side of the aircraft, reverse is locked out on the coincidental engine on the other side. So asymetrical reverse will not happen, that's a given.

And as David L has said, this is not much different than a scenario with twins. Reverse, even in a contaminated runway condition, does not factor greatly with the stopping distance charts. Modern carbon brakes and anti-lock systems are quite capable of stopping an aircraft, even under the worst circumstances.

However, it is up to the pilot to avoid having the aircraft land in such conditions, and to make sure the aircraft performance is adequate enough to land on any runway dependent on the present condition, with thrust reversers or without. That's why we often check the runway condition when going into an airport where weather is a known factor.

I can seriously imagine the day when thrust reversers will be obsolete with the way braking technology is going. Most airports we fly into, we're only using a low autobrake setting, along with idle thrust reverse. As carbon brakes don't wear with pressure or heat (they wear only by the number of applications), this isn't a problem with brake wear.

However, in reference to turnaround times, heavy braking can cause high temps, which need to be sorted before the aircraft departs. But again, as materials research is advanced, I'm sure this problem will go away also.
 
Matt72033
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 5:59 am

Quoting BuckFifty (Reply 24):
If one engine has failed on either side of the aircraft, reverse is locked out on the coincidental engine on the other side. So asymetrical reverse will not happen, that's a given.

not on our 747-400's! the MEL states that the aircraft can depart with one thrust reverser u/s, 2 u/s is a no go.....so if you have one go tits up and you lock out the corresponding engine on the other wing, your not going anywhere!
 
buckfifty
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 6:14 am

I suppose I should have clarified my statement above.

I'm not sure what the 744's MEL states. However, on the 340, if we are to have a reverser locked out, for example on engine 1, then engine 4's reverser is unusable above idle, and is locked. Therefore we're only left with two reversers, on 2 and 3. If a reverser is inop on the inboard engines, the other can still be used, but may have to use differential braking to offset any directional stability issues.

And it is possible for us to dispatch a 340 without any reversers, as long as certain requirements are met.

[Edited 2006-07-06 23:17:14]
 
tepidhalibut
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 6:16 am

Quoting Jetlagged (Reply 21):
Rolls Royce for one have always supplied complete engine/nacelle assemblies on their turbofans, including reversers. I confess to not knowing whether this is true for the A380 Trent engines, or the Engine Alliance equivalents.

I'm going to have to call you on that. For example, the Trent 700. The TRU is the responsibility of Airbus / Aircell, and not part of the engine (as defined in DIS part 1.) I believe the Production engines are passed off with a slave C-Duct rather than a pukka TRU unit. The engine is shipped to Toulouse, where the engine is "podded", and that's where the engine gets it's fiirst sniff of a TRU.

On the older engines (RB211-535, 524 etc) the TRU was a Rolls-Royce supplied item, tho' I don't think they were DIS Pt 1 bits.
 
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Starlionblue
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 6:30 am

Quoting Jetlagged (Reply 21):
Quoting Starlionblue (Reply 18):
If there was a problem with this scenario, don't you think the FAA, the JAA and the operators would have insisted on 4 reversers? As Mel says, I'm sure Airbus has done the math.

I'd love to share your confidence. History shows that sometimes "the math" is not done properly, and unforseen accident scenarios occur from time to time. It may not have been considered with an inboard engine failure wiping out half the reverse thrust and introducing asymmetry. I'm only speculating. I don't suppose the rules envisage a situation with only half the engines having reversers, normally it is either all or none.

Even if the math were not done properly tests would uncover the deficiency. That's why they test.

Of course the rules envisage a situation with only half the engines having reversers. Otherwise the 380 would have no hope of certification.

Quoting Jetlagged (Reply 21):
Quoting Starlionblue (Reply 18):
On an icy runway, stopping distance is just calculated as longer. This calculation is not affected by the presence of reversers.

Much, much longer stopping distance for wheel brakes only on ice. On icy runways, the only really effective braking you have is aerodynamic and reverse thrust. Friction for wheel braking is minimal.

And why would you land on that short icy runway with an inoperative reverser? Your calculations will simply extend the runway requirement (and factor in friction) and you would have to find one long enough.

Also, anti-lock braking is pretty efficient. Avro 100s land on icy runways all winter long without skidding off the end.

Quoting Jetlagged (Reply 21):
Quoting Starlionblue (Reply 18):
Also, two engines would certainly not "still be in forward thrust". They would be idle.

They certainly will be in forward thrust, at idle, which is a significant component of forward thrust nevertheless.

Idle is idle. Means that the forward thrust is insignificant in this regard. Idle thrust is more than compensated by any form of braking..

Quoting Jetlagged (Reply 21):
Quoting Starlionblue (Reply 18):
Maybe, maybe not. 747 outboards are several meters further inboard. Consider that the wingspan of the 380 is 8 meters more than the wingspan of the 747.

747 outboards are about 140 feet apart, centerline to centerline, which means the outer side of the nacelle will be roughly over the edge of a 150 foot wide runway. Apparently the FOD argument didn't also apply to the 747, which is why I think it is spurious.

FOD is one of the stated reasons, but I don't really care since the reversers are a bonus anyway. Also your argument is a bit weak without a comparison to the 380. How far outboard are the 380 engines?

Quoting Jetlagged (Reply 21):
Quoting Starlionblue (Reply 18):
Too much weight increase and not a lot of redundancy increase.

How much is too much? Do you know the reverser weight? Redundancy would be doubled by adding outboard reversers. What about the RTO scenario after an engine 2 or 3 failure for a very heavy A380? You will certainly want max reverse then, and I'd rather the outboards were not still producing forward thrust, even at idle.

As I said, redundancy would not be doubled. Wheel brakes on a whooping 8 axles provide redundancy in themselves.



There are plenty of jets out there with no reversers and they seem to be doing just fine with just wheels and aerodynamic brakes, even in icy conditions. Sure, they tend to be smaller jets, but they have correspondingly smaller brake discs and airbrakes.

[Edited 2006-07-06 23:32:28]
"There are no stupid questions, but there are a lot of inquisitive idiots."
 
David L
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 6:49 am

Quoting Starlionblue (Reply 28):
Wheel brakes on a whooping 8 axles

Ahem... 10 axles?  Smile
 
2H4
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 6:51 am




Quoting Starlionblue (Reply 28):
Also, anti-lock braking is pretty efficient. Avro 100s land on icy runways all winter long without skidding off the end.

I agree with the point being made, but Avro flights are also cancelled from time to time due to their occasional inability to cope with shorter icy runways.




2H4


Intentionally Left Blank
 
David L
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 7:19 am

Quoting 2H4 (Reply 30):
but Avro flights are also cancelled from time to time due to their occasional inability to cope with shorter icy runways

But aren't Avros more likely to fly into airports where the ice-removing infrastructure isn't as great?

Whose turn is it now?  Smile
 
2H4
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 7:32 am




Quoting David L (Reply 31):
But aren't Avros more likely to fly into airports where the ice-removing infrastructure isn't as great?

I can't speak to the accuracy of that....I was just relaying what I've observed (and heard from some Mesaba friends) regarding AZO ops.




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vikkyvik
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 7:42 am

Quoting Matt72033 (Reply 25):
not on our 747-400's! the MEL states that the aircraft can depart with one thrust reverser u/s, 2 u/s is a no go.....so if you have one go tits up and you lock out the corresponding engine on the other wing, your not going anywhere!

So if one reverser is unusable, would the opposite reverser be deployed on landing or not? I don't really understand the logic behind having 2 unusable reversers be a no-go.

Quoting Starlionblue (Reply 28):
FOD is one of the stated reasons, but I don't really care since the reversers are a bonus anyway. Also your argument is a bit weak without a comparison to the 380. How far outboard are the 380 engines?

It's hard to tell exactly where the aircraft is in the following photo, but I'm pretty sure it's toward the far end of the runway (which, I believe, is 46 m or 150 ft wide). That said, it looks like centerline-to-centerline of the outer engines is on the order of 160-170 feet.

Quoting David L (Reply 31):
Whose turn is it now?

Not mine, but I'm jumping in here anyway  Smile

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
2H4
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Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 7:46 am




Quoting Vikkyvik (Reply 33):
So if one reverser is unusable, would the opposite reverser be deployed on landing or not?

Would procedures not differ from one aircraft type to another?




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prebennorholm
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RE: Why The Airbus A380 Has Only Two Trust Reverser?

Fri Jul 07, 2006 7:58 am

Quoting 747400sp (Thread starter):
...how are the pilots going to stop a 1,200,000 lb plane on a takeoff roll with only two trust reversers.

With the wheel brakes.

In many airports like my local CPH the use of thrust reversers is prohibited for environmental reasons - except in case of an emergency, e.g. failing wheel brakes.

Thrust reversers are never counted on when runway performance data are certified for a plane type. They are only considered a noisy gadget to save wear and tear on the wheel brakes.

The A380 also has less need for thrust reversers since with its generous wing size is has a somewhat lower take-off and landing speed than other long range planes like B744, MD-11, A346, and also the later, heavier versions of B707 and DC-8.

If the proposed, stretched and heavier A380-900 some day takes to the air, then its landing speed will grow to the same level as present day long range planes. It is assumed to use the same wing with some structural reinforcements only. Or rather likely the same wing as the 1,300,000 lb A380F.

And who knows, maybe the A380-900 will be fitted with four thrust reversers?

Like the A380 also the VC-10 had somewhat lower takeoff and landing speeds than the competitors, and most of them were also operated with only two thrust reversers. It had, however, hardly any FOD issue. It was only for weight saving and maintenance saving, since most international airports had had to have their runways extended to accommodate B707 and DC-8 with their much inferior high lift devices compared to the VC-10.
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vikkyvik
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 8:29 am

Quoting 2H4 (Reply 30):
Would procedures not differ from one aircraft type to another?

Sorry - I was referring specifically to Matt72033's statement regarding his company's B744s.

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Starlionblue
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 8:29 am

Quoting David L (Reply 25):
Quoting Starlionblue (Reply 28):
Wheel brakes on a whooping 8 axles

Ahem... 10 axles?

Oops...

Quoting 2H4 (Reply 26):
Quoting Starlionblue (Reply 28):
Also, anti-lock braking is pretty efficient. Avro 100s land on icy runways all winter long without skidding off the end.

I agree with the point being made, but Avro flights are also cancelled from time to time due to their occasional inability to cope with shorter icy runways.

Indeed. Which is why I keep saying that if the landing distance is not enough (including friction considerations) the aircraft will divert anyway. The number of reversers has no impact on this decision.


The only time you NEED the reversers (not counting turning off earlier) is when the wheel brakes have failed. This seems a pretty unlikely occurence. Besides, if it did happen, I would think the pilots would go for a long runway, burn off/dump fuel to get the landing speeds down, touch down early, use the spoilers and all that jazz anyway. Eventually the plane will stop (presumably on the runway if your numbers are right) regardless of reversers or not.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 12:56 pm




Quoting Starlionblue (Reply 33):
Which is why I keep saying that if the landing distance is not enough (including friction considerations) the aircraft will divert anyway.

Ah. I guess I was under the impression that aircraft equipped with reversers, and with comparable runway length requirements were getting in and out, while the Avros were not. I probably misunderstood something along the way....




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n8076u
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 2:09 pm

Quoting Vikkyvik (Reply 29):
So if one reverser is unusable, would the opposite reverser be deployed on landing or not? I don't really understand the logic behind having 2 unusable reversers be a no-go.

I can't really answer your question, as it never made that much sense to me either, but I can tell you that at UA, the MEL stated the same for our 747-400's, two inop reversers is a no-go, non-deferrable.

I did ask one of the pilots once, after we deactivated his #3 reverser, and he said they'd only use the two symmetrical good ones (#1 and #4) above idle thrust, and that he himself wouldn't bother reversing #2, but that "some of the fellas" might go to idle reverse on #2.

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Matt72033
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 3:00 pm

Quoting Vikkyvik (Reply 32):
Sorry - I was referring specifically to Matt72033's statement regarding his company's B744s.

i'll check the MEL again at some point today, but i know ive definately locked out one reverser without locking out the opposite!
 
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 4:58 pm

Quoting Matt72033 (Reply 36):
but i know ive definately locked out one reverser without locking out the opposite!

I don't think opposite T/R deactivation would be called for.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 9:18 pm

Quoting 2H4 (Reply 34):
Quoting Starlionblue (Reply 33):
Which is why I keep saying that if the landing distance is not enough (including friction considerations) the aircraft will divert anyway.

Ah. I guess I was under the impression that aircraft equipped with reversers, and with comparable runway length requirements were getting in and out, while the Avros were not. I probably misunderstood something along the way....

No, I was unclear. I simply meant that reversers are hardly a necessity for airliner operation. No runway length comparison involved.
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OldAeroGuy
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Fri Jul 07, 2006 11:44 pm

Quoting Prebennorholm (Reply 31):
Thrust reversers are never counted on when runway performance data are certified for a plane type. They are only considered a noisy gadget to save wear and tear on the wheel brakes.



Quoting Starlionblue (Reply 33):
The only time you NEED the reversers (not counting turning off earlier) is when the wheel brakes have failed.

Under today's takeoff performance rules, neither of these statements is correct. Current EASA/FAA rules (FAR 25 Adment. 98 for the FAA) allow the use of reverse thrust when calculating required TOFL. This is particulary useful for wet runway takeoff computations. Airplanes like the A346 and the 773ER field length performance benefit from this change in the rules.

The A380-800 will suffer somewhat from not having four reversers, but as Prebennorholm points out, its large wing will allow low takeoff speeds so the point is somewhat moot. The situation will be more acute on the A380-900.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sat Jul 08, 2006 1:33 am

Quoting OldAeroGuy (Reply 39):
Under today's takeoff performance rules, neither of these statements is correct. Current EASA/FAA rules (FAR 25 Adment. 98 for the FAA) allow the use of reverse thrust when calculating required TOFL. This is particulary useful for wet runway takeoff computations. Airplanes like the A346 and the 773ER field length performance benefit from this change in the rules.

Good point thanks. I will revise my statement: Reversers are not needed for safe operation, but if they don't exist required runway length is increased.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sat Jul 08, 2006 4:10 am

Quoting 747400sp (Thread starter):
Why The Airbus A380 Has Only Two Thrust Reverser?

The major reason is for FOD ingestion prevention. The two pics make it obvious why this is the case.


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This was discussed a while back, and I was pretty stubborn in believing that such an enormous aircraft only had two reversers. I was then shown documentation and pictures to prove otherwise.

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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sat Jul 08, 2006 7:28 am

Quoting Matt72033 (Reply 3):
OH MY GOD!!!!! a huge design flaw by the Airbus engineers there, they'd better go back and re-design the whole aircraft!!!!!

i mean seriously......do you not think they thought about this when designing the aircraft? do you seriously think they would let it fly if it was going to be a problem?

whats a trust reverser anyway? some kind of unit to reverse somebody's trust?

you are funny but there is no need to take the p*ss out of him...he was just curious..he didn't say there was a design flaw
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 09, 2006 9:47 pm

Quoting Starlionblue (Reply 24):
And why would you land on that short icy runway with an inoperative reverser? Your calculations will simply extend the runway requirement (and factor in friction) and you would have to find one long enough.

Also, anti-lock braking is pretty efficient. Avro 100s land on icy runways all winter long without skidding off the end.

All I'm saying is that having the outboards with reversers would increase the options, or reduce the diversions. Avro RJ 100s have additional air brakes and flight/ground idle selection to reduce forward thrust significantly (if you don't switch to ground idle stopping distance increases markedly).

Anti-lock brakes only maximise the use of the available runway friction, they can't increase it. On ice, friction coefficient is 0.2 or less. Dry is 0.6 or more.

Quoting Starlionblue (Reply 24):
Idle is idle. Means that the forward thrust is insignificant in this regard. Idle thrust is more than compensated by any form of braking..

Idle is definitely a significant forward thrust component. It is certainly not zero thrust. It can be up to 5% of TO thrust. Assuming the net inboard reverse thrust is about 20% of TO thrust, that component from the outboard reduces it effectively to 15%. It cannot be ignored.

Quoting Starlionblue (Reply 24):
Also your argument is a bit weak without a comparison to the 380. How far outboard are the 380 engines?

Why is it weak? No-one is disputing the A380 outboard engines will be over the grass, I'm just pointing out that the 747 engines are also up to a standard runway edge, so can impinge on the grass area too.

Quoting Starlionblue (Reply 24):
As I said, redundancy would not be doubled. Wheel brakes on a whooping 8 axles provide redundancy in themselves.

Reverser redundancy would double, not overall stopping redundancy as you well know. Qantas severely damaged a 747-400 in a runway overrun at BKK partly because reversers were not deployed fully, so reversers are not irrelevant. That runway was merely wet, not icy. I guess "whooping" is a typo. The 747 has eight pairs of braking wheels but still has four reversers. The large number of wheels spreads the load, it's not to increase redundancy as tyres tend to blow together. The wheel brake redundancy comes from multiple brake hydraulic sources and systems.

Quoting Starlionblue (Reply 24):
There are plenty of jets out there with no reversers and they seem to be doing just fine with just wheels and aerodynamic brakes, even in icy conditions.

These are very much in the minority in the air transport category, however you count them. Also types like the F.28 and BAe146/RJ have additional air brakes at the tail as well as wing mounted spoilers. Thrust reversers are the norm.

I repeat, on an icy runway aerodynamic braking and thrust reverse are the largest stopping forces available. Wheel braking is limited by the available runway friction, which will be very low. Stopping distance with brakes alone can be three or four times longer on ice than a dry surface. You should be able to stop in time with brakes alone, but it will push the limits, and other adverse factors may be in play (as tends to happen at accidents).

I'm sure the A380 is designed fully in accordance with current FAA and JAA requirements. I'm simply pointing out my own misgivings about the deletion of the outboard reversers for whatever reason. If nothing else, reversers increase safety margins, but only having two out of four on the A380 is not much of a margin when you count the two lots of forward idle thrust. I'd be interested in hearing the opinion of pilots on this issue.
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zeke
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 09, 2006 10:17 pm

Quoting Jetlagged (Reply 43):
Qantas severely damaged a 747-400 in a runway overrun at BKK partly because reversers were not deployed fully, so reversers are not irrelevant.

If they touched down in the correct speed in the zone that would have been a good start.

Quoting Jetlagged (Reply 43):

I repeat, on an icy runway aerodynamic braking and thrust reverse are the largest stopping forces available. Wheel braking is limited by the available runway friction, which will be very low. Stopping distance with brakes alone can be three or four times longer on ice than a dry surface. You should be able to stop in time with brakes alone, but it will push the limits, and other adverse factors may be in play (as tends to happen at accidents).

On the A340, the reversers only reduce stopping distance by 26/27% on ice.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 09, 2006 10:35 pm




Quoting Zeke (Reply 44):
On the A340, the reversers only reduce stopping distance by 26/27% on ice.

That sounds fairly significant. It would be interesting to examine a chart that compares this percentage for all reverser-equipped aircraft.




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zeke
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Sun Jul 09, 2006 11:08 pm

Quoting 2H4 (Reply 45):

That sounds fairly significant. It would be interesting to examine a chart that compares this percentage for all reverser-equipped aircraft.

On the 340 landing with a 10 kt of tailwind and full reverse should give you the same distance as nil wind and no reverse.

The numbers in my previous post were for the -300, on the -600 its about 35% reduction.

Our charts are all for no reverse, the reverse is a bonus. The percentage reduction using full reverse on an icy or dry runway is the same. The difference is our stopping distance required on a icy runway is about double a dry runway.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 10, 2006 3:01 am

Ok let's have a poll. Would any of our pilots such as Philsquares, SlamClick, Wing and so forth have any reservations about flying the 380 with only 2 reversers based on that fact alone?

I still don't buy the argument that the plane is less safe without outboard reversers. Since the stopping distances change so little, it's not really closing off a huge bunch of diversion options. Most runways at major airports are more than long enough anyway.

As was said, in the case of the Qantas 744 over-running the runway. It should have been able to land without reversers at all. They cannot be singled out as the cause of the problem.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Mon Jul 10, 2006 4:20 am

Quoting Boeing Nut (Reply 41):
The major reason is for FOD ingestion prevention. The two pics make it obvious why this is the case.

Very Nice Picture Examples.Clearly the Engines Extend Beyond the Runway Width.
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RE: Why The Airbus A380 Has Only Two Thrust Reverser?

Thu Jul 13, 2006 6:11 am

The A380 has only has two reversers. Yes and No!

If we call Airbus and order an A380 with four reversers, then I'm pretty sure they will say "Thank you Sir".

But present customers have correctly noticed that the A380 has a somewhat lower takeoff and landing speed than the other large quads, 744 and 346. So where the A380 is planned to go, runway length is hardly an issue, but runway width is. And to minimize risk of FOD, only the #2 and #3 engines...

I'm pretty sure it is no more complicated than that. It's the same old story as the VC-10, two or four reversers, operator's choise.

And when they are not needed, then the weight of those reversers can be directly converted into available payload = more profit to the shareholders. Maintenance costs can be put directly into the pockets of the shareholders.

And remember, there is only one reason for airline companies to exist: To please their shareholders.

Did I say something wrong? No, I don't think so.
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