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Reverse Thrust On The A380.
Why does Airbus only design the A380 to have the reverse thrust operating on the two inboard engines (# 2 & 3). Is it to do with the outboard engines sometimes being close to the edge of the runway and therefore there could be debris blown around the engines? You would think that the weight of the A380 could use all the reverse thrust it could get, yet the two engines which have it seem to do the job adequately, although observing several landings today at LHR, the A380s kept their reverse thrust on the longest time of any other aircraft.
RE: Reverse Thrust On The A380.
Indeed, you would think so.
But the A380 was grossly overweight. To reduce weight, the outboard thrust reversers were removed, as were FOUR main wheel brakes.
Terrifying, huh?
Not really, I suppose. They just extended the landing distance requirements to correspond to the missing aft brakes, and reverse thrust isn't allowed to be used in calculating landing distance anyway, so on paper, there was no "penalty" for removing half of them.
But I'm with you. If I, in my armchair, were ordering a fleet of A380's, I'd be inclined to spec them with four reversers and all main gear having brakes. Airbus would do it for me, too, because I'm a great CEO.
But the A380 was grossly overweight. To reduce weight, the outboard thrust reversers were removed, as were FOUR main wheel brakes.
Terrifying, huh?
Not really, I suppose. They just extended the landing distance requirements to correspond to the missing aft brakes, and reverse thrust isn't allowed to be used in calculating landing distance anyway, so on paper, there was no "penalty" for removing half of them.
But I'm with you. If I, in my armchair, were ordering a fleet of A380's, I'd be inclined to spec them with four reversers and all main gear having brakes. Airbus would do it for me, too, because I'm a great CEO.
RE: Reverse Thrust On The A380.
| Quoting audidudi (Thread starter): Why does Airbus only design the A380 to have the reverse thrust operating on the two inboard engines (# 2 & 3). Is it to do with the outboard engines sometimes being close to the edge of the runway and therefore there could be debris blown around the engines? |
That is the explanation as I understand it.
| Quoting audidudi (Thread starter): You would think that the weight of the A380 could use all the reverse thrust it could get, yet the two engines which have it seem to do the job adequately, although observing several landings today at LHR, the A380s kept their reverse thrust on the longest time of any other aircraft. |
The A380-800 performed her Rejected Take-Off at a weight of 575t, which is the current maximum take-off weight. The RTO test is performed with brakes worn to the absolute minimum legal limit. Considering the Maximum Landing Weight of an A380-800 is 395t, the reverse thrust is completely unnecessary to bring the plane to a safe stop even if the brakes were worn to the minimums.
| Quoting AA737-823 (Reply 1): But the A380 was grossly overweight. To reduce weight, the outboard thrust reversers were removed, as were FOUR main wheel brakes. |
As I understand it, the A380-800 was never designed to have reverse thrust on the outboard engines.
And I believe the passenger model was always designed to have 16 main wheel braking units. The A380-800 Freighter, due to it's higher MTOW (590t), would have had 20 main wheel braking units. Should Airbus introduce a passenger A380-800 with a higher Weight Variant than the 575t WV008, I would expect it to have all 20 braking units.
[Edited 2013-02-16 14:52:26]
RE: Reverse Thrust On The A380.
Pardon my ignorance here, but does that mean that the inboard engines on the A380 are somewhat different from the outer ones? I mean, if the inboard engines of one of the A380's were to be taken off, could they be installed in an outboard position? Or are engines designed for a specific position only, on the wing?
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- chuchoteur
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RE: Reverse Thrust On The A380.
| Quoting Stitch (Reply 2): Quoting AA737-823 (Reply 1):But the A380 was grossly overweight. To reduce weight, the outboard thrust reversers were removed, as were FOUR main wheel brakes. As I understand it, the A380-800 was never designed to have reverse thrust on the outboard engines. |
| Quoting Stitch (Reply 2): And I believe the passenger model was always designed to have 16 main wheel braking units. |
Indeed. The reason being that the 2 rear wheels of the body landing gears are directional. For the Freighter, they would have had brake assemblies but not the directional capability (as I understand it)
| Quoting Gr8Circle (Reply 3): Pardon my ignorance here, but does that mean that the inboard engines on the A380 are somewhat different from the outer ones? |
The engines are identical, the outer nacelles are not equipped with thrust reverser doors and the throttle controls do not have thrust reverser levers for the 1 & 4 engines.
[Edited 2013-02-16 15:50:50]
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- Starlionblue
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RE: Reverse Thrust On The A380.
| Quoting audidudi (Thread starter): observing several landings today at LHR, the A380s kept their reverse thrust on the longest time of any other aircraft. |
Probably just idle reverse for much of that time.
| Quoting AA737-823 (Reply 1): Not really, I suppose. They just extended the landing distance requirements to correspond to the missing aft brakes, |
Even so, the 380 has at least as good landing performance as a 747. But yes as you say since reversers "don't count" deleting the outer two probably didn't count either.
| Quoting Gr8Circle (Reply 3): if the inboard engines of one of the A380's were to be taken off, could they be installed in an outboard position? |
They are identical, but the nacelles are different. Certainly interchangeable.
RE: Reverse Thrust On The A380.
| Quoting Starlionblue (Reply 5): Even so, the 380 has at least as good landing performance as a 747. But yes as you say since reversers "don't count" deleting the outer two probably didn't count either. |
Reversers do count and are taken into consideration on a wet or contaminated runway.
Just when you need them most and brakes are least effective.
If you have a situation where one of the inboard engines has to be shut down you are down to one reverser, or with both inboards out, none, not a good situation and shortsighted on the part of Airbus.
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- Starlionblue
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RE: Reverse Thrust On The A380.
| Quoting Max Q (Reply 6): Quoting Starlionblue (Reply 5): Even so, the 380 has at least as good landing performance as a 747. But yes as you say since reversers "don't count" deleting the outer two probably didn't count either. Reversers do count and are taken into consideration on a wet or contaminated runway. |
Quite.
What I wanted to add before leaving for a long lunch was that regulators don't typically just go "ah, you found a loophole that allows you to comply with the letter of the law so that's ok with us". They looked at the aircraft in its entirety and then decided that it complied with their safety concerns and regulations with two reversers only.
RE: Reverse Thrust On The A380.
I remember reading somewhere (probably on A.Net) that Airbus wanted to build the aircraft without thrust reversers to save weight, but were unable to due to regulations.
Is there a requirement for commercial airliners to have reverse thrust?
Is there a requirement for commercial airliners to have reverse thrust?
RE: Reverse Thrust On The A380.
| Quoting chuchoteur (Reply 4): Quoting Gr8Circle (Reply 3):Pardon my ignorance here, but does that mean that the inboard engines on the A380 are somewhat different from the outer ones? The engines are identical, the outer nacelles are not equipped with thrust reverser doors and the throttle controls do not have thrust reverser levers for the 1 & 4 engines. |
| Quoting Starlionblue (Reply 5): They are identical, but the nacelles are different. Certainly interchangeable. |
Ah, okay....thanks....
RE: Reverse Thrust On The A380.
| Quoting ba6590 (Reply 9): I remember reading somewhere (probably on A.Net) that Airbus wanted to build the aircraft without thrust reversers to save weight, but were unable to due to regulations. Is there a requirement for commercial airliners to have reverse thrust? |
This is my understanding also, except I understood that the A380 met ALL regulatory requirements (according to Airbus) with NO reverse thrust but the FAA (& the report said the FAA, not EASA) simply would not accept it, so the inboards only was an agreed comprise.
Gemuser
RE: Reverse Thrust On The A380.
| Quoting David L (Reply 8): Quoting Max Q (Reply 6): If you have a situation where one of the inboard engines has to be shut down you are down to one reverser, or with both inboards out, none, not a good situation and shortsighted on the part of Airbus. Isn't that also the case for all twin-engined aircraft? |
Yes. So just consider it a twin with two extra engines.
If I remember correctly, The A380 has quite good (read: slow) approach speeds for an airplane of its size (even for a smaller airplane). So no, I wouldn't be worried one bit. It states here:
http://www.airbus.com/fileadmin/medi...ata/AC/Airbus-AC_A380_20121101.pdf
...that the approach speed is 138 kt for MLW at ISA conditions.
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- Starlionblue
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RE: Reverse Thrust On The A380.
| Quoting David L (Reply 8): Quoting Max Q (Reply 6): If you have a situation where one of the inboard engines has to be shut down you are down to one reverser, or with both inboards out, none, not a good situation and shortsighted on the part of Airbus. Isn't that also the case for all twin-engined aircraft? |
I'll be devil's advocate here and say that those are lighter.
| Quoting gemuser (Reply 11): Quoting ba6590 (Reply 9): I remember reading somewhere (probably on A.Net) that Airbus wanted to build the aircraft without thrust reversers to save weight, but were unable to due to regulations. Is there a requirement for commercial airliners to have reverse thrust? This is my understanding also, except I understood that the A380 met ALL regulatory requirements (according to Airbus) with NO reverse thrust but the FAA (& the report said the FAA, not EASA) simply would not accept it, so the inboards only was an agreed comprise. |
That is my understanding also. The point is that the FAA and EASA are fine with deceleration capabilities in all conditions, same as they are with a 777. The fact that Airbus managed to achieve this without the outboard reversers is, I suppose, a testament to good engineering.
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- chuchoteur
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RE: Reverse Thrust On The A380.
| Quoting HAWK21M (Reply 14): Anyone having more info on this part......Which 4 MW Brakes were removed.Locationwise. |
As I mentioned in my post, it is the 2 rear wheels of the body landing gear assemblies.
The brakes were not "removed", those wheels are unbraked as they are directional units.
The consideration for putting brake assemblies on them for the Freighter version included removing the directional component of the BLG.
RE: Reverse Thrust On The A380.
This is a subject that comes once every six months or so..
1/- the landing distance the A380 requires at Max landing weight is inférior to the 748 at its MLW.
That's due to - as one poster has alreadu said - to its low landing speed and the brakes on 16 wheels.
Braking is a function of the brake assembly available torque, the contact surface with the ground for each wheel... the friction coeff of the runway surface... etc...
the A380 having bigger, wider tyres at a lower pressure than the 748, it follows that , with a higher loading per tyre, its braking " power" will be greater.
2/- The choice of two reversers vs four could be discussed ad nauseam. the decision was on fewer reversers but with a more fiable system : the first electro-magnetic T/Rs on a commercial airplane.
With this, the airplane fulfills - and way beyond- all requirements for certification
3/- The argument about FOD from the sides of the runway is no joke : the distance between the centerlines of the outer engines is some 51.5 m. With an average runway width of 45 m, each engine hangs more than 3 meters outside the runway.
The A380 ACAP states :
"The A380 outer engines are high enough above ground to prevent the ingestion of typical loose
objects, which can be found on ground at the edge of runways/taxiways paved areas (loose gravels
for example), in the following conditions:
- at usual taxiway thrust (i.e. up to the breakaway power setting), even if the loose objects are
below the A380 outer engines.
- at usual take-off thrust (i.e. up to the maximum take-off power setting), if the loose objects are
beyond 3 meters from the A380 outer engines centreline."
... which means that all objects blown by an outer reverse will be swallowed.
Is that what you want ?
The two rear pars of the 2 x six wheel body landing gear. They're used for steering.
1/- the landing distance the A380 requires at Max landing weight is inférior to the 748 at its MLW.
That's due to - as one poster has alreadu said - to its low landing speed and the brakes on 16 wheels.
Braking is a function of the brake assembly available torque, the contact surface with the ground for each wheel... the friction coeff of the runway surface... etc...
the A380 having bigger, wider tyres at a lower pressure than the 748, it follows that , with a higher loading per tyre, its braking " power" will be greater.
2/- The choice of two reversers vs four could be discussed ad nauseam. the decision was on fewer reversers but with a more fiable system : the first electro-magnetic T/Rs on a commercial airplane.
With this, the airplane fulfills - and way beyond- all requirements for certification
3/- The argument about FOD from the sides of the runway is no joke : the distance between the centerlines of the outer engines is some 51.5 m. With an average runway width of 45 m, each engine hangs more than 3 meters outside the runway.
The A380 ACAP states :
"The A380 outer engines are high enough above ground to prevent the ingestion of typical loose
objects, which can be found on ground at the edge of runways/taxiways paved areas (loose gravels
for example), in the following conditions:
- at usual taxiway thrust (i.e. up to the breakaway power setting), even if the loose objects are
below the A380 outer engines.
- at usual take-off thrust (i.e. up to the maximum take-off power setting), if the loose objects are
beyond 3 meters from the A380 outer engines centreline."
... which means that all objects blown by an outer reverse will be swallowed.
Is that what you want ?
| Quoting HAWK21M (Reply 14): Which 4 MW Brakes were removed |
The two rear pars of the 2 x six wheel body landing gear. They're used for steering.
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- travelavnut
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RE: Reverse Thrust On The A380.
| Quoting Pihero (Reply 16): |
Thank you for inserting some hard needed facts in this thread Pihero!
| Quoting AA737-823 (Reply 1): But the A380 was grossly overweight. To reduce weight, the outboard thrust reversers were removed, as were FOUR main wheel brakes. Terrifying, huh? |
You got any sources to back that up?
| Quoting Max Q (Reply 6): not a good situation and shortsighted on the part of Airbus. |
So having T/R's hanging over the grass should have been the way to go?
RE: Reverse Thrust On The A380.
| Quoting Pihero (Reply 16): which means that all objects blown by an outer reverse will be swallowed. |
...by all engines.
This is an illustration of the subject :
Photo © Gareth harvey
These outer engines hang 1.90 m over the runway edge, so that the engine centerline is at some 3.5 m over it.
RE: Reverse Thrust On The A380.
| Quoting gemuser (Reply 11): reverse thrust but the FAA (& the report said the FAA, not EASA) simply would not accept it, so the inboards only was an agreed comprise. |
This is a strange statement. I thought that all airplane (A380 would be no exception) have to be certified to be able to stop with T/R in-op with brakes at the maximum wear and weight a max take off.
T/R's are meant to reduce brake pad wear. Although some here have stated that with the newer brakes and wear life, the cost of the pads replacement are not as bad.
One thing about the max rejected take-off is that I'm not sure is whether it is performed on a wet surface. I'm also not sure about the T/R requirement for wet landing/stopping. Consider that in a rejected max take off, the most worrisome item is the brakes catching fire because it gets too hot. With a wet runway, you would think the water would keep the bakes cooler. Although the cooler brakes would probably not help the stopping distance as the weak link is between the rubber and the runway.
bt
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- longhauler
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RE: Reverse Thrust On The A380.
| Quoting ba6590 (Reply 9): Is there a requirement for commercial airliners to have reverse thrust? |
No. There are many airlines not equipped with reverse thrust. The BAe 146 for example. And there are a few that did not have reverse on the inboards, like the Super VC-10.
The A380 is not treading new territory in this regard.
[Edited 2013-02-18 08:15:40]
RE: Reverse Thrust On The A380.
| Quoting bikerthai (Reply 19): One thing about the max rejected take-off is that I'm not sure is whether it is performed on a wet surface. |
No. It's done on dry, hard surface.
| Quoting bikerthai (Reply 19): I'm also not sure about the T/R requirement for wet landing/stopping. |
For wet landing, means other than wheelbrakes, i.e. spoilers and thrust reversers }can be used when they are safe and reliable.
That's the reason why in many instances wet landing distances are shorter than dry ones. ( yeah! strange paradox )
| Quoting bikerthai (Reply 19): Although the cooler brakes would probably not help the stopping distance as the weak link is between the rubber and the runway. |
That's a very vague statement. You seem to consider wet as contaminated
Anyway, before each takeoff, we would have checked the takeoff performance regarding the actual conditions of the runway and the weather. We wouldn't start the takeoff before we're sure that the accelerate-stop distance is within the runway length.
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- Starlionblue
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RE: Reverse Thrust On The A380.
| Quoting Pihero (Reply 22): Anyway, before each takeoff, we would have checked the takeoff performance regarding the actual conditions of the runway and the weather. We wouldn't start the takeoff before we're sure that the accelerate-stop distance is within the runway length. |
This ties in with my earlier statement about regulators (and pilots!) caring about real world conditions and not just "the letter of the law". The FAA and EASA look at actual operational requirements. Having three or four brand new airliners run off the end of the runway would look pretty bad for the regulators who certified them even if they fulfilled the formal requirements.
RE: Reverse Thrust On The A380.
| Quoting Starlionblue (Reply 23): The FAA and EASA look at actual operational requirements. |
Just found a pilot test report on Aviation on Line, talking about the "bvrake-to-vacate" feature and what it brings :
"Airbus test pilot Armand Jacob recalled
an extreme test of BTV that addressed the
issue of an emergency return :
“We landed the A380 at 1.3 million pounds,
about 40 tons over the certified maximum
takeoff weight and 202 tons above maximum landing weight, all using normal
procedures through BTV with maximum
reverse. We found that the maximum brake
temperature was about 400 degrees C, far
below the 800 degrees C that would have
deflated the tires.” "
That very informative article can be found here
[Edited 2013-02-18 16:08:57]
[Edited 2013-02-18 16:09:31]
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- Viscount724
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RE: Reverse Thrust On The A380.
| Quoting longhauler (Reply 21): Quoting ba6590 (Reply 9): Is there a requirement for commercial airliners to have reverse thrust? No. There are many airlines not equipped with reverse thrust. The BAe 146 for example. And there are a few that did not have reverse on the inboards, like the Super VC-10. |
Reverse thrust was also optional on the Embraer 135/140/145 and many don't have it.
RE: Reverse Thrust On The A380.
| Quoting Pihero (Reply 22): You seem to consider wet as contaminated |
No, as I look at all the interface with respect to the aircraft wheels braking from brake pads to pavement, I assume that the interface between the tires and pavement would be the first to slip (lowest coefficient of friction with wet pavement more likely to slip than dry). Although the anti-lock system would probably prevent the tire from slipping.
bt
RE: Reverse Thrust On The A380.
| Quoting bikerthai (Reply 28): the interface between the tires and pavement would be the first to slip (lowest coefficient of friction with wet pavement more likely to slip than dry). Although the anti-lock system would probably prevent the tire from slipping. |
"Lower" would be more precise. I agree with th rest of your post...
...but :
We are somehow into some murky fiels as the FAA didn't recognize "wet" performance, the EASA did... and Boeing has, like Airbus included wet performances in the AFM... These performances are now "Part 25.113" of airplane certification
That said, for us, we consider that the "wet" friction coeff of any given airplane should be the "dry" one divided by 2. In other words, wet is half dry... That statement is transparent to us as we just follow the performance graphs / tables.
The anti skid system, then will trigger brake release at a far lower braking power than on a dry runway in order to prevent a tyre lock... the resulting effect is only in stopping distances.
[Edited 2013-02-19 07:38:11]
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