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Old 28th Dec 2012, 21:59   #1 (permalink)
 
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a380 thrust reverser

I noticed during landing only #2 & 3 T/R's deployed. Is this how the aircraft is equipped or was this possibly due to a narrow rwy and FOD?
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Old 28th Dec 2012, 22:11   #2 (permalink)
 
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That's all they have. Gotta save weight and Euros!
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Old 29th Dec 2012, 02:09   #3 (permalink)
 
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Since #1 and #4 oft overhang, there's substantial risk of FOD/erosion/etc. if reversers were employed there.
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Old 29th Dec 2012, 06:22   #4 (permalink)
 
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Had me question if simply 2 reversers were sufficent in design, 1 and 4 are quite high. FOD was my first thought, how does this effect landing distance if 2 or 3 are inop on such a large/heavy aircraft. I wonder if the aircraft was able to control with a 1 or 4 on MEL (being enabled) given the thrust of the engines? Thanks for the answers, just seemed a bit odd.

Grounded
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Old 29th Dec 2012, 07:03   #5 (permalink)
 
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Landing Distances are not calculated with thrust reversers deployed. The figures only takes into account, landing in the touchdown zone, max anti-skid braking, and ground spoilers deployed.

There is an exception in some aircraft such as the G550 that when landing on a wet runway, the wet landing distances numbers do incorporate TR's.

To be more specific if you care, below is the information right out of the G450 OM. But is basically the same across all aircraft I have flown.

Landing Field Length

The landind distance shall be the horizontal distance required to land and come to a complete stop from a point at a height of 50 feet above the landing surface. Landing distances are shown as a function of flap position, anti-skid functions (operative or inoperative), landing gross weight, airport pressure altitude, wind condition, spoiler deployment (automatic or manual), and possible speed increase (delta V) to the threshold speed (Vref). The threshold speed increase (delta V) correction grid provides landing distance increments which would occur on occasions such as Category II approaches or when the pilot elects to use autothrottle all the way to touchdown.

Normal Flap Landings:
Conditions:

The following conditions apply for normal flap landings:
1. All engines operating at high idle before touchdown (high idle automatically established with gear down, flaps 39) and reduced automatically to ground idle about 5 seconds after touchdown.

2. Flaps 39 and gear down.

3. Landing distances based on 3.0 glide path at 50 feet and 6 FPS sink rate at touchdown.

4 Distances are shown for both anti-skid operative and inoperative. Either ground spoilers armed and automatically deployed on touchdown, or manual speed brakes selected after touchdown. For a complete ground spoiler / manual speed brakes deployment failure, a correction factor is presented at the top of the anti-skip operative chart.

So as you can see, no where is Thrust Reversers factored into landing distances.

Hope this helps in some small way
J
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Old 29th Dec 2012, 18:30   #6 (permalink)
 
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Back when it was still the A3XX (in the late 90's) Airbus raised the subject of reverser's at a Configuration Working Group meeting and produced a paper showing the pro's and con's of zero, 2 or 4 reverser's including projected runway length requirements. Among the con's were weight, complexity, maintenance costs, etc and the fact the no reverser version was projected to require an excessive runway length for a wet/slippery runway whereas the 2 reverser version had a more reasonable required length. The airlines attending the meeting recommended the 2 reverser option.
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Old 29th Dec 2012, 21:18   #7 (permalink)
 
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Quote:
Back when it was still the A3XX (in the late 90's) Airbus raised the subject of reverser's at a Configuration Working Group meeting and produced a paper showing the pro's and con's of zero, 2 or 4 reverser's including projected runway length requirements. Among the con's were weight, complexity, maintenance costs, etc and the fact the no reverser version was projected to require an excessive runway length for a wet/slippery runway whereas the 2 reverser version had a more reasonable required length. The airlines attending the meeting recommended the 2 reverser option.
" The airlines attending the meeting recommended the 2 reverser option. "

Of course they would, It's cheaper and meets the certification requirements.,

In fact zero reversers is even better unless you ask a pilot who always wants more margin.

It's in the book.

The worst combination is when you teach pilots to land with reversers and then take it away without unlearning them.

Shouldn't be a problem with an A380 where pilots are only qualified with two reversers
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Old 29th Dec 2012, 22:30   #8 (permalink)
 
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IMO, residual forward thrust is more important a factor in wet/icy runway conditions than the availability of any significant amount of reverse thrust. I have been in one situation where idle reverse was the only thing that kept me from going off the end of the runway in a 747, and I know of at least 1 other case in our airline where the non-use of reverse caused an overrun at low speed.

Bigger engines with bigger fans have significant idle thrust, and with poor braking action it may well overcome the ability of the brakes to slow the airplane below taxi speed. At least the A380 negates the thrust of about 2.5 engines with 2 in idle reverse...
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Old 30th Dec 2012, 01:21   #9 (permalink)
 
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Bigger engines with bigger fans have significant idle thrust,....
But that doesn't have to go hand in hand does it?

I mean the majority of the thrust could well be in the fan speed and the fan speed doesn't always have to match a significantly high compressor speed in idle .

I wonder if anybody has any relative data for the latest high-bypass generation engines including a 3 spooler or a geared fan?
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Old 30th Dec 2012, 22:35   #10 (permalink)
 
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Technically, the constraints on idle thrust are -

o Flight idle - limited by the time required to accelerate (throttle burst) to 90% of TO thrust (for GA).

o Ground idle - probably limited by the ability to carry gearbox HP loads and/or bleed air requirements.
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Old 31st Dec 2012, 04:31   #11 (permalink)
 
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Quote:
Technically, the constraints on idle thrust are -

o Flight idle - limited by the time required to accelerate (throttle burst) to 90% of TO thrust (for GA).

o Ground idle - probably limited by the ability to carry gearbox HP loads and/or bleed air requirements.
That sounds like gas generator energy and not necessarily a significant relationship as a comparative indicator of idle propulsive thrust
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Old 31st Dec 2012, 05:27   #12 (permalink)
 
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I second Intruder.

Todays big fans have huge idle thrust. Simply consider the fuel flow (almost 1t on a GE90) and you realise what I mean.
On the T7 upon push-back some tractors can't cope with the residual forward thrust ......

The 380 engines are not much smaller and imho it is not very astute building an aircraft with that much residual forward thrust and not being in the position to cancel it out completely on slippery surfaces.

But then again: It is certified, so who cares ......
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Old 1st Jan 2013, 17:08   #13 (permalink)
 
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That sounds like gas generator energy and not necessarily a significant relationship as a comparative indicator of idle propulsive thrust
Correct, nonetheless significant.

There is an operability parameter known as acceleration fuel margin. It is the difference between the fuel required to hold a steady-state speed (say, ground idle) and the capacity of the engine to accept more fuel at that speed. For a successful accel, this must always be a positive margin, preferably a healthy margin.

Now, load up the gearbox with SHP extraction (hydraulics and BIIIG gererators), and bleed out a lot of pneumatic load; The steady-state fuel required will increase, eating into the accel fuel margin.

If we were to adjust ground idle downwards, the steady-state fuel flow will decrease (but not much), but the accel limit line (related to stal/surge margin) will creep downward more rapidly. All this time, idle thrust is decreasing, but so is the ability to accelerate.

At some point we'll find the beast will refuse to budge in response to the throttle!
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Old 4th Jan 2013, 15:08   #14 (permalink)
 
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There are some methods (mostly theoretical) for reducing idle thrust in a turbofan. One I have seen proposed is a bleed valve between the HP and LP turbine. A beautiful concept, reducing the driving gas flow for the fan shaft system; but the implementation in that temperature/pressure environment requires a metallurgical breakthrough known as unobtainium.
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Old 4th Jan 2013, 15:17   #15 (permalink)
 
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We suffer more from a lack of brake fans than we will ever do with only reversers installed on 2 and 3..!!!!!!!!!!!!!!!!!!!!!!!
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