Gbass21
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Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 2:58 am

Hi guys, I have this doubt: I've always wondered if it is better and A380 in hot and high conditions than a 77W for more than 14 hours flying, Particularly, I'm talking about emirates starting Dxb-Bog/Mex. I'm pretty sure that if Bog or Mex wouldn't have hot and high conditions, they definitely would have been there because an 777 will struggle in those conditions. But, what about an A380 doing those routes? I definitely see Mex capable of filling an A380 to dxb (20 millions inhabitants just in Mexico City area) and for Bogota, Colombia has nearly 50m inhabitants and its economy is one of the strongest of S. America). Cheers and hope answers about it.
 
jfrworld
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 3:17 am

I don't know the exact answer to your question, but AF flies the A380 from MEX. My assumption is that the A380 can takeoff from MEX on the 10-11 hour segments just fine. I don't know in comparison to the 77W and on the 14+ hour segment, but an interesting question.
 
rbavfan
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 3:28 am

Would likely be yes as the A380 looses 1 engine on T-O they still have 3 to use. 777 loses 1 engine they have 1 engine thrust for T-O. T-O distances are based on 1 engine out performance for safety reason.
 
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NameOmitted
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 5:47 am

rbavfan wrote:
Would likely be yes as the A380 looses 1 engine on T-O they still have 3 to use. 777 loses 1 engine they have 1 engine thrust for T-O. T-O distances are based on 1 engine out performance for safety reason.


If the question was binary, CAN the 777 do it, your argument would be solid. The question is about how well it can work, which I think works the in the other direction for your question. Because a twin needs to have the power to be able to work with a single engine out, under normal operation they have twice the power they need available to them. A quad without an engine has 3 others, which means they need are built with significantly less reserve power. Each engine needs to only produce a third of the total amount of thrust, not the entirety of the thrust, which means that with all 4 engines running, they only have a third more capacity.

Using your logic, under normal operation the twin should be able to provide more power than the quad.
 
mjoelnir
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 8:29 am

I would say that the A380 should have the advantage. The A380 and the 777-300ER have about the same length take off run at MTOW. The A380 has the lower landing and take off speeds and does not run into the problems with wheel rotation speeds. The A380 has the longer range so it would not to be as near to MTOW as the 777-300ER.
 
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LA704
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 10:27 am

mjoelnir wrote:
I would say that the A380 should have the advantage. The A380 and the 777-300ER have about the same length take off run at MTOW. The A380 has the lower landing and take off speeds and does not run into the problems with wheel rotation speeds. The A380 has the longer range so it would not to be as near to MTOW as the 777-300ER.


Totally Agree.
Add the lower wing loading to the mix, especially if not close to MTOW.
There is a reason airlines like to operate the 380 to JNB, MEX, etc instead of other types.
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smi0006
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 10:32 am

mjoelnir wrote:
I would say that the A380 should have the advantage. The A380 and the 777-300ER have about the same length take off run at MTOW. The A380 has the lower landing and take off speeds and does not run into the problems with wheel rotation speeds. The A380 has the longer range so it would not to be as near to MTOW as the 777-300ER.


Forgive me, I’ve heard reference to it before - what’s the limit of the wheel rotation speed? What’s the impact?
 
tommy1808
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 10:55 am

smi0006 wrote:
mjoelnir wrote:
I would say that the A380 should have the advantage. The A380 and the 777-300ER have about the same length take off run at MTOW. The A380 has the lower landing and take off speeds and does not run into the problems with wheel rotation speeds. The A380 has the longer range so it would not to be as near to MTOW as the 777-300ER.


Forgive me, I’ve heard reference to it before - what’s the limit of the wheel rotation speed? What’s the impact?


There are two limitations:

Tire speed: Once you reach that limit, you can´t go any faster down the runway. If that speed is not enough to get airborn, you can´t take off no matter how long the remaining runway is. The 77W runs into that limit quickly at high temperatures or altitudes (376 kph). At ISA +33°C (48°C on sea level) that happens after just short of 14k feet runway and the MTOW under that condition is just below 330 tons, quite a bit short of the models MTOW. At MEX on an ISA +15°C day, sounds about right for Mexiko, maximum Take of Weight is ~300 ton.

Breaking Energy limit: at all times down the runway the brakes have to be able to absorb the whole kinetic energy to make the plane stop. I don´t think that is a meaningfull limit for the 77W, but for some other planes.

Chapter 3.14: http://www.boeing.com/resources/boeingd ... 2lr3er.pdf

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Waterbomber
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 11:57 am

NameOmitted wrote:
rbavfan wrote:
Would likely be yes as the A380 looses 1 engine on T-O they still have 3 to use. 777 loses 1 engine they have 1 engine thrust for T-O. T-O distances are based on 1 engine out performance for safety reason.


If the question was binary, CAN the 777 do it, your argument would be solid. The question is about how well it can work, which I think works the in the other direction for your question. Because a twin needs to have the power to be able to work with a single engine out, under normal operation they have twice the power they need available to them. A quad without an engine has 3 others, which means they need are built with significantly less reserve power. Each engine needs to only produce a third of the total amount of thrust, not the entirety of the thrust, which means that with all 4 engines running, they only have a third more capacity.

Using your logic, under normal operation the twin should be able to provide more power than the quad.


It gets more complicated than that though: for safety margins, the twin is required to achieve a gross climb gradient of 2.4% with one engine inop after V1, while a quad is required to achieve 3.0% gross with one engine inop. So a quad operating at 75% thrust available due to an engine INOP at or after V1 needs to be able to climb 25% steeper than a twin with 50% of thrust available.
Things are thus not as clear-cut as that.
In addition, for operational planning you calculate weights and performance based on an engine failing at V1, regardless of how remote the chance is, so you can't touch "the reserve thrust" portion after V1. So yes, a twin will have more excess power up to a presumed engine failure at V1, but after that the quad flies away with more excess power and more margin for obstacle clearance.

So instead, I think that there are other, more important parameters to compare.

Off the top of my head, hot and high performance would be dependant on:
-wing loading, so weight to wing area ratio
-excess thrust at given relative payloads, so thrust to weight (loaded) ratio
-the aerodynamic efficiency of the aircraft, so lift to drag ratio.
-payload-range charts

Comparing each of them,
-in terms of wing loading, the A380 wins big time
-At each aircraft's MTOW, the B77W has more thrust to weight ratio up to the presumed engine failure at V1 at 0.29 vs. 0.22 for the A380, from which point the roles are inversed and the B77W will have a slightly harder time accelerating and clearing obstacles at 0.15 vs. 0.16. However, aything less than MTOW and things get blurry.
-L/D ratio should be slightly in favor of the A380
-Payload range sould be slighly in favor of the A380

In this case, it's too close to call, so you would need to compare take-off performance charts for equivalent relative payloads and also look at the runway length available, you would need to see what weights you end up with according to the range and payload, etc...
 
WIederling
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 12:46 pm

taking the ACAPS data and ISA conditions for MEX 3.9km/12.7kft runway elev: 7300 :
I get
~50t (525t TOW) derate for the A380 ( more runway with diminishing returns but no hard limits.)
~63t (290t TOW) derate for the 77W ( tire speed limits would set in beyond 13.5kft )
However you dice it the A380 loses less.
Murphy is an optimist
 
airbazar
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 1:37 pm

I remember reading a post here a while ago that even the A340 performed better for CX out of JNB than the 77W. If true then I would expect the A380 to also perform better.
 
DylanHarvey
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 4:09 pm

Quads generally preform better in hot and high conditions. 757's are outlier's for twins with their hot and high performance.
 
ADOB
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 4:14 pm

Always wondered...can an 4-engine plane like A380 maintain flight if...2 engines simultaneously fail?
 
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767333ER
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 4:33 pm

Part of the issue for the 777-300ER is how much weight it carries for its wings. It is a stretch of a shorter and lighter 777 unlike the A380whihc has a rather huge wing that works well for its weight. The wing does ,are a big difference.
DylanHarvey wrote:
Quads generally preform better in hot and high conditions. 757's are outlier's for twins with their hot and high performance.

Or the A319 or the 737-700 or the 767-300ER or the CS100 or maybe even the 777-200LR.
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zeke
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 4:55 pm

smi0006 wrote:
Forgive me, I’ve heard reference to it before - what’s the limit of the wheel rotation speed? What’s the impact?


Most airliner tyres are limited to a ground speed of 204 kts. However for takeoff we need enough airflow to go over the wings to generate lift, we measure this as the indicated airspeed in the cockpit.

At high elevation airports and under hot conditions, what would normally be say 6,000-8,000 ft above sea level has a density height of over 10,000 ft. For example at MEX with an elevation of 7316 ft, temperature of 21 deg C, QNH of 1007 would result in a density height of 10,000 ft. A 204 kt ground speed at 10,000 ft would be indicated as 172 kts, which is below the rotation speed of a heavy weight 77W. Under those sort of conditions a 777W would have to be 60 tonnes below MTOW to get out, that does not even take into account required climb gradients once airborne.

At these higher density heights the indicated airspeed required to rotate and get airborne is at a higher ground speed than 204 kts. Therefore to get airborne payload has to offloaded until the indicated airspeed at that density height is below a ground speed of 204 kts.

Twins often use a technique called V2 overspeed to improve single engine after liftoff climb performance, again by hitting the ground speed limit of 204 kts impacts on their ability to carry payload.

Modern quads generally have lower minimum control speeds and therefore takeoff speeds than twins, if a 77W rotates at 180 kts an A380 might rotate at 160 kts. That means the quad at a given hot and high condition would run into tyre speed limiters well before a quad. Also the obvious loss of one engine they still have a three engines producing thrust.
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zeke
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 4:58 pm

ADOB wrote:
Always wondered...can an 4-engine plane like A380 maintain flight if...2 engines simultaneously fail?


This all depends on weight and altitude/temperature, if they are light enough they can even fly with 3 out of 4 engines failed.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
mjoelnir
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 5:16 pm

767333ER wrote:
Part of the issue for the 777-300ER is how much weight it carries for its wings. It is a stretch of a shorter and lighter 777 unlike the A380whihc has a rather huge wing that works well for its weight. The wing does ,are a big difference.
DylanHarvey wrote:
Quads generally preform better in hot and high conditions. 757's are outlier's for twins with their hot and high performance.

Or the A319 or the 737-700 or the 767-300ER or the CS100 or maybe even the 777-200LR.



The 777-200LR should run into similar trouble at MTOW as the 777-300ER, tire rotation speed.
 
DylanHarvey
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 6:32 pm

767333ER wrote:
Part of the issue for the 777-300ER is how much weight it carries for its wings. It is a stretch of a shorter and lighter 777 unlike the A380whihc has a rather huge wing that works well for its weight. The wing does ,are a big difference.
DylanHarvey wrote:
Quads generally preform better in hot and high conditions. 757's are outlier's for twins with their hot and high performance.

Or the A319 or the 737-700 or the 767-300ER or the CS100 or maybe even the 777-200LR.


763ER struggles, especially on KSLC-EHAM, and KSLC-LFPG and some others. 319 and 737-700 are good performers, 772LR definitely on FAOR-KATL. and the middle east to US and Australia/NZ routes. Yea forgot about the CS100.
 
DylanHarvey
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 7:20 pm

mjoelnir wrote:
767333ER wrote:
Part of the issue for the 777-300ER is how much weight it carries for its wings. It is a stretch of a shorter and lighter 777 unlike the A380whihc has a rather huge wing that works well for its weight. The wing does ,are a big difference.
DylanHarvey wrote:
Quads generally preform better in hot and high conditions. 757's are outlier's for twins with their hot and high performance.

Or the A319 or the 737-700 or the 767-300ER or the CS100 or maybe even the 777-200LR.



The 777-200LR should run into similar trouble at MTOW as the 777-300ER, tire rotation speed.


It does, not as much as the 77W though, 772LR has better performance than the 77W out of JNB from what I've heard. Both are extremely capable aircraft. Does anyone know how the a359 is out of DXB and DOH?
 
jetblueguy22
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 8:34 pm

zeke wrote:
smi0006 wrote:
Forgive me, I’ve heard reference to it before - what’s the limit of the wheel rotation speed? What’s the impact?


Most airliner tyres are limited to a ground speed of 204 kts. However for takeoff we need enough airflow to go over the wings to generate lift, we measure this as the indicated airspeed in the cockpit.

At high elevation airports and under hot conditions, what would normally be say 6,000-8,000 ft above sea level has a density height of over 10,000 ft. For example at MEX with an elevation of 7316 ft, temperature of 21 deg C, QNH of 1007 would result in a density height of 10,000 ft. A 204 kt ground speed at 10,000 ft would be indicated as 172 kts, which is below the rotation speed of a heavy weight 77W. Under those sort of conditions a 777W would have to be 60 tonnes below MTOW to get out, that does not even take into account required climb gradients once airborne.

At these higher density heights the indicated airspeed required to rotate and get airborne is at a higher ground speed than 204 kts. Therefore to get airborne payload has to offloaded until the indicated airspeed at that density height is below a ground speed of 204 kts.

Twins often use a technique called V2 overspeed to improve single engine after liftoff climb performance, again by hitting the ground speed limit of 204 kts impacts on their ability to carry payload.

Modern quads generally have lower minimum control speeds and therefore takeoff speeds than twins, if a 77W rotates at 180 kts an A380 might rotate at 160 kts. That means the quad at a given hot and high condition would run into tyre speed limiters well before a quad. Also the obvious loss of one engine they still have a three engines producing thrust.

Most informative hot and high explanation I've read yet, saved me from starting a thread. Thank you.
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DocLightning
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Re: Does an A380 works better than a 77W in hot and high conditions?

Mon Feb 12, 2018 8:50 pm

A quad will always have better T/O specs in a hot/high environment.

However, if you have enough pax to load up a 77W to a 75% LF but an A380 only to a 65% LF and your break-even LF is 70%, then the A380 isn't a good choice, is it?
-Doc Lightning-

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Rookie87
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Re: Does an A380 works better than a 77W in hot and high conditions?

Thu Feb 15, 2018 4:41 pm

jetblueguy22 wrote:
zeke wrote:
smi0006 wrote:
Forgive me, I’ve heard reference to it before - what’s the limit of the wheel rotation speed? What’s the impact?


Most airliner tyres are limited to a ground speed of 204 kts. However for takeoff we need enough airflow to go over the wings to generate lift, we measure this as the indicated airspeed in the cockpit.

At high elevation airports and under hot conditions, what would normally be say 6,000-8,000 ft above sea level has a density height of over 10,000 ft. For example at MEX with an elevation of 7316 ft, temperature of 21 deg C, QNH of 1007 would result in a density height of 10,000 ft. A 204 kt ground speed at 10,000 ft would be indicated as 172 kts, which is below the rotation speed of a heavy weight 77W. Under those sort of conditions a 777W would have to be 60 tonnes below MTOW to get out, that does not even take into account required climb gradients once airborne.

At these higher density heights the indicated airspeed required to rotate and get airborne is at a higher ground speed than 204 kts. Therefore to get airborne payload has to offloaded until the indicated airspeed at that density height is below a ground speed of 204 kts.

Twins often use a technique called V2 overspeed to improve single engine after liftoff climb performance, again by hitting the ground speed limit of 204 kts impacts on their ability to carry payload.

Modern quads generally have lower minimum control speeds and therefore takeoff speeds than twins, if a 77W rotates at 180 kts an A380 might rotate at 160 kts. That means the quad at a given hot and high condition would run into tyre speed limiters well before a quad. Also the obvious loss of one engine they still have a three engines producing thrust.

Most informative hot and high explanation I've read yet, saved me from starting a thread. Thank you.


I agree! Zeke is king in my book for anything operations related with his experience!

From everyone’s responses, it’s amazing to see how much better of a performer the A380 is to the 77W so that it can consistently be scheduled on those routes.

Doclightning, the choice would still be the A380 UNLESS the airline can only consistently fill the aircraft (whichever of the two to just above 70% LF). But then, how many business and economy seats at higher yields are in that LF? Do they consistently fill up with profitable cargo? The A380 would still be the better choice since it would give the airline more breathing room and if not, something smaller than the 77W would be ideal according to your example.

Cheers
 
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767333ER
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Re: Does an A380 works better than a 77W in hot and high conditions?

Sun Feb 18, 2018 4:01 am

DylanHarvey wrote:
767333ER wrote:
Part of the issue for the 777-300ER is how much weight it carries for its wings. It is a stretch of a shorter and lighter 777 unlike the A380whihc has a rather huge wing that works well for its weight. The wing does ,are a big difference.
DylanHarvey wrote:
Quads generally preform better in hot and high conditions. 757's are outlier's for twins with their hot and high performance.

Or the A319 or the 737-700 or the 767-300ER or the CS100 or maybe even the 777-200LR.


763ER struggles, especially on KSLC-EHAM, and KSLC-LFPG and some others. 319 and 737-700 are good performers, 772LR definitely on FAOR-KATL. and the middle east to US and Australia/NZ routes. Yea forgot about the CS100.

Any plane will “struggle” when loaded up like that. Those that fly the 763ER say that it is almost as good as the 757 for performance, just more often is it’s payload and range capability more utilized.

The 77L can do 0-60 mph when empty in about 6 seconds which is really impressive for a massive jet. Again often these wide bodies are being really loaded up so that they are heavy. A319s and 737-700s can do shorter transatlantic flights, but they hardly ever do use that much of their capability.
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VSMUT
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Re: Does an A380 works better than a 77W in hot and high conditions?

Sun Feb 18, 2018 9:50 am

NameOmitted wrote:
rbavfan wrote:
Would likely be yes as the A380 looses 1 engine on T-O they still have 3 to use. 777 loses 1 engine they have 1 engine thrust for T-O. T-O distances are based on 1 engine out performance for safety reason.


If the question was binary, CAN the 777 do it, your argument would be solid. The question is about how well it can work, which I think works the in the other direction for your question. Because a twin needs to have the power to be able to work with a single engine out, under normal operation they have twice the power they need available to them. A quad without an engine has 3 others, which means they need are built with significantly less reserve power. Each engine needs to only produce a third of the total amount of thrust, not the entirety of the thrust, which means that with all 4 engines running, they only have a third more capacity.

Using your logic, under normal operation the twin should be able to provide more power than the quad.


Yes, under normal operation the twin might be able to provide more power. The problem is, you can't use it to take more payload with you. You need to plan for the worst case scenario, meaning just a single engine available.

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