dennys From France, joined May 2001, 792 posts, RR: 1 Posted (2 years 7 months 2 weeks 6 days 13 hours ago) and read 4657 times:
It is amazing seeing an A340-500 or -600 using so much runway's length to take off for such midium hauls like CDG-DOH or CDG-AUH ! A 772 flying CDG-SGN takes less runway 's lenght to get airborne .
Any comments will be welcome
SchorschNG From Germany, joined Sep 2010, 442 posts, RR: 0 Reply 1, posted (2 years 7 months 2 weeks 6 days 13 hours ago) and read 4585 times:
The one engine out requirement need to be regarded.
The twin must achieve its V1 earlier (in terms of distance) to have enough reserves if an engine fails at/after V1.
The quad loses only 25% of thrust, therefore acceleration is less compromised in case of engine out.
Therefore, quads can usually chose a lower thrust setting (engine de-rate). Another issue can be 2nd segment climb. That is, an obstacle in the flight path might reduce the Take-Off Distance Available for the twin, as his second segment climb with one engine failed is smaller.
This take-off performance considerations might really ruin the day of performance engineers.
And actual aircraft loading is hard to guess: for example cargo weight.
And consequently: when no engine fails the twin will lift off earlier.
cpd From Australia, joined Jun 2008, 4879 posts, RR: 44 Reply 4, posted (2 years 7 months 2 weeks 6 days 8 hours ago) and read 4305 times:
Quoting SchorschNG (Reply 1): And consequently: when no engine fails the twin will lift off earlier.
You certainly see massive take off performance when a B777-312/ER cranks its engines all the way up for a proper takeoff. YSSY has a pretty decent length 34L runway, and the B777-300/ER needs little more than a quarter of that.
The derate/flex doesn't give you a good example of what the A340/A380 can really do.
SchorschNG From Germany, joined Sep 2010, 442 posts, RR: 0 Reply 6, posted (2 years 7 months 2 weeks 6 days 2 hours ago) and read 4107 times:
Quoting sccutler (Reply 5): The 340 is sound, well-crafted and safe. It does make me chuckle, though, when I fly on it and observe that it is climbing more slowly than I climb in my Bonanza...
The A340-300 has similar wing area as the A330, but a 40t higher MTOW. Its initial climb climb rate isn't too sporty indeed, while the time-to-climb to ICA is about 29 minutes. In the end, the A340-300 is in no way worse than a fully loaded B747-400. I think the A340 is more often used at maximum payload.
A closer look at those issues reveals that twins also have some disadvantages.
SchorschNG From Germany, joined Sep 2010, 442 posts, RR: 0 Reply 9, posted (2 years 7 months 2 weeks 1 day 13 hours ago) and read 3078 times:
One reason might be ... but that is just a guess ... that the used CFM56-5C engines were pushed hard to deliver the take-off thrust. The Max Climb thrust setting might then be smaller compared to TOGA-thrust. Usually the difference between TOGA and MaxClimb is about 5%. If you increase this distance you may save engine life, but climb performance will suffer.
As the used CFM56-5C are rated at close to 36klbs that might be an issue.
Depending on the runway characteristics the take-off thrust may be de-rated quite a bit. An A340-300 on a 3400m runway with some headwind will have plenty of runway.
That was the purpose of the 340 variant in the 330/340 family. Common wing and fuselage with one long to ultra haul (340( and one medium to long haul (330). Airbus' idea of making a common twin/quad family was inspired, and much cheaper than developing two separate aircraft.
The idea was that the 340 would become more efficient with increasing distance, and it does. And there are still things that quads do better, for example hot and high airports.
While many nowadays poo-poo the 340 as inferior to the 777, it still gives sterling service to first rate airlines. For example CX uses the 340 on many routes, and they also fly 777s. One advantage of the 340 is that it leverages CXs investment in 330s, with common pilot pool and much common maintenance.
Undoubtedly the 777 is superior in capabilities, but it came along several years later. The 777 could not have been built when the 340 was. Conversely, if 777-class engines had been on the drawing board 8-10 years earlier, the 340 might not have come into being.
[Edited 2010-10-09 19:18:58]
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
Speedbird741 From Portugal, joined Aug 2008, 654 posts, RR: 0 Reply 11, posted (2 years 7 months 2 weeks 11 hours ago) and read 2673 times:
Quoting SchorschNG (Reply 6): In the end, the A340-300 is in no way worse than a fully loaded B747-400. I think the A340 is more often used at maximum payload.
Another curious thing about the A340, is how they always climb to a higher altitude at MTOW than an 747 or 777 at MTOW. A fully loaded 747 is sometimes stuck at 28 or 29 thousand feet for its initial stage of the cruise while an A340 can go up to 34 thousand or more! Quite interesting.
Boa noite Faro, Air Portugal 257 climbing flight level 340
SchorschNG From Germany, joined Sep 2010, 442 posts, RR: 0 Reply 12, posted (2 years 7 months 1 week 6 days 13 hours ago) and read 2463 times:
The A340 is one generation newer in wing design.
The wing loading is a bit lower than on the B747[-400 and before].
And drag is slightly lower.
Finally, the B747 has a lower maximum lift coefficient of buffet onset.
We must remember that the B747 is 1960ies heritage. A great design in many respects, but aerodynamics did see some progress. And the B747 was designed for higher Mach numbers (up to M.89), which resulted in some different design decisions.