Originally Posted by BOH
As I understand it, it's mainly due to the A340 having four engines rather than two.
The 343 & 342 jus didn't get the engine they were originally designed for - the P&W geared turbo fan. So Airbus had to settle for 737-class engines (CFM56/IAE 2500) instead which made the birds underpowered.
Also to be fair, the A340s supposedly have flex power configuration so the inflight computers use whatever thrust is required given load, length of runway and weather conditions to minimise engine wear at takeoff. This may account for the feeling that the 340 won't get off the ground - the a/c is using as little thrust as it can safely do so.
IIRC an AC pilot posted some time ago that a fully loaded 744 has an equal (or even slightly worse) take-off and climb performance to a fully loaded A343. Of course only the Airbus would get slated for it though
These conditions seem to afflict the 343 more than a 744 in the real world. SQ dropped the 343 because of poor climb performance out of SIN. Basically the a/c couldn't get to cruise altitude over the Indian ocean (avoiding storms and trying to get into the flight paths with 744s whizzing past overhead). And Airbus wings are supposed to be designed for climb (while Boeing supposedly optimises for cruise).
The 340 series got engines more suited to what they required with the Trents used in the -500 and -600.