Originally Posted by
tdracer
When they incorporated the stretched upper deck for the 747-300 (and later the -400), the longer upper deck improved the area ruling enough that it compensated for the increased weight of the upper deck. In fact KLM had the longer upper deck retrofit to their 747-200s - and was pleasantly surprised to find it didn't affect the fuel burn at all. They basically got some extra seat miles for no increase in operating costs.
Thanks - Sadly what was the Achilles heal of the -200SUD conversions, and also the 300 series, was that the upper deck floor was not strong enough for the new wave of big heavy electric F/J/C Class seats that were becoming de rigueur with all the legacy airlines - The seats simply were too heavy for the UD floor plus channelling the electrics posed issues.
The upper deck was of course more popular for F and J/C seats therefore the resale and re-use values of these two somewhat ''orphan fleets'' was not very good.
The -400 did not suffer from this UD floor issue.