"Clark W. Griswold, Jr." schrieb:

What's the difference between the -400 and -400D??

Some structural parts have been beefed up to handle the additional number of
cycles that happen when operated in a domestic route structure, and the winglets
have been removed.

As background, specing out an airplane is a balancing act between reducing
weight and maintaining an economical life. Overall cycles are more important
than hours on an airframe (engines can be replaced).

Long haul aircraft see 1-2 takeoff/land cycles per 24 hours, but average 10-12
flying hours in that same period. Cycles are a factor in engine life, but are
more of a concern to the airframe due to pressurization/depressurization stress
as well as the impact of landing.

Aircraft that operate domestically will see their daily cycle counts go up by a
factor of 4 or more and their overall flying hours staying the same or dropping.

They learnt from the 737 cabrio effect (Aloha 1988).

http://myadvertiser.com/2001/Jan/18/118localnews1.html

Regards, ULF