[Martin Georg/EDDF]

Hallo,

Warum das so ist weiß ich nicht, der Realität entspricht es leider nicht.

Das ist nicht richtig. Ich darf mal aus dem B737-400 Operations Manual (Volume 8, page 8.15.1/3) von British Midland zitieren (zur Info: Als Vorlage für die DF737 diente eine B737-400 von Midland):

AUTO-MANUAL SWITCH
(alternate action)

AUTO (illuminated white) - Tuning is accomplished by the FMC. Pressing the switch will change the function from AUTO to MANUAL

MANUAL (illuminsted white) - Tuning must be accomplished manually by rotating the frequency Selector.

Dazu passt auch die page 8.5.14/7:

Radio tuning (-300/-400 series aircraft)

with the VHF Nav Radios selected to AUTO, the FMC automatically tunes stations having DME capability (DME or VOR/DME). The stations tuned are seöected based upon the best available signals for updating the FMC position, unless a specific station is required by the flight plan.

Auto tuning utilises any of several radio modes. The primary mode tunes radio No.1 and No.2 to separate DME-capable stations (when available). If required, a single radio can cyclic-tune (agility tuning") between two DME-capable stations. The lowest priority mode is the use of both VOR and DME from a single station. The FMC does not use VOR information beyond a range of 25 NM.

...

With the radios selected to MAN, the FMC continues to update its position using manually tuned DME or VOR/DME stations if the tuned stations meet the FMC position updating requirements. ILS frequencies can only be tuned manually (although the FMC will auto tune a DME associated with an ILS frequency).

Das Verhalten der DF737 entspricht also recht genau dem Original.