B737NG DES NOW
 

Hi as we know, the DES NOW function, is used to commence an early descent, before reaching the TOD.

However, on certain routes, the aircraft continues to maintain, the existing CRZ altitude, till it reaches the TOD. Even if all altitude restrictions on the legs page is deleted.

B737NG FLT & Landing ALT.
 

1. What happens if we climb above the Flight Altitude?
Example: FLT ALT is 30,000 FT
We have climbed to 40,000 FT without resetting the FLT ALT in the PX Panel.

2. What happens if we descent below the Landing Altitude?
Example: LAND ALT is set to 7000FT
We have descended to SL now. without resetting the LAND ALT in the PX Panel.

I have flown 737s for a short while (3 years) and I have never come across a situation where DES NOW doesn't start an immediate descent.

As for your second post,
B737NG FCOM Vol 2. Air Systems Page 2.40 onwards.

Try using ALT ITVN instead, although I think it performs the same function.

- computer programme or real aircraft? You are re-setting the MCP, aren't you?

It's in the real aircraft. When flying from VABB-VAUD you select the ARADO Transition. The A/C will refuse to descend before the TOD. There are no altitude constraints in the legs page.

The same happens when flying from VABB-VOCB, PEXEG Transition.

This could theoretically be the case if the 1000fpm descent path would require a level flight segment before an altitude constraint (if the normal idle descent path would never be intercepted)... but I have never seen this happen.