Sorry Swedish Steve, but you are wrong! (at least with regards to RB211 powered -400's)
The EEC will maintain idle speed by increasing the fuel flow
Misleading rubbish I'm afraid.
FACT:
When NAI is selected ON, the EEC automatically selecte APPROACH IDLE N1, which if you had IDLE selected prior to selecting NAI on would increase thrust produced by the engine.
This is to guarentee sufficient hot air for the system to operate correctly.
Totalbeginner:
then what is the point of entering the altitude at which NAI is expected to be used in the descent forecast page, if it has no impact on the descent gradient?
It does, you are correct!
Ref FCOM 1/ TECH, Section 20:
EEC Idle selection:
The EEC selects minimum idle or approach idle. Minimum idle is a lower thrust setting than approach idle (N1). Approach idle is selected when:
- Nacelle anti-ice in ON
and/ or
- Flaps are in the landing position (25 or 30)
Approach idle decreases acceleration time for go-around, and is maintained until 5 seconds after touchdown, when minimum idle is automatically selected.