Actually the FMA always does not show what's behind the scene. I had posted earlier from the Sochi accident that when THR CLB/ OP CLB engages in FMA aircraft actually acts initially through Speed VS of +8000ft/mt. Only when the thrust goes past a certain N1 then it goes to THR CLB and OP CLB. Similarly in DES mode the thrust is not really idle but retains some thrust to prevent large deviation if anti ice is used. I had also noticed contradictory statement in FCOM "The aircraft may deviate from the DES profile while DES mode is engaged if:‐
Unexpected wind conditions are encountered
‐Anti-icing is turned on‐
The lateral flight plan is changed"
and FCTM says "The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the descent path if engine anti-ice is used or if winds vary. This explains THR DES on the FMA". Perhaps it meant not fully caters but partly. However according to me there's no THR DES in A320/21. OP DES should always be idle except perhaps the repressurization segment.
Last edited by vilas; 29th Jul 2021 at 15:33.