rob ginger
I thought that
35YearPilot in Post #1751 had explained rather well the Alternate/Direct Law relationship that gave rise to the engine protections adopted. The Eng 2 first mentioned in the following, seems to be a typo, and he was referring to Eng 3, i.e. ATHR was not available and protections were applied.
Eng 2 in ALTN Mode (N1 with -4% thrust protection) (Due EEC having insufficient valid signals to calculate TPR)
1+4 EEC errors logged. Degraded mode is purely due to loss of sensor inputs/EEC errors. Think of it as the engine in Direct Law (no thrust protections!) (Due EEC not able to calc TPR demand. So N1 commanded as a function of TRA and altitude)
One engine in a degraded mode does not cause another symmetrical engine into degrade mode. Airbus never matches thrust - it's against their philosophy "manual thrust is manual thrust!" - not like pseudo Boeing "manual thrust".
So 1 & 4 in Degraded (think Direct Law) Mode (no protections)
3 in ALTN mode (think Alternate Law with protections)
2 was in Degraded mode (not surprising given there were so few sensors left)
NO engines were in a normal mode.
This is why 1,2,4 had N1 centric displays (Degraded) and 3 had an TPR centric display (ALTN) (though thrust set via N1 with 4% pad).
Thrust control on 1,3,4 all 100% correct (but differing protections)
Auto Thrust inop due to MORE THAN 2 eng in Degraded Mode
1+4 thrust levers held constant to: (not in any manuals)
permit very accurate (vernier) total thrust -> speed control on finals
minimise thrust missmatch->yaw->roll->flight control demand on finals with thrust changes (ESSENTIAL when little excess roll control available)
1+4 WAS NOT held at constant thrust due to them being in Degraded Mode (illogical).
Conclusion: The three level 380s thrust system has incredible fail operational capability.
From memory, I believe that Capt Evans said that a decision was made to use No.3 for speed control as less yaw correction would be required and Nos.1 & 4 were matched.
EDIT:: See
Post #1931 for
35YearPilot's update.
mm43