Engine %N2 > 56% AND EEC selected ON AND Loss of signal from either DEU OR AND Invalid Pt signal from ADIRU (*) -> soft 15" delay.
Engine %N2 > 56 % AND EEC not manually selected ALTN AND EEC in soft ALTN > 15" AND TLA < 19 degrees -> hard
EEC manually selected ALTN -> hard
(*) If Pt (total pressure) data from the ADIRU is missing (e.g. pitot tube frozen or blocked), the EEC in soft mode calculates Pt using TAT and Ps (static press).
If both EEC revert to ALTN at the same time, you are most probably dealing with a DEU failure. Each DEU sends signals to both EEC's so loss of signal from either DEU causes both EEC to revert to soft alternate. other indications associated with a DEU failure are DSPLY SOURCE annunciation and a SPD LIM flag.
The EEC remains in soft alternate mode until hard alternate (reversionary) mode is entered by either retarding the related thrust lever to idle (TLA < 19 degrees) or by selecting ALTN via the EEC flight deck switch.
DISPATCH WITH EEC IN ALTN
Refer to MEL 73-11 [Electronic Engine Control (EEC)]
Select both EEC to ALTN. The EEC control switches are located on the aft OVHD panel or (as installed) in the E&E compartment.
Use full takeoff thrust :
- do not derate engine thrust, except for (700) under specified conditions
- do not use the assumed temperature method reduced takeoff thrust.
Do not use FMS calculated takeoff V-speeds. V1 and VR with EEC in ALTN are slightly higher, as the EEC uses a fixed 30 degrees C - 86 degrees F outside temperature.
FMS takeoff N1 values are valid, except for thrust level 26K or above. For the latter retrieve N1 from PI [Alternate Mode EEC Max Takeoff %N1]
The Autothrottle may be used for takeoff, except for thrust levels 26K and above, or except if not in line with company SOP. With manual thrust setting or with the autothrottle in [THR HLD], N1 indications may increase up to 7% during takeoff run, due to windmilling : DO NOT ADJUST !