Originally Posted by
EXDAC
Many years ago I investigated a problem with 777 display of engine limits. I was eventually given data from one of the early flight test aircraft that explained the problem and I reported my findings. I don't think the problem was ever fixed.
Basic problem was that EEC was required to provide limits data to AIMS. Because of a system level requirements defect EEC stopped sending the data before AIMS was ready to receive it. It I remember correctly re-powering the aircraft did not fix the problem because every power up had the same timing and AIMS could not receive the required data.
I think the solution was to delay EEC power up until after AIMS power up was complete. There was an alternative work-around but I don't remember any details. I think it required some action on the MAT.
On the 777, we started getting engine limits from the EEC (aka FADEC) - prior to that, the limits were hard coded in EICAS, meaning that a simple change to an engine limit (which are not particularly uncommon) required an EICAS software roll (cost starts at seven figures to roll EICAS software). Hence the change in the 777 (and subsequent Boeing installations).
On Boeings, the EEC is not normally powered prior to engine start (putting the fuel switch to run or start switch to start provides aircraft power to the EEC since the PMA doesn't come alive until ~8% N2/N3) - and no one should
ever be attempting an engine start before AIMS is fully booted and running. However, if someone leaves the EEC ground test power ON (contrary to SOP) (flight deck switches used for troubleshooting without have to run the engine), then the scenario you describe is a possibility. The cure is to turn off EEC ground test power, and follow the SOP to not power up the EEC until after AIMS is fully up and running.