B777 Back-up electrical system - TCAS

[MickG0105]

As I understand it if the power to one or both of the main AC busses is lost the backup electrical system automatically powers one or both transfer busses as required. To reduce electrical loading on the backup generator, the TCAS is not powered and is therefore inoperative.
Does shedding power to the TCAS have any impact on other Mode S transponder functions? (eg is ATC SSR impacted?)

[Amadis of Gaul]

If you lost both main busses, I'd think the transponder would be the very least of your worries.

[MickG0105]

If you lost both main busses, I'd think the transponder would be the very least of your worries.
If you're looking to ATC for a vector it's helpful if they can actually see you.

[NSEU]

The Left ATC Transponder is on the Standby Bus. Surely that would be working (along with most of the essential inputs).

[Amadis of Gaul]

If you're looking to ATC for a vector it's helpful if they can actually see you.

Indeed. Still, I suspect it won't be problem #1.

[MickG0105]

Indeed. Still, I suspect it won't be problem #1.
Your suspicions and prioritisation of problems aside, can you help answer the question, Does shedding power to the TCAS have any impact on other Mode S transponder functions? (eg is ATC SSR impacted?)

[MickG0105]

The Left ATC Transponder is on the Standby Bus. Surely that would be working (along with most of the essential inputs).
Thanks for that. What I'm trying to understand though is the impact of ELMS load shedding to the TCAS on other S band functions such as ATC SSR.

[Amadis of Gaul]

Your suspicions and prioritisation of problems aside, can you help answer the question, Does shedding power to the TCAS have any impact on other Mode S transponder functions? (eg is ATC SSR impacted?)


My conjecture would be that if the manual was specific enough to tell you that you'd lose TCAS, it would also mention that you lose the entire transponder, if that were, indeed, the case. Then again, how much power does TCAS really take?

[sleeve of wizard]

In flight if both transfer busses are unpowered the APU starts automatically regardless of APU switch position. 2 Backup generators (considered to be 1 generator) plus the APU generator will provide sufficient power to satisfy most electrical loads.

[MickG0105]

In flight if both transfer busses are unpowered the APU starts automatically regardless of APU switch position. 2 Backup generators (considered to be 1 generator) plus the APU generator will provide sufficient power to satisfy most electrical loads.
Thank you, SoW.