B777 ADS-B & ADS-C question

[agg_karan]

I have following questions regarding b777 on
GPS
ADS B
ADS C
FMCF


Q1) With a complete GPS failure, ADS-B is lost (as per QRH) . Is ADS-C also lost?
ADS-C features various contract events which are independent of ADS-B operation. So is ADS-C avail with GPS inop?!

Q2) our MEL says - The Automatic Dependent Surveillance - Contract (ADS-C) Function is hosted in the FMCF
If there is DUAL FMC failure (FMCF's to be more precise) can I say I have lost ADS-C?!

And what happens to ADS-B when both FMCF's fail?

THANKYOU and regards

[extricate]

Hi,

1) ADS-C is part of CPDLC system when user logon to the AFN. If GPS is down, as long as there's VHF coverage, it will be a means of transmission. Referencing to MEL item 23-15-01

2) I'm not sure. Manuals don't talk much about ADS-C.

I would think ADS-B and FMS are independent but need confirmation

[STBYRUD]

1) ADS-B is a transponder feature that takes its position input directly from the MMRs - ADS-C uses the FMC computed position, thus a GPS failure will not affect it directly - the inertial position will still be available, possibly even a composite position through radio updating. However, VHF coverage is of course not a requirement either - it uses any available data connection, be it VHF, SATCOM or HFDL.
2) I would assume a dual FMC failure means you won't be able to use ADS-C anymore, the MEL is unclear about it because FMC inop dispatch is not permitted at all (at least according to the one I have on hand) - ADS-B is unaffected.