Can any one explain why Approach can not be armed in a Dual RA failure.
I guess it's because during the "AP/FD common modes" (FCOM 1.22.30. AP/FD ) Approach modes on the ILS, "LAND FLARE & ROLL OUT" are armed. Since there is no RAD ALT information - the FMGC can't compute LAND & FLARE so "APP" is rejected.
On a previous aircraft (L1011) the AP/FD "Approach" mode was separate to the "A/L" (autoland) mode, so we could do an auto coupled approach to CAT 1 minima with no RAD ALT information. PFM.