Originally Posted by
galaxy flyer
Not having GPS equipment is probably the least important factor in this incident. Plenty of planes fly safely w/o GPS.
True enough if they don't have RNAV. But, the early RNAV FMS birds that do not have GPS are subject to small inaccuracies when the DME geometry is not good. Those small inaccuracies can become a significant factor flying an FMS visual, especially down low and close in.
We do not know if that happened in this case. But, the potential is there using DME updating instead of GPS for position determination, particularly as the airplane gets low and loses good DME geometry.
With the tight spacing of Runways 28L/R and especially 28R and Taxiway Charlie, this 28R FMS Visual should mandate GPS. Look at the RNAV IAPs to 28R. They state "DME/DME RNP-0.30 not authorized." The FMS 28R Visual should have the same restriction, particularly since it uses RNP 0.30 from F101D inbound.