Originally Posted by
CaptainSandL
The 737 A/P’s are dual-dual and we only have 2 FCC’s, IRS’s, ILS receivers and Rad Alts. I don’t see where a third would fit into the system, unless it was a backup for Rad Alt #1.
Have any other 737NG operators noticed if they have these blank extra Rad Alt aerials? They could have been standard fit in anticipation of a development or other reason that has since become redundant.
The manufactures have sometimes leased back a new airplane for flight testing, and they could have added the third radalt. If so, they must have decided to leave the antenna mounts in place, rather than remove them and plug the holes.
Slight clarification: the Cat IIIa autoland, such as the MD-80 and 737 are simply dual, not dual-dual. They don't have the full redundant circuitry within each Fright Guidance Computer, as they are fail passive. If there is a failure or mis-compare, they give the plane back to the pilot. A Cat IIIb autoland will continue the approach and landing after a single failure, which requires dual-dual circuitry. Note, that does not mean 4 sensors each, but just two. The Triplex autoland uses 3 relatively simple Flight Control Computers and 3 sensors each.
There was no point making the 737 dual-dual or triplex, until it had a fail operative rudder.
GB