Originally Posted by
CaptainSandL
Thanks Greybeard, they certainly look like provisions for a third Rad Alt – same size, same distance apart & the wedges give them the same vertical directionality.
The question now is why were they put there? I was not aware of the requirement for a third Rad Alt for triplex autoland but the 737 only has 2 autopilots. The latest versions are Cat IIIb capable with a rudder channel (see other threads) but I am sure a third Rad Alt is not required, it doesn’t appear in the MMEL. Can anyone operating the 737 to Cat IIIb confirm this?
Are you sure your plane doesn't have a third radalt?
The radalt has become a more vital part of safety fixes over the years, such as making sure the plane is on ground before the reversers deploy: Lauda 767. The Boeing triplex philosophy for critical systems is really a 2 of 3 voting system.
3 ILS receivers, 3 radalts, and 3 IRU feed 3 Fright Control Computers, who do a continuous voting during approach. In the 757/767, the voting is carried all the way to the control surfaces, where the hyd power units play in unison in detent. If one computation/command is out of unison with the others, it pops out of detent, and the approach is continued with dual command.
Airbus, OTOH, has the dual-dual architecture, as did the first commercially successful Cat IIIb, the L-1011, and the inept DC-10 and less inept MD-11. It takes much higher integrity sensors, ILS, IRU and radalt, to assure safety in a dual-dual configuration.
I'm not really familiar with the 737-7 series, but they possibly use triplex sensors to feed the dual fright control computers. It would be a step toward full triplex.
GB