Tech question about a 738 autoland requirements !?
 

Hi guys

I'm doing a little study on the autoland on the 738, and wondering if anyone can throw a little light on the matter as I can't really find a definite answer myself !

Why can an autoland (cat 3) not be done if we only have one generator powering the electrics !?
Why do we need 2 sources of electrics to carry out a cat 3 autoland ?

Thanks for any help given :)

Simple redundancy? What happens if you lose the remaining generator?
Same reason why you need
- two autopilots
- two NAV receivers
- two hyd systems
- two pilots :)

Hello,


The reason why you need both generators in the B737 for performing an autoland, is when you want to execute an CATIII approach you need both autopilot systems A & B switched ON. Each autopilot system is coupled with it's own electrical circuit, so they are seperated from each other and not fed by only one generator. Normally when you aren't able to perform an autoland anymore with the B737 you gonna switch to an fail-passive automatic landing system.

Tech question about a 738 autoland requirements !?
 

Normally when you aren't able to perform an autoland anymore with the B737 you gonna switch to an fail-passive automatic landing system.

That's still an autoland?!?!

The Cat IIIa certified 737s have passive autoland capability, which means that in the event of a failure, no significant mistrim or deviation from previous flight path / attitude will occur, but the landing has to be complete manually (or missed approach, of course).

As said before, with one generator inop (assuming APU generator not supplying power to the respective generator bus) the aircraft is left with only Cat I capability due to loss of redundancy of electrical system.

At glide slope capture the 2 DC busses are automatically isolated from each other by the cross bus tie relay opening.

As geo said, you need two completely isolated electrical systems, each with its own autopilot, receivers etc for redundancy and for cross monitoring of signal and command input integrity.

It's not about , it is about redundancy. The cross bus tie relay opening is a red herring in this case. The DC busses will be just as isolated with one generator supplying the power to both transfer busses as they are with two. However, there will be no redundancy should that generator fail, that is the issue.

Most 737s are just fail passive, as said above. The fail operational 737s can actually do a successfull autoland and automatic rollout if one generator fails below alert height (200ft). They can also do a fail operational autoland to CAT IIIa minima on one engine provided the APU can be started and is powering the other bus.

Putting it another way, for the fail passive aircraft (the majority of all 737s) the fall back is the pilot. For the fail operational aircraft it is the other autopilot, however only below the alert height. Above that it is the pilot. And for those using the HUD only and not any autoland capability it is the pilot to begin with that is doing the low vis landing and the fall back as well.