B737 1CH app. Why doas it need radio altimeter?

[relax.jet]

Hello fellow pilots,

other day, we had radio altimeter on r/h side (B737NG) inop.
I was just wondering, for what does one autopilot (1ch app) need radio altimeter?
(Just an theoretical question. It would be nice to know why?)

DDPG says: With radio altimeter(s) inoperative, do not use the associated autopilot or autothrottle for approach and
landing.

That`s clear, but why?

(Isn`t it just some kind of selftest? Autopilot is probably doing it like for dual ch app....?)

Thnx, for you judgement.

be relaxed

[sjm]

toga? on classic requires rad alt to be 2000' or less to be armed, linked to autopliot engaged so if no rad alt info, no auto throttle/FD command.

All i can think of without books

[BOAC]

I'm pretty sure that the steering response to ILS deviation is reduced as radalt reduces to avoid 'hairy' banking as the needles move as you get lower down. No radalt removes this protection.

[relax.jet]

Originally Posted by BOAC

I'm pretty sure that the steering response to ILS deviation is reduced as radalt reduces to avoid 'hairy' banking as the needles move as you get lower down. No radalt removes this protection.

Yes, that`s one of the reason, it makes sense.
But, I saw in DDPG, that with both rad.alt. INOP the autopilot will command only 8deg. bank. Would it be the same with only one rad alt inop, wouldn`t be?

thnx for your ideas