For Boeing, during ILS landing, I notice many pilots in YouTube tutorials activate CMD B at co pilot side, but didn't explain the reason. Why CMD B but not CMD A?

This is a special feature only available in approach mode, and activates both autopilots (normally pressing CMD B deactivates CMD A and vice-versa).   It is so that one autopilot can monitor the other (fail passive) or even take over if a failure occurs (fail operational).  It is essential in order to perform an autoland.

Also on the busses, by the way, and for the same reason.

 

Edited July 13, 20205 yr by lzamm

8 hours ago, Billcoke said:

For Boeing, during ILS landing, I notice many pilots in YouTube tutorials activate CMD B at co pilot side, but didn't explain the reason. Why CMD B but not CMD A?

Who is in charge for the approach?

Normal practice to have the AP2/CMD B when FO Is in chargé 

Michael Moe

8 hours ago, lzamm said:

This is a special feature only available in approach mode, and activates both autopilots (normally pressing CMD B deactivates CMD A and vice-versa).   It is so that one autopilot can monitor the other (fail passive) or even take over if a failure occurs (fail operational).  It is essential in order to perform an autoland.

Also on the busses, by the way, and for the same reason.

 

For safety procedure, does that mean both the NAV1 (captain side) and NAV2 (co pilot side) need to be tuned to the same ILS frequency too?

As for approach, I notice Youtube pilots activate Des Now in their FMC, is this step optional? didn't see this step in some of the videos.
 

Edited July 13, 20205 yr by Billcoke

13 minutes ago, Billcoke said:

For safety procedure, does that mean both the NAV1 (captain side) and NAV2 (co pilot side) need to be tuned to the same ILS frequency too?

Yes.  Also both course selectors have to be on same setting (the runway direction).

15 minutes ago, Billcoke said:

As for approach, I notice Youtube pilots activate Des Now in their FMC, is this step optional? didn't see this step in some of the videos.

DES NOW is only needed if you need to start to descend before the TOD (top of descent) calculated by the FMC.  Normally for fuel efficiency you want to stay high as long as possible, but you may be asked to descend early by ATC.  In many simulation aircraft VNAV is not very accurate (and the winds may not be correct) so adding a few track miles to the descent may be a sensible precaution.

 

 

 

11 hours ago, lzamm said:

Yes.  Also both course selectors have to be on same setting (the runway direction).

DES NOW is only needed if you need to start to descend before the TOD (top of descent) calculated by the FMC.  Normally for fuel efficiency you want to stay high as long as possible, but you may be asked to descend early by ATC.  In many simulation aircraft VNAV is not very accurate (and the winds may not be correct) so adding a few track miles to the descent may be a sensible precaution.

 

 

 

I see, many thanks