AP CMD A and B / Autothrottle Master Lights

[NoslenYL]

Bit of a disclaimer here, I've been flying /W and /A (primarily the 732) for basically my entire simming career up to now, so all the automation is new to me. I got the 737 yesterday, and I was going through the tutorial flights to learn all the new systems before taking it online. I finished the second tutorial flight today, but I noticed that in-flight the master light for the flight directors went out, and I couldn't get it to come back on again (turned what was previously the master FD off, turned both off, turned only 1 on, etc). Just wanted to confirm that's supposed to happen? Also, I'm a little confused about CMD A vs CMD B on the autopilot. I had always thought that they were just two different channels - they could do the same thing, but there are 2 in case one fails, and both are always slaved to the master flight director. But today I was in VOR/LOC mode with CMD A active and tracking a radial outbound from Nav 1, and I switched over to CMD B just to play around with the buttons again. The plane banked right and looked like it was trying to intercept the course I had inputted for Nav 2. I read something in the tutorial documentation that CMD B can't track Nav 1, but wouldn't that just revert to CWS-R? Why did it start tracking Nav 2? Wouldn't I have to set the First Officer's FD to master first for that to happen? Also, if for some reason I feel like flying using VORs and I wanted an easy transition, could I theoretically switch from tracking the Captain's nav source to the First Officer's nav source by just turning off the captain's FD? That should switch the First Officer's to master, meaning the autopilot would then track Nav 2? I get a feeling these are somewhat stupid questions, but any help would be appreciated...

 

Note - Also, I had a weird glitch earlier where I hit LVL CHG to start descending and the speed somehow got set ridiculously high (as in at the top of the speed gauge - caused an off-the-guage descent rate) before then being set to 50 knots or something really low. I didn't touch the speed dial at all, so I'm almost certain that's a bug unless I did messed something up big time...

Edited June 20, 2016 by NoslenYL

[Litjan]

Hi and thanks for the report!

The channels A and B are exclusively LEFT and RIGHT. There is no crossreferencing - even when you have both on (dual channel approach) every autopilot is tracking his onside ILS signal. We have a quirk which will be fixed with the next update: The FlightDirector MAster lights (MA) should stay on, even if the AP is in "command" (= working). So that might have added a bit of confusion.

The FD that gets turned on first will be MA(ster) - unless an autopilot gets engaged, this will supersede the FD Master allocation. The channel A (left side) can only track stuff tuned on VHF NAV 1 and vice versa. Both can reference the FMS (LNAV).

You can also see this at work when flying an ILS - the onside FD will give commands, while the offside FD will disappear (unless the ILS is also tuned on that side). However, the FMA (flight-mode-annunciation) will always be the same on both sides.

I am not sure what happened on your FL CHG, it does sound quirky. If you can reproduce it and (ideally) make a short video of how it happens, you would help uns very much!

Thanks for the feedback, Jan

 

[Broeselmappe]

I think my problem is a small one and must not start a new thread...

I can´t enable da 2CH Approach... Everytime I turn on AP2, AP1 goes off...

Sequence: ILS Freq. In NAV 1, Course left and right, AP1, LOC, (when aligned) APP, AP2....

Where is my "silly" (I think) fault...?

Thomas

[Litjan]

Hi Thomas,

this is needed for the 2nd AP to stay on:

Both ILS set on respective VHF receivers

Altitude > 800´ RA

Both hyd systems powered

Both gen buses powered

And probably a few more I didn´t think about - but most likely it is the second ILS not being set on VHF Nav 2...

Jan

 

[Broeselmappe]

That´s it...

As I said... Silly mistake... Thx for the quick offtopic-answer.

Thomas

[NoslenYL]

Hi Jan,

Thanks for the help. That clarifies a lot in terms of the two autopilot channels. I was wondering if it was SOP for certain airlines to switch from CMD A to B for easier tracking of VORs? I noticed in the tutorial flights that you used the heading mode to track VOR 2 using the FO's PFD. Are there risks to switching channels like that?

I'll try reproducing the LVL CHG glitch either tonight or tomorrow night. After the speed went AWOL I reset and tried the flight again and it worked fine, so it may be a one time thing; don't know if I'll be able to get it to happen again.