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ilya1502
1st Dec 2022, 03:15
Hello to everyone!

This morning I was preparing our A320 sim for training and peeked into the cockpit of the 737NG where students were already flying. I saw a student disengage the A/P by pressing the CMD button and the CFI yelling at him that this was not a proper technique. I've heard of it many times, I understand that when you want to transition to manual flight, pressing the yoke A/P disc button is a better idea overall, but I decided to see what Mr. Boeing had to say. Having looked through FCOM and FCTM I couldn't really find any recommendation on this. There are instructions on how the A/P can technically be disconnected, but are there any recommendations on how to properly do this that I have overlooked? The only place I remember as a credible source is the 737 CBT. There, yes, it was mentioned that the proper technique is the yoke button, the disc bar is not the common way, but any docs?

Besides, is it really possible to disconnect A/P on a real 737 by pressing the CMD button for the second time? I realised I've never tried. FCOM doesn't list such a function in the description of the button.

172_driver
3rd Dec 2022, 17:23
Besides, is it really possible to disconnect A/P on a real 737 by pressing the CMD button for the second time? I realised I've never tried. FCOM doesn't list such a function in the description of the button.

It is possible. As for any method being "a better method" that comes down to personal preference. The yoke disconnect switch is conviniently positioned, that's all. You could run the trim wheel also.

Fursty Ferret
3rd Dec 2022, 20:29
Nothing saying you can’t because it’s a really terrible idea. If the aircraft isn’t in trim then at least if you use the instinctive disconnect your hand is on the controls.

Denti
3rd Dec 2022, 23:01
I started flying on 737-300/500s which had already the AP/CWS buttons and the disengage bar. Using that one could leave it in a position where it was ever so slightly displaced and prevented any autopilot engagement without it being an obviously visible cause. Therefore it was drilled into us not to use the MCP disengagement bar. The resulting mode confusion and then the resulting confusion why the autopilot was not working was simply too distracting for many pilots.

The NG initially had a similar setup until it was replaced by the updated MCP with a disengagement button instead of the bar that had to be pushed downwards. Of course, using the CMD buttons a second was always possible, but not used as a standard method either.

ilya1502
4th Dec 2022, 02:32
The NG initially had a similar setup until it was replaced by the updated MCP with a disengagement button instead of the bar that had to be pushed downwards.
This is quite a surprise for me since every NG I've seen had the bar. However, as you say, I was taught that this bar was to be used only when other means haven't worked. I can see your point regarding the rest.

stilton
4th Dec 2022, 02:52
It is possible. As for any method being "a better method" that comes down to personal preference. The yoke disconnect switch is conviniently positioned, that's all. You could run the trim wheel also.


Not flown the 737 but spent a couple of decades operating the 727, MD80, 757 and 767


Not sure why you’d ever want to disconnect the AP with the MCP switch, you really want to be holding the yoke and looking at the attitude indicator when you disconnect, and the yoke mounted disconnect switch makes that as easy as possible

Thats why it’s there, using the MCP switch would be an unnecessary distraction

Denti
4th Dec 2022, 11:34
This is quite a surprise for me since every NG I've seen had the bar. However, as you say, I was taught that this bar was to be used only when other means haven't worked. I can see your point regarding the rest.

Yes, it was a change made pretty early during the NG production when they changed to the Honeywell to the Rockwell Collins MCP which enabled, among other things, fail operational CAT IIIb (nowadays CAT III without DH) operation if the customer bought the servo for the rudder, which my back then employer did. According to Chris Brady's website in early 2003. By the way, that is one of the best resources on the web and getting his book is in m view mandatory for any 737 pilot, and i don't get paid for this, since i do fly the A320 now for quite some time ;)

Jump Complete
5th Dec 2022, 08:39
I agree that using the MCP button is in the ‘why would you want to?’ category and deliberately using the trim to disconnect isn’t a good practice. Either technique would probably be questioned, both on line or in the sim.
Re the disconnect bar, the -800’s I fly (mix of older NG’s and almost new) all have the bar. I seem to remember an internal report that a crew couldn’t engage the autopilot until they pushed the bar, which was fractionally out of place.
Anyway, heres one from a zoomed in bit of a photo I took a few months ago.
https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/750x1334/94cc1d83_0393_49f5_948a_cdf199e7ea4a_498d05a04fb66a07535a608 4a7c6268e813cb68c.jpeg

Specaircrew
7th Dec 2022, 17:03
Hello to everyone!

This morning I was preparing our A320 sim for training and peeked into the cockpit of the 737NG where students were already flying. I saw a student disengage the A/P by pressing the CMD button and the CFI yelling at him that this was not a proper technique. I've heard of it many times, I understand that when you want to transition to manual flight, pressing the yoke A/P disc button is a better idea overall, but I decided to see what Mr. Boeing had to say. Having looked through FCOM and FCTM I couldn't really find any recommendation on this. There are instructions on how the A/P can technically be disconnected, but are there any recommendations on how to properly do this that I have overlooked? The only place I remember as a credible source is the 737 CBT. There, yes, it was mentioned that the proper technique is the yoke button, the disc bar is not the common way, but any docs?

Besides, is it really possible to disconnect A/P on a real 737 by pressing the CMD button for the second time? I realised I've never tried. FCOM doesn't list such a function in the description of the button.

I'm not 737 type rated but I think the advantage of using the instinctive cutout on the yoke is that a second press cancels the warnings whereas other methods require you to cancel the warnings in the usual way?