View Full Version : B737 Autopilot Disengage Bar

16th Jun 2004, 23:17
On the newer MCP the Autopilot Engage Paddles have been replaced by switches;
under those switches there is an Autopilot Disengage Bar.

What is your policy concerning that Bar?

The Boeing manuals leave us alone, for a change, on this matter.

We have two philosophies, and the partisans of each one is crusading against the other, so I'd like to know....

1) Keep the bar down; when you want to engage the AP lift it up and press the engage switch.

When you want to disengage the AP push the bar down.
The reason?
Now that we don't have paddles anymore, a simple circuit "short" or malfunction might engage the AP when least needed, on short final for example.

2) The others say: keep the bar up.
There's no danger of an unwanted engagement, and if you forget to raise it, you won't be able to engage the AP.

Personally I think the first one is correct, but we don't have any official source to refer to...

Or maybe yes?


17th Jun 2004, 11:45
LEM, take care with any assumptions that you make in the use of the “Autopilot Disengage Bar”.
I suspect that the 737 is very similar to, if not the same as the Avro RJ autopilot engage panel (the RJ has a Honeywell design based on 737). The RJ Disengage Bar is specifically an “emergency” disengagement system for use in the event of the normal methods failing. Thus, the bar has no involvement in either normal engagement or disengagement and should remain up at all times. If the bar is down in the RJ there is a red / black stripped warning to indicate the bar’s abnormal position and I recall that a down selection sets a maintenance fault.

Also, disengaging the autopilot with the bar in the RJ, invokes different functions. So that if the control-wheel disengage buttons fail or the autopilot computer rejects that input, then the bar will by-pass the normal operation and cuts engagement power the autos. This action, from memory, also removes power from the controlling pitch, roll, and yaw servos, although I think not for pitch trim or yaw damper – Avro RJ operators check the manuals.

The Avro RJ has engagement buttons in lieu of ‘paddles’, but their use differs from the 737’s selection of A or B channel (or similar, I don’t recall), in that the RJ has a single (integrated dual lane) autopilot where the buttons switch between sensor inputs – Nav 1 / Nav 2.

17th Jun 2004, 12:02
Thanks for the reply, safetypee, bar up at all time.

Hope to hear from other pilots, too.

17th Jun 2004, 12:28
LEM, something puzzles me. We have been at loggerheads before about the technical questions you keep asking for an experienced 737 operator. Please don't get me wrong, I'm not having a shot at you but in my previous life, any time a new type or upgrade was introduced into our fleet, we were rostered for ground school instruction by our technical training department. It really concerns me that I might be flying in the oldest part of Europe with operators that are not fully conversant with their equipment.:uhoh:

17th Jun 2004, 12:49
Hi HD!

Long time...

What can I say, you seem to live in a perfect world...

Thanks God, the world I live in today is not so perfect...
and there's still some room left for the individual.

I have lived in perhaps in the most perfect country in the world many years ago, but the suicide rate in that society is also one of the highest!

I continue to smell what I smelled in the beginning, in your post, and you know very well what I'm talking about...

No, my world is not perfect, but rest assured you would pax in very good hands, down here.


17th Jun 2004, 12:59
Bar up, normal position. There is actually no danger of accidental engagement of autopilot.As safetypee said ,the bar is there for emergency use only.
Why the use of two manouvres to engage autopilot?
And what would be the risk of a not commanded engage of the autopilot? What will hapen in such a case? PF will be with hands on controls,so no problem at all.
And,maybe such a use of the bar,for which was not designed,will damage in time the switch.
It says nowhere in the books to use it as your nr 1) ,only as a disconect option.
Ang HotDog,nobody knows it all,even the technical dep,so let the man ask whatever he want's. I look forward for his questions ,most were always interesting questions to ask .
You find out about different SOP's ,better or worse .
Sometimes I had to open the books to anwer them,so it's a positive thing.
Brgds Alex

17th Jun 2004, 13:26
LEM,Thanks God, the world I live in today is not so perfect...and there's still some room left for the individual

Do you mean to say that you approve of individual SOPs?:confused:

17th Jun 2004, 16:32
Thanks, Alex, for your reply.

You are probably right, funny thing is our head of tech dept., the guy giving us lessons on the ground (who is an incredibly deep engineer, btw), thought us philosophy n°1.

Excellent engineer, as I said, but also a failed commercial pilot, and you can almost immediately spot a subtle resentment buried very deep in his personality.

There's no escape, that damn missing stripe on one's shoulders definitely cannot be forgotten, as someone on this forum keeps showing.



18th Jun 2004, 04:41
Very subtle LEM, you should have been a comedian! For your info, I'm willing to bet that my three stripes as a senior check airman were worth heaps more than your four. If you really have them?:rolleyes:

square leg
18th Jun 2004, 07:04
Once the bar is down (in flight) you need WoW's to be able to reset it (i.e. you have to land again manually).

As said the bar is an emergency AP off switch on the Avro. The Avro's MCP is almost identical to the 737-3/4/5's MCP and even the NG's.

18th Jun 2004, 13:23
I would definitely not recommend using the disenga bar to control the autopilot on/off status. On the newer Boeings one you disable the AP in this fashion you may also lose other potections passively provided by the AFDC's....eg Bank Angle Protection etc.:cool:

18th Jun 2004, 16:49
Your concerns on this issue were raised at the time Honeywell was selected as the vendor with this design.
The answers to your worries are apparent if you look closely at the operation of the panel switches.
When a mode engages the LED's in the switch illuminate. That is not something you can prevent. It doesn't matter if you touch the switch or not. It's software/hardware controlled from the DFGC and not a part of the switch function.
When you press your column disengage switch power is interrupted to the interlock system and when the mode disengages the final indication to you, the pilot, is the LED extinguishing (plus, of course the horn). The LED's are their own system unto themselves and are in fact independant of the switches.
The disengage bar is a "positive disengagement" mechanism that physically removes power from all of the DFCS flight control commanding subsystems. It's there in the unlikely event that your column switch fails in the "continuous contact" mode.
You'll notice that it takes at least twice the force to raise the bar as to depress it. That's to ensure that you don't accidentally raise it once you've used it in a problem situation.
The bar is not considered a normal means of disengagement of the DFCS and was designed to be left in the raised position.
One other little bit of trivia you may or may not be aware of; the two yellow panels behind the bar are illuminated. Not brightly enough to be seen in normal light but if you have a dark cockpit have a look and you'll see the light...:-)

Blue above the rest below

Paolo de Angelis
14th Sep 2006, 13:04
Taken from a 744 bulletin:

Background Information
Boeing has received reports from operators of uncommanded autopilot
engagement and flight mode changes. The reports document at least one rejected takeoff and one...

From now on, I'll keep it down.