737, manual override in dual channel approach

[Flutter speed]

I had a long discussion today with two 737 pilots about the following:

On a B737 NG, should the pilot be able to override the AP with the control column / wheel -force when on a DUAL channel approach (with FLARE armed). I know that when on a single channel approach, there is an FCC option to revert from CMD to CWS if the control force exceeds a certain threshold. However, we were not certain about this for a DUAL channel approach.

The FCOM is not helpful: " ... This manual pitch override is inhibited in the APP mode with both A/Ps engaged.". Which bascially says that the systems dont revert to CWS. But it doesnt tell me if the AP disconnects or just stays connected. So the only way to disconnect from the A/P in a dual channel approach would be to press A/P disconnect.

I tested it in the simulator, and the A/P didnt disconnect. But this is for me not a final verdict. I am sure you guys know what behavior can be expected.

Thanks for the thoughts :-)

[Don Bobbio]

Hello Flutter speed,

I am afraid I cannot answer your question. But why would you want to know this as a pilot?

Since your making a dual AP approach you are aiming for an autoland. If the autopilot does not behave as desired I would either disconnect the AP and continue manually or go around.

Regards

[Flutter speed]

Hi Don Bobbio,

I agree, I would say the first thing you do is press TO/GA when on a misbehaving dual channel approach, or disconnect the AP. It is just that nobody seems to know what actually should happen IF you pull the column. In training scenario's they try everything, including this :)

I am not a pilot myself. But I spend my days in and around full flight simulators as an engineer. I work with pilots on a daily basis, and sometimes I am surprised about the amount of discussion of correct aircraft systems behavior, though same goes for the lack of clarity of the FCOM at times. The latter is clearly not written for engineers ;-)

G'day

[nick14]

I believe I am right in saying that the ap will disconnect should you apply control column force during dual channel approach.

Also agree with the above

[Denti]

Since your making a dual AP approach you are aiming for an autoland.

Not really true, depending on SOPs. For example we do every ILS/GLS as dual channel approach, even if we plan to disconnect the autopilot later on for a normal manual landing. It is just a longstanding (>20 years on 737s) SOP which does prevent single channel aileron hardovers. However, disconnecting the AP is easier done with the normal switch instead of overpowering it, never really tried to wrestle with the autopilot on short final, there are better situations for that.

[safetypee]

In most modern aircraft /recently certificated designs, the autopilot should disconnect if the pilot applies an overpowering force.
In other aircraft, depending on system design, the overpowering force could be interpreted by the autotrim as being out of trim, thus it will attempt to trim the aircraft away from the datum condition with potentially hazardous outcome. There is a history of such accidents and incidents relating to loss of control.
Do not overpower automatic systems, disengage and fly the aircraft manually; and similarly ensure that the aircraft is in trim before engaging the autopilot.

[Centaurus]

For example we do every ILS/GLS as dual channel approach, even if we plan to disconnect the autopilot later on for a normal manual landing

Now that's a new theory. I must say it seems like jumping at your own shadow. Surely the autopilot airworthiness people as well as the aircraft manufacturer would have considered the probability factor of an obscure defect such as an aileron channel runway and catered for that in the FCTM - if of course there was a problem in the first place.

[BOAC]

Now that's a new theory - not really - 'recommended'/'encouraged' by BA (up until 2004 anyway) and I believe many other airlines.

[RAT 5]

Was this practice not to allow an Autopilot G/A? It does lead to the nose up trim if disconnected after 450', and that might be problem, but manageable.

[Denti]

That is a nice side-effect of course, but as far as i remember not the main reason. We did have a few autopilot hardovers before that SOP came into force and when the company talked to boeing they just asked "why don't you use a dual channel approach?".

[BOAC]

Yes, Rat, but also for the 'redundancy' aspect. The trim change was trained in the sim.

[Flutter speed]

Thanks for all the valuable comments! I understand that there are different SOPs in how to disconnect from the AP on a approach. I think my question is however a more specific one for the B737 NG.

I found this in our FCOM:

Option - CWS deactivated on approach
Note: During a dual autopilot approach and after FLARE ARM annunciation,
any attempted manual override of the autopilots will result in an autopilot
disconnect.

So I am just wondering, if I yank the wheel in DUAL CMD, what will happen, provided I dont have the above option in my aircraft.

Thanks!

[737ngpilot]

From my FOM......During a dual autopilot approach and after FLARE ARM annunciation, any attempted manual override of the autopilots will result in an autopilot disconnect.