[AP] Switching to HDG / VS during APP

[mokny]

Hi,

no clue if it's a bug or not - I already searched here but I did not find sthg.

Scenario:

• LOWI LOC/DME East 26
• VOR LOC 111.1/255° and GS established, descending with the glide to 3700 (Single Channel) 
• Now the the visual runway 08 -> Heading preselected for the left turn

Now I clicked HDG SEL but nothing happens. APP stays active. The same for ALT HLD. The only way was to disable the AP completely and continue hand flying (more fun, but that was unexpected :))

Tried a second time, same result. Am I doing sthg wrong?

Cheers,

Till

[Alpha Floor]

That's normal.

The only way out of APP mode below 1500 feet radio is cycling BOTH Flight Directors, i.e., both FDs to off and then back to on. That or hitting the TOGA buttons on the thrust levers to start a automatic Go-Around.

As a side-note: A Dual Channel ILS approach below 1500 feet is a very "unique" autoflight system situation. The rudder pedals will be backdriven, for example, so the pilot feels what the AP is doing to the rudder. Usually the rudder pedals are not backdriven.

Of course you can also get rid of all automation, take over and hand-fly the visual procedure into runway 08. More fun!

Edited May 2, 2016 by Alpha Floor

[mokny]

Wow, that was quick!

Thank's a lot. Yes handflying it is really more fun, but usually I disconnect the AP at the downwind! Thanks for clarification!

[Alpha Floor]

Well, I just read you were flying SINGLE CHANNEL. Made me check the FCOM because I was unsure, hehe

So apparently, during a Single Channel ILS approach, there's three ways of interrupting it. Here it doesn't make any difference above/below 1500 feet:

• TOGA switch
• Tuning a different frequency in the NAV receiver
• Overriding the control column/wheel into CWS

During a Dual Channel approach and below 1500 feet it's:

• TOGA switch
• Tuning a different frequency into either NAV receiver
• Disengaging AP and turning both FDs off

As in many occasions, I believe the FCOM is a bit ambiguous here. At one point it says:

But then there's the different ways of exiting the APP when in Dual or Single... hmmm

[mokny]

Ok, in that special scenario a high workload in single pilot operation, in case you want to make the turn with the A/P engaged. I did not think about the CWS-Option. Will try that. Thanks.

[Alpha Floor]

Yeah but overriding the controls into CWS doesn't seem too "elegant" to me. Also, is CWS the mode that you're really looking for at this stage? I don't think so. In other aircraft, overriding the controls will disengage the AP. In the 737, which has CWS, the logic of the system is "well, you're forcing the controls so we're going to let go, but to protect you a bit, we will go into CWS at least..."

My humble advice: Reach the "break-off" point, I believe it's an NDB, with everything (gear, flaps, lights, etc) set for the visual segment and landing. In the heading bug on the MCP set the downwind heading for reference. The idea here is that you don't have to release the hands from your hardware while handflying. Turn both FDs off, disengage AT and AP. And fly!

If you still want to turn on the autopilot, try cycling both FDs off and the on and see what happens!

[mokny]

 Will do so

16 minutes ago, Alpha Floor said:

If you still want to turn on the autopilot [...]

Not by all means. Handflying that plane is great - No need to discuss that. I was just curious what's happening. Due to struggeling with the AP I overshot the point for the leftturn and was just staggered about what's going on. Next time I will be prepared.