Trouble deactivating MCP functions

[Nevian 0]

When dealing with the MCP there is always the possibility of hitting a button next to the one you were actually aiming for. This brings me to the question: when inadvertently hitting the FLCH or one of the HOLD buttons, these won't simply switch off when pressed again. Often activating one function deactivates another, such as the altitude HOLD and FLCH buttons, but I'm nevertheless left with one unwanted function being engaged. What is one actually supposed to do when this happens?

[downscc 1,327]

Depends on what you want to do. Change to the mode you  want.

 

You'll have to be more specific.

[Nevian 0]

Depends on what you want to do. Change to the mode you  want.

 

You'll have to be more specific.

 

Just now I was arming LNAV/VNAV whilst on the ground, and I happend to press FLCH. So now it's lit - and I don't want it to be. Nothing more to it really...

[downscc 1,327]

That's an easy case to remedy. Turn both F/D off then on, which resets everything.

[Nevian 0]

That's an easy case to remedy. Turn both F/D off then on, which resets everything.

 

That works, thanks. I guess making it an ON/OFF button was just too effective and functional :dry:

[downscc 1,327]

It is intended to work same as real aircraft...and they had their reasons.

[scandinavian13 5,192]

That works, thanks. I guess making it an ON/OFF button was just too effective and functional :dry:

 

I get that this might make sense to you, but to a pilot, this makes no sense at all.

 

The autopilot, as a tool, is a way for the crew to transition some of the workload from themselves to the automation. With that being said, if I as a pilot tell the plane "fly yourself," I am expecting it to continue to fly itself. With the AP on, most of the time, you're going to have the plane in a lateral and a vertical mode. The last thing I want is to see that the AP is on, but somehow, something or someone accidentally hit one of the MCP buttons which turned it off, and we're now in some sort of dangerous situation because the AP is simply on without any specific direction to follow.

 

In other words, if I tell the AP that I want it to pay attention to vertical and lateral paths, it shouldn't be able to drop a vertical or lateral mode until I'm ready to assume control of that mode manually myself, or I want to change modes. You can change between modes, but you shouldn't be able to just drop out of FL CH with nothing to fall back on.

 

If it were possible and you hit FL CH to "turn it off," what is the AP now doing? What are you expecting it to do now that it has no mode to follow? Who is in control of the vertical mode and thrust now? You? The AP and AT? Physics alone? This isn't a good position to be in. One of the reasons there's an AP disconnect switch on the yoke is so that a pilot can have a positive hold on the flight controls to positively transfer flight controls from the automation to the pilot. In other words, there is not a moment where nobody is in control of the plane (nobody is holding a yoke, and the AP isn't flying). Similarly, there should be a positive transfer of control between AP modes. The AP should not allow a mode drop to be sure that the aircraft is always being controlled by some set of AP rules. In order to get out of FL CH, you must select another vertical mode like VNAV or SPD + V/S.

 

Before casting aspersions on someone's design, it may be worth looking into the reasons why someone might be doing it the way that it's being done.