MSFS Mysterious AUTOPILOT VERTICAL HOLD

[eric_marciano]

Hello,

I am working on an autopilot that uses the standard MSFS autopilot. When I engage the ALT mode to reach a given altitude in V/S mode, it works well, the aircraft climbs to the target altitude using the V/S I have set, perfectly. The problem is that during this climb (or descent), I read the AUTOPILOT VERTICAL HOLD SimVar and it is always 0 (false). This Boolean variable is supposed to indicate when the V/S mode is engaged but it remains false, as if the V/S mode was never engaged, while the aircraft correctly climbs/descends to its target altitude.
Isn't it strange?

 

[DragonflightDesign]

IF MSFS has the same base code as FSX and P3D then no, it is not strange. Vertical Hold has been broken since FS2002 and according to Lockheed-Martin, the bug is buried too deep in the code to fix. They tried. So my guess is that Asobo have implemented an override that looks something like this (at least, this is my implementation of a fix for FSX and P3D):

//**************************************************************************
// Vertical speed fix
//**************************************************************************
void toggleVerticalSpeed(bool onoff, double rate = 0)
{
    if(onoff == true && rate != 0)
    {
        if (rate > 0)trigger_key_event(KEY_AP_ALT_VAR_SET_ENGLISH, 100000);  // Positive VS = negative pitch demand
        else trigger_key_event(KEY_AP_ALT_VAR_SET_ENGLISH, 0);                       // Negative VS
        trigger_key_event(KEY_AP_VS_VAR_SET_ENGLISH, (UINT32)rate);             // Set required VS
        trigger_key_event(KEY_AP_ALT_HOLD_ON, 0);                                           //Set alt hold on
    }
    else trigger_key_event(KEY_AP_ALT_HOLD_OFF, 0);                                       //Set alt hold off

    return;
}

In this case AUTOPILOT VERTICAL HOLD always shows zero.

 

[eric_marciano]

Thank you for the info, I didn't know this, and I wonder why Asobo kept this in the SDK if it doesn't work, removing it would so much better.
I thank you for sharing the code. In my case, it is not necessary because the target altitude is set before the V/S mode is engaged. My code works fine but the V/S mode flag is wrong, now I know why. I will just create my own boolean variable to replace the SimVar that doesn't work, it shouldn't be too complex.