Autopilot Alt_mode_slot

Dear Asobo, Can we get a better explanation about what Autopilot
“ALT_MODE_SLOT_INDEX” do? the current description is very vague, I can see the
AP behavior changes with different values but we are lacking what posible
values this parameter can be set as, what each index would do, etc., It is
very hard to configure our aircraft without such detailed descriptions.

Kind Regards, Simbol

Hi, The alt mode slot index is the index on the simvar AUTOPILOT ALTITUDE LOCK
VAR which altitude hold mode will track when captured. There are 4 altitude
slots/indexes (interchangeable terms), with slot 0 being a bit of a special
slot. The behavior is a bit funky (not really changed since FSX) so I’ll do my
best to describe it. When this config parameter is set to 1, 2 or 3, the
altitude capture will by default watch the altitude set at that index, unless
overridden by changing the currently observed slot using the key event
AP_ALTITUDE_SLOT_INDEX_SET. If the currently observed slot has been changed,
when altitude is captured, the set altitude in the currently observed slot
will be copied to the alt_mode_slot_index slot and the currently observed slot
will also be changed to that slot. When this config parameter is set to 0, the
altitude capture will always use whatever the currently observed slot is set
to and will not change slots or write to any slots on capture. Related to
this: AUTOPILOT ALTITUDE LOCK VAR at index 0 will always read the currently
set observed slot. So if you set the currently observed slot to 2, then index
0 will read whatever you set index 2 to. If you write to index 0, it will
write to all slots (0, 1, 2, and 3). Thanks, Matt

@Nocturne FYI

Hi Matt, Ok, many thanks for the insight… Could you explain also why the
Vertical speed behavior of the autopilot also changes with this value? for
example with Alt_mode_slot = 0, the VS speed never resets when you capture the
altitude. Where as with Alt_mode_slot=2 the VS reset to zero when you reach
the altitude. In addition, it seems when using Alt_mode_slot=0 the AP starts
immediately yo change altitude when the AP alt set is moved… where as with
Alt_mode_slot=2 this doesn’t happen, you are on altitude SET… you change the
desired altitude and it stays… untless you change the VS speed or the PIT…
then the AP will descend or ascent to the armed altitude.
@FlyingRaccoon , Sylvain, it seems the
Alt_mode_slot has a big impact on AP behavior and it would be good to document
all this behaviours properly in the SDK so we developers can understand what
to do, etc. Specially when different AP can behave different from others in
the real world. Best, Raul

Hello Raul. That’s why I tagged Nocturne, he’s following the discussion and
will complete the doc :wink: Regards, Sylvain

Many thanks Sylvain…

Could you explain also why the Vertical speed behavior of the autopilot also
changes with this value?

I think, unfortunately, the reasons are probably lost to the sands of time;
you’d have to ask the ACES folks about that. The logic in this area is
unchanged from previous sim generations (important for compatibility). The
Flight Simulator AP historically has modeled the KAP140 most closely, so
perhaps there was some mode of that that operated close to the behavior that
you see.

In addition, it seems when using Alt_mode_slot=0 the AP starts immediately
yo change altitude when the AP alt set is moved.

You actually need to be careful of this even with a different source slot
configured. If you change directly the observed slot altitude while in alt
hold (as opposed to changing slot 0 or some non-observed slot), the autopilot
will immediately begin to try to track the set altitude (quite aggressively
so). It just so happens that setting the source slot to 0 sorta observes them
all, in a sense, so you always get this behavior. I must confess that even
though I’ve spent a good deal of time in this code I can’t actually think of a
good use case for using alt_mode_slot=0. I’m sure there must have been a good
reason back in the day for setting it up in this way. Thanks, Matt

Thanks Matt, Turns out if you don’t set any value for this variable (which can
happen if you doing a new project) it sets to zero, causing all these
behaviours… and the reason why I started testing trying to understand why my
AP was behaving erratically. Your explanations bring some more understanding
to the logic behind. I noticed most Asobo default AP are set to
Alt_mode_slot=2, can you or Asobo think why is this the case? advantages,
etc.? Best, Raul