Question regarding the autopilot unable to hold speed/ vertical speed

summerman

So I might be doing something wrong even though I checked the manuals and the tutorial videos. After takeoff I engage the AP from the center console, it engages and I engage VS and start climbing. I then engage SYNC and set power accordingly. No warnings or anything here.
But when I turn LNAV on, things start to go south real fast. The plane turns to follow the GPS path so I imagine nothing wrong there, however it does not keep the VS stable and goes into a nosedive while turning.

After this point the AP never works properly again, VS goes up and down and up again, it does not hold VS at all even though it is engaged. Until the plane finally stalls or overspeeds. I tried correcting it and got it down to correct speeds and power levels but even then the VS does not seem to hold. And the nosedive happens again if i try to follow the GPS path. What gives?

FrancoisH

@summerman I'm sorry if I ever state the obvious but you didn't mention you armed the ALT HOLD just aside of your altitude selector ?

LNAV afaik is another lateral mode like LOC or HDG. You should also have V/S, IAS or ALT HOLD activated alongside as vertical mode.

As there's no autothrottle, you can maintain cruise speed either with the TMS set on SYNC or TGT (turbine gas temperature), SYNC is syncing engines N1 or N2 on either the number 1 or number 2 thrust leveler, and TGT is syncing the turbine gas temperatur with the value set and the 3 dials at the bottom of the TMS.

But looks like you just forgot the ALT HOLD

summerman

@francoish Thank you for the reply, you might be right I don't think I armed ALT HOLD, but I was climbing, doesn't ALT HOLD keep the altitude stable? Could I have VS armed to 2000 ft/mn and ALT HOLD armed to keep climbing?
edit: I think you actually said I should either have VS or ALT HOLD armed, I did have VS armed

FrancoisH

@summerman no, you should have V/S or IAS as active and ALT HOLD as armed or ALT HOLD active.

But I just wonder... When you said SYNC was active, did you mean on the TMS or the sync mode of the AP... Becaus as you know, if you activated the AP sync mode, it's not handling the commands until you leave the mode (as it's designed to quickly set the V/S and capture it when exited.)

I didn't encounter your issue.

b3lt3r

My experience is that VS or IAS has no effect unless ALT HOLD is armed. I have seen exactly the behaviour described where LNAV works but the plane descends.

ALT ARM muscle memory is the way :-)

summerman

@francoish Something is definitely going on but I cannot figure out what. With SYNC I did mean the TMS mode.

Vertical speed holding issue persists when trying to descend. I am trying to descend to 3000ft, AP engaged, VS engaged, LNAV engaged, ALT ARM 3000ft, airspeed around 250, I am trying to hold -2000ft per minute.

Vertical speed goes up and down all over the place. All the way up to 0 and above. I have no idea what the issue is.

summerman

I have mostly figured it out thank you guys for the input. I believe what it comes down to is; AP gets thrown a bit easily, it could well be how it is supposed to be in the real aircraft. I think it does have to do with the lack of autothrottle too.

Another thing i found: it is important to be quick with the SYNC on AP.
If you take too long it throws the AP. I am not %100 sure about this but I think you can not just push SYNC and have it off for many seconds and push it again to resume it. Somehow things go south. I am not sure if that is how SYNC should work. But in my experience it responded well enough if i push it, set pitch and push it again within a couple of seconds. However, when i took longer it did not respond as well.

So, for future reference for anyone who might read here the moral of the story is: keep a close eye on your throttle as well as your speed and be quick with the SYNC on AP.

FrancoisH

@summerman

Don't know if this will be of any use but...

Autopilot sync mode can now be controlled using the SET ELT assignment, allowing you to hold-then-release the button in addition to the existing on/off toggle (TOGGLE AFTERBURNER)

This was in the 1.3 patch notes. Rather than pushing twice, you bind a button you must hold until you adjust your Vs then release it.

ALH

@summerman The only way to keep climbing with constat speed is using IAS hold. In this way the autopilot controls the vertical speed to maintain it constant adjusting the vertical speed. if you use VS all the way up, somewhen the airplane will stall and the climb is further not possible.
After takeoff fly by hand looking to accelerate until 210 or 220 Kt and then pres IAS hold and engage AP. It should work fine.