Arming LNAV or VNAV and A/T

[Rhinozherous]

Hello! 

Since the Autopilot on the 300 is a bit older I have a few questions. 

1. Is it possible at all to arm any roll or pitch mode before activating AP after takeoff? On my flights the modes do not arm on ground. 

2. In the tutorial PDF it says we should arm Autothrottle before takeoff (on gate or while taxi) This is impossible because the switch advances my virtual throttles into takeoff mode.

3. Is the Autopilot in general modeled realistic or are there IXEG specific quirks or comfort functions? 

Thank you! 

Edited February 20, 2019 by Rhinozherous

[Litjan]

1.) Well, you COULD engage the autopilot on the ground in certain modes (like HDG) and it would try to steer the aircraft (with the ailerons, so it wouldnt work). I am not sure if engaging LNAV would work. "Arming" LNAV does not work in the 737 classic, unlike in the NG. So you have to take off with the TOGA flight-director mode (HDG and speed pitch mode) and then engage LNAV when airborne.

2.) You need to ARM the autothrottle (move the little switch to UP), but not ENGAGE it (big difference, as you have found out). Since there is some danger of accidentially ENGAGING the autothrottle, it is procedure to not ARM it before entering the take-off runway.

You may want to watch the tutorial videos I made, I explain a lot of stuff like this in them.

Cheers, Jan

 

[Rhinozherous]

2 hours ago, Litjan said:

1.) Well, you COULD engage the autopilot on the ground in certain modes (like HDG) and it would try to steer the aircraft (with the ailerons, so it wouldnt work). I am not sure if engaging LNAV would work. "Arming" LNAV does not work in the 737 classic, unlike in the NG. So you have to take off with the TOGA flight-director mode (HDG and speed pitch mode) and then engage LNAV when airborne.

2.) You need to ARM the autothrottle (move the little switch to UP), but not ENGAGE it (big difference, as you have found out). Since there is some danger of accidentially ENGAGING the autothrottle, it is procedure to not ARM it before entering the take-off runway.

You may want to watch the tutorial videos I made, I explain a lot of stuff like this in them.

Cheers, Jan

 

Thank you very much! I will watch your videos! 

The thing with A/T is, I know that the switch should just arm Autothrottle. It is activated with the switches on the throttle. But on my flight the switch on the MCP alone activated the autothrottles... This confused me. 

Edited February 20, 2019 by Rhinozherous

[Litjan]

4 hours ago, Rhinozherous said:

 But on my flight the switch on the MCP alone activated the autothrottles... This confused me. 

Most likely you have a "combined" autoflight mode active - this will actively "seek" the assistance of the autothrottle - for example FL CHG (it asks full throttle or idle power) or the TOGA mode (asking full throttle). If a mode like this is active, "arming" the autothrottle will immediately "activate" it as well.

But also simply being in ALT HOLD will still engage the autothrottle if you arm it.

Think of it this way: When I flew the 737, there was not a single minute during my flight where the autothrottle was not armed. It was NEVER off. Always at least armed. That way it was always "ready" to help when the autoflight system needed it - especially in an emergency, like a go-around (TOGA mode) or during a speed reversion (preventing stall or overspeed).

Jan

 

Edited February 20, 2019 by Litjan

[Rhinozherous]

48 minutes ago, Litjan said:

Most likely you have a "combined" autoflight mode active - this will actively "seek" the assistance of the autothrottle - for example FL CHG (it asks full throttle or idle power) or the TOGA mode (asking full throttle). If a mode like this is active, "arming" the autothrottle will immediately "activate" it as well.

But also simply being in ALT HOLD will still engage the autothrottle if you arm it.

Think of it this way: When I flew the 737, there was not a single minute during my flight where the autothrottle was not armed. It was NEVER off. Always at least armed. That way it was always "ready" to help when the autoflight system needed it - especially in an emergency, like a go-around (TOGA mode) or during a speed reversion (preventing stall or overspeed).

Jan

 

Thank you! Sorted all out on my last flight, all is working well! 

[Alpha Floor]

On 2/20/2019 at 12:31 PM, Litjan said:

 

Think of it this way: When I flew the 737, there was not a single minute during my flight where the autothrottle was not armed. It was NEVER off. Always at least armed. That way it was always "ready" to help when the autoflight system needed it - especially in an emergency, like a go-around (TOGA mode) or during a speed reversion (preventing stall or overspeed).

Jan

 

Jan that's a very interesting point. I had a little "accident" regarding an armed A/T in manual flight and will open another thread to discuss it. But I think this is pretty much down to airline SOP as I've been told by other 737 pilots that it's their SOP to "disarm" (not just "deselect") the A/T when in manual flight.