Been flying the NGX for a while and generally speaking it's working very well. There is one thing though that I have not been able to

figure out yet. Here is an example.

 

Cruising along with LNAV-VNAV engaged, at some point start the descent by using VS and so far so good. Suddenly Controller

gives me an altitude and speed restriction which necessitates I disengage Autothrottle. Later I no longer require manual throttle control

and want Autothrottle to become active and control speed, so I turn on Autothrotlle but it doesn't seem to be able to control the speed

set in the MCP even though I'm flying level with AP on, LNAV on.

 

What is the correct way to get the autothrottle to engage and effectively control the speed?

 

Thanks

First, there's really no wrong way to use the autopilot to get what you need to get done. This is one of the main reasons we still have pilots instead of computers flying the planes.

 

Second, why are you descending using VS? It's not necessarily wrong, but you're better off in VNAV, as it's slightly more efficient (and less work intensive). If you're going to VS because the controller gives you your descent instructions, pay attention to what he/she is saying:

-If it's "descend pilot's discretion," or "cross [point] at [altitude]," you may descend whenever you want. In this case, leave it in VNAV and let the plane take it down at the calculated T/D.

-If it's "descend and maintain," it means 'now'. In this case, you can still leave it in VNAV and hit "DES NOW" on the descent page of the CDU.

-If you really want to use V/S, you don't need to disengage A/T. When you hit V/S, the MCP will enter speed hold mode, so it will hold the throttles to keep whatever speed is in that window. If it's a steep descent, it'll put them at idle. If it's shallow, it will set them to hold the speed.

 

Third - and likely your issue - is that A/T needs instructions other than just "turn on." Flipping the A/T switch turns it on. After that point, you need to re-engage a throttle mode. Even though it may say 240 in the window, you need to tell it to follow that 240. This can be done by selecting VNAV (even if your plane is already following VNAV with the A/P, select it again to tell your A/T to follow it now as well), or SPEED.

 

Always, always, always check your PFD. At the top, all of the active modes will be displayed. I'm guessing yours is like this:

 

Look at the top. You can see ARM | HDG SEL | VNAV PTH

This means that the A/T is armed, but no mode is active, the plane is following the HDG SEL, and it is following a VNAV PATH.

 

Next time, watch your modes. Select VNAV once more, or click SPEED and you'll see that mode annunciation change to an active mode. These are somewhat explained in the tutorials, so I suggest having a look there (particularly Tutorial 2).

Cruising along with LNAV-VNAV engaged, at some point start the descent by using VS and so far so good. Suddenly Controller

gives me an altitude and speed restriction which necessitates I disengage Autothrottle

 

No need to disengage the AT, simply select SPEED mode, set the speed ATC assigned, and set the VS so that your 'green banana' is on the fix your altitude clearance is given for., If its just a climb/descent clearance, set the new altitude on the MCP and set VS to 1500 or so and enjoy the ride. :-)

No need to disengage the AT, simply select SPEED mode, set the speed ATC assigned, and set the VS so that your 'green banana' is on the fix your altitude clearance is given for., If its just a climb/descent clearance, set the new altitude on the MCP and set VS to 1500 or so and enjoy the ride. :-)

No need to touch the SPEED button. If one is in V/S, the window should be open with a speed already selected.

Scandinavian13 - I am quite conversant with ATC both in Vatsim and in the real world.

Over the years I have had a chance to speak to a number of pilots flying Boeing aircraft

and the general feedback I received is that descent by VNAV is not preferred by many

pilots. Also aware that if you change from LNAV/VNAV to VS the AT will continue to

work perfectly well and that speed can be increased or reduced without any issues.

 

So the answer to my question I gather from your reply is that I should engage VNAV to arm

the AT, so thanks for that. In previous versions of this aircraft (FS9) the AT used to work

independently from VNAV when required, which is why I was confused.

 

The reason for AT disconnect is in situations when speed/altitude restriction

cannot be achieved by AT/VNAV mode of operation. In such a case placing

manually throttle to Idle and raising nose of aircraft to induce drag. Grant you

there aren't many ocassions when this will happen, but I used it as an example

to give background to my question.

 

Thanks to everyone for their replies.

If you have read Mike Ray's books and his old articles in Computer Pilot magazine (a retired United airline Captain), he would recommend NOT using VS to descend unless it is just a small altitude change. He considers it to be dangerous if used improperly at high altitudes.

 

I would suggest another alternative, such as Lvl change as opposed to VS. Just make sure you set the altitude to where ATC said to fly.

 

Bill Clark

The reason for AT disconnect is in situations when speed/altitude restriction

cannot be achieved by AT/VNAV mode of operation. In such a case placing

manually throttle to Idle and raising nose of aircraft to induce drag. Grant you

there aren't many ocassions when this will happen, but I used it as an example

to give background to my question.

What would be an example of this situation? Are you on a STAR or an approach? Idle is idle whether you're in VNAV or V/S.

What would be an example of this situation? Are you on a STAR or an approach? Idle is idle whether you're in VNAV or V/S.

 

Yeah, I was confused as well. I kinda see what he's going for (idle to cut speed before descending in a high drag situation), but I can't vouch for how effective it would be in a 73.

 

Speaking of, Matt, can you comment on the V/S being preferred over VNAV?

Yeah, I was confused as well. I kinda see what he's going for (idle to cut speed before descending in a high drag situation), but I can't vouch for how effective it would be in a 73.

 

Speaking of, Matt, can you comment on the V/S being preferred over VNAV?

 

On approach, some guys will pop the nose up, get dirty and then drop the nose again. You can get a pretty steep descent at F40 and you can fix problems that way. It's not very stabilized, however. Some use it regularly, I use it as a last resort. I'll go for Landing Gear earlier, if I see the problem coming and try to fix it before it becomes that much of an issue.

 

Your friends at SoCal are pretty famous for slam-dunking us, and you need to pop the nose up to get down sometimes. OAK VISual 29 is a good example.

 

As for V/S, I see some guys use it regularly. I think the former-MD-80 guys might use it more. If you get crossing restriction that we might be high for, they'll roll in a V/S that matches the "green banana" to the fix and just watch the airspeed so that it doesn't get out of control. V/S tends to be more stable in pitch.

 

I prefer using VNAV and SPD INT with a higher airspeed to fix being high. I don't trust myself not to screw up crossing restrictions if I choose V/S.

What would be an example of this situation? Are you on a STAR or an approach? Idle is idle whether you're in VNAV or V/S.

 

While on a Vatsim fly-in this last weekend flying a STAR, I had to resort to disconnecting the AT and creating drag to try

and reach a sudden and unexpected and steep restriction by the controller to avoid traffic and some chaos caused

by some pilots who were unable to fly their arrivals properly. I am aware that when VNAV goes into RETARD the AT goes to idle, but I prefer to disconnect it and apply power manually when required at a later stage to stabilise aircraft, once done, then set the required speed on the bug and engage AT. My problem was that I wasn't aware on the NGX you need to engage VNAV to activate AT as explained y Scandinavian13.

 

As regards use of VS, you will find many differing opinions. Personally I use it most of the time, because I have found

over the years that VNAV hasn't always been able to fly the approach correctly even with speed/alt restrictions on the FMS. - Just a personal preference. I use level change sometimes also.

 

Regards

Gotcha - thanks!

 

Your friends at SoCal are pretty famous for slam-dunking us, and you need to pop the nose up to get down sometimes.

 

Haha - sounds like a good time. The closest I'll say I've been to any sort of slam dunk is the approach over the mountains to a west SAN landing, in a UAL DC-10 ages ago.

 

I'll have to give one of the guys I know a hard time when I see him in Jan.

I am aware that when VNAV goes into RETARD the AT goes to idle, but I prefer to disconnect it and apply power manually when required at a later stage to stabilise aircraft, once done, then set the required speed on the bug and engage AT.

If you're going to do that, you might as well not use VNAV. When you say you apply power later to stabilize, what problems are you having?

 

A/T is compatible with other modes, not just VNAV.

Let's start again.The reason for my post was to find out how you can re-arm and engage the AT in the event it is disconnected for whatever reason. I found that on the NGX it doesn't work

like it does on the 747 or other aircraft I own. Simply turning the AT throttle on the MCP doesn't make the AT respond to the MCP panel speed setting. So far I've had many flights where

any of these combinations have been used: LNAV/VNAV, LVCHG\LNAV, VS/LNAV, MANUAL TH/LNAV, or just manual flying. I don't normally use a nose-up/drag attitude, only in extreme

circumstances have I done so. When for example trying to rapidly descend and lose speed from say 20000 to 10000, once the aircraft begins to get close to its speed/altitude target,

I set the speed on the mcp at say 240, then begin to pull down nose slowly and engage AT, this is simply done to allow a safety margin and avoid a stall. The only other type of situation

I have ever used this, has been during landing approaches where speed was a bit above where it should be, AT not applicable because it happened in the interval between disarming

AT and landing. So far Scandinavian13 gave me an answer to the question stating that engaging VNAV would fully enable AT. If there is any other way of fully enabling the AT besides

VNAV, I really would like to know and be very grateful.

 

Thanks

If there is any other way of fully enabling the AT besides VNAV, I really would like to know and be very grateful.

 

If you select any speed mode, you should be back in business.

 

The issue is that the A/T isn't a device with long term memory. If you shut it off, when you turn it back on, it has long since forgotten what its last task was:

"Oh hi, I'm the A/T, I don't think we've met before."

 

So you just point it back in the direction in which it needs to go.

"Yeah, we've met, back to work holding my SPEED please."

 

...and off it goes.

 

 

(Sorry for the elementary explanation - had to get up early to head to the FAA downtown, and then get back to get more work done before heading to the Tech Center 4 hours away tomorrow. My brain is a little tired of advanced details by 20:40. Not trying to take a shot at your intelligence - it was more for me than anyone else.)

Thanks Kyle, much appreciated.