Sign in to follow this  
Captain2000

747-400F VNAV Descent Issue

Recommended Posts

First off, thanks in advance for the great product line and support that you guys put behind it!I am having an issue when descending in VNAV that I didn't have on the 744 before installing the F.Shortly after the aircraft has begun its descent and settled in on the path (by the way, nice transition from cruise, much smoother than before) the throttles decide to advance to near full power, rather than staying back and maintaining the descent speed. Then, once the plane reaches overspeed, the engines throttle down, and then back up again. This same problem happens when using FLCH for a descent as well, the engines never pull back to idle, instead they advance to max power.When I disconnect the autothrotle and set thrust manually it behaves itself. Haven't gone back to the pax version to see if the problem is there as well.I certainly hope that I'm overlooking something new in this update that I didn't have to do in the original B744.Any suggestions would be appreciated!

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

Check the PMDG options menu for A/T overide.................Randy J. Smith................A PROUD MEMBER OF THE PMDG BETA TEAM[h4]Evolution is a process that results in heritable changes in a population spread over many generations[/h4]

Share this post


Link to post

Randy,I have it checked. Pretty sure I've had it this way since is was added. Is this correct?

Share this post


Link to post

I don't get this logic. One of the most advanced airliners in the world and the A/T won't operate like they do in almost every other aircraft? What good is A/T?

Share this post


Link to post

No no, you missed my point. Why would the A/T not prevent overspeed like it seems to do in the 737. I know they're different, but it seems an Autothrottle should at least prevent going over the MCP or FMC calculated speed even it's it basically idle for descent. I find that logic odd.

Share this post


Link to post

I am running into a similar situation. I walk away from the aircraft in cruise and when I come back it is in overspeed. The only way to get the thing to slow down is to disconnect AT and retard the throttles manually. This also seems to happen if I leave it paused for a while. I've never run into this before and I'm very familiar with fms/at/ap opperaton so I'm a little confused. This never happened in the pax version.Scott

Share this post


Link to post

The path that VNAV PATH calculates is based on idle thrust at a specific airspeed. Therefore, at TOD, thrust should go to idle. Watch that left FMA like a hawk. That's your thrust mode. It should annunciate "IDLE" ("IDLE" then "HOLD" with the new version?). What's it annunciating when things go haywire? Also, what is your target airspeed (above the speed tape) before and after the thing goes nuts. An FMA __ left-thrust / right-pitch __ and target airspeed (magenta airspeed above the speed tape) 'blow by blow' might be helpful too. Are you using the DES NOW function? (Please, no!)Just for fun: . . . With the AT engaged, move your controller's throttle. Can you adjust thrust with your throttle? Keep checking and un-checking boxes till you CANNOT control thrust with your controller's throttle when the AT is engaged. That'd take one variable out of the troubleshoot.

Share this post


Link to post

Actually, in my problem mentioned above, the A/T did protect against overspeed. What was strange is that it would go to max power, accelerate to overspeed, and then throttle back, then repeat that cycle.Randy mentioned the Throttle Override option. Despite reading the manual and a number of related posts I'm not quite sure I understand what that option does.Can anyone please explain it for me? Why do I want to use it?

Share this post


Link to post

Sam,Getting late, but here's the play by play as I remember it:1) Frindly FMC warning to check MCP altitude. I do so.2) At TOD, the throttle reduces, IDLE is annunciated, target speed as determined by the FMC is 313 knots.3) Plane begins gentle descent, nice and smooth.4) Within 30-40 seconds, throttle goes to the wall, HOLD is annunciated. Still reads 313 for the target.5) I try changing the target by changing the speed on the MCP. No go, still full power.6) I switch to FLCH, now have IDLE back, but no change in throttle.7) Time to disengage the A/T, setting thrust manually works fine with FLCH. Too terrified to bring back VNAV.I'll try to grab some screenshots tomorrow to help tell the story. Thanks for your input!

Share this post


Link to post

4) Within 30-40 seconds, throttle goes to the wall, HOLD is annunciated. Still reads 313 for the target.That might be it. "HOLD" is now an active VNAV mode in the new model. "HOLD" will allow a thrust adjustment without disco-ing the AT. The HOLD mode may be allowing some external input to control the throttles. Do you have the reg'd FSUIPC? (See where I'm going?) Try the throttle override experiment with the left FMA annunciating anything BUT "HOLD". I'll bet you can override thrust with your controller. That would be good, because we probably found the problem . . . or not . . . that would be bad because we still haven't found it. 6) I switch to FLCH, now have IDLE back, but no change in throttle.If so, this is a glitch. "IDLE" should allow the AFS to retrieve throttle control from . . . whatever. However, maybe once you give control to a controlling controller, it may not want to give it back. You know how that goes.

Share this post


Link to post

Guys,You're not understanding what HOLD and the A/T Override option do and I believe that's the source of the problem here:The real 744's throttles are connected to a mechanical servo that moves them when the A/T is in SPD or THR mode. When it goes into HOLD mode, that servo disconnects, allowing the throttles to be manually repositioned. VNAV PTH is exactly such a mode where this occurs - it's there so that the pilot can make slight adjustments to the A/T in order to compensate for slight under and overspeeding. Why couldn't it just stay in SPD mode all the time? Ask Mr. Boeing. HOLD is not, however, a "VNAV mode" as someone said above - it is an autothrust mode and doesn't happen only in VNAV.Anyhow, the issue here is that unless you're a crazy home cockpit builder with tons of cash, you don't have a servo in your throttle moving it under the AT's commands. If this is the case, you need to manually position your throttle to the setting that corresponds with the commanded N1 value. This is not easy to do with a small sensitive joystick and if you happen to have your throttle firewalled when the A/T goes to HOLD, it will go to the N1 limit and you'll overspeed. For that reason I recommend UNCHECKING A/T Override, which gives total control to the panel and does not allow your throttle to affect thrust in HOLD mode. You may under or overspeed slightly in descent, but it will be only by +/- 5 knots or so usually.

Share this post


Link to post

Thanks Ryan,I'm guessing that might explain the behavior I'm getting in cruise also. I firewall the throttle on departure and let AT take over. If it goes to hold, it would overspeed.Scott

Share this post


Link to post

Just as an interesting discussion point, my impression was that "HOLD" was to let the pilot make small adjustments in path (i.e., rate of descent) with thrust. Why? A VNAV PATH descent is a speed-on-PITCH mode. Airspeed is controlled with the pitch of the airplane. VNAV SPD (up), FLCH SPD (up or down) or Glideslope (down) are all speed-on-pitch modes too. Speed is controlled by Pitch and the rate of climb or descent (in this case, the descent PATH) is controlled by Thrust. But back to the problem at hand. If clicking around in the PMDG options menus doesn't get the controller

Share this post


Link to post

Gents-Ryan's advice is valuable- so let me restate a few things in hopes of helping out:Take a look at your PMDG/Options menu:IF YOU HAVE THROTTLE OVERRIDE CHECED:=====================================The A/T will take it's thrust ques from the AFDS/FMS unless HOLD is displayed in the FMA.When HOLD is displayed in the FMC- the A/T is going to stop looking to the AFDS/FMS for thrust information- and instead will look to your joystick throttle setting.IF YOU DO NOT HAVE THROTTLE OVERRIDE CHECKED:=============================================The A/T will always take it's thrust ques from the AFDS/FMS.Now for an example:Assume you have THROTTLE OVERRIDE checked, and you advance your joystick throttle for takeoff- then engage TO/GA. During climb you engage VNAV/LNAV and use them throughout the flight, allowing the A/T to handle the engine power settings.When your reach Top of Descent, the airplane begins a descent- and as the airplane enters descent it's energy state becomes high enough that it begins calculating a reduction in required thrust.Since the optimal descent path will always be an idle powered descent- the airplane will eventually get to the point where the A/T will come back to idle power- and HOLD will annunciate in the FMC.Suddenly- your A/T is going to stop taking it's thrust cue from the AFDS/FMS- and instead will look to your JOYSTICK THROTTLE.Since you are a good airline pilot- you have been happily sipping your favorite beverage and monitoring things closely- but you haven't touched that joystick throttle setting since takeoff- and if you glance down right now- you'll notice that it is about 70%..which is where you had it when you engaged TO/GA.So what's going to happen?You guessed it! The A/T is now taking cues from that throttle- which is set at 70%.... So the thrust is going to come back up- and since you have manual control of the throttles- you are going ot overspeed in the blink of any eye...So.... If you want the ultimate in realism- check the box- but guard your throttles....If you want a realistic operation without the concern for the manual throttle trimming ability when HOLD is displayed- UNCHECK THIS BOX.

Share this post


Link to post

>Just as an interesting discussion point, my impression was>that "HOLD" was to let the pilot make small adjustments in>path (i.e., rate of descent) with thrust. Why? A VNAV PATH>descent is a speed-on-PITCH mode. Airspeed is controlled with>the pitch of the airplane. VNAV SPD (up), FLCH SPD (up or>down) or Glideslope (down) are all speed-on-pitch modes too.>Speed is controlled by Pitch and the rate of climb or descent>(in this case, the descent PATH) is controlled by Thrust.This is INCORRECT. VNAV PTH is not a pitch for speed mode. The airplane is following a vertical path angle through the air, not a variable pitch to hold a speed like FLCH or VNAV SPD does. The speed target that's there in VNAV PTH is something to shoot for as an economy speed, but the plane can and will deviate from it to hold the path. Think of it as LNAV flipped on its side.GS is also not a pitch for speed mode. In the vertical componet of flight, it's functionally almost the same as VNAV PTH except it's a mathematically calculated glideslope inside the FMC instead of a radio signal. You have a glideslope, which is by definition a constant angle (usually 3 degrees) to the runway. The plane grabs that signal and holds the three degree angle. It does not care what the speed is, it will always hold that path through the air - try it, firewall the throttle while in GS mode.In summary:Pitch for speed modes:VNAV SPDFLCHVertical path modes:VNAV PTHGSRate of climb mode (variable angle depending on what's needed to achieve that rate):V/S

Share this post


Link to post

Robert,1 question then:For the ultimate realism (check the box), how do you resume A/T-A/P operation after hold mode?I'm assuming that real crews will want to resume A/T operation after the hold mode at some point, right?When I tried as you describe, I had to do a magical "finger" dance with the MCP and did get it to resume, but I was in such a panic at that point I do not how I got it all back. When I did resume A/P operations again down the ILS, then after disco'ing everything manually at the touchdown point (200' RA), I could not get the power to retard at all.It's all a little confusing for someone like me, who's been used to the 37 for so long and then flying the 47 prior to the first service pack being issued, then trying to fly the 47 after the service pack and cargo addition.That's a heck of a toungue twister there, but I think you might be able to decipher what I'm getting at. Thanks :-)

Share this post


Link to post

Jeff,There are conditions when the active thrust mode will revert back to SPD in VNAV - basically if you get too far below the speed target, it'll come in to bring you back up. Also in VNAV PTH as soon as you enter the approach phase, it will lock into SPD mode with no more HOLD.We have a PMDG Type Course document in the works on this stuff that'll explain it all in detail.

Share this post


Link to post

Thanks Ryan, I thought you were going to say that. It did not revert when I tried it.In the 37 it does, so to speak without the hold mode as you know.This is where I'm kind of at a loss. I'm sure it's in the new VNav logic for the 47, it's just a matter of figuring out why it's not reverting automatically like it should. :-)

Share this post


Link to post

>So.... >>If you want the ultimate in realism- check the box- but guard>your throttles....>>If you want a realistic operation without the concern for the>manual throttle trimming ability when HOLD is displayed->UNCHECK THIS BOX.Robert,That's just the description I was looking for, and explains my "issue" entirely. Was this A/T hold behavior modled before the 400F? I have had the box checked since it's been there, if I remember correctly because I read somewhere on the forum that this would allow me to yank the throttles to abort a takeoff as necessary. If the box is unchecked, what is the correct procedure? Disengage the A/T and then yank back for RTO to kick in?

Share this post


Link to post

Sam,Experiment in progress. With the Throttle Override box unchecked, all is well. When it IS checked, once HOLD is annunicated the engines go to wherever I left the joystick throttle at. Guess I need to start paying attention to where I leave that thing.I'll do a complete flight this evening to verify the tests. Doesn't seem to be a problem with erratic control inputs (CH Yoke in good shape and registered FSUIPC that behaves).I think at the end of that day here is a good reason why Mr. Boeing won't let me move to Seattle to play in his birds. But maybe in a few weeks when I have mastered the realistic A/T interface...

Share this post


Link to post

>Since you are a good airline pilot- you have been happily>sipping your favorite beverage and monitoring things closely->but you haven't touched that joystick throttle setting since>takeoff- and if you glance down right now- you'll notice that>it is about 70%..which is where you had it when you engaged>TO/GA.Which is probably why I haven't noted the problem. I never trust computers ;-), so I always checks whether the autothrottle is engaged by reducing my joystick throttle to zero./ Jesper

Share this post


Link to post

Thank you for the explanation Robert. That cleared up my questions about this subject also. I don't have the CH throttle but is this a better option than using joystick throttle? I have a MS Sidewinder- Joe

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this