747-400F VNAV Descent Issue

[Guest D17S]

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

[rsrandazzo 6,353]

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.

[Tabs 2,790]

>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

[Jeff Nielsen 1,385]

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 :-)

[Tabs 2,790]

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.

[Jeff Nielsen 1,385]

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. :-)

[Captain2000 0]

>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?

[Captain2000 0]

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...

[Guest wisborg]

>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

[jtsjc1 3]

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