Descend problems

[Guest Ray CYYZ]

Haha, thats too funny, thats exactly why I asked ... everyone complains about Vnav in our sims yet the very same problem exists in the real plane :-lolRay

[Guest pschartau]

Thanks!- Phil

[Guest Murf]

While we are discussing the topic of descents, I have a question about the 800/900 that perhaps a member of PMDG or the Beta Team can comment on.Using the 600/700 in VNAV PATH DES, with winds entered in the forecast page, you will still occasionally get a DRAG REQUIRED or be under speed and manual application of thrust is necessary.Now, I can correct the DRAG REQUIRED problem with the application of the speed brakes. But, I am unable to manually add thrust to correct the under speed situation.One other question

[Mats_J 2]

Hi Murf,I'll try to answer your questions. Though beware I might be wrong (It happens once in a while)! ;-)>Now, I can correct the DRAG REQUIRED problem with the>application of the speed brakes. But, I am unable to manually>add thrust to correct the under speed situation.This is something that I would like to see implemented in later models. A possibility to adjust the thrust when A/T is in ARM position. It is quite intricate though as there is no feedback from FS to the throttle levers (via servos) in the sim.I think the NG A/T will adjust the throttles when the speed drops low enough to compensate the lack of user thrust.>One other question

[Guest Murf]

>Hi Murf,>>I'll try to answer your questions. Though beware I might be>wrong (It happens once in a while)! ;-)It's been known to happen with me on rare occasions as well.>>Now, I can correct the DRAG REQUIRED problem with the>>application of the speed brakes. But, I am unable to>manually>>add thrust to correct the under speed situation.>>This is something that I would like to see implemented in>later models. A possibility to adjust the thrust when A/T is>in ARM position. It is quite intricate though as there is no>feedback from FS to the throttle levers (via servos) in the>sim.It would certainly be nice if CH or another manufacturer would come out with a product which did move the throttle lever(s) accordingly. However, I find with my CH yoke that wehn I disconnect the Auto Throttle on approach the N1 remains as it was last set by the auto throttle. Once I touch the throttle N1 spools up or down to where the lever was positioned.It would be nice if the manual intervention could work like this. Don't know if this is possible but it sure would be nice to have this feature available.>I think the NG A/T will adjust the throttles when the speed>drops low enough to compensate the lack of user thrust.Must kick in at quite a low speed as I have not yet seen it happen, but then I am usually not that far below target.>>One other question

[Guest HepburnUGA]

>>>Now, I can correct the DRAG REQUIRED problem with the>>>application of the speed brakes. But, I am unable to>>manually>>>add thrust to correct the under speed situation.>>>>This is something that I would like to see implemented in>>later models. A possibility to adjust the thrust when A/T is>>in ARM position. >>I think the NG A/T will adjust the throttles when the speed>>drops low enough to compensate the lack of user thrust.>>Must kick in at quite a low speed as I have not yet seen it>happen, but then I am usually not that far below target.Gentlemen -- Its funny this should come up, because I have been meaning to ask this question for weeks (only in a different way).1. My observations...(all these items are VNAV PTH)On approach but above 10,000ft the airplane will not add thrust (as far as I have seen) to compensate when speed drops below target descent speed. Is this correct? It seems like the plane would add power to correct.2. The above observation leads me to the following question...Initial Cleanance: Delta 425 descend at pilots discretion, cross LOMAC at 14-thousand feet, maintain 280 knots.Next clearance (before LOMAC): Delta 425, increase speed 300 knots.QUESTION...If I wanted to use the newly implemented speed intervention during a VNAV PTH descent to increase speed temporarily to 300 knots, then would the autopilot increase power to accomplish this increase without changing path angle?Hope this wasn't too confusing :)Thanks!

[Mats_J 2]

Hi Jeff>1. My observations...(all these items are VNAV PTH)>On approach but above 10,000ft the airplane will not add>thrust (as far as I have seen) to compensate when speed drops>below target descent speed. Is this correct? It seems like>the plane would add power to correct.Let's remember that a VNAV PATH DES will sacrifice speed for path. Meaning you as the pilot has to manage the speed. As this is not possible with the technology today the A/T will try to compensate for very low speeds during this descent. AT least that's what I've been experiencing.>2. The above observation leads me to the following>question...>>Initial Cleanance: Delta 425 descend at pilots discretion,>cross LOMAC at 14-thousand feet, maintain 280 knots.>>Next clearance (before LOMAC): Delta 425, increase speed 300>knots.The above situation looks a bit tricky. If I opted to go for a SPD DES I wouldn't easily know I would make 14000 feet at LOMAC. And with a PATH DES I wouldn't have full control of the speed. (Remember PTH DES doesn't give a dang about speed).V/S and SPD could probably be an option here. Adjusting V/S to get the green banana on LOMAC provided I set my MCP alt to 14000. And SPEED set to 280. Adjusting for the next clearance would mean alter the speed to 300, but that would change the green banana so I would have to realign it to LOMAC once again. If not able to do that I would have to state that would be unable to comply with ATC request.Any thoughts?>QUESTION...>If I wanted to use the newly implemented speed intervention>during a VNAV PTH descent to increase speed temporarily to 300>knots, then would the autopilot increase power to accomplish>this increase without changing path angle?I guess, as I haven't tested this scenario yet. That a SPD INTV would revert PTH DES to a SPD DES and then go with your speeds.>Hope this wasn't too confusing :)Well, I hope I wasn't too confusing answering! ;-)Cheers,

[zubart 5]

Try setting your Vnav Path speed to something more realistic. I'll bet your not even cruising at .810. I would never plan a path decent at this kind of speed in using Vnav Path descent because theres no room for error if forcast winds are off. The airplanes autopilot may pitch down to increase airspeed and once it gets close to the maximum speed, Vnav will disengage. A more reasonable airspeed might be .79/320 if you gota go fast. Going fast at this point in your flight waste fuel anyway unless it is ATC request.Floyd

[Guest HepburnUGA]

@ZubartWas that response directed towards me or the oringinal poster?

[Guest HepburnUGA]

>(Remember PTH DES doesn't give a dang about speed).Thanks for the reply Mats,I disagree with the above statement in some cases. Below 10,000 feet if the aicraft falls below target speed, the autopilot will add power. I am just curious to know what is correct in the real-deal. I can modify my easily modify my practices accordingly. Thanks for the reply!

[zubart 5]

The original poster.Floyd

[Mats_J 2]

Ah ok,The real-deal as I understand it doesn't care about speed at any flight level as the pilot has the ability to adjust the throttle levers when A/T is in ARM mode.On the other hand, the PMDG NG doesn't have this ability to let the pilot adjust speed (Other than decreasing speed using speed brakes.), and the cause is what I described in a previous post here. We lack the feedback loop from FS to our throttle levers.That's why the PMDG NG will add power if the speed drops too low. Below 10.000 feet the limits for speed is narrower than above 10.000.Anyway, this is how I've understood it. Maybe someone else has some other bright ideas?Hope it's a bit clearer now?

[Guest pschartau]

Thanks. I've figured out now that it is quite useless. I occasionally cruise at above mach .80 just to be that little bit faster, but from now on I'll use PATH DESCEND and make life easier for myself...cheersPhil

[crvm 0]

>Hi Murf,>>I'll try to answer your questions. Though beware I might be>wrong (It happens once in a while)! ;-)>>>Now, I can correct the DRAG REQUIRED problem with the>>application of the speed brakes. But, I am unable to>manually>>add thrust to correct the under speed situation.>>This is something that I would like to see implemented in>later models. A possibility to adjust the thrust when A/T is>in ARM position. It is quite intricate though as there is no>feedback from FS to the throttle levers (via servos) in the>sim.>>I think the NG A/T will adjust the throttles when the speed>drops low enough to compensate the lack of user thrust.> Ah one does not need to concern themselves with the drag message since it is perfectly OK to go well beyond the target SPD in a pth descent ABOVE the airport restriction (240/10000). Like you said, if you want to keep the speed (why?) then put out the boards. Per incresing speed when a tad low, the AFS SHOULD help but I was at -20 and it seems to not allow lower than this but did not try to increase to target which sounds ok to me. Good news is that I can use the keyboard (F4) to INCREASE throttle in ARM mode to come back to des target manuallt, try it Mats! works like a charm here but unsure on the 600-700 but think it must be too because I do not recall anything on this area worked on.[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

[Guest Ray CYYZ]

>> ISA DEV should be entered as average ISA deviation for descent according to Bulfer. But this value has a negligible effect on the calculations.I would suggest you don't consider it to be negligible when flying up north in the winter ... ISA is 15 C, it will have you way low on one of our lovely -40 C winter days. Not good to break out of the snowstorm to find yourself hundreds of feet below where you think you are. Altimeters are affected by both pressure and temperature, not just pressure, neglecting one can really ruin your day.Ray