737NG VNAV PATH DESCEND

[Maxthrust 0]

Hi,I made a few VNAV PATH Descend tests at ISA conditions. (no wind, std temp etc.)I programmed the FMC for a descend on direct course to RWY TDZ, from FL310 to 162ft. Based on B737 FCTM, it should took ~105 nm from TOD to TDZ. But FMC Calculated TOD point is only 82 nm out from TDZ. I think VNAV PATH descend TOD comes a bit late than normal.I'm used to check my descend rate with "3nm for every 1000ft alt" rule, and now I feel a little weird with 737.Do you feel yourself late for descend or feel high when descending too?

[Guest rcarlson123]

I've noticed that normally my descents just "feel" like they are starting late. I fly on VATSim a lot, and more than once I have had a controller call me to say "Delta 1392, be advised you were cleared to descend at pilot's discretion..." meaning they expected me to have started my descent already.The plane normally gets down to the target altitude just in time, sometimes requiring some drag if the winds aren't optimum, but it still makes me wonder if I'm setting up the descent profile right.

[Maxthrust 0]

Hi Ross,Infact I'm very happy with 737NG's descend performance. It forgives a lot. May be it's just me, you know if you have PIC767, it's very sleepy.Anyway, FMC Calculates only 82nm for FL310 to sea level descend and that seems a bit less to me.

[crvm 0]

In the real world there is no *one* or *set* TOD/DIS, it's based on a few things, CRZ Height etc. One BIG factor is your CI (not modelled here right now) so just what part of the FCTM you are getting that from I am not sure but please point me to the page that states you will get 105 miles in this particular case. The TOD is built from the runway back through the route, the FMC calculates for a IDLE DESCent thus your TOD point. If ATC desires you to DESCend sooner, simply use the DES NOW feature or any other method you perfer. Same thing happens in the real world all the time. You would never descend without ATC clearance in the first place. And most important here, it is never *late*, (Unless you do not have an E/D or have not reset the MCP) it descends right when it's suppose to based upon the FMC's calculations, nothing else.. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest A340-300 fan]

my descend started 400 miles out at FL310. is this normal?adamYou cant truley understand flying until you have actually tasted and felt it.

[crvm 0]

hehehe well I can say that is incorrect BUT are you asking me if the FMC made an error or the pilot ;)? [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest ba747heavy]

Hehe, I have had controllers do that to me all the time. I most especially love the 'Meridian 528, please expidte your descent' when I can clearly see, based on my ND information, that I will arrive at the proper alt at the proper time. Hrph ;)

[Guest rcarlson123]

I haven't yet had a controller ask me to expedite descent ... I've only had them call to make sure I remembered that I was clear to descend, because I hadn't started descending yet, when they had expected me to. I've never had a problem making the crossing restriction though, so it's all good ... it just seems that the descent profile that the FMC calculates isn't typical, at least from ATC's point of view.

[crvm 0]

Sure but then one would ask, "How does ATC know where my TOD point is unless I tell them?". ATC will clear you to descend if you request or they need you to des now but this is never because they know our TOD point on thier radar screens. The only thing we must be concerned with is the clearance to.....[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest rcarlson123]

We are in agreement. I just find it curious that the PMDG plane calculates a descent profile that is uncommon, and I'm forced to wonder why. I personally don't care, since I haven't yet had any problems meeting the altitude restrictions given to me by ATC, but I'm wondering if the PMDG calculates a "last minute" descent by default, which wouldn't really leave room for error due to a tailwind, etc. I have had to use the spoilers on several occasions ...Maybe I'll see if I can determine how to program a more gradual descent profile ... I haven't tried yet.

[crvm 0]

With the advent of FORECASTS winds in SU2, the TOD is *dynamic*. The FMC goes into the DES phase not at the *last minute* but at it's calculated *time*. I will assume that on-line ATC most be use to add-ons that do it incorrectly (maybe not PIC) but most all other add-ons.[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest rcarlson123]

Does that calculated time include any margin for error in the forecast winds, or any other unforseen factors that may slow a descent? If not, then it is in fact last minute. (Not that that's a problem.)I would imagine the VATSim ATC aren't really used to any particular add-on's behavior, just what they see most pilots doing. So you could say that pilots do it wrong, but there really is no right or wrong here, provided ATC crossing restrictions are met.My bet is that most online pilot's don't use an FMC that fully handles VNAV for them, and instead they just start a descent using v/s or alt hold, starting it right around the time they get the "pilot's discretion" descent clearance from ATC, and they end up at their altitude restriction relatively far ahead of the fix, whereas the PMDG plane would level off right at the fix, or just before.Personally I prefer the way the PMDG FMC does it ... seems more accurate and predictable that way. Besides, who's to say how much margin for error in the TOD calculation would be appropriate ...

[Maxthrust 0]

Well I must say FCTM doesn't say "3nm for 1000ft" for "FMC" TOD calculations but it gives you an idea about how to calculate it, in case FMC data not available. Calculation is based on .78/280/240 speed scheulde and you can find it at Descend preparation section.There is also a small descend table ( for 738 but I don't think It'll change calculations a lot) in QRH, if I recall correctly. I'll try to post a shot of the table.I think the real question is, since the VNAV PATH DESCend uses a fixed PATH for descend ( because CI not modelled yet ?) , is it descending with correct angle OR a bit high angle than normal ? ( for idle thrust descend )

[crvm 0]

Well another question, who's to say it's high or low? By what measure do we judge? If the aircraft is on the FMC's calculated PATH and the deviation scale is pretty well centered, then naturally it's only relitive to it's own PATH whether it be high or low on the descent profile. Dynamics play a roll in unforecast winds sure, but if one knows to a certain degree what these are likely to be, and the pilot makes the proper inputs for descent winds, you will start your descent based upon the current information provided to the FMC. If the aircraft happens to drift away from that path (Which I am sure happens all the time in the real world because winds do change) you still have the means availible to get back towards it. So the calculation is itself correct (Idle descent with KNOWN conditions) but the pilot monitoring should be ready to make adjustments as nessary if those conditions warrent..[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Guest rcarlson123]

Yup, makes sense to me. And that's how it played out in the VATSim flight I just finished. I was on descent to KLAS and ran into an unforecast tailwind, and had to dirty up the plane to stay on profile.