VNAV Functionality in "Real" World

[Guest MSMITH6]

Guys-Satisfy my curiosity. I can set up a beautiful flight plan with all the speeds and altitude planning I want, however, since I'm flying an IFR plan, ATC seems to have different ideas! What exactly is the point of having predetermined TOC and TOD if ATC is assigning altitudes to you during all phases of flight? I can perhaps see it for a SID or STAR but even then, with MSFS ATC or Radar Contact, you basically have to employ "cheats" to do them. But enroute, what good is a TOC if ATC is telling you when and how far to climb or a TOD if ATC has you coming down at their discretion?Maybe I'm missing the obvious?Thanks-

[Guest Iz]

Mike --Situational awareness! It's good to be aware where your optimum descent path is, when you're below it, but also when you're above it (BUG: Today on a PMDG flight, I kept the MCP altitude at cruising level beyond TOD and instead of the APFD switching to MCP SPD | ALD HOLD, it stayed in VNAV PTH). ATC doesn't always impede your most economic profile. Especially when it's night time and it's quiet in the skies, you can fly your descent profile pretty much all the way to touchdown. (The climb profile is not really that important, that's why a VNAV climb is not a path, but a given speed setting)Hope this answers your question!

[Guest MSMITH6]

Iz-In regards to ATC allowing a descent, that's assuming you ask for permission to descend and they give it to you. I'm still unclear about your reference to situational awareness when you are at the discretion of ATC of whether or not you get to utilize the economical descent or not. It still seems to me that your VNAV computations are a "crapshoot" of being allowed to do what you want, when you want. If you are told to descend at any point other than the exact pre-flight calculated TOD, all your BEST computations are out the window!

[SAS263 1]

Hi.You

[Tabs 2,790]

If you go to the DES page on the FMC there's a prompt that says DES NOW - if you press that, the plane goes into a shallow descent (1000FPM I think) until it captures the VNAV path. This is really good in the event you get descended by ATC prior to your TOC. What I tried to do when I controlled center on VATSIM was give a "Descend pilot's discretion, cross X at Y, report leaving FLXXX." That way they could use the fully optimal VNAV path. Real ATC usually doesn't micromanage every single thing a plane does the way FS ATC does unless traffic or SOP in the area warrants it.

[Guest Iz]

Tord, I am a real pilot in one of the busiest airspaces of Europe, The Netherlands. I've flown to, from and around Europe, Asia, Africa and the USA. Even the busiest airspaces turn quiet at some time during the day (night). You can basically ask ATC anything and they usually attempt to coordinate with other sectors, but when it's busy, you won't get much done in the form of significant shortcuts, descent profiles etc. (the longest shortcut I ever got was 1800nm when I asked the Los Angeles center controller if we could fly direct to Cleveland, jokingly. He approved it)Mike, situational awareness is number one in aviation, without it, all else is gone. You still have your VNAV path in the FMC, even if you descend before or after your TOD. If ATC tells you to cross a waypoint at or below (or above) a certain altitude and/or airspeed, your VNAV can quickly help you with that. Even if you do not adjust the VNAV path during your descent, you can still judge how you're doing on energy management while you're descending. You may decide to descend a little slower or a little faster, fly slower or faster, use speedbrakes, start configuring for landing.So you see, it's all about situational awareness :)In real life, about 85% of the time, the controller will give you a descent before you ask for it, if not, you do indeed ask for it.Iz

[brucek 38]

Hi Mike,Here in the US, pilots often get the clearance "Descend at Pilot's Discretion". The pilot can then let the FMC and the TOD calculation do it's thing, and all that is required is to let the controller know when vacating the prior altitude. But as Iz says below, situation awareness is the key, and to blindly trust an FMC to determine where TOD is (without varifying first yourself) is a short cut to the unemployment lines :).We also have a "cruise clearance" which allows a pilot to deviate at any time (without reporting) between the cleared altitude and the MEA. It's kind of like a block altitude clearance, just a big block! I've never heard it, but suspect that at 3AM when the freight dogs are the only ones up there, that this is the time when you might expect that clearance :).Thanks,Bruce.

[Ckeatin 0]

Here's a great post from a while ago. It was posted in the 767PIC forum, but I think it applies in theory to the 737 also. One of the best explanations I've ever read to your question (Thanks Ryan)http://forums.avsim.net/dcboard.php?az=sho...g_id=4143&page=

[imoore 1]

I don't know about Radar Contact, but with the stock ATC, you can just set your cruise altitude (most of the time) in to the MCP, take off, and start your VNAV ascent. As long as your in contact with control about 1000 feet before your ATC "assigned" altitude, you're cleared to a higher altitude immediately, all the way to TOC.During busy times, when the ATC is talking to all sorts of aircraft, this doesn't always work and you might find yourself decending back down so they can clear you higher.Not really as real as it gets.