VNAV, A little confused

[Robert Bernstein 1106839]

Hi all. I have discovered the wonderful world of VNAV. Set the FMC to your waypoints and set the altitude too and voila! My question is: If the SID chart says you are supposed to be at a certain altitude and you pre-set that in the FMC.. how do you reconcile that when live ATC clears you to usually 5,000 or 6,000 and then up to your final altitude? Thanks. -Bob.

[Ryan Geckler]

If ATC manually gives you an altitude, that automatically erases any published altitudes on the chart.

 

For example, look at the TERPZ1 departure out of BWI. Say you were on the LDN transition, so the chart says to cross WONCE at or above 11,000. Now, I as a controller could say, "SWA2, cross WONCE at or above 11,000, climb and maintain 17,000." This therefore mandates the crossing restriction as well as gets you climbing. Now, if I just said "SWA2, climb and maintain 17,000", then you do not have to follow the restriction as I have canceled it.

[Robert Bernstein 1106839]

Thank Ryan. I understand that ATC goes before the published info. So, I guess regardless of what the SID says, if ATC says climb to 5,000 and then 18,000... that's what you do on takeoff, but follow the SID waypoints. Usually, ATC says "fly runway heading"... and then they contact you and tell you to start on the SID at which point you engage the flight director so the plane can follow the FMC? -Bob.

[Ryan Geckler]

It all depends on what the SID says. If you look at the TERPZ1 again, the instructions are to fly runway heading and expect vectors to TERPZ to then join the departure. But if you look at any RNAV SID out of Atlanta, you'll immediately begin to navigate the departure as soon as you get to the specified altitude.

 

Long story short, read what each departure says for you to do.

[Robert Bernstein 1106839]

Ok, but all the published instructions are [Mod - Happy Thoughts]uming there is no live ATC? I would usually want to set the various altitudes in the FMC for each waypoint based upon the chart. But, if live ATC is online, they just tell you, 5,000, expect 18,000 10 minutes after departure. And then once you reach 5,000, or even before, they tell you, radar contact go on up to 18,000. So, there is no point in setting the waypoint altitudes in the FMC at all, right? -Bob.

[Charan Kumar]

Hi Bob, the initial 5k, expect 18k is a lost comm procedure like you said for r/w. And almost always, you will get "radar contact, c/m 18k" immediately. But in the rare event of conflicting tfc or a busy freq, you will be reqd to stay at 5k and not climb automatically. That's why it's there. The controller will [Mod - Happy Thoughts]ume that in the absense of any other instruction you are going to remain at 5k and will work around it, till he can get a break to give you further instructions. So set 5k and be ready to turn it up to 18k, but don't [Mod - Happy Thoughts]ume you will get it right away. I have had situations both as a pilot and ctrlr having to restrict it further due to tfc reasons:

"AWE456, maintain 3000, wind XX@x, rwy XXL, CTO", now you gotta turn the knob back to 3k and remain there until further advised, unless other metal or mother nature get in the way

[Robert Bernstein 1106839]

Sounds reasonable Charan. I just wonder why are the altitudes published on the charts at all,, when in Real Life, you Always have ATC to tell you the altitudes? -Bob.

[Tony Koch]

In the event of a communications failure.

[Robert Bernstein 1106839]

Understood. -Bob.

[Jason Baxter 920557]

To lower communication. In real life ATC may need you to fly at a specific climb gradient to avoid arrivals and departures. Works similarly on VATSIM as I send planes over the departure area at 8000 since their [Mod - Happy Thoughts]igned altitude is 7000.