Vnav

[steve mc]

Hi,

Could someone please shed some light on the general procedure for using VNAV. ie. Can it be selected alone or does it have to be coupled with autopilot or autothrottle?

Many thanks

[Tonic Please]

The VNAV ensures that altitude and speed constraints are observed, speed limits at altitudes are observed, and the pilot specified vertical profile is followed. VNAV automatically commands the autopilot to set target speeds and target altitudes in order to follow the selected flight plan during various phases of flight. It captures and levels the aircraft at the flight plan altitude constraints, and begins a descent at a planned location. During the descent, the VNAV computes a geographical path to each waypoint that has an altitude constraint. If the plan has multiple altitude constraints, the descent path is automatically adjusted so the aircraft descends smoothly and accurately through each altitude at each waypoint. This allows the pilots to confidently meet altitude and airspeed restrictions along any phase of the flight. Required changes are made through the CDU if necessary. The VNAV mode is coupled with the autopilot by pressing the VNAV button on the FGP. This couples the FGS with the FMS, and steers the aircraft in accordance with the vertical portion of the flight plan.

Smooth Skies,

Dan

[steve mc]

Excellent! Thanks very much for taking the time to explain it.

Regards,

Steve Mc

[FatFlyer]

On a 737-300 you can use vnav without the autopilot, and manually follow the flight director though i don't think anyone would do this unless autopilots were inop.

[Daysleeper]

Although they dont mention that on the 757 its about as much use as a chocolate teapot and consistently has you high and fast in descent.
The other cracker is when you are nicely on the profile and ATC (bless them) give you a direct route, cut 30 miles off your track and instantly your 8000 feet high using max speedbrake to get down.
Still it sounds like a good idea.