Re: 7x7-EFIS question

Date:         31 Dec 99 02:09:32 
From:         "Chris Dahler" <>
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> How well does the 757/767 conform to the ideal descent profile with
> VNAV engaged with strong tail winds? I have heard FLCH is a better
> tool in such circumstances?

It's basically the same as with any other computer operation: garbage in,
garbage out.  The DES page has a FORECAST selection which allows you to
enter up to four different altitudes in the descent with the forecast winds
at those altitudes.  It also allows you to enter the expected altitude for
activating engine anti-ice.  If all this info is entered prior to the
descent and the wind forecasts weren't wildly inaccurate, VNAV does a pretty
good job in any wind situation.  If you decline to enter this info (it is
not required), then you are basically hoping the winds don't change much on
the way down.  In this case, the FMC will compute a top of descent point
based on the winds at your cruise altitude only and assume the winds will
remain the same all the way down.  VNAV can get a little screwy then.  FLCH,
of course, just pulls the throttles to idle and maintains whatever airspeed
you dial into the speed window through pitch alone.

> Also do you HDG SEL onto the LOC, or LNAV? I assume because of
> Map Shift you HDG SEL..

It depends on how far I am from the airport when ATC turns me onto the
localizer.  If I'm much more than 15 miles from the runway, I'll arm LNAV
and let it fly the extended centerline initially, mainly just to avoid the
swings of the localizer so far out.  The extended centerline is rarely more
than a dot off the center of the localizer, and it's usually right on
center.  It's not so much a result of a map shift that one uses HDG SEL to
intercept a localizer course, it's more just a result of ATC providing
vectors, and there is no way to tell the FMC to just fly a heading.

email: dahler at bigfoot dot com