Archived from groups: alt.games.microsoft.flight-sim (
More info?)
Erik wrote:
> What´s the difference between a GPS and a VOR approach. I know how
to
> perform a VOR approach, but I´m not quite sure about the GPS
approach
The difference is the navigational device you are authorized to use for
lateral navigation along the approach. The VOR approach requires the
VOR to be used as the primary navigational means, whereas the GPS
approach requires the use of the IFR-certified, panel mounted GPS to be
used as the primary navigational means.
Beyond this major difference, the two non-precision approaches are
flown roughly the same way (let's disregard WAAS-enabled GPSs for a
moment). You keep the VOR or GPS-sourced CDI needle centered, while
you monitor your progress along the approach. As you cross
pre-definited intersections (either DME or VOR) or GPS waypoints, then
step-down to the next published altitude you get from the approach
plate.
At some point, you cross the final approach fix and step down to the
final minimum altitude allowed (called the MDA, or minimum descent
altitude), then fly on course until you see the airport or cross the
MAP (missed approach point), whichever comes first.
To prepare for either approach, you start with the appropriate approach
plate. Brief it to set up all required comm and nav frequencies. If a
VOR, you will be setting your primary nav radio with the VOR frequency,
then ensure your NAV/GPS switch is set to NAV so that the VOR is
driving the CDI needle. The GPS can be used in place of a DME to
indicate mileage, but it cannot be used as primary navigation along a
VOR approach (too bad, too, since the CDI needle is more solid, making
it easier to fly).
GPS approaches are a little easier to brief, because once you pull the
approach from the GPS database (PROC button on the Garmin series), you
simply activate it when you are cleared to fly it, then fly the course
and step down the altitudes. You don't have to set all of the
different NAV radios.
Additionally, when flying a GPS approach, you need to watch your GPS
when you get within 2 miles of the final approach fix. At this point
the GPS needs to switch from TERMinal mode to APPRoach mode (which
means that sensitivity of the CDI has changed from 1 mile to 0.3 miles
full deflection). If the GPS cannot switch, you will get a RAIM alert
(an internal integrety alarm) and you must fly the missed and decide
what your next gameplan is. Of course, RAIM errors are not modeled in
the sim (AFAIK), so this is merely procedural.
To speed the creation of GPS approaches, the US gov't took many
existing VOR approaches and "overlaid" GPS approaches to these. In
other words, when flying a VOR/GPS combined approach, you are
authorized to use either VOR or IFR-certified, panel-mounted GPS to fly
the approach. Step-down altitudes and minimums are the same for either
one.
However, "fresh" GPS approaches are also being created, and these
usually are nicer since they are simple T-shaped approaches, rather
than approaches with procedure turns in them (as are many VOR
approaches).
I'll give you the lowdown on GPS WAAS approaches later, which are
precision approaches one can fly very similar to ILS approaches using
the GPS.
--
Peter
Facebook
Twitter
Instagram