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Navigation
Resolving ambiguity
The automatic direction finder (ADF), long found on most civil
aircraft panels, doesnt aid navigation inan entirely
straightforward way.
Although the ADF needle points directly at low-frequency
non-directionalbeacons (NDB), it doesnt indicate a heading to the
station. To get themagnetic bearing to the station, the ADFs fixed
compass card requires the
pilot to take the relative bearing to or from the station (the
angle between
the aircrafts nose or tail and the NDB), and then add the
relative bearingto the current magnetic heading to get the magnetic
bearing to the
station.
An ADF with a rotatable card partially solves this problem by
allowing thepilot to rotate the card to match the heading on the
heading indicator.
While this provides a magnetic heading to the station, the pilot
must reset
the heading indicator to the magnetic compass in level flight
every tenminutes or so, adding an additional manual task.
RMI defined
The radio magnetic indicator (RMI) is one solution to the
ADF's
shortcomings. The RMI combines three components: a fluxgate,
a heading indicator, and a relative bearing indicator.
Some RMIs have two needles like the one pictured on this
page;
others only have one needle. The RMI can be used for
VORnavigationas well as ADF navigation. Most single-needle RMIs
have a switch that allows the pilot to select either an ADF or
VOR station to which the needle canpoint.
In Flight Simulator, all of the RMIs are dual-needle and only
provide information for the ADF and VOR
2. You cannot select a different radio for each needle. The ADF
needle is yellow and the VOR needle is
green. The VOR needle points to a VOR station and the tail of
the needle indicates the current radial.
The fluxgate
The fluxgate is a detector that is sensitive to magnetic north.
It is mounted in a relatively non-magnetic place in the airplane to
minimize magnetic interference. The fluxgate constantly and
automatically updates the RMIs heading indicator, eliminating
the task of turning a compass card or
correcting the RMIs heading indicator to the magnetic
compass.
With the magnetic bearing indicator pointing at an NDB, the
current heading to or from the station is
readily apparent. The ADF needle indicates the magnetic heading
to the station and the tail of the
The Radio Magnetic Indicator
RELATEDLINKS
Using the Radios
Automatic Direction Finder
What You Need to Know
About VOR
Green needle: VOR
Yellow needle: ADF
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pointer indicates the magnetic heading away from the station.
Depending on whether the pilot wants
to fly to or from the station, he just turns the aircraft to the
heading indicated by the ADF pointer.
NDB approaches and RMI
An RMI can simplify flying NDB approaches by eliminating the
need to add magnetic heading
calculations into the IFR task load. The aircrafts position
relative to the station is always clear,
whether flying to or from the station.
In the example here, the pilot is
flying outbound for a coursereversal. With the tail of the
ADF
needle centered at the top of the
RMI, the magnetic heading is thereciprocal of the final
approach
heading. The final approach courseis 157 degrees, the
outbound
heading is 337 degrees. A right-
hand procedure turn will make theheading 022 degrees and the
tail of
the RMI needle will point to 337
degrees.
Upon reversal of the procedure turn,the heading is 202 and the
head of
the RMI needle points to 157. As the
needle moves closer to the leftwingtip the pilot turns the
aircraft
onto the final approach course,centering the RMI needle on the
final approach heading of 157. Remember that the pointer always
points at the station and the RMI compass card displays the
current magnetic heading. If the needle
points ahead of the wing the station is ahead of the aircraft.
Similarly, if the needle points behind thewing the station is
behind the aircraft.
RMI and DME Arcs
Flying a DME arc can also benefit from reference to the RMI.
Until reaching the radial that represents
the final approach course, the pilot fl ies the arc by keeping
the aircraft a specified distance away froma VOR/DME station.
In the example pictured here, there is anarc seven nautical
miles (7 DME) from a
VOR and the final approach course is 109
degrees. The pilot flies the arc bykeeping the RMI needle
pointed towards
the left wingtip while flying a series of
short straight legs.
A DME arc provides an 8-mile wide
corridor, but the goal should be to stay
within one nautical mile of the arc.
As the pilot flies a straight line tangentialto the arc, the DME
distance begins to
increase and the needle moves behindthe wingtip. When the needle
is 10
degrees behind the wingtip, the pilot
turns 20 degrees in the direction of thearc, which moves the RMI
needle 10
degrees ahead of the wingtip. The pilot
holds that heading until the needle fallsbehind the wingtip
again, and then
repeats this procedure until it is time toturn inbound on the
final approach
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