Navaid Devices AP-1, Manual

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Head toward a VOR and flip the Mode Switch rightward to Wing Leveler. Zero the CDI
needle and, after a delay of about half a minute, flip the Mode Switch to the far left to
change to TRACK mode. The airplane should track in over a VOR.
The purpose in the delay in switching to Track mode is to allow the charge on some
capacitors in the AP-1 to stabilize. If the switch is made as soon as the needle is centered,
the machine will "remember" that the needle was off, "think" that it must correct, and
bank the airplane off course. If the airplane is improperly trimmed when Track mode is
initiated, the CDI needle will shift away from zero deflection as the autopilot flies the
aircraft as necessary to establish a stable off-course condition, a condition that generates
an error signal that cancels the trim error. If you are tracking a GPS or LORAN, the
aircraft settles on a course parallel to the desired course. If you are tracking a VOR, the
CDI needle settles down to a different radial from that selected at the OBS and still flies
over the VOR station.
The autopilot always makes the airplane follow the needle. If the CDI needle wanders
around, the airplane will too. This is a VOR problem that usually occurs at low altitude
over hilly terrain. If the cruising altitude is 4,000 or more feet AGL, the autopilot should
track the VOR without much S-tuming, or scalloping, back and forth across the track.
The autopilot works best tracking a GPS or LORAN because of the higher quality signal.
The error signal for a given distance off course stays constant in a GPS or LORAN, but a
VOR signal may scallop, and signal sensitivity varies over a wide range as a function of
the distance from the station.
TRIM WHILE TRACKING
The necessity for putting the TURN CONTROL in the dead zone has been discussed in
TRIMMING FOR TRACK MODE on the previous page. However the "dead zone" does
not necessarily command a perfect zero turn rate — but it is close. Therefore do not be
surprised if the airplane drifts off a little, probably less than a mile, after following the
foregoing procedure and switching to TRACK mode. This residual error, or any error that
later occurs as a result of a change of airspeed or uneven fuel loading in the wings, can be
corrected by turning the TRIM trimpot toward the error, i.e., turning the trimpot
clockwise to correct for a CDI needle that has shifted to the right.
Sometimes you may want to track with a parallel offset from a given line. If you are
tracking with GPS or LORAN that does not have this feature built in, just trim in the
direction of the desired offset.