T-6A INSTRUMENT NAVIGATION
Note the difference between the indicated altitude and the known field elevation.
If the difference is greater than 75 feet, the altimeter is not acceptable for IFR flight.
The accuracy of pressure altimeters is subject to the following factors: nonstandard temperature
of the atmosphere, aircraft static pressure systems (position error), and instrument error.
Aircrews should disregard the effect of nonstandard atmospheric temperatures except that low
temperature need be considered for terrain clearance purposes.
The range indicator, commonly referred to as DME, displays slant range distance from the
aircraft to a selected ground station in NM. The presentation may be separate or integrated with
a bearing instrument. The aircraft DME transmitter sends an interrogating pulse, triggering a
ground station response pulse. The aircraft's DME measures round-trip time and converts it to a
display representing nautical mileage on the range indicator. The interrogation/reply cycle is
continuous and the indicator constantly shows slant range. When the aircraft is directly above
the ground station, altitude is shown in NM. DME should be considered unusable unless it tests
within 1/2 NM or 3% of the distance to the station, whichever is greater. The appearance of the
warning bar/flag signifies an unusable or unreliable signal and value.
The Electronic Horizontal Situation Indicator (EHSI) displays aircraft heading along with
navigational bearing data. It consists of a rotating compass card and two bearing pointers. The
compass card is actuated by the aircraft's compass system so it continually displays aircraft
magnetic heading. The aircraft's current magnetic heading is displayed on the compass card
beneath the top index. The bearing pointers display VOR magnetic bearings to the selected
navigational station or GPS bearing to the selected waypoint. Radial position is displayed under
the tail of the bearing pointers.
Bearing pointers do not function in relation to ILS signals.
VOR and TACAN navigation receivers, as well as GPS equipment, electronically measure the
magnetic bearing for display by the bearing pointers. Therefore, if there is a malfunction in the
compass system or compass card, the VOR, TACAN, and GPS bearing pointers do not point to
the station/waypoint; however, they may still indicate proper magnetic bearings. When a
compass malfunction is known or suspected to exist, the VOR, TACAN, and GPS radial displays
must be considered unreliable until verified by other means. When navigating with VOR/DME,
TACAN, or GPS, distance from the ground station/waypoint is displayed on a range indicator
The T-6A is not equipped with a TACAN receiver; however, all other
aircraft you fly will be equipped and function as described.
2-4 INTRODUCTION TO AIRCRAFT FLIGHT INSTRUMENT