Instrument Flight
Flight Procedures
TACAN AND VOR/DME PROCEDURES
Adding range information to instrument navigation enables you to perform several other navigational
procedures, including flying an arc around a station, proceeding point-to-point, or determining ground
speed, if the ground speed indication on the HSI display is not displayed.
Ground Speed Checks
You must recall two facts to perform a TACAN ground speed check: first, your DME from the station must
be greater than or equal to your altitude in thousands of feet (in order to limit the impact of slant range on
your calculation); second, you must be flying either directly to or directly from a station to get an accurate
ground speed check. If you are arcing or cutting across radials, your check will be inaccurate. The GINA
also calculates ground speed. But unlike the ground speed check you do, the aircraft does not have to
proceed direct to or from the station for the system to calculate a valid ground speed. The time-to-go in
the TACAN data block is computed from slant range to the station and computed ground speed.
Therefore, the time-to-go displayed in the TACAN data block is only valid if the DME from the station is
greater than or equal to your altitude in thousands of feet. The time-to-go in the waypoint data block is not
based on slant range so it is always valid.
To perform a ground speed check, start your timing when the DME displays a whole number. After a
predetermined time (in minutes), record the DME that has elapsed.
To calculate your ground speed, divide the distance (in nautical miles) by the elapsed time (usually 1 or 2
minutes) and then multiply the quotient by 60. The product of this calculation will be your ground speed in
knots.
For example: 12 nm/2 min = 6 X 60 = 360 kts
You can also time for a longer period of time to find the average miles per minute and then multiply by 60.
This will increase the accuracy of your ground speed calculation.
For example: 12 nm in 3 minutes = 4 nm/min = 240 kts
NOTE: ATC can also provide you with ground speed from radar.
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