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, and determining ground
speed.
Ground Speed Checks
You need to remember 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 ft (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.
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 elapse 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 knots
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 knots
NOTE: ATC can also provide you with ground speed from radar.
Intercepting an Arc from a Radial
The key to intercepting an arc precisely at the desired DME lies in performing an accurate lead point
calculation (LPC) to determine the correct lead point DME to initiate the interception turn. For radial to arc
intercepts, you will determine the lead point in miles (DME) instead of radials, which are used in arc to
radial intercept calculations discussed later in this chapter.
The turn to intercept an arc from a radial will normally be at approximately 90 degrees. When intercepting
an arc, you have to calculate the lead point at which you initiate the turn in order to intercept it at the
correct distance. To determine the lead point, use 1 percent of your ground speed. For example, whether
flying inbound or outbound at a ground speed of 250 knots, your lead point will be 2.5 DME prior to the
desired arc. When inbound to the arc, add the 1 percent to the arc DME and when outbound, subtract 1
percent from the arc DME when calculating the lead point.
When you reach the lead point, initiate a 1/2 SRT turn in the proper direction and maintain it until the
bearing pointer nears the wingtip position. You may have to modify your turn rate/AOB somewhat in order
to arrive on the arc at the proper DME.
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