Instrument Flight

Flight 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.

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.

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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