General Procedures

T-39 FLIGHT PREPARATION

STUDENT GUIDE

(1)

Level flight

(2)

Stabilized mach number

(3)

Radial tracking

(4)

DME greater than the thousands of feet of aircraft altitude

Once these conditions are met, there are two methods of computing ground speed. One

is to check the DME at 1, 2, or 3-minute intervals. Since the DME readout is digital, the one-

minute check is accurate and ground speed can be determined by multiplying the DME flown

by 60. Another method is to take a 36-second check and add a zero to the end of the DME

difference. If this check is continued to the 1-minute mark, ground speed in both knots and

nautical miles per minute can be obtained without calculation (and can be used for cross-

checking).

At the completion of the first ground speed check, and every leg thereafter (once the

GSP readout is valid), the student must give an updated estimated time of arrival (ETA) at

the next point and estimated fuel remaining at the IAF.

b. Wind analysis

The headwind/tailwind component is determined by taking the difference between TAS

and ground speed. The crosswind component is determined by the crab angle: the amount

of wind that equates to one degree of crab can be determined by dividing the TAS by 60. If

the TAS is 420 kts then one degree of crab equates to 7 kts of crosswind. This is called the

Guide number. A quick method of determining wind is to take all of the larger component

and half of the smaller to determine velocity and use vector analysis to determine direction

(See the Trainee Guide for Visual Navigation CNATRA P-811 or P-812). At 420 kts TAS,

for example, if it takes 7 o of left crab to maintain a course of 360 with a ground speed of

390 kts, the wind can be determined this way:

7 kts/degree of crab x 7 degree of crab = 49 kts crosswind

390 kts GS - 420 kts TAS = 30 kts headwind

The resulting wind is 300 at 65 knots (rounded to the nearest 5 kts).

c. Lead points

During flights, all turns greater than 30 o (including point-to-points) will be led. To

calculate the lead point for a 90 o turn, use Minimum DME +1 percent of ground speed over

NAVAIDS, and 1 percent of ground speed at fixes. Consequently, 45 and 30 turns are

led by one half of 1% and one third of 1% of ground speed, respectively.

EXAMPLE: Calculate the lead point for an aircraft at FL 350, 450 kts ground speed, making a

90 o turn over a NAVAID.

Minimum DME

35,000/6,000

5.8 DME

1% of ground speed

0.01 x 450

4.5 DME

Lead Point

10.3 DME

The pilot would be directed to turn to the outbound heading upon reaching 10.3 DME

from the NAVAID or at an appropriate number of radials approaching a point to point.

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