T-39 FLIGHT PREPARATION
Stabilized mach number
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-
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.
1% of ground speed
0.01 x 450
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.