t-39 - Chief of Naval Air Training
t-39 - Chief of Naval Air Training
t-39 - Chief of Naval Air Training
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T-<strong>39</strong> FLIGHT PREPARATION STUDENT GUIDE<br />
(1) Level flight<br />
(2) Stabilized mach number<br />
(3) Radial tracking<br />
(4) DME greater than the thousands <strong>of</strong> feet <strong>of</strong> aircraft altitude<br />
Once these conditions are met, there are two methods <strong>of</strong> computing ground speed. One<br />
is to check the DME at 1, 2, or 3-minute intervals. Since the DME readout is digital, the oneminute<br />
check is accurate and ground speed can be determined by multiplying the DME flown<br />
by 60. Another method is to take a 36-second check and add a zero to the end <strong>of</strong> the DME<br />
difference. If this check is continued to the 1-minute mark, ground speed in both knots and<br />
nautical miles per minute can be obtained without calculation (and can be used for crosschecking).<br />
At the completion <strong>of</strong> the first ground speed check, and every leg thereafter (once the<br />
GSP readout is valid), the student must give an updated estimated time <strong>of</strong> arrival (ETA) at<br />
the next point and estimated fuel remaining at the IAF.<br />
b. Wind analysis<br />
The headwind/tailwind component is determined by taking the difference between TAS<br />
and ground speed. The crosswind component is determined by the crab angle: the amount<br />
<strong>of</strong> wind that equates to one degree <strong>of</strong> crab can be determined by dividing the TAS by 60. If<br />
the TAS is 420 kts then one degree <strong>of</strong> crab equates to 7 kts <strong>of</strong> crosswind. This is called the<br />
Guide number. A quick method <strong>of</strong> determining wind is to take all <strong>of</strong> the larger component<br />
and half <strong>of</strong> the smaller to determine velocity and use vector analysis to determine direction<br />
(See the Trainee Guide for Visual Navigation CNATRA P-811 or P-812). At 420 kts TAS,<br />
for example, if it takes 7 o <strong>of</strong> left crab to maintain a course <strong>of</strong> 360 with a ground speed <strong>of</strong><br />
<strong>39</strong>0 kts, the wind can be determined this way:<br />
7 kts/degree <strong>of</strong> crab x 7 degree <strong>of</strong> crab = 49 kts crosswind<br />
<strong>39</strong>0 kts GS - 420 kts TAS = 30 kts headwind<br />
The resulting wind is 300 at 65 knots (rounded to the nearest 5 kts).<br />
c. Lead points<br />
During flights, all turns greater than 30 o (including point-to-points) will be led. To<br />
calculate the lead point for a 90 o turn, use Minimum DME +1 percent <strong>of</strong> ground speed over<br />
NAVAIDS, and 1 percent <strong>of</strong> ground speed at fixes. Consequently, 45° and 30° turns are<br />
led by one half <strong>of</strong> 1% and one third <strong>of</strong> 1% <strong>of</strong> ground speed, respectively.<br />
EXAMPLE: Calculate the lead point for an aircraft at FL 350, 450 kts ground speed, making a<br />
90 o turn over a NAVAID.<br />
Minimum DME = 35,000/6,000 = 5.8 DME<br />
1% <strong>of</strong> ground speed = 0.01 x 450 = 4.5 DME<br />
Lead Point 10.3 DME<br />
The pilot would be directed to turn to the outbound heading upon reaching 10.3 DME<br />
from the NAVAID or at an appropriate number <strong>of</strong> radials approaching a point to point.<br />
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