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CHAPTER FIVE
INSTRUMENTS FLIGHT PLANNING
502.
DME/GS RESTRICTIONS
Your fuels and times are always a function of your GS; therefore, it is important to know when
DME/GS is reliable and when it is not.
In previous sections, we discussed the unreliability (for purposes of GS checks) of DME when
within a radius of the TACAN station equal to your altitude in thousands of feet. That restriction
still holds true. Other restrictions are:
1.  Level Flight To accurately accomplish a GS check, you cannot be in a climb or a descent
because part of your airspeed has a vertical component. In addition, as you climb or descend,
your IAS/TAS varies according to temperature and pressure altitude.
2.  Steady Airspeed Do not start a GS check until leveled off and stabilized at a cruising
airspeed. Additionally, if you are taking a GS check and the pilot accelerates or decelerates, the
GS check is invalid. Airspeed must be steady throughout the check.
3.  Radial Tracking You must be on or close to a radial, tracking inbound or outbound to a
station. If you managed to get off course and are correcting laterally back to course, be wary of
performing a GS check. For corrections of a few degrees it may be permissible; however, the
greater your degree of lateral movement, the more in error your GS check will be.
4.  Correct TAS It is important to always know your TAS. TAS is a function of IAS or
Indicated Mach Number combined with temperature and pressure altitude. If your IAS is in
error, your TAS is wrong. Since GS equals TAS + wind, taking a GS without being at your
correct TAS will not aid wind or course control computations.
5.  Time The time required for DME/GS checks is a minimum of one minute. If any of the
aforementioned conditions restrain your ability to do this however, any GS check is acceptable.
If a 36 second or one minute GS check is not feasible, there is one other accurate method. You
must attempt at least one GS check per leg.
Figure 5-1 is a short segment of V35E between Gainesville and Cross City. If you were
transiting at 16,000 feet MSL and a TAS of 210 kts, you might have trouble getting a GS check.
To overcome this problem, you could employ the lessons learned in earlier sections. Since you
are responsible for logging your MARK-ON-TOP times at Gainesville and Cross City, you can
come out with an accurate GS check.
5-2 IN-FLIGHT FUEL AND TIME ANALYSIS


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