DME/GS Restructions

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