Refer to figure.
Some questions might give you a range of wind values and TAS values at various altitudes and a true track and ask for the average groundspeed. In these cases, the TAS and W/V to use on the Flight/Nav computer is as follows:

• For Climb problems - the TAS and W/V at the altitude 2/3 of the cruising altitude.
• For Descent problems - the TAS and W/V at the altitude 1/2 of the descent altitude.

The aircraft climbs from MSL (0 ft) to FL270. The 2/3 of the cruising altitude is at:

• 0 ft + [2/3 x (27 000)] ft = 18000 ft

For the wind velocity at 18 000 ft then, we would usually have to interpolate 3/5^ths of the way from 15 000 ft to 20 000 ft, but the winds are the same so no interpolation necessary. We can use the wind velocity 310º(T)/45 kt.
For OAT, the ISA temperature at 18 000 ft is 15 - 2(18) = -21ºC, and the OAT is ISA +5ºC, so OAT = -16ºC

To find the groundspeed, we first need to calculate the TAS. (We have used a CRP-5 and the wind-down method here)

• On the computer side, in the air speed window, line up 18 000 ft with the OAT of -16ºC
• On the computer's inner scale, find the CAS of 280 kt, and read off the uncorrected TAS of 373 kt on the outer scale. As the TAS is over 300 kt, this needs to be corrected.
• In the COMP. CORR. window, calculate and apply the small correction in the direction of the arrow (only 0.7 divisions).
• Go back to the CAS of 280 kt on the inner scale, and read off the new TAS of 369 kt on the outer scale.

The Groundspeed can be found using the wind side of the computer.

• Set wind direction 310ºT under the "TRUE HEADING" index at the top.
• Set the centre point on the TAS of 369 kt *
• Mark the wind velocity 45 kt down from the centre point.
• Initially set the True Track 045°T under the "TRUE HEADING" index. Note that this heading would give 6º drift and a track of 051º, so reduce the heading value under the index until the heading plus the drift gives a track of 045º. This occurs at a heading of 038º with 7º Right drift.
• The groundspeed of this track is approximately 370 kt

Therefore the average groundspeed for the climb is 370 kt, closest answer 373 kt.

Calculating the TAS with E6B;

On the computer side, in the air speed window, line up 18 000 ft with the OAT of -16ºC. On the computer's inner scale, find the CAS of 280 kt, and read off the uncorrected TAS of 375 kt on the outer scale. As the TAS is over 300 kt, this needs to be corrected. However, there is no compression correction calculation in this part of the E6B flight computers. Therefore, we need to find the compression ratio from the compressibility correction table on the wind side of the flight computer (refer to figures). It is approximately 0.98.

TAS is 375 x 0.98 = 368 kt