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The following information is for an aircraft in the cruise at 11:50 UTC.

Cruise level: FL80
Track Direction: 343°T
OAT: ISA +10°C
CAS: 145 kt
W/V: 163°T /15 kt

What will be the Groundspeed?

  • A
    181 kt
  • B
    166 kt
  • C
    151 kt
  • D
    194 kt

Refer to figure.
Determine OAT temperature at FL80:
ISA temperature at FL80 = 15 - 2 x 8 = -1ºC
OAT: ISA +10°C

  • OAT = -1ºC + 10ºC = 9ºC

Using the navigation computer, convert 145 kt CAS to TAS:

1) Align the pressure altitude of 8 000 ft window with the OAT of 9ºC;
2) Once the wheel is aligned, find the CAS of 145 kt on the inner most scale. Read the corresponding TAS of approx. 166 kt on the outer scale.

Finding ground speed using the Flight Computer:

(1) Place the wind direction (163º) under the true index.
(2) Mark wind velocity upwards from the centre. It can be done from arbitrary positions, only magnitude is important (i.e. vW = 15 kt if centre is on position 166, mark wind on 181).
(3) Rotate the disk until your course or track (343º) is under the true index.
(4) Slide the grip up or down until the wind track (end of the line) reaches TAS arc (166).
(5) Ground speed is indicated by the centre of the rotating disk (181 kt)

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