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An aircraft descends from FL310 to FL050. Given the following information, which statement is correct?

Pressure altitude 31 000 ft: Descent distance 93 NM in 20 min
Pressure altitude 5000 ft: Descent distance 10 NM in 4 min
Headwind component during descent: 25 kt

  • A

    The ground distance is 79 NM.

  • B

    The air distance is 83 NM.

  • C

    The groundspeed is 345 kt.

  • D

    The TAS in descent is 321 kt.

Refer to figure,
Note: The values appear to be taken from a descent table in which the distances are given as air distance, as shown in the figure. Therefore, the NM values should be interpreted as NAMs rather than NGMs.

Calculate the time for descent from FL310 to FL050

  • Time = Time for descent from FL310 - Time for descent from FL050
  • Time = 20 min - 4 min
  • Time = 16 min

Calculate the air distance for descent from FL310 to FL050

  • Air Distance = Air distance for descent from FL310 - Air distance for descent from FL050
  • Air Distance = 93 NM - 10 NM
  • Air Distance = 83 NM

Calculation of True Airspeed

  • TAS = Air Distance / Time
  • TAS = 83 NAM / 16 min
  • TAS = 311 kt

Calculation of Groundspeed

  • TAS 311 kt
  • Headwind in descent: 25 kt
  • Groundspeed = 311 kt - 25 kt = 286 kt

Calculation of ground distance

  • Time = 16 min
  • Groundspeed = 286 kt
  • Ground distance = 286 x 16 / 60 = 76 NM

Therefore, the correct answer is: “The air distance is 83 NM”.

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