FAA Sport Pilot Airplane (SPA)Airspace and NavigationHard
An aircraft departs on a cross-country flight with a true airspeed of 105 knots. A 15-knot headwind reduces the groundspeed to 90 knots. If the total distance to the destination is 180 nautical miles, how long will the flight take?
- A2 hours 15 minutes
- B1 hour 45 minutes
- C2 hours 0 minutes
- D1 hour 30 minutes
Show answer & explanationAnswer & explanation
Correct answer: C. 2 hours 0 minutes
Time en route = Distance ÷ Groundspeed = 180 NM ÷ 90 knots = 2.0 hours. Since the headwind reduces the groundspeed to 90 knots (not the 105-knot true airspeed), the correct groundspeed must be used for the time calculation, giving exactly 2 hours 0 minutes.
Why the other options are wrong
- A. This would occur only if the groundspeed were slower than 90 knots.
- B. This is close but doesn't match the exact math using 90 knots groundspeed.
- D. This would result from mistakenly using a groundspeed higher than 90 knots.
Time En Route Calculation
Time en route is found by dividing the distance to be flown by the groundspeed, not the true airspeed, since groundspeed accounts for wind effects.
- Time (hr) = Distance (NM) ÷ Groundspeed (kt)
- Always use groundspeed, not TAS, in this formula
- Headwinds reduce groundspeed, increasing flight time
Memory trick: Distance Divided by Ground speed gives Duration — 'D over G equals D-uration'