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A light aircraft has a landing speed of 70 knots. A wind of 25 knots is blowing over the airfield. What is the ground speed of the aircraft when it touches down, a) directly in the wind?

Why is it that this is 70-25? For the 25knots wind, is this horizontal or vertical, I think horizontal?

Herbert
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  • You’re right. It’s horizontal. Thus 70-25. – Jim Apr 05 '22 at 17:11
  • Think of it as paddling upstream into a current. You're paddling at a running speed across the water, but because the water is moving the other way, your actual progress relative to the shoreline (or the river bottom) is more like a walking speed, as the current going against you subtracts from your water speed. If you paddle with the current, the opposite happens and you're now passing people on bicycles on the shoreline as your paddling speed is adding to the current speed. Paddle at 90 deg across the current, and it's a wash. Flying in the "air current" (the wind) is exactly the same thing. – John K Apr 05 '22 at 18:43
  • Groundspeed can be more, or less that airspeed. See this similar question: https://aviation.stackexchange.com/questions/92189/how-can-ground-speed-be-higher-than-airspeed/92190#92190 – Michael Hall Apr 06 '22 at 01:34
  • Re "Why is it that this is 70-25?" - why would you think it might be otherwise? – quiet flyer Apr 06 '22 at 20:04
  • Re "For the 25 knots wind, is this horizontal or vertical, I think horizontal?" - 25 knots is about 2500 feet per minute. The min sink rate of a Cessna 152 w/ power at idle (in still air) is about 700 feet per minute. So if the wind were vertical, and were upwards rather downwards, landing under such circumstances would be "interesting". Pop quiz for you, to see if you understood the concepts behind the upvoted answer to your question - how rapidly could a Cessna 152 gain altitude with the engine at idle, without losing airspeed, under these circumstances? – quiet flyer Apr 06 '22 at 20:21
  • PS if the wind were vertically downwards, the landing would also be "interesting", just in a different way. – quiet flyer Apr 06 '22 at 20:23

1 Answers1

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Assuming, for simplicity, that the plane is landing directly into the wind:

Aircraft speeds like landing speeds, stall speeds, etc. are given in KIAS (Knots Indicated Air Speed), because so much about the airplane's operation is derived directly from how the wings are interacting with the air.

Ground speed is just the plane's movement in reference to the ground, and has no direct influence on the aircraft (the aircraft's situation determines the ground speed, the ground speed doesn't determine anything by itself).

So if the aircraft has a landing speed of 70 KIAS, that means air is going over the wings at 70 knots - that's how much air the engine is propelling the plane through. If 25 of those knots are consumed just keeping pace with the headwind, then that leaves 45 knots of forward motion relative to the ground, aka the ground speed.

This is because both the wind speed and the ground speed are measured relative to the ground. It's just the aviator's version of the "Train A leaves station 1 at 100kph, Train B leaves station 2 on the same track in the opposite direction at 75kph, assuming both operators are asleep, at what speed do the trains collide?"

William Walker III
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  • They don't. After a few minutes, the driver vigilance device kicks in and brings the train safely to a stop. – Mark Apr 06 '22 at 00:16
  • @Mark Are you trying to tell me my 5th grade math teacher lied to me?!?!?! – William Walker III Apr 06 '22 at 12:26