The groundspeed of a plane refers to its speed relative to the ground, while a headwind is a wind that blows in the opposite direction of the plane's motion. Therefore, the phrase "Groundspeed of Plane in 10km/h Headwind: 90km/h" means that the plane is moving at a speed of 90km/h relative to the ground while facing a headwind of 10km/h.
A headwind will decrease the groundspeed of a plane because it is pushing against the plane's forward motion. In this case, the groundspeed of the plane is 90km/h, but it would have been 100km/h if there was no headwind.
Yes, the groundspeed of a plane will always be lower in a headwind because the wind is acting against the plane's motion. The strength of the headwind will determine how much the groundspeed is reduced.
Yes, it is possible for a plane's groundspeed to be higher than its airspeed if it is facing a tailwind. A tailwind is a wind that blows in the same direction as the plane's motion, therefore, it will increase the groundspeed of the plane.
Headwind can significantly affect a plane's flight time. In this case, the headwind of 10km/h will slow down the plane's groundspeed, which means it will take longer to cover a certain distance. This is why pilots often try to avoid headwinds to reduce flight time and save fuel.