A pilot has two airplanes (red and yellow). He wants to find out which one of them is faster. He first flies the red plane to a point 30 miles away and back again at absolute still air. He measures the total travel time to determine the plane's speed.
He completes the same route with the yellow plane. This time, a tailwind occurs on the trip out, and there is a headwind on the return trip.
Question: Can he make a reliable judgement to determine which is the fastest airplane?
Our pilot won't be able to make a decision, as the wind has skewed his test plan!
Perhaps you considered that a tailwind on the trip out, and a headwind on the way back might cancel each other out. This is not true.
Let's suppose that the speed of the plane (measured at still wind) and wind speed are both 125 mph. The pilot will arrive at his destination in half the time, but he will never be able to return to the airport.
If the wind speed is less than 125 mph, he surely will make the return trip, but he will need more time on the return flight (due to the headwind). In fact, he will need as much time as he saved on the outbound flight with a favorable tailwind.