People who have travelled on a two-way trip from Port of Spain to Miami will have observed that the return flight takes less time, even though the aircraft speed on both flights is the same over the fixed ground distance of 1,394 nautical miles or 1,604 statute miles.

This is because of the winds the aircraft encounters during the flight.

On the flight from Port of Spain to Miami, the aircraft encounters headwinds which blow against the direction of the aircraft, resulting in its moving forward in a body of air that is moving backwards.

On the return flight, the aircraft encounters tailwinds moving in the same general direction as the aircraft, resulting in its moving forward in a body of air that is also moving forward.

The difference in the flight times in the two scenarios gives rise to the concepts of "airspeed" and" groundspeed," which are both expressed in knots.

The term “knot,” like other aviation terms, was imported from the maritime industry, which existed long before the aviation industry. The knot is a unit of speed equal to one nautical mile per hour, and was first used by sailors to calculate the speed of ships using a device called a “common log.”

The cruising speed of Boeing 737-800 aircraft is approximately 460 knots. Unlike miles per hour there is no such term as knots per hour, because a knot is defined as one nautical mile per hour.

A statue mile equals 5,250 feet. A nautical mile equals 6,076 feet, because it is based on the earth’s longitude and latitude co-ordinates, with one nautical mile equalling one minute of latitude, or one sixtieth of a degree of latitude. Therefore, in the aviation industry, distance is expressed as nautical miles and speed is expressed as knots.

The use of latitude and longitude co-ordinates is more practical for long-distance travel, as the curvature of the earth is a critical factor in accurately measuring distances. Aeronautical charts use latitude and longitude, as it is easier for aviators to measure distance with nautical miles, which follow the earth’s curvature.

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When that aircraft travelling at an airspeed of 460 knots experiences a 15-knot headwind, the actual groundspeed of 445 knots is derived by subtracting the 15-knot headwind from the 460-knot airspeed. If the headwind for the entire flight of 1,394 nautical miles is constant at 15 knots, the flight time would be three hours and eight minutes.

On the other hand, if there is a constant 15-knot tail wind, the airspeed will remain at 460 knots, and the actual groundspeed of 475 knots is derived by adding the 15-knot tailwind to the 460-knot airspeed. In this case, the flight time would then be two hours and 56 minutes.

Therefore, the basic rule is that headwinds increase the flight time and tailwinds decrease the flight time.

The concept is similar to the moving platforms at major airports. If a passenger remains stationary on the platform, the passenger moves at the speed of the platform. If the passeng