I know nothing about flying an aircraft, but on a recent trip on a Dreamliner to the Carribean I found myself regarding the very helpful maps and flight information accessible through the in flight entertainment system and puzzling over the expressions used such as ground speed, tail wind, head wind and wind speed. Cross winds weren't mentioned but I'm sure they could well have been.
Here's my layman's theory which made sense to me during the flight:
For passengers, the aeroplane's speed relative to the ground is the most important. It is ground cover we want after all, isn't it, to get us from home to our holiday destination and back in the most direct and efficient way possible.
However it seems to me that a moving aeroplane has two states:
- a terrestrial state: when it is still in contact with the ground
- airborne state: when it has left the ground
When moving forward for take off with its wheels still in contact with the runway a head or tail wind or indeed a side wind or an oblique wind will affect its motion as it would a car or any other terestrial vehicle designed to cover ground while remaining in contact with it.
However, once an aircraft has left the ground i.e. no longer has its undercarriage in contact with the runway, it is entirely at the mercy of the body of air in which it is suspended as it travels though it at several hundred mph. Here an aircraft experiences no more pressure on its nose cone at a constant speed through the air whatever the speed at which and direction in which the air itself may be travelling; head, tail or any other 'winds' surely become irrelevant; although the speed at which the body of air may be travelling is extremely relevant.
For example, if the body of air supporting the aeroplane is travelling at 50 mph in the same direction as the aircraft which is doing 400 mph the ground speed will be cumulative - namely 450 mph but the plane will still only be doing 400 mph relative to the body of air in which it is traveling. If the body of air has a terestrial speed of say 100 mph in the opposite direction to an aircraft travelling at 400 mph, the aircrafts speed relative to the ground - its ground speed - will be subtractive - only 300 mph.
If the body of air in which the aircraft is suspended is moving in a sideways direction' either directly or obliquely, relative to the terestrial course the plane needs to follow in order to describe a relatively direct route from take off to landing the plane won't be 'blown' off course as such but if no corrections are made and if all speeds are steady then the aircraft will arrive at a different destination from the one intended.
For example if an aircraft maintains a steady speed of 500 mph with respect to the ground i.e. has no help or hinderance from the body of air which is supporting it moving in a favourable or unfavourable direction but has a direct sideways motion of say a steady 20 mph left to right and the distance of terestrial travel is 2000 miles, after the plane has travelled for the 4 hours necessary to cover the 2000 miles it will arrive 80 miles to the right of its desired destination.
In these same steady conditions the plane would need to aim at a point 80 miles to the left of its desired destination and then it would arrive at its desired destination. The tail, head and cross winds are a different story though once the (for all intents and purposes) static ground is about to come up and meet the aircraft at considerable speed and as it prepares to leave it's airbourne state and assume its terrestrial state.
It's then, I guess, that the boffins invoke their equations for approach speed relative to the ground, air movement speed - be it head, tail or side wind and runway length in order to make the transition to a smooth landing.
I know this forum is to answer a specific question; please accept this as the offering of a complete layman but it does seem to me to be scientifically sound - indeed is my own answer to a question I have asked myself but I think this would appear to cast some light on the role head or tail winds play in aviation scenarios.