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Computing GS takes into account the wind and how fast you are actually traveling through the air-TAS

For example, if you are flying east bound through a mass of air (the "wind" refered to in your question) that is moving from east to west (opposite direction to your flight path) at 100 knots (pretty big headwind) and your true airspeed is 250 knots (your actual speed through the air) your ground speed (speed over the ground) will be 150 knots. (TAS - Headwind: 250 knots - 100 knots = 150 knots GS)

So, without applying any other variables, the time from your departure point to your destination will be based on 150 knots ground speed. If your destination is 150 nautical miles from your departure point, your enroute time (time interval in your question) would be 1 hour.

So, why do you need to know your TAS?

During preflight planning you need to know what your planned TAS will be (from your aircraft's performance information/tables) based on the altitude you plan to fly, temperature, power setting, etc. Then, after consulting the forecasted enroute weather (e.g. wind direction, wind speed, temperature, etc.) that you will encounter along your route, you can compute your expected ground speed. So, knowing what your TAS will be is a necessary step in determining what your ground speed will be.

Simple as that. (Ground speed) "GS accounts for the wind" as you say in your question.

Computing GS takes into account the wind and how fast you are actually traveling through the air-TAS

For example, if you are flying east bound through a mass of air (the "wind" refered to in your question) that is moving from east to west (opposite direction to your flight path) at 100 knots (pretty big headwind) and your true airspeed is 250 knots (your actual speed through the air) then your ground speed (speed over the ground) will be 150 knots. (TAS - Headwind: 250 knots - 100 knots = 150 knots GS)

So, without applying any other variables, the time from your departure point to your destination will be based on 150 knots ground speed. If your destination is 150 nautical miles from your departure point, your enroute time (time interval in your question) would be 1 hour.

So, why do you need to know your TAS?

During preflight planning you need to know what your planned TAS will be (from your aircraft's performance information/tables) based on the altitude you plan to fly, temperature, power setting, etc. Then, after consulting the forecasted enroute weather (e.g. wind direction, wind speed, temperature, etc.) that you will encounter along your route, you can compute your expected ground speed. So, knowing what your TAS will be is a necessary step in determining what your ground speed will be.

Simple as that. (Ground speed) "GS accounts for the wind" as you say in your question.

Computing GS takes into account the wind and how fast you are actually traveling through the air-TAS

For example, if you are flying east bound through a mass of air (the "wind" refered to in your question) that is moving from east to west (opposite direction to your flight path) at 100 knots (pretty big headwind) and your true airspeed is 250 knots (your actual speed through the air) your ground speed (speed over the ground) will be 150 knots. (TAS - Headwind: 250 knots - 100 knots = 150 knots GS)

So, without applying any other variables, the time from your departure point to your destination will be based on 150 knots ground speed. If your destination is 150 nautical miles from your departure point, your enroute time would be 1 hour.

So, why do you need to know your TAS?

During preflight planning you need to know what your planned TAS will be (from your aircraft's performance information/tables) based on the altitude you plan to fly, temperature, power setting, etc. Then, after consulting the forecasted enroute weather (e.g. wind direction, wind speed, temperature, etc.) that you will encounter along your route, you can compute your expected ground speed. So, knowing what your TAS will be is a necessary step in determining what your ground speed will be.

Simple as that. (Ground speed) "GS accounts for the wind" as you say in your question.

Computing GS takes into account the wind and how fast you are actually traveling through the air-TAS

For example, if you are flying east bound through a mass of air (the "wind" refered to in your question) that is moving from east to west (opposite direction to your flight path) at 100 knots (pretty big headwind) and your true airspeed is 250 knots (your actual speed through the air) your ground speed (speed over the ground) will be 150 knots. (TAS - Headwind: 250 knots - 100 knots = 150 knots GS)

So, without applying any other variables, the time from your departure point to your destination will be based on 150 knots ground speed. If your destination is 150 nautical miles from your departure point, your enroute time (time interval in your question) would be 1 hour.

So, why do you need to know your TAS?

During preflight planning you need to know what your planned TAS will be (from your aircraft's performance information/tables) based on the altitude you plan to fly, temperature, power setting, etc. Then, after consulting the forecasted enroute weather (e.g. wind direction, wind speed, temperature, etc.) that you will encounter along your route, you can compute your expected ground speed. So, knowing what your TAS will be is a necessary step in determining what your ground speed will be.

Simple as that. (Ground speed) "GS accounts for the wind" as you say in your question.

If you are flying east bound through a mass of air that is moving from east to west (opposite direction to your flight path) at 100 knots (pretty big headwind) and your true airspeed is 250 knots (your actual speed through the air) your ground speed (speed over the ground) will be 150 knots. (TAS - Headwind: 250 knots - 100 knots = 150 knots GS)

So, without applying any other variables, the time from your departure point to your destination will be based on 150 knots ground speed. If your destination is 150 nautical miles from your departure point, your enroute time would be 1 hour.

During preflight planning you need to know what your planned TAS will be (from your aircraft's performance information/tables) based on the altitude you plan to fly, temperature, power setting, etc. Then, after consulting the forecasted enroute weather (e.g. wind direction, wind speed, temperature, etc.) that you will encounter along your route, you can compute your expected ground speed. So, knowing what your TAS will be is a necessary step in determining what your ground speed will be.

Simple as that. (Ground speed) "GS accounts for the wind" as you say in your question.

Computing GS takes into account the wind and how fast you are actually traveling through the air-TAS

For example, if you are flying east bound through a mass of air (the "wind" refered to in your question) that is moving from east to west (opposite direction to your flight path) at 100 knots (pretty big headwind) and your true airspeed is 250 knots (your actual speed through the air) your ground speed (speed over the ground) will be 150 knots. (TAS - Headwind: 250 knots - 100 knots = 150 knots GS)

So, without applying any other variables, the time from your departure point to your destination will be based on 150 knots ground speed. If your destination is 150 nautical miles from your departure point, your enroute time would be 1 hour.

So, why do you need to know your TAS?

During preflight planning you need to know what your planned TAS will be (from your aircraft's performance information/tables) based on the altitude you plan to fly, temperature, power setting, etc. Then, after consulting the forecasted enroute weather (e.g. wind direction, wind speed, temperature, etc.) that you will encounter along your route, you can compute your expected ground speed. So, knowing what your TAS will be is a necessary step in determining what your ground speed will be.

Simple as that. (Ground speed) "GS accounts for the wind" as you say in your question.