True Airspeed

Featuring Bob Nardiello

Subscriber Question:

"True Airspeed. Is it the same as GPS ground speed? How do you calculate? Why is it important?" - Mo S.

Bob:

"No, true airspeed is not the same as ground speed shown on the GPS. True airspeed (TAS) is important because performance charts in your Pilot Operating Handbook (POH) are based on true airspeed.

True airspeed is a calculated number depicting how fast the aircraft is moving through a still air mass. It is computed using indicated airspeed, pressure altitude, and temperature.

(Remember that indicated airspeed drops off as we climb due to lower air density at altitude not having the same impact on the pitot tube)

By using an E6B flight computer we can arrive at true airspeed.

Some airspeed indicators have a true airspeed ring around them that can be set for outside air temperature and pressure altitude, allowing you to read true airspeed off a scale on the ring.

Glass cockpit aircraft typically have what is called an air data computer which processes the above information and displays it for you.

Ground speed on the GPS represents how fast the airplane is moving across the ground. Ground speed is affected by winds aloft. If flying into a headwind, groundspeed will be slower than true airspeed.

Ground speed is used to determine how long to a destination. True airspeed is used with the POH to determine rates of fuel consumption over that time period."