Altitude’s affect on GS

May, 27, 2021 by

Altitude can have a big affect on ground speed (GS). The speed shown directly on the map is the aircraft’s speed in GS, not IAS.

6.14.2 — Wind: if there is a tailwind of 20kts, that means there is 20kts of wind pushing the aircraft along in addition to its normal speed, so 180kts IAS will become 200kts GS. Similarly, if there is 20kts of headwind, that means there is 20kts of wind pushing against the aircraft, so 180kts IAS becomes 160kts GS.

Here are good ballpark figures to use as a guide when controlling a radar frequency, or to reference as a local controller.

6.14.3 — Altitude: as the altitude of aircraft increases, so will the difference between IAS and GS. Below are some very “ball-park” figures which can be used for reference. With the aircraft flying at 250kts IAS with no wind:

3000ft ~ 260kts GS (+10)
6000ft ~ 270kts GS (+20)
9000ft ~ 290kts GS (+40)
12000ft ~ 300kts GS (+50)

Websites like windy.com can be great to reference to see what the winds aloft are like at specific altitudes near the airport you are controlling or flying to.

But wait! You don’t need to guess because both the aircraft’s ground speed (GS) and indicated airspeed (IAS) are shown in the pilot’s information on the flight strip.

Kyle Boas is the Founder of the IFATC Education Group. He is an IFATC Supervisor and Infinite Flight Appeals team member. — More