Reminder that speed instructions are in IAS

July, 24, 2021 by

Speed instructions from all controllers are sent in indicated airspeed (IAS) not ground speed (GS). Here is an excerpt from the ATC Manual.

6.14.1 – There is a common misconception between the relationship between Indicated Airspeed (IAS) and Groundspeed (GS). For simplicity the reasons behind the difference are not covered, however there are two things that will ultimately affect the relationship: wind and altitude!

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.

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)

Controllers can still view an aircraft’s GS, IAS and Mach Speed in the pilot’s information and on the radar.

References: 6.14 of the Infinite Flight ATC Manual

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