When ATC tells an A/C to maintain a certain airspeed


Just curious, I’m only flying props at this time so I haven’t been issued this request, but when I hear ATC request an aircraft to maintain 210kts until such and such fix, do they mean IAS? I’d assume they don’t mean TAS or ground speed.


When they ask you your speed or give you a minimum/maximum speed, it’s KIAS.


For jets, they also might ask an aircraft to maintain a specific airspeed during the climb until changeover to mach. Like maintain 300kts or greater until changeover to mach and then maintain .75 or greater.


As answered earlier (KIAS) but for ATC purposes a +10/-10 window is allowed.

Example: ATC assigns you 170K, any speed between 160K to 180K is ok.

One other thing, if for any reason you cannot follow the assigned speed, tell ATC so they won’t be getting a big surprise and you won’t be getting a call.

Don Craig - aka Crude - formerly CG @ D10


I would modify that slightly to say that they want exactly 160 knots, but you aren’t in violation of the regs if you are +/- 10 knots. If they are requesting this airspeed for separation (the typical reason) and get the lead plane 10 knots slow and the trailing plane 10 knots fast, separation could become a problem. In that case they could ask one or both of the planes for an additional airspeed adjustment.


We take the +10/-10K IAS into account when we assign speeds for separation.

Crude aka CG (formerly @ D10)


What’s a CG and what’s D10?

I would tend to agree with CAFlier. If they say 170, you’d better be at 170 unless you want to get bitched at. At least that’s how it’s works in the northeast!


Hey, CG…go ahead and try to explain how ARTS handles airspeeds…


CG were my operating initials when I used to do ATC.

D10 is the identifier of the DFW Tracon.

I would tend to agree with CAFlier. If they say 170, you’d better be at 170 unless you want to get bitched at. At least that’s how it’s works in the northeast!

Crude - is that better?


ARTS doesn’t do airspeeds, what we get shown on the radar displays is the aircraft’s groundspeed. Calculated per sweep by distance from the last sweep.

Centers also have groundspeed display, but theirs is an average over the last 40 to 60 seconds.

When aircraft are at higher altitudes, especially with a tailwind, we would get to see some pretty speeds around 600K. Loved watching the speed wind up on the SST once they got out of 10,000’. You ought to see the GS on the shuttle!

When aircraft approach the radar site and then move away from it the groundspeed readout can vary greatly too, used to scare the hell out of trainees when I would show them that 250K overtake with the lead only 5 miles away. That had to do with the slant range error.

How’d I do?

Don Craig - aka Crude - formerly CG @ D10


Incredibly I was asked to ‘reduce speed ten knots’ in a Warrior going into KAUS last week!


Indianapolis told me one time in the Pilatus that we were showing 100kts over the RJ in front of us. I bet the RJ guys were surprised when then cleared the runway and saw a PC12 behind them!


I was in an Aerostar one time and passed a SWA on final to the parallel runway. They had an app speed of 140ish and I passed them on about 3 mile final. They were over the marker while I was still with the approach controller. I’m guessing they were 5 out when I was 8. I was doing about 180 kts until 3 miles. Amazing airplane.


Always based on airplane equipment. Indicated airspeed, just like headings are always magnetic.

Not everybody uses a GPS on the plane, so they won’t know ground speed or ground track.



You wouldn’t want to run over that Tomahawk now would you?

Either that or my bud Snoopy was working!

Don Craig - aka Crude - formerly CG @ D10