ATC vs Pilot Airspeed discrepancy

I can’t seem to find an answer to this question. So apologies if this subject has already been covered.

I’m often given instructions to not exceed a certain airspeed by ATC i.e. do not exceed 220kts even when my airspeed indicator is 210kts or less.

I’m not sure if ATC see my groundspeed rather than my airspeed or whether there’s a time-lapse issue.

Thanks,

Jay

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Sometimes they may know but its an instruction to not exceed it at any point. Speed at your discretion is when you can decide speed. It really helps if you always follow atc commands, even though obviously in this case you were 😁

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ATC is given ground speed. So if you’re at 210 knots airspeed, you might be 242 ground speed. That’s why it is “reduce” to 220 knots when you’re at 242 knots ground speed.

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I wonder why ATC are given pilots ground speed rather than airspeed?

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So the display on top left states ground speed?

They’re given ground speed because it is more accurate then airspeed. If you’re flying at 200 knots airspeed with 30 knots headwind - then your ground speed would be 170 knots. That’s a big difference. :)

Yes, it’s in ground speed.

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Ok before this gets more cloudy then it is already let me clarify. ATC gives you air speed instructions. ATC sees the ground speed so you are correct with being at 210 airspeed the fault lies with the controller not being able to distinguish the diffrence in ground vs airspeed.

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Thanks Brandon - that makes sense

That’s not entirely true. The “reduce” and “speed up” is based on groundspeed, so if you’re at 210 knots airspeed (242 ground speed) and ATC tells you to go to 220 knots - it is correct to say “reduce speed”, because ATC follows ground speed in that case.

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@Henrik_B we see ground speed but issue airspeed so for aircraft more then 3000 ft MSL the ground speed starts to climb past the airspeed and must be compensated for when issuing speed commands

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This is super cloudy haha. So we need a calculator to issue airspeed as ATC then. Because pilots (in IF) will take it as airspeed.

You are issuing airspeed so that’s how they should take it. I haven’t found a calculation for it yet but i know when I’m flying at 200 kts air speed @ FL100 my ground speed is about 250 but at 3000 ft it’s about 210

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Ah right I see, so it doesn’t matter, yes I’ve caught up. The only issue is when a pilot is doing below the asked for speed and you then say reduce based on the gs displayed. So as long as you remember to reduce the indicated speed for approach guidance there is no problem

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What I do is issue the plane in front of the plane I’m trying to slow down one speed so they will either speed up or maintain and then issue the following aircraft 10 knots slower airspeed.

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as long as the controllers know that they are issuing AS but can only see GS then that’s ok. I don’t want to be ghosted for not following ATC instructions if they ask me to reduce speed to below my stall speed! That only happened once, to be fair was on playground and couldn’t be ghosted by was at 8000ft, 220 KIAS and ATC asked me to slow down to 250 kts…

Just fly the airspeed your instructed to and if you get ghosted snap shot the log and instruments at the time. @David_Lockwood

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I’ve been asked to slow down too (more than once) on Advanced Server even though my airspeed was below what they were asking me to slow down to. I was concerned that the Advanced controller may think I was ignoring them and ghost me. I ended up slowing down to almost stall speed!

@Tyler_Shelton he has had to explain this in the past but maybe it needs to be touched on again.

ATC issues speed in KIAS. Pilots adjust KIAS to assigned speed. ATC’s radar shows aircraft speed in ground speed, as it does in real life, because it is important for us to see how fast you’re moving over the ground. For example, an aircraft on final at KASE is at 150KIAS, which is 168GS (give or take based on wind and altitude). It is important for ATC to see their actual ground speed for separation and spacing.

The pilot should only be concerned with KIAS. Pilots do not fly assigned airspeed as ground speed. “Maintain at or below 200kts”. The pilot will change KIAS to 200kts although their GS will be higher.

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