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Airspeed limitations (KIAS)

Asked by: 6750 views ,
Airspace, FAA Regulations

Why are the airspeed limitations at lower altitudes based on indicated airspeed?  It seems that groundspeed would have been more of an important factor when establishing these airspeeds.

For instance, flights below 10,000 feet MSL are limited to a max indicated of 250 knots. 

If there was a direct tailwind of 50 knots, the groundspeed would be 300 knots.  As far as aircraft separation is concerned, it seems as if groundspeed would be the real factor here.  It also is my understanding that ATC's radar scopes display groundspeed.

Therefore, I'm back to my original question: Does anybody have any idea why airspeed limitations are based on indicated rather than groundspeed?

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2 Answers

  1. Kent Shook on Dec 24, 2010

    Several reasons:
    1) If that reg was written a fair amount of time ago, it would be difficult to get an accurate groundspeed. Sometimes in this day and age of GPS, we forget that the system isn’t based on GPS and it hasn’t been too awfully long since there was no groundspeed readout.
    2) Presumably, the reg was written to allow both controllers and pilots to maintain separation in the busier airspace down low where most VFR traffic is. If you look at closure rates, there isn’t any difference between using groundspeed and airspeed.
    3) Finally, and perhaps most importantly, basing the limit on groundspeed would cause more headaches for controllers – “If I assign that guy a 090 heading, is his airplane still going to be able to fly?” In some cases, a strong tailwind would cause an airplane to stall at more than 250 knots groundspeed. Airspeed is what makes airplanes fly, so maintaining a consistent limit on airspeed makes more sense than basing it on groundspeed.

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  2. Brian on Dec 24, 2010

    …Qt: Kent Shook…In some cases, a strong tailwind would cause an airplane to stall at more than 250 knots groundspeed…
    Just so we are clear, the regulation accounts for this as currently written. Though I think your #3 is right on par with the main reason for this.
    Think how much would it stink to have an aircraft that cruises at 240 and with each change of heading have to check groundspeed just to ensure you don’t get violated? What about days when the wind is strong at 3, calm at 6, and 50 degrees different and strong again at 9. Talk about a fun decent profile to calculate. 🙂
    One last kink for this groundspeed versus airspeed. What about fighter pilots? They could legally fly a 500+ knot decent or climb right through class Bravo so long as they did so at an angle that gave a ground speed below 250. Such as straight up or down. 

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