Generally we fly under the rule of QNH which means our IA is above mean sea level. And as we all know airpace limits are charted in MSL. Then, why the definitions of airspaces are they (class C, D and E) depiceted in AGL?

  • $\begingroup$ Can you give an example of where you see this? In Germany, only the airspaces directly touching class G are in AGL, all other airspaces start in MSL. The reason is that class G goes up to 2500ft AGL, so all touching airspaces need to start in AGL as well. $\endgroup$ Jan 31 '15 at 7:57
  • $\begingroup$ @SentryRaven In the AIM, class C airspace is explained like this `Generally, that airspace from the surface to 4,000 feet above the airport elevation ...' this where I was confused. $\endgroup$
    – Mun
    Jan 31 '15 at 9:53
  • 1
    $\begingroup$ Which AIM is that? Control Zones in Germany have a MSL upper limit. But we don't have class C CTRs, only class D. $\endgroup$ Jan 31 '15 at 10:34
  • $\begingroup$ It's America's. $\endgroup$
    – Mun
    Jan 31 '15 at 11:33
  • $\begingroup$ Well, if you have airports at different elevations, and you want a 4000 feet high CTR around each airport how else are you going to define it? $\endgroup$
    – Radu094
    Jan 31 '15 at 15:44

You're confusing the airspace height with airspace altitude.

The definition of Class C (in the US) is

3-2-4 (a) Generally, that airspace from the surface to 4,000 feet above the airport elevation (charted in MSL)

If you look at chart for the Class C for Colorado Springs (elevation 6,187), the surface area of the airspace goes up to 10,200 ft MSL, approximately 4,000ft above the airport elevation of 6187.

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