Class A airspace begins at 18,000 feet above sea level known as mean sea level, or msl, and goes up to 60,000 feet msl. This airspace covers both the 48 contiguous states and Alaska and extends offshore 12 nautical miles.
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Class A airspace generally begins from 18,000 feet mean sea level up to and including 60,000 feet. Operations in Class A are generally conducted under Instrument Flight Rules and primarily used by higher performance aircraft, airline and cargo operators, etc.
Class A airspace is a fairly straight forward type of airspace to understand, it covers the entire globe, from 18,000' msl or FL180 (flight level 180) up to and including FL600 (or 60,000' msl).
Class A: All Airspace above 18,000 ft. Anybody flying here must receive a clearance from, be talking to, and be controlled by ATC. Class B: Airspace within approximately 30 miles and 10,000 feet of the ground around the busiest airports in the US.
In the 1900s, Hungarian physicist Theodore von Kármán determined the boundary to be around 50 miles up, or roughly 80 kilometers above sea level. Today, though, the Kármán line is set at what NOAA calls “an imaginary boundary” that's 62 miles up, or roughly a hundred kilometers above sea level.
The upper limit of the airspace should not exceed 10,000 feet MSL. However, high airport field elevation, adjacent high terrain, or operational factors may warrant a ceiling above 10,000 feet MSL.
Class A airspace is generally the airspace from 18,000 feet (~3.4 miles, 5.5 km) mean sea level (MSL) up to and including flight level (FL) 600 (~11.4 miles, 18.3 km), including the airspace overlying the waters within 12 nautical miles (NM) (~13.8 miles, 22.2 km) of the coast of the 48 contiguous states and Alaska.
There are two categories of airspace or airspace areas: Regulatory (Class A, B, C, D, and E airspace areas, restricted and prohibited areas). Nonregulatory (military operations areas [MOA], warning areas, alert areas, controlled firing areas [CFA], and national security areas [NSA]).
The biggest reason for flying at higher altitudes lies in fuel efficiency. The thin air creates less drag on the aircraft, which means the plane can use less fuel in order to maintain speed. Less wind resistance, more power, less effort, so to speak.
In short, the real purpose of Class F is to allow flights to remain IFR in uncontrolled environments. Since this is a sort of mix between Class E and Class G airspace, there is no Class F inside the United States.
There's one more thing to keep in mind when you're operating in Class B airspace: in general, you need to be at least a private pilot to enter the airspace. Student, sport and recreational pilots can enter specific Class B airspaces, but only after they receive training and an endorsement from an instructor.
You should advise ATC before any altitude change when operating VFR-on-top. This ensures ATC can provide accurate traffic information and separation. VFR-on-top is not permitted in Class A airspace.