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→Stall speed and \(V_{NE}\)
The point where the stall speed (IAS) corresponds to the never exceed speed (TAS) because of a decrease of density gives the theoretical ceiling. This is the theoretical maximum altitude at which the aeroplane can fly. If you fly at this altitude, you must fly at this speed precisely, or you will either stall or overspeed.
The Lockheed U-2, which flies at very high altitudes, have very notable problems when the ceiling is reached. For a U-2 in cruise, the stall speed and the never exceed speed is less than 10 knots apart on the ASI. This calls for very accurate handling by the pilot. The same applies to glider pilots who wish to fly at high altitudes: you must remember that the airspeed window in which you can fly is reduced, and, by flying higher, the red mark on the ASI must gradually move inwards.
=== Altimeter ===