What must you do when you climb above 18,000 feet into Class A airspace?

What must you do when you climb above 18,000 feet into Class A airspace?

• A. Set the altimeter to 29.92
• B. Set the altimeter to QNH
• C. Set transponder to 1200
• D. Reduce speed to 200 knots

Answer: (A)

When you climb into Class A airspace above 18,000 feet, you switch the altimeter to the standard pressure setting. That standard is 29.92 inches of mercury (1013.25 hPa). Using this fixed reference aligns your altitude readings with flight levels, which are used for vertical separation at high altitude. If you left the altimeter on local QNH, the reading would track current surface pressure and vary with weather, causing inconsistencies with ATC’s flight level assignments. The other options relate to different procedures (transponder codes or speed limits) and don’t address the altitude reference required for Class A airspace.

When you climb into Class A airspace above 18,000 feet, you switch the altimeter to the standard pressure setting. That standard is 29.92 inches of mercury (1013.25 hPa). Using this fixed reference aligns your altitude readings with flight levels, which are used for vertical separation at high altitude. If you left the altimeter on local QNH, the reading would track current surface pressure and vary with weather, causing inconsistencies with ATC’s flight level assignments. The other options relate to different procedures (transponder codes or speed limits) and don’t address the altitude reference required for Class A airspace.