What is the meaning of 'transition altitude'?

• A. The altitude at which the standard 29.92 inHg pressure setting is used (in the U.S., typically 18,000 ft).
• B. The altitude at which the aircraft transitions to cruise power.
• C. The altitude where flight levels begin.
• D. The altitude at which landing gear is retracted.

Answer: (A)

Transition altitude is the altitude at which the altimeter setting switches to the standard pressure of 29.92 inHg. Above this level, all aircraft use 29.92 so altitude readings are referenced to a common baseline, allowing consistent flight-level separation in the upper air. In the U.S., this standard setting is typically used starting at about 18,000 feet. Below that altitude, pilots use the local altimeter setting (QNH) so the reading reflects actual height above mean sea level as atmospheric pressure changes with weather. The idea that flight levels begin is related but separate: flight levels start at the transition level, which comes after the transition altitude, and is defined by when the pressure setting changes in response to weather conditions. The other options describe unrelated concepts—cruise power, the exact start of flight levels, or landing gear operation.