A member of an internet forum brought up an interesting question: “Why is it that you are supposed to set your altimeter to 29.92 when reaching 18,000 ft? Do you wait for ATC to tell you to do so, or is it automatic?” Read on for the low-down on transitioning to flight level flying.
When flying below 18,000 feet, pilots are expected to keep their altimeters adjusted to a local setting as reported by a station within 100 nautical miles. For IFR traffic and participating VFR traffic, ATC makes this task easy by continually updating pilots of new altimeter settings as they fly from one ATC sector to another.
Upon reaching 18,000 feet, pilots must reset their altimeters to the standard pressure setting of 29.92″ Hg. This is done to ensure altitude separation of all traffic. Furthermore, aircraft typically operate at high speeds above flight level 180, and according to Jeppesen, it would be impractical to reset the altimeter every 100 miles.
The astute reader may be wondering why the standard pressure setting is reserved for high flying traffic only. After all, if a standardized altimeter aids in altitude separation, why not apply that standard to everyone? The simple answer is that terrain does not respect standardized altimeter settings. For low flying aircraft, it is far more important to be able to precisely measure one’s height above obstructions than to ensure vertical separation of other airplanes.
To recap, pilots are required to transition to 29.92″ Hg automatically and without ATC direction as they reach 18,000 feet. Likewise, the transition to a local altimeter setting should be accomplished during the descent through flight level 180 regardless of whether ATC has advised the pilot of a local altimeter setting. After all, you were monitoring ATIS before you started down, weren’t you?
For even more information, see Wikipedia’s article on the transition altitude.