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.

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.

“N5678, cross LTOWN at and maintain one-zero thousand feet.” Center just issued you a crossing restriction. You are expected to plan and initiate your descent so as to cross some point at a given altitude. You are cruising at 300 knots 100 miles from LTOWN at flight level 240. When will you initiate your descent? Just how quickly do you need to come down?

The Rule-of-Thumb
Part 1: Compute your Top of Descent:
First, find out how many feet you need to lose:
24,000 – 10,000 = 14,000.
Now, discard those zeroes and multiply by 3:
14 x 3 = 42.
Your top of descent is 42 miles from LTOWN.

Part 2: Compute your Rate of Descent:
Begin by determining your ground speed.
Multiply your ground speed by 6:
300 x 6 = 1,800.
Your rate of descent is 1,800 fpm.

At 42 miles from LTOWN, you will have to maintain 1,800 fpm to meet the crossing restriction.

Why you should not (necessarily) descend right away.
Many a pilot would be tempted to initiate a descent immediately upon receiving a crossing restriction, however one should be patient as there are other factors at play.
Recall that fuel efficiency increases with altitude, particularly for turbine engines. By descending too early, the pilot will be shorting himself on fuel. Furthermore, it is usually easier to avoid unwanted weather conditions at higher altitudes. By descending into the muck too early a pilot may be inviting turbulence, icing, and other nastiness into the flight. Also consider the effect of altitude on true airspeed: as altitude increases, true airspeed increases. A premature descent can lead to a lower true airspeed and longer flight times.