Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network.

Aero-Tips 06.30.06

Most aircraft are required to have an altitude sensing device, and in most cases the altimeter must be adjustable for variations in air pressure (Note: common altimeters measure air pressure and convert that to an altitude display). If the air pressure changes, and it does, the pilot must manually calibrate the altimeter by "dialing in" the local air pressure. This is called altimeter setting.

Our friends the Federal Air Regulations, of course, address altimeter setting as well. They tell us:

Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating --

• Below 18,000 feet MSL, to-
• • The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
  • If there is no station within 100 nautical miles, the current reported altimeter setting of an appropriate available station; or
  • If the aircraft is not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or
• At or above 18,000 feet MSL, to 29.92" Hg.

Note that the altitude where "standard" (29.92" Hg/1013.2 mb or hectoPascal) varies from country to country.

Altimeter Error

Altimeters (under US rules) must be accurate +75 feet with the current altimeter setting. Say you are sitting at on the ground at a known elevation above mean sea level (MSL; at the point on the airport where the official elevation was measured), dial in the current altimeter setting and note, for instance, that your altimeter is reading 50 feet high, do you --

1. Leave the altimeter set as it is and accept the 50-foot error;
2. Change the altimeter to the known elevation and, for all subsequent altimeter settings in flight, dial in the setting and then adjust the altimeter indication by 50 feet; or
3. Change the altimeter to the known elevation, note the difference in altimeter-indicated air pressure from the setting window needed to make this adjustment, and then apply this correction to all altimeter settings for the remainder of your flight?

The FARs don't give us any guidance. My personal opinion is that the margin for error both in reported altimeter setting and measured field elevation is large enough that I would (and do) accept this within-tolerance altimeter error, avoiding the workload of mental altimeter math in flight.

Another note

Again, the rules don't say so, but it's good to double-check altimeter setting as part of your approach checklist if you are flying an instrument approach and/or at night.

Aero-tip of the day: Set your altimeter properly before flight, and regularly update it as you proceed.

FMI: Aero-Tips