Harmon Technologies, Inc.

Altimeter

The altimeter tells how high the airplane is above mean sea level.  To see how high the airplane is above the ground, you would have to know the elevation of the land.  This is easy to obtain from aviation charts.  The Altimeter works by measuring the air pressure (just like a barometer) and reading out the answer in feet of air instead of inches of mercury.  Because the barometric pressure changes with the weather, the altimeter must be adjusted for local weather conditions.  All aviation weather reports and control towers provide an "altimeter setting" which the pilot can dial into the altimeter to provide this correction.  Except at very high altitudes, pilots must keep a setting from an airport within 100 miles of the airplanes position.  When flying along and switching to a new controller's airspace, they often provide their current altimeter setting as a convenience to the pilot.

Since height above the ground is not important above 18,000 feet and since many planes at that altitude go very fast, the requirement is changed and everyone just dials in a standard number.  This means that everyone up there (mostly airliners) may be wrong but they are all wrong together so the system works.  At those altitudes, the heights are called "flight levels" instead of altitudes since they don't actually represent measured heights.

In N4907J we don't have to worry about that.  Our service ceiling is about 12,000 feet and our most efficient cruise altitude is around 7,000 to 8,000 feet.

The altimeter reads like a clock except that there are 10 major divisions instead of 12.  The long hand represents hundreds of feet (one division is 20 feet, the "5" represents 500 feet).  The medium hand is thousands of feet and thin line with the triangle represents 10,000 feet.

To help keep air traffic organized, there are rules about what altitudes to use.  For cruising, instrument flights use odd thousands if going east bound and even altitudes if going west.  Visual flights use these altitudes plus 500 feet.  So if we are going to the islands on an instrument flight plan, we are likely to cruise at 5,000 (or 7,000) feet.  Coming back, 6,000 (or 4,000) might be used.

In addition to telling the pilot the altitude, the instrument sends the information to the transponder so that air traffic controllers can see the altitude information on their radar screens.

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