by Jonathan M. Stern
The altimeter is the only instrument in the airplane that shows how high the airplane is above some level. The altimeter has two hands like a clock and a third small indicator that appears near the numbers on the outer ring of the gauge. The large hand indicates hundreds of feet. The small hand indicates thousands of feet. The small indicator reflects tens of thousands of feet.
The altimeter uses an aneroid barometer to display pressure in feet above sea level (mean sea level), not above ground level. The altimeter cannot work accurately unless the pilot sets it to the current altimeter setting, which is the pressure at sea level under existing atmospheric conditions. To set the Flight Simulator altimeter to the current setting, press the B key.
Alternatively, on the Cessna, the mouse can change the altimeter setting. The numeric setting, however, is not displayed, so this option can only be used by setting the altimeter to whatever setting results in the display of a known elevation. There is also a menu item that allows calibration of the altimeter (Sim/Calibrate Altimeter).
The Federal Aviation Regulations require pilots of radio-equipped airplanes to keep the altimeter set to the current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft. The rationale for this requirement should be obvious. As an aircraft flies through areas of varying pressure, the reading on the altimeter is incorrect if the barometric setting is not adjusted. For each 1" Hg variation in pressure, the altimeter reading varies by 1,000 feet. The potential consequences, both in terms of hitting terrain or obstructions and mid-air collisions, explain the rule.
The procedure is somewhat different, however, for aircraft flying in Class A Airspace, which is the airspace over the United States between 18,000 feet and 60,000 feet. In Class A Airspace, altimeters must be set to the standard setting of 29.92" Hg. As long as all of the aircraft above 18,000 feet have their altimeters set to the same barometric setting, they should be properly separated by altitude. The rationale for this different procedure is that the aircraft in Class A Airspace are typically so fast-moving that they would have difficulty keeping up with all of the altimeter setting changes.
There is a saying that pilots use to help them remember specifically the consequences of flying with an incorrect barometric setting on their altimeter. From high to low, look out below. In other words, an aircraft that flies from a high-pressure area to a low-pressure area flies at a lower altitude than intended if the altimeter is not reset. Version 5.0 of Flight Simulator, however, was improperly programmed in this respect.
The altimeter error in version 5.0 that occurs from improper setting of barometric pressure is backwards. Flight Simulator version 5.0 pilots should look out below when flying from low pressure areas to high pressure areas.
The altimeter, when set to the current barometric setting, must be within 75 feet of airport elevation or the aircraft may not be flown on an instrument flight rules flight. There is no accuracy requirement for visual flight rules flight. To test the altimeter, set the current barometric setting while on the ground and compare the indicated altitude to the known elevation.