WEATHER By Jack Williams LEADVILLE, COLORADO (LXV) at 9,927 feet is the highest elevation general aviation airport in the country. VITAL BUT NOT ALWAYS TRUSTWORTHY YOUR FRIEND THE ALTIMETER t first glance an aircraft’s altimeter seems pretty simple. It tells you how high you are. What could be easier? It’s not as simple as that. An aircraft altimeter is supposed to show the height above mean sea level. A new pilot could ask: “I’m flying out of Scottsbluff, Nebraska, where the runway is 3,967 feet above sea level. Why should I care about how high I am above sea level?” A ATMOSPHERIC PRESSURES LOWER THAN THE SURROUNDING PRESSURE ARE ASSOCIATED WITH FOUL WEATHER. Using mean sea level as the zero altitude is more than an arbitrary rule; it makes the most sense. You would never want to try to remain at a particular distance above the ground, especially when flying over mountains. And the air traffic control system would not work without controllers being able to assign particular altitudes that aren’t changing based on a distance from the hills and valleys below. To measure the height above the ground, you need a radar altimeter. Its antenna sends microwaves down and uses the time it takes for them to reflect back at the speed of light to calculate height above the ground. Radar altimeters work well only relatively close to the ground. Air carriers use radar altimeters for low-visibility landings and as a part of autoland systems. Here a radar altimeter “tells” the autopilot when to begin flaring for a landing and controls the autothrottles. Military pilots use radar altimeters to follow terrain as they fly mere feet above the ground or water to avoid radar detection. The frequencies used for radar do not follow the curve of the Earth. The lower an aircraft is flying the closer it can approach a radar antenna while remaining below the radar’s horizon. 44 / FLIGHTTRAINING.AOPA.ORGhttp://FLIGHTTRAINING.AOPA.ORG