How High Are We Flying?

Have you ever wondered what keeps pilots "on track" in the sky? A pilot must know how high a plane is flying as well as its direction of travel. Key in measuring altitude, or the plane's elevation, is the altimeter, since the aircraft must maintain a safe distance from terrain and other obstacles. How does the altimeter function and how does the information it provides help the pilot safely conduct the flight?

Depending on the reference point used, there are several types of altitude. The most commonly used is true altitude, or an aircraft's height above mean sea level. Height "above ground level" (AGL) is, quite simply, the absolute or actual altitude of an aircraft. Pressure altitude, also used in flight planning, represents the height of the aircraft above standard sea level pressure. Aircraft flying above 18,000 feet use "flight levels" (FL), which represent pressure altitude in hundreds of feet. For example, FL 210 represents a pressure altitude of 21,000 feet, FL 240 is 24,000 feet, and so on.

The pressure at sea level, about 15 pounds per square inch, is caused by the weight of the atmosphere. As altitude increases, the associated pressure decreases, although not in a linear manner; decreases occur in smaller and smaller amounts. Atmospheric pressure decreases by one-half for every 20,000-foot rise in altitude. At 20,000 feet the pressure is about half of that at sea level, so at 40,000 feet (cruise altitude) the pressure is approximately one-quarter of that at sea level. The height of the aircraft can be determined by measuring this decrease in pressure.

As high and low pressure systems move overhead, changes occur in the pressure at sea level. So why is the altimeter not tricked by these constant changes? The pilot is able to set the altimeter to the existing sea level pressure; the altimeter then indicates how high the aircraft is above this pressure level. Air traffic controllers are constantly monitoring the changing barometric pressure and relaying this information to pilots.

The aneroid altimeter is very similar to a barometer. Inside, sealed wafers expand as the pressure around them decreases. The pressure inside the altimeter is vented to a static port on the outer surface of the aircraft so that it can measure the outside pressure.

A highly accurate device known as a radar altimeter is used when the aircraft is close to the ground. A radar signal is bounced off the terrain below to determine the aircraft's AGL height. This information is used in a ground proximity warning system that alerts the pilot before the altitude becomes dangerously low. Future systems will benefit from the global positioning satellite system (GPS) because an airborne GPS receiver can determine position as well as altitude. A computer database that is part of the system stores the elevation of terrain and obstacles and constantly monitors the safe separation of the aircraft.

Because maintaining a safe distance from obstacles is of utmost importance to aviation safety, jet transport systems use three independent altimeter systems. Altitude information is backed up by a radar altimeter and ground proximity alerting system. Even though the current system is incredibly safe, future innovations using satellites and computers will undoubtedly enhance air safety even further.

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