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How A WAAS / LPV Approach Counteracts Temperature Effects

September 2012

The altimeter reading in an aircraft with Air Data Computers displays the altitude based on barometric pressure. Temperature plays a role in the accuracy of this information. When the air is cold, it is condensed and the altimeter reading will be less than the true altitude. Conversely, in hot weather, the uncompensated altitude will bring the aircraft higher than the published alt constraint. That is why settings must be adjusted by the crew based on the readings provided by Air Traffic Control (ATC) or the controlling agency for the area, in which they are flying. This adjustment can be 100-300 feet depending on the temperature.

During an approach with Wide Area Augmentation System (WAAS) / Localizer Performance with Vertical Guidance (LPV), the initial vertical guidance is computed by the Flight Management System (FMS) based on the current altitude at which the aircraft systems say it is flying. At some point during the approach, the vertical guidance computed by the Air Data Computers will transition over to vertical guidance from GPS satellites. Once this transition occurs, the Air Data Computers are removed from the equation and the altitude error, caused by extreme temperatures, is no longer a factor. With GPS, the descent is merely a mathematical equation based on current GPS altitude and selected three dimensional approach path.

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