Altimeter Errors.pdf

NASA Technical Note D-465 measures the errors of service altimeter installations during landing-approach and take-off operations. The study analyzed data from 196 airplanes during 415 landing approaches and 70 airplanes during 152 take-offs. The average height above the runway was approximately 280 feet for landings and 440 feet for take-offs.

The overall error of the altimeter installations had a probable value of ±36 feet and a maximum probable value of ±159 feet with a bias of +10 feet during landing-approach. During take-off, the overall error had a probable value of ±47 feet and a maximum probable value of ±207 feet with a bias of -53 feet.

The study found that the overall errors of military airplanes were generally larger than those of civil transports in both landing-approach and take-off conditions. The bias values of the error distributions appeared to represent a measure of the hysteresis characteristics and friction of the instrument and the pressure lag of the tubing-instrument system.

The test method involved comparing the altimeter indication with the correct pressure altitude at the elevation of the airplane, determined from the geometric height of the aircraft above a ground station. The geometric height was computed from the length of the wing span on photographic film, the actual wing span, and the focal length of the lens. The results provided a measure of the accuracy of service altimeter installations during routine landing and take-off operations.