Digests of Papers on Aircraft Operations

Abstract

Use of Barometric Pressure for Setting Sensitive Altimeters in Airport and Airivay Traffic Control. The increasing complexity of the problems involved in the control of airport and airway traffic has necessitated the adoption of a uniform scheme for the setting of sensitive altimeters throughout the country. This scheme, following the proposals of the Weather Bureau at a Conference on Airport and Airway Traffic Control held on November 12-14, 1935, under the auspices of the Bureau of Air Commerce, Department of Commerce, contemplates the use of the pressure measuring instruments employed in the airway weather service (both mercurial and aneroid barometers) for the purpose of obtaining pressure settings pertinent to the various airports, such that if a sensitive (Kollsman) altimeter in an airplane is set accordingly, the airplane can land at an airport to which the setting pertains just as the altimeter hands indicate the surveyed elevation of the landing field above sea level. In this paper the details of the above-mentioned proposals are presented. It is first to be pointed out that, assuming radio communication between an observer at the ground and the pilot in the airplane, the use of sensitive altimeters in blind landing and blind cross-country flying presents four distinct problems: (1) the determination of the barometric pressure at the level of the altimeter when the airplane is just at the point of landing on the ground, (2) the precise calibration of the altimeter in accordance with the Standard Atmosphere relation of pressure to height, (3) the determination (or the knowledge) of the elevation of the terrain above sea level in the general neighborhood of the airplane and with reference thereto, and (4) the determination of the mean temperature of the air column between the airplane and the ground beneath. In regard to the determination of barometric pressure near the ground, it is shown that the mercurial barometer is generally superior both to the aneroid barometer and to the altimeter owing to its greater precision, stability and relative freedom from the various errors inherent in instruments which depend for their indications on the expansion or contraction with change of air pressure of evacuated diaphragm capsules or similar devices. The mercurial barometer thus forms a more suitable standard than either of the other instruments. The aneroid barometer when used in conjunction with a mercurial barometer is found to be advantageous for the purpose of obtaining the desired results expeditiously during intervals intermediate between the times when the readings of the former instrument are compared with the readings of the latter and corrections to the aneroid are thus obtained. The pressure measuring equipment involved here is already available at a number of stations in the airway weather reporting service. It is pointed out that with the maintenance of the customary precision employed in the calibration of sensitive altimeters, the procedure under consideration which involves the use of this equipment may be conveniently applied to the checking of altimeters in the field. This is seen to provide an important advantage over the methods hitherto used. The fundamental mechanical characteristic of the sensitive altimeter is then outlined, viz., that given an appropriate setting therefor, an altimeter can be made to indicate any preassigned altitude reading when brought to a given air pressure, and thence it is shown how the desired setting may be easily determined with the aid of the Standard Atmosphere tables once the air pressure just above a landing field is known and an altitude is preassigned thereto. It is demonstrated that for the sake of speed in procedure it is best to prepare a table of these settings in advance using as the sole argument the air pressure just above the field at the average level of altimeters in landing airplane, viz., about 8 feet.