Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 25. Altimeters

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXIII - AIR SPEED INDICATORS

116. Purpose and use.- a. AItimeters are used for two distinct purposes in aircraft:

(1) To measure the elevation of the aircraft above some point on the ground regardless of its elevation above sea level. This method of altitude measurement is called the "field elevation pressure" system. The "field elevation pressure" is the existing atmospheric pressure at a point 10 feet above the mean elevation of the runway and is obtained by applying a suitable correction to the station pressure. It is assumed that the altimeter in an airplane is 10 feet higher than the landing surface.

(2) To measure the elevation of the aircraft above sea level. This method of altitude measurement is called the "altimeter setting" system. The "altimeter setting" is the atmospheric pressure in inches of mercury and is the existing station pressure reduced to sea level in accordance with the U. S. Standard Atmosphere (N. A. C. A. Report 538).

b. The specific uses of the field elevation pressure system are to

(1) Determine accurately the vertical distance between the air-craft and objectives on the ground when performing tactical missions.

(2) Indicate the elevation of the airplane above the runway for coordination with other instrument indications when making instrument landings.

c. The specific uses of the altimeter setting system are to

(1) Indicate at all times the elevation of the airplane above sea level so that it can be compared with strip maps for the purpose of clearing critical points and mountain peaks safely.

(2) Use advantageously the meteorological data supplied by weather stations such as wind velocities and directions for various elevations, and elevations of cloud and storm formations that are to be avoided in flight.

(3) Observe and correctly follow airways traffic regulations.

(4) Furnish information for changing the blade settings of controllable pitch propellers.

117. Description.-a. All altimeters used on military aircraft are of the sensitive type shown in figure 65(1), utilizing an aneroid mechanism. This mechanism is very sensitive and well-balanced, jeweled bearings being used at all principal pivots to reduce friction. A temperature compensator is included in the mechanism and partially corrects for the mechanical error resulting from changes in temperature. Figure 66 shows a typical sensitive altimeter mechanism wherein D is the aneroid and F the temperature compensating unit which connects to the aneroid by T1 and T2. The expansion of the aneroid D through linkage causes the rocker shaft R to turn. Sector S, which is part of rocker shaft R, engages a pinion and gear causing the movement of the indicating hands over the engraved dial.

(1) Sensitive altimeter.

(2) Altimeter scale correction card
FIGURE 65

b. Standard models of altimeters for tactical operation have a calibrated range of from 1.000 feet below sea level (-1,000 feet) to 35.000 feet above sea level (+35,000 feet). By use of a multiple pointer system, the instrument can be read accurately to at least one-half the smallest unit graduation on the scale. The later types of altimeters have one altitude scale, one barometric scale with an index marker, two reference markers. and three pointers as shown in figure 67. The altitude scale, having major divisions of 0 to 10, is fixed and the pointers, reference markers, and barometric scale rotate and indicate with reference to it. A setting knob located at the bottom front of the instrument case drives two pinions in opposite directions. One of these pinions rotates the barometric scale and reference markers and the other pinion rotates the aneroid mechanism assembly of which the pointers are a part. The three pointers or indicating hands are concentrically arranged and indicate on the one common dial. The long pointer is referred to as the "minute hand," the intermediate pointer as the "hour hand," and the small pointer as the "second hand." The minute hand makes one revolution for a change of 1,000 feet. the hour hand makes one revolution for a change of 10,000 feet, and the second hand would indicate 100,000 feet for one revolution if it were not restrained. To cover the full range of the insetrument, the minute hand makes a total of 36, the hour hand 3.6. and the second hand 0.36 revolutions. The position of the reference markers are read on the same dial as the pointers. The outer one which rotates on the periphery of the dial makes one revolution for 1,000 feet and the inner one makes one revolution for 10,000 feet. The standard range for the barometric scale is 28.1 to 31.0 inches Hg. with unit graduations of 0.02 inches of Hg. When the limit of the range of the barometric scale is reached at either extreme, a shutter blanks out the indication of the barometric dial and the barometric pressure is read from the position of the reference markers by computation. By introducing a limited range barometric scale, the actual unlimited possibilities of setting barometric pressure by means of the reference markers is not in any way affected.

FIGURE 66.- Internal mechanism of the sensitive altimeter.

c. The entire mechanism is enclosed in a two-piece bakelite case which is provided with a vent in the rear case wall for connection to the static line of the pitot static system. Since this instrument must be sealed airtight, its only opening is through the static vent. Due to the fact that the mechanism is mounted in a bakelite case which is a nonconductor of electricity, a suitable means is provided to bond electrically the mechanism to the airplane structure. This is accomplished by attaching a phosphor-bronze spring to the brass insert on the back of the case, which takes the nipple for the static connection and makes a sliding contact with the mechanism when it is rotated by the setting knob. The present standard sensitive altimeters are provided with a 3-volt ring light and reflector, the receptacle being molded integral with the instrument case. Later type sensitive altimeters eliminate the 3-volt ring light and instead are designed for fluorescent lighting.

118. Operation.-a. The altimeter being an absolute pressure-indicating instrument, its operation is obtained by the changes in the existing atmospheric pressure. This makes the instrument practically automatic in operation. The gear ratio of the hands being ten to one gives a multiple pointer system of extreme sensitivity together with the high ratio multiplying mechanism in the linkage system. When installed in an airplane, it is essential that the aneroid

be subjected to the same existing atmospheric pressure as that through which the airplane is flying and the air must be undisturbed (static). This is especially true on aircraft utilizing a sealed cabin wherein pressures are carried in excess of the altitude through which the craft is flying. This extreme change in pressure, if allowed to leak into the static line, is capable of causing the altimeter to indicate not the pressure through which the craft is flying, but rather the pressure within the craft. Consequently the altimeter must be vented to the static line and the entire installation must be free of all leaks.

b. Operation of the altimeter will depend upon the nature of the work and the mission that the airplane is to perform. There are two systems that are adaptable when using the sensitive altimeter and with each system there are two methods of setting off variation; that is, in feet of altitude with the reference markers or in inches Hg. on the barometric scale. The reference markers are used only on tactical missions such as bombing. Operation of either system is dependent upon radio contact with the ground stations which are equipped with either station altimeters or very accurate barometers.

119. Installation.  All general points on the installation of air-craft instruments that are applicable to altimeters are given in section III. The altimeter must be connected to the static line of the pitot static system. This will insure correct operation as long as the pitot static tube is correctly alined with the direction of flight, provided there are no leaks in the entire system.

120. Maintenance.-a. The general points on inspection and maintenance of instruments given in section II are applicable to altimeters.

b. The following specific points and procedures are applicable to standard altimeters:

(1) Just prior to take-off, the altimeter is set for use on either the field elevation pressure or altimeter setting system. For flight of a local nature or practice in instrument landings, the knob is turned so that the pointers indicate zero. On this setting, the reading indicated on the barometric scale will be the local barometric pressure in inches Hg. Where cross-country flights are anticipated, the knob is turned so that the pointers indicate the surveyed elevation of the field from which the take-off is to be made. On this setting, the reading indicated on the barometric scale will be the local pressure reduced to sea level altimeter setting.

(2) Check setting knob for free and easy turning, also see that all pointers, reference markers, and the barometric scale move when setting knob is turned.

(3) To check the zero setting error, obtain the reading from the station altimeter. Set pointers of the airplane altimeter to zero; vibrate panel two or three times while making this setting, then check reading of reference markers. Their reading should be the same as the pressure altitude on the station altimeter plus or minus 30 feet. If the error exceeds the permissible tolerance of plus 30 feet, loosen small screw located just to the left of setting knob. (This screw does not come out; excessive pulling will seriously damage the instrument case.) After the screw is loosened, push it and lock pin to which it is attached all the way to the left as far as it will go. Then pull out on setting knob and turn, so that the reference markers correspond with the pressure altitude obtained from station altimeter. Push knob in, reseat lock and screw. It is to be noted that when checking and setting the reference markers, the corrections on the scale correction cards of the airplane and station altimeters must be taken into consideration.

(4) To check the case for leaks, disconnect static connection at the instrument; place a short piece of rubber tubing over the vent marked "S"; with mouth pressure only, decrease the altitude indication by only 500 feet. Pinch off the tubing and watch the pointer for 10 seconds. The pointer position should not change more than 20 feet during this interval.

c. (1) The aneroid mechanism of an altimeter is also affected by expansion and contraction due to changes in temperature. Even though each instrument is compensated for temperature changes, an error remains which varies at different temperatures throughout the range of the instrument. These errors must be within the tolerances specified in the Technical Order for the particular instrument.

(2) Figure 65(2) shows a scale correction card which contains scale corrections for a particular altimeter. The card affords a visual means of determining the magnitude of the remaining errors due to instrument temperature change. The scale corrections are scale errors with the signs reversed, to be added to or subtracted from (as indicated by the sign) the reading of the altimeter. For example, if the altimeter reads 2,000 feet, a correction of 35 feet must be made for a temperature of +25° C., and accordingly, the corrected altitude is 2,035 feet. For instrument temperatures between the values of -25° C. and -35° C. (temperature limits shown on correction card), interpolation is necessary. For example, the scale correction at 2,000 feet is +35 feet at +25° C. and is +50 feet at -35° C.; therefore there is a variation in the scale correction at this altitude between

these two temperature of 50 minus 35, or 15 feet. Accordingly, an approximate correction of 15/60 or 1/4 foot is necessary for each degree change in temperature. If the temperature at 2,000 feet were 0° C,. a correction of  1/4 x 25, or approximately 6 feet. would be necessary in addition to the value of 35 feet, or a total of 41 feet: therefore the corrected altitude is 2,041 feet. This method is applied for any altitude and temperature within the ranges specified on the scale correction card.

(3) The scale correction card is placed in a holder as near as is praticable to the altimeter to which it refers. When calibrating the altimeter in an airplane, the scale correction card should be consulted. When an altimeter is replaced, the scale correction card is removed from the holder and attached to the instrument.

(4) Only those scale errors determined by an accurate mercurial barometer at the time the altimeter is calibrated for acceptance or after repair are used in determining scale corrections for entry on the card.

(5) Should it become necessary to use the old type altimeter, complete information may be secured in section V. paragraph 3b. Technical Order 05-30-1,

FIGURE  67.- Altimeter setting system.