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A321-26 · Mercury
1970–1972
Object Overview
Presented here is an Altimeter Setting Indicator, Type 1130-038, dating from 1970–1972 and made by the American firm Kollsman to the order of the Brazilian Ministry of Aeronautics. It is a ground-based precision aneroid barometer for an airfield weather station. The instrument gives a direct and continuous indication of the altimeter setting, the so-called "Kollsman number", which is radioed to aircraft. Its task was to give the duty observer or air traffic controller an exact pressure value by which aircrews set their altimeters.
A metal nameplate on the back of the case bears the company name: KOLLSMAN INSTRUMENT CORPORATION / ELMHURST NEW YORK. The name reflects the firm's eventful history. It was founded in Brooklyn in 1928 by Paul Kollsman, an engineer who had emigrated from Germany, together with his brother Otto. Production soon moved to Elmhurst (Queens, New York), and a branch was later opened in Glendale, California. In 1940 the firm was bought by the Square D Company of Detroit, and throughout the 1940s its instruments were marked as products of the Kollsman Instrument Division of Square D. In 1951 Square D sold the division to Standard Coil Products of Chicago, a maker of television tuners. From then on Kollsman Instrument Corporation existed as a New York corporation, a subsidiary of the holding company Standard Kollsman Industries. On 29 December 1972 the holding company was merged into Sun Chemical Corporation, and Kollsman became one of its divisions. The plant at 80-08 45th Avenue in Elmhurst was still in operation in the 1960s.
self-elastic stacked capsule movement
aluminium, brass, copper-beryllium, enamel, glass, steel
The number 1130 designates a family of Kollsman ground-based barometric instruments. The firm's 1954 catalogue describes three models of the indicator: 1130-013, 1130-014 and 1130-015. All of them have scales in inches of mercury, and each covers its own station-elevation range of 3,400 feet. The Brazilian Type 1130-038 appears to be a metric version of the lowest-range model for low-lying airfields, with a millibar scale and inscriptions in Portuguese.
The calibration of barometric altimeters is based on the standard atmosphere. This is a conventional model in which pressure and temperature change with height according to a strictly defined law. The International Standard Atmosphere takes the pressure at mean sea level as 1013.25 mbar (29.921 inches of mercury) at a temperature of +15 °C. Temperature falls with height by 6.5 °C per kilometre, or roughly 1.98 °C per 1,000 feet. An aircraft's barometric altimeter is essentially an aneroid barometer whose scale is graduated not in units of pressure but in the heights that correspond to them in the standard atmosphere. When the actual atmosphere matches the standard one, the altimeter shows what is known as pressure altitude, and this equals the true height above sea level.
The real atmosphere seldom matches the standard one, and this is where errors arise. For example, an unadjusted altimeter at a sea-level airfield under standard conditions will sense a pressure of 1013.25 mbar (29.92 inHg) and indicate a pressure altitude of 0 feet. At an airfield 1,000 feet above sea level it will sense the standard pressure for that height, 977.2 mbar (28.86 inHg), and indicate 1,000 feet. Now suppose the pressure at the sea-level airfield drops to 1012.2 mbar (29.89 inHg). The altimeter will then show the airfield to be about 30 feet above sea level, an error of +30 feet. If the pressure instead rises to 1014.2 mbar (29.95 inHg), the altimeter will show the airfield to be about 30 feet below sea level, an error of −30 feet.
Errors arise in exactly the same way in flight whenever the pressure at a given height departs from the standard value. The standard pressure at 5,000 feet, for instance, is 843.1 mbar. If an aircraft flying at that height enters an area where the pressure has fallen to, say, 837 mbar, the altimeter will read about 5,190 feet. This is precisely the dangerous case: when flying from an area of high pressure into one of low pressure, the aircraft is lower than the instrument shows.
To compensate for errors caused by changes in atmospheric pressure, altimeters are fitted with a manual adjusting device. By turning the setting knob, the pilot shifts the pointers relative to the mechanism and at the same time sets a barometric scale in a small window in the dial. As a result, readings in flight still correspond to heights in the standard atmosphere but are now referenced to the actual pressure at the ground. The required pressure value is passed to the crew by radio by the controller, who in turn reads it from an instrument like the one described here, installed in the airfield's weather station or control tower.
Curiously, both the altimeter and the ground indicator that serves it are linked to the same name. The sensitive barometric altimeter was created in 1928 by Paul Kollsman, founder of the firm that made this instrument. On 24 September 1929, at Mitchell Field on Long Island, Lieutenant James Doolittle made the first flights relying solely on instruments, and the Kollsman altimeter was among the key instruments on board. Shortly afterwards the U.S. Navy ordered 300 of these altimeters. Later models were given a device for conveniently setting the local pressure, and the small window with its barometric scale has been universally known ever since as the "Kollsman window". By the mid-1930s Kollsman altimeters had all but cornered the aviation market. The word took such firm root in professional language that by 1940 pilots in most air forces and airlines, on approaching an airfield, would simply ask the ground for the "Kollsman number".
Setting altimeters to the pressure prevailing at flight levels and airfields is part of flying technique. It is essential for maintaining separation between aircraft and for safe terrain clearance during take-off and landing. Three Q-code groups are used in connection with it:
QNH is exactly what the instrument described here provides. An ordinary station barometer shows the pressure at station level, and the observer has to reduce it to sea level each time. The altimeter setting indicator does this by itself. The station elevation is set once on the small scale at the bottom of the dial, after which the pointer continuously shows the already-reduced pressure, ready to be passed to the aircraft.
The case is cylindrical and cast, apparently in aluminium alloy, and finished in black paint with a textured surface. At the front it widens into a broad flange with holes for panel mounting: the instrument could be set into a controller's console alongside the other standard tower instruments. At the back, a round wall-mounting plate with four screw holes is screwed to the case. The same arrangement allowed the instrument to be hung on the wall of a weather station. The front bezel ring of the flange is fixed to the case by screws around its circumference.
The dial is protected by a thick, flat mineral glass set into the bezel ring on a light-coloured gasket.
At the bottom of the case, on a vertical cast boss, are two ports, one above the other, each closed by a protective plug. One is for connecting a static line; the other gives access to the zero-adjustment screw. Their purpose is described in more detail below.
The dial is large and easy to read. It is a brass disc coated in deep matt black enamel. The scale and inscriptions appear to have been applied by screen printing in yellow-green paint. The combination of a matt black ground and light yellow-green markings gives high contrast, produces no glare and reads well under artificial light, which mattered in a round-the-clock weather station and a darkened control room. A shaped window is cut in the lower part of the dial, through which the station-elevation scale can be seen.
The pointer is long and slender and painted yellow. Its sharp, knife-edge tip reaches the scale graduations, allowing precise readings free of parallax. On the opposite side of the arbor the pointer continues as a long tail that counterbalances it.
Around the dial runs a concentric barometric scale calibrated in millibars from 950 to 1050 mbar. The scale spans about 300 degrees of arc and is divided into one hundred divisions of one millibar each. Every fifth division is lengthened and numbered with its last two digits: 55, 60, 65 … 95, then 05, 10 … 45. The three key marks, 950, 1000 and 1050, are shown in large full figures. At this radius each division occupies about 4 mm of arc, so a reading can confidently be taken by eye to a fraction of a millibar.
The chosen range covers practically the entire real-world spread of sea-level pressure. According to the Kollsman catalogue, the rated accuracy of indicators of this family is ±0.02 inHg, or about ±0.7 mbar, at 25 °C at any point on the scale and at any elevation setting. Across the whole temperature range from 10 to 40 °C, an additional deviation of no more than the same amount is allowed. To give a sense of scale: near the ground, 1 mbar corresponds to roughly 8 m of height.
The name of the unit of measurement, "MILIBARES", is printed large in the centre of the dial. Below the pointer arbor is a block of inscriptions naming the customer:
The station-elevation scale is at the bottom of the dial. Mechanically it works as follows. The scale, graduated in metres, is printed on a separate black disc fixed to a very large fine-toothed gear wheel, which also serves as the base plate of the entire mechanism. This gear meshes with a small pinion connected to the setting screw on the back of the instrument. When the screw is turned, the small pinion rotates the large gear, and with it the whole mechanism together with the pointer, relative to the fixed barometric dial. Shifting the pointer through a given angle is equivalent to adding to the station pressure a constant correction corresponding to the station's elevation, so that the instrument shows pressure reduced to sea level. At the same time, the corresponding section of the elevation scale moves past the window in the dial, and the required elevation is set against it.
The window shows numbered marks at −25, 0 and +25 with divisions of 5 m. Below the window are a fixed index and the inscriptions "5 METROS" (the value of one division) and "ALTITUDE". Lower still, along the arc, is the inscription "GAMA −30 A 1000 METROS", meaning "range −30 to 1000 metres". This is the full range of station elevations to which the instrument can be set. The negative values are needed for stations below sea level, or at sea level once the height at which the instrument is mounted is taken into account.
The Kollsman catalogue explains how the elevation should be set. This is done before the instrument is mounted: the outer sealing screw on the back is removed, and the setting screw is turned until the required value appears in the window. The setting is the elevation of the instrument's location (airfield elevation plus the instrument's height above the airfield) less 10 feet, or about 3 m. The deduction is made because an aircraft's altimeter sits roughly that height above the ground, and it should read the airfield elevation at the moment the wheels touch the runway.
The instrument's principal operational merit is a continuous and highly accurate indication of the altimeter setting. It needs no resetting before each reading, unlike a station barometer or an altimeter, which had to be continually readjusted as the pressure changed. Friction in the mechanism has been reduced to the point that there is no need to tap the case before taking a reading, as was customary with other aneroids.
These qualities are the result of the mechanism's design. The sensing element is a stack of three evacuated aneroid capsules of a copper-bronze colour. Judging by the colour, and by the fact that the capsules work without an external spring, they are made of a resilient copper alloy, most likely beryllium bronze. A light-coloured seam rim runs around the circumference of each capsule. In a classic aneroid, a separate steel spring keeps atmospheric pressure from crushing the evacuated capsule. Here the catalogue specifically stresses that the capsules are springless: the restoring force is provided by the corrugated walls themselves. These are what are known as self-elastic aneroid capsules. This frees the mechanism from an extra link with its own hysteresis and temperature error. Combining three capsules in a stack adds up their travel and increases sensitivity.
The stack is embraced by a U-shaped bracket, from which two thin tapered stays converge on a central boss on the top capsule. They apparently keep the moving centre of the stack from shifting sideways without hindering its travel. The movement of the stack's centre is transmitted through a vertical link to a horizontal lever. The lever carries a cylindrical counterweight on an adjustable clamp, which balances it and reduces the effects of the instrument's position and of vibration. From there the motion passes through a high-precision gear train, mounted in a massive upper bridge, to the pointer arbor.
A key feature of the design, to which the Kollsman catalogue attached particular importance, is the abandonment, in part of the mechanism, of conventional pivots in bearings and jewelled bearings. In their place is a suspension of original design using flat elastic strips: the moving links hang on thin spring plates and rotate by flexing them rather than through the friction of pivots in holes. Such a suspension has neither sliding friction nor play, and the overall friction in the mechanism becomes negligible. This accounts for the instrument's main characteristics: the pointer responds to pressure changes of a fraction of a millibar and does not "stick", and tapping becomes unnecessary.
Another feature noted in the catalogue concerns calibration. The mechanism is calibrated to a standard printed dial rather than being individually matched to each dial, as earlier instruments required. This ensured interchangeability of parts and simplified servicing. The same feature presumably allowed the firm to produce metric dials with inscriptions in the customer's language without difficulty.
One of the two ports at the bottom of the case is an outlet with a ⅛-inch female pipe thread for connecting a static line. Atmospheric pressure inside a room often differs noticeably from the pressure outside. This is especially true of a heated control tower on a cold, windy day: the wind creates zones of higher and lower pressure on the walls of the building, and heating and ventilation add a pressure difference of their own. Even a small discrepancy of a fraction of a millibar produces an error of several metres on the altimeter of a landing aircraft. The catalogue therefore strongly recommends connecting the barometer directly to the outside atmosphere.
To do this, the protective plug is removed from the port, a tube fitting is screwed into the thread, and the tube is led outside. The end of the static line is placed in relatively undisturbed air and protected from sun and rain. In this way the aneroid stack senses the true outside pressure, regardless of conditions inside the building. Atmospheric air from the airfield reaches the inside of the case through the tube.
The second port, located next to it, closes off access to the zero-adjustment screw. This screw is used for final calibration of the mechanism at the factory and for minor readjustment should the calibration shift slightly over the years as the mechanism ages. The catalogue expressly warns that it must not be confused with the station-elevation setting screw. To reach it, the plug is removed and the mechanism is rotated with the setting screw on the back of the case until the adjustment screw lines up with the hole.
The Kollsman Altimeter Setting Indicator brings several stories together. It is an instrument made by the firm of a man whose name became a byword in aviation: the Kollsman window is found on every analogue altimeter, and for decades pilots all over the world asked the ground for the Kollsman number. It is an example of mid-twentieth-century American precision instrument-making: a springless aneroid stack, a suspension on elastic strips instead of pivots and jewels, calibration to a standard printed dial, and an ingenious way of allowing for station elevation by rotating the entire mechanism. And finally, it is a document of Brazilian administrative history. The heading on the dial names bodies that existed in this combination for only about two years, and so dates the instrument more precisely than any factory mark.
The instrument's task was modest yet responsible: to show, continuously and without any intervention by the observer, a single number that the controller passed to an aircraft on approach. On the accuracy of that number depended whether the crew's altimeter would show the true elevation of the airfield at the moment of touchdown.