In the fog you might think you were approaching the airfield at a greater altitude than you actually were ... then .........
The reporting of altimeter setting information by an airport traffic control tower and/or by ATC Regional Centers is a routine requirement for safe en route (altitude separation) and terminal area (approach) navigation and has been a standard procedure for many years. Presently, each control tower has one or more precision aneroid or bourdon tube type barometric pressure sensing altimeter setting indicators (ASI) having accuracies on the order of ± 0.020 to ± .040 inches of mercury. Each sensor has to be calibrated periodically in accordance with its own calibration characteristic and this calibration "number" must be "greasepencilled" periodically on the face of the instrument and taken into account in the computation of the reported altimeter setting number. This task is difficult, time consuming and subject to human error. The altimeter setting indicator reading from such instruments is not readily converted into digital format for transmission throughout an airport for remote indication. It is often the situation that different calibration numbers must be added or subtracted from the instrument reading with respect to the various instruments at the airport, often resulting in discrepancies between altimeter settings communicated to aircraft from different sources at the same airport.
Schiphol airliner crash blamed on altimeter failure, pilot error
Investigators in The Netherlands have announced that last month's crash of a Boeing 737-800 was caused by a combination of an instrument failure and pilot error. Turkish Airlines Flight 1951 crashed upon approach to Amsterdam's Schiphol Airport, killing nine and injuring eighty of the 135 passengers and crew on board.
Pieter van Vollenhoven, head of the Dutch Safety Board, said yesterday that as the aircraft was at 1,950 feet the faulty altimeter indicated that the airliner's height was at minus eight feet. Because the autopilot and autothrottle were running from this flawed data the plane automatically reduced engine power as it would in the final seconds before landing.
The sensitive altimeter
The sensitive altimeter is the cockpit instrument that indicates the aircraft's altitude. The instrument is a refined aneroid barometer with a dial indicating height above a pre-set level rather than atmospheric pressure. The main component of such an instrument is a small, flexible, corrugated metal capsule from which the air has been partially evacuated — fitted with a metal closure or diaphragm. There is a spring within the capsule that applies a constant force to the bottom of the diaphragm, while atmospheric static pressure applies a counter force to the top, so that the diaphragm moves as atmospheric pressure changes. The movement of the pressure-sensing capsule is transferred and magnified — via a mechanical linkage or piezo-quartz component — to a dial pointer or pointers, or a digital display, which indicate the altitude reading. The static pressure is drawn from the aircraft's static vent, which may induce slight position errors due to aerodynamic effects around the vent.
The level in the atmosphere at which any particular pressure occurs is also dependent on temperature — as we saw in the 'Airspeed and the properties of air' module — but the altimeter does not sense the air temperature. Consequently, all altimeters are calibrated in accordance with the International Standard Atmosphere [ISA] model, which utilises a standard temperature lapse rate with height of 6.5 °C per km. The atmosphere in any region rarely corresponds to the ISA, so aneroid altimeters do not indicate totally accurate height. This is not that important, as true altitude can be calculated, in the rare circumstance that it is needed for terrain clearance purposes. There is no problem with air traffic management, in that all aircraft in the same region, with properly set (and functioning) altimeters, will be out by the same amount.
It is, of course, desirable to set the current local surface pressure into the altimeter by setting that reference pressure into a pressure-setting scale (known since the 1930s as the 'Kollsman Window'), which in turn resets the position of the height-indicating pointers against the dial. Or, if the aircraft is on the ground, the same result is achieved by turning the pressure-setting scale until the altimeter indicates the known airfield elevation. The altimeter in the image indicates an altitude of 1400 feet with the baro-scale set at 29.9 inches of mercury [in/Hg] — equivalent to 1013 hPa. If the altitude was 11 400 feet, the pointer with the inverted triangle on the end would be past the figure 1 on the image, indicating +10 000 feet.
In Australia, all barometric pressures are reported in hectopascals [equivalent to millibars]; and in the USA in units of inches of mercury [in/Hg =33.86 hPa]. The sub-scale setting range provided in modern altimeters is from 850 to 1050 hPa.
Altitude encoding