Altimeter
A barometric altimeter consists of a barometric capsule linked to a pointer by a suitable mechanical or electronic system. The pointer moves across the dial in response to changes in barometric pressure. The dial is calibrated in feet, or (less commonly) in metres.
Barometric altimeters are provided with a pressure setting control and sub-scale (Kollsman window) so that the altimeter may be calibrated according to the appropriate pressure setting to indicate flight level, altitude above mean sea level, or altitude above ground level.
The altimeter provides an output to the transponder system to enable the transmission of the flight level or altitude to the air traffic control.
Barometric altimeter displays may be of two main types:
· Conventional analogue display;
· Electronic display.
The main types of conventional altimeter which have been used in aircraft are:
· Three-pointer altimeter;
· Drum-pointer altimeter;
· Counter-pointer altimeter; and,
· Counter drum-pointer altimeter.
The three-pointer altimeter and the counter drum-pointer altimeter are illustrated below.
It has been found that the displays of three-pointer altimeters, drum-pointer altimeters and counter-pointer altimeters are capable of being mis-read and several accidents have been attributed to this cause. Accordingly, counter drum-pointer altimeters are the the only type currently approved for use in commercial aircraft. In modern aircraft, conventional instruments are used mostly as standby instruments.
Modern aircraft are usually equipped with composite Electronic Flight Instrument System displays which combine the functions of several conventional instruments into one. Presentations vary according to the manufacturer's design philosophy. The illustration below shows a typical EFIS display in which the altitude is depicted on a vertical tape to the right of the attitude indicator. In the illustration, the altitude is 5100 ft. The altimeter pressure setting (29.92 in Hz) is depicted in green below the altitude tape.
