The presentation of altitude information has undergone many changes in recent years, principally as a result of altimeter misreading being the proven or suspected cause of a number of fatal accidents. In consequence, several methods are to be found in altimeters currently in use, the most notable of which are the triple-pointer, single-pointer and digital counter, and single-pointer and drum presentations. The triple-pointer method is the oldest of presentations and is the one which really made it necessary to introduce changes. This method is used in the altimeter shown in Fig 3.3 the susceptibility of its predecessor to misreading of 1,000 ft and 10,000 ft, has been overcome to a large extent by giving the pointers a more distinctive shape, and by incorporating the trace disc referred to earlier. In addition, some versions incorporate a yellow and black striped disc which serves as a low-altitude warning device by coming into view at altitudes below 16,000 ft.

An altimeter can measure two types of altitude. They are as follows:
i) Indicated altitude,
ii) Pressure altitude.
(i) Indicated altitude:
An altimeter, which measures the pressure above the existing sea level, this pressure measurement is called indicated altitude.
(ii) Pressure altitude:
The altimeter, which measures altitude above standard sea level, this pressure (29.92 in. Hg or 1013 millibars) measurement is called pressure altitude.
The setting of altimeters to the barometric pressures prevailing at various flight levels and airports is part of flight operating techniques, and is essential for maintaining adequate separation between aircraft and also terrain clearance during take-off and landing. In order to make the settings flight crews are dependent on observed meteorological data are requested and transmitted from air traffic controls. The requests and transmissions are adopted universally and form part of the ICAO ‘Q’ code of communication.
There are three code letter groups are normally used in communication with altimeter settings and they are defined as follows:-
(i) QFE: – Setting the pressure prevailing at an airfield to make the altimeter read zero on landing and take-off.
(ii) QNE: – Setting the standard sea-level pressure of 1,013.25 mbar (29.92 in Hg) to make the altimeter read the airfield elevation.
(iii) QNH: – Setting the pressure scale to make the altimeter read airfield height above-sea level on landing and take-off.

The altimeter is the only instrument specifically singled out in the Federal Aviation Regulations as requiring a test. Every 24 calendar months, every altimeter and every static system of airplanes used for instrument flight rules (IFR) flying must be checked according to the tests prescribed in F.A.R 43, appendix E. These tests include;
3.6.1 Scale error:
The barometric scale is set to 29.92 in. Hg and the instrument subjected to pressure corresponding to a series of test altitudes. The instrument must not have a scale error in excess of allowed in table 3.1.
3.6.2 Hysteresis:
This test is made to determine that the instrument will be within tolerance between a reading taken when the altitude is increasing and one taken when the altitude is decreasing. Hysteresis is essentially a lagging of the indication caused by the deflection of the metal in the diaphragms not keeping up with the pressure changes.
3.6.3 After-effect:
This error shows up by the altimeter not returning to its original reading after the hysteresis test has been performed. It is the effect of the “set” the diaphragms have taken.
3.6.4 Friction:
All non-servo altimeters have enough friction that some form of vibration is needed for their accurate reading. This test determines just how much friction the instrument has. A reading is taken before and after the case is vibrated.
3.6.5 Case leak:
The case is tested at 18,000 feet pressure to be sure it does not leak more than 100 feet in one minute.
3.6.6 Barometric scale error:
This test determines that the movement of the barometric scale has the proper effect on the pointers.

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Author: Sir Godfrey Gregg

Sir Godfrey Gregg is one of the Administrators and managing Director of this site
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