The altitude indicator tells the pilot how high the aircraft flies with regard to a set barometric pressure or level. This is usually mean sea level (MSL or QNH) but when flying at flight levels the level is 1013 hPa/mbar or 29.92 inHg (QNE) and around mountain areas field elevation is sometimes used (QFE).
As the aircraft is moving in three dimensions the altitude indicator or altimeter is primarily important for obstacle clearance by the pilot. Setting the correct barometric pressure is therefore equally important as a difference of 1 hPa/mb is some 27 feet change in altitude.
Thus for safety reasons all aircraft in the same area should set the same QNH in the sub window.
Here we are going to take a look inside this altitude indicator and see how it operates and what limitations these instruments may have that you must be aware off.
In the image to the right we can see that the basic altimeter is barometric pressure sensor calibrated to indicate altitude. It contains one or more vacuum aneroid capsules, a sealed case and a leak free connection to the static port.
During climb the pressure in the instrument case changes and the aneroid capsule will expand (climb) and this change is shown on the instrument face as a change in altitude. Setting the correct barometric pressure with the knob rotates the internal mechanism driving the three pointers.
Most instruments have three pointers and can indicate up to 20000 ft. Two pointer instruments go up to 10000 feet. The subscale settings is either in inHg / hPa or even with dual scales should you wish such a type. Some altimeters have a striped sector indicating that you are below 10000 feet./
Altitude indication is usually in feet but some indicators are also calibrated in meters. Most commonly gliders and some foreign east European aircraft.
The knob on the altitude indicator sets the barometric pressure known as: QNH, QNE or QFE. The MSL setting (QNH) is very helpful as all obstacles on maps and chart are also noted in regard to mean sea level pressure. It is thus very easy to determine our height above ground when the altimeter is set to MSL and this helps to remain clear of obstacles.
When flying from a local or rural airport the altimeter is set to local QNH, either obtained from flight information, ATIS or tower. When this information is not available set the altitude of the airport (source AIP) with the subscale knob. As soon as you are airborne contact the nearest flight information service (FIS) within 100 nm and obtain the regional QNH.
This is the standard flight level setting of 1013 hPa or 29.92 inHg. With this setting the altimeter shows pressure altitude or altitude in the international standard atmosphere (ISA). The altitude where you need to set QNE depends on the country where you fly, check the AIP for that.
This is the setting at any other datum than QNH. Setting QFE gives you your height against a certain datum, usually the mountain airport you are flying to or from. After landing the altitude indicator will show 0 (zero). Mostly used in mountain areas but it is not very commonly used outside those areas.
The altimeter is sensitive to pressure changes, as such any outside pressure change will affect the instrument. The pilot will not know it as he is following a set level which moves up and down together with changes in outside pressure. Keep in mind that every hPa change is some 27 feet altitude difference, hence the importance to regularly get updated QNH settings.
When flying from a high pressure area to an area of lower pressure the aircraft will slowly descent and loose absolute height without showing that on the altimeter. This is a so called over reading and can be dangerous. Thus: from high to low, look out below, and reset your QNH setting!
Changes in outside air temperature (OAT) will also affect the instrument as the pressure lapse rate is slower in warm air than in cold air. Warm air expands and the pressure levels are further apart. When flying from an area with higher temperature to lower temperatures the aircraft will descent. Again: from high to low, look out below!
Other errors have their origin in mechanical issues as: manufacturing imperfections, static vent position error and even instrument lag as their is some inertia in the mechanics moving the pointers with rapid pressure changes as happens while climbing or descending rapidly.