Altimeters

A number of different types of pressure altimeter are manufactured; however, they differ in detail depending on the altitude band covered, the accuracy of the instrument, and the method by which the altitude is displayed. Types of display vary from multi-needle to needle plus digital counters, with accuracy varying from 100 feet at 0 feet to 1,000 feet at 35,000 feet in early models, to 35 feet at 0 feet to 600 feet at 60,000 feet in later models.

The Simple Altimeter

The altimeter is an instrument that is designed to measure static pressure and, using the conditions of the standard atmosphere, convert that pressure into a value of altitude, eg. if the pressure measured is 472 hPa, then a calibrated altimeter will indicate 19686 feet.

A simple or non-sensitive altimeter comprises of a partially evacuated barometric (Aneroid) capsule, a leaf spring, a mechanical linkage and a pointer, as shown below.

These components are all housed in a container, which is supplied with static pressure from the static vent system. As an aeroplane climbs the static pressure will decrease, which will cause the capsule to expand and drive the pointer to indicate a higher altitude. As an aeroplane descends, the capsule will compress due to the increasing static pressure, and the pointer will indicate a lower altitude.

The leaf spring in the instrument is designed to prevent the capsule from collapsing and acts as a balance between the capsule and the static pressure. The altimeter is normally calibrated at ISA +15°C and 1013.2mb (hPa) until it reads zero, although the datum can be adjusted via the sub-scale dial setting knob. This allows the instrument to be adjusted for different values of mean sea level (msl), and heights above an aerodrome at which the altimeter will read zero.

Datum Sub-scale Settings

The setting of altimeters to datum barometric pressures forms part of the flight operating procedures, and is essential for maintaining adequate separation between aeroplanes and terrain clearance during take-off and landing. These settings have been adopted universally and form part of the ICAO "Q" code of communication, which consists of three-letter groups, each having "Q" as the first letter. The codes normally used in relation to altimeter settings are:

QFE

The pressure prevailing at an airfield, the setting of which on the altimeter sub-scale will cause the altimeter to read zero on landing and take-off.

QNE

Setting the standard mean sea level pressure of 1013.25 hPa to make the altimeter read the airfield elevation, i.e. the equivalent height in ISA above the 1013.25 hPa pressure level. When QNE is set, the altimeter will indicate the pressure altitude, which is the reported flight level. Flight levels occur at 500 ft intervals and are calculated by dividing the pressure altitude by 100, e.g. a pressure altitude of 10,000 ft will equate to FL 100.

QNH

This is the actual msl pressure. Setting the pressure scale will cause the altimeter to read the airfield altitude above sea level on landing and take-off. This setting is also handy for checking the height above terrain or a radio mast.

If the aeroplane remains at the same altitude winding on hecto Pascals or millibars will wind on more height and vice versa.

Design Errors

Altimeters suffer from the following errors:

Instrument Error

Since capsule movements must be greatly magnified, it is impossible to ignore the effect of small irregularities in the mechanism. Certain manufacturers' tolerances thus have to be accepted, and errors generally increase with altitude.

Pressure Error

Pressure errors arise because the true external static pressure is not accurately transmitted to the instrument. A false static pressure arises because of disturbed airflow in the vicinity of the pressure head or static vent.

Pressure error is negligible at low altitudes and speeds, but becomes more significant with increasing airspeed. Correction for pressure error takes the form of a correction, which has to be applied to the indicated altitude, and must be determined by calibration. Air Data Computers are designed to compensate for this type of error.

Time Lag

Because the response of the capsule and linkage is not instantaneous, the altimeter needle will tend to lag behind whenever the altitude changes. Subsequent over-indication during descent could be dangerous, but should be allowed for in rapid descents. Time lag is virtually eliminated in the servo-assisted altimeter.

Hysterisis Error

The capsules suffer from hysterisis, which can cause a lag in the instrument reading during a climb or descent.

Errors due to Calibration

The calibration of the altimeter, i.e. the conversion of ambient (barometric) pressure to readings in feet, is normally based on ISA conditions. If the real atmosphere however differs, the altimeter will not indicate the true vertical distance above the sub-scale datum. The most significant errors are:

Flying into areas of low pressure is therefore potentially dangerous since the altimeter will over-read, which will result in the flight crew over estimating their clearance from obstacles. Conversely if the aeroplane flies from a low-pressure area into a high-pressure area the altimeter will under-read. When flying at low altitude it is thus good practice to periodically reset the altimeter to minimise the barometric error.

Barometric Error

Barometric error occurs when the actual datum level pressure differs from that to which the subscale has been set. It is caused by the changing ambient barometric pressure experienced during transit and with the passage of time. If the aeroplane flies from an area of high pressure into an area of low pressure it will descend even though the altimeter reading will remain constant.

The figure above illustrates the effect if the subscale is set to 1030 hPa. A subscale error of 1 hPa is equivalent to an indicated altitude error of 28 to 30 feet. The QNH has reduced to 1010 hPa, which will represent a altitude change of approximately 600 feet. The subscale datum will now be at a point that is effectively 600 feet below sea level, and this is the level from which the altimeter is measuring.

Ornographic Error

Differences from standard may occur when air is forced to rise/descend over hills or mountains. Low pressure tends to occur in the lee of mountains with high pressure on the windward side. Additionally, vertical movement of air can result in change of temperature from ISA, which will tend to induce further errors into the altimeter readings.

Temperature Error

Temperature error arises whenever the mean atmospheric conditions below the aeroplane differ from the standard atmosphere. If the actual temperature lapse rate differs from the assumed one, then the indicated altitude will be incorrect.

In general, if the air below the aeroplane is warmer than standard, the air will be less dense (low pressure) and the aeroplane will be higher than indicated. Conversely if the air is colder than standard, it will be more dense (high pressure), and the aeroplane will be lower than standard.

Blockages and Leakages

If the static tube or vents become blocked, the pressure within the instrument case will remain constant and the altimeter will continue to indicate the altitude of the aeroplane when the blockage occurred.

Leaks can also take place either inside or outside the pressure cabin. Within the pressure cabin the cabin pressure altitude will be shown rather than the true altitude. In some aeroplanes, an emergency source inside the fuselage is available. The static pressure inside an aeroplane is however normally different from that external to the fuselage, since it is influenced by blowers, ventilation, etc, so that a different correction for pressure error is necessary. This is normally given in the Aircraft Flight Manual.