Atmospheric pressure is the force per unit area exerted by the atmosphere on any surface in contact with it. If pressure is considered as the weight of a column of air of unit cross sectional area above a surface, the pressure at the upper surface will be lesser than that at the lower surface. Thus, atmospheric pressure will decrease with an increase in height.
Reduction of Pressure to Standard Level
Reduction to aerodrome level (QFE) The barometer reading with corrections applied gives the atmospheric pressure at the cistern level and to get the pressure at the aerodrome level, this value should be increased at the rate of 01 hPa per 30 feet of the elevation of the cistern from the aerodrome level. This value of the atmospheric pressure at the aerodrome level is called QFE and finds universal application in aviation.
Reduction to Standard Level (QFF) To compare atmospheric pressure over a wide region, all pressure values are reduced to a standard level, such as the mean sea level. The sea level pressure is the reading denoted by the barometer if a tunnel is bored at the station down to sea level and the barometer is lowered to that level. The same result is achieved in a much simpler and practicable way by adding to the station level pressure a value equal to the weight of column of the air of unit cross section and of length equal to the elevation of the station depending upon the average density and therefore average temperature of the air column. The correction to be applied to reduce the value to mean sea level, depends upon the dry bulb temperature.
Reduction to Mean Sea Level (QNH) QNH is defined as the pressure over the airfield reduced to mean sea level assuming that the rate of variation of pressure between the airfield level and sea level is the same as in the ISA. Thus, an ICAN altimeter whose sub-scale is set to the current value of QNH, will read, while on ground, the elevation of the airfield above sea level. While flying in the immediate neighbourhood of the airfield, the altimeter will indicate reasonably true altitudes. Current values of QNH are supplied to aircraft in flight by ATC in hectaPascal, inches or millimetres as required.
Calculation of QFE, QFF & QNH
Table – I
|Bar hPa / Attached thermometer reading ° A||960||980||1000||1020|
|-To be subtracted-|
Note: The table supplied by IAF includes the height correction at the rate of 8.33 meter per hPa, where ever the difference is more than 1 metre between datum level and aerodrome level.
Height of the station = 95ft (28 m) AMSL.
Attached thermometer reading = 293.5°A
Bar as read = 995.0 hPa
Index correction = -0.1 hPa
Correction of temperature, gravity and height from table I = -3.8 hPa
∴QFE = 995.0 – (0.1 + 3.8) = 991.1 hPa.
To obtain QNH, a correction at the rate of 0.03 hPa per feet for the height of the station above mean sea level is added to the QFE. Thus for the height of every 95ft 3.0 hPa to be added to QFE.
QNH = Aerodrome level pressure (QFE) + Height correction.
= 991.1 + 3.0 = 994.1 hPa.
Calculation of QFF from QFE
Table – II
|Dry Bulb Temp°F||QFE in hPa|
|-To be added-|
For stations whose elevations are between 800 – 2300 GPM
QFE = 995.5 hPa
Dry bulb temperature = 68.5°F
Correction to be added to QFE as per table II= 3.3 hPa
∴ QFF = 995.5 + 3.3 = 998.8hpa.
For stations with higher elevation, the geo-potentials of nearest standard pressure level (850, 700, and 500 hPa) shall be reported as follows:
(a) Stations whose elevation is in the range of 800 – 2300 Geo Potential Meter (GPM), shall report in GPM, the geo-potentials of 850 hPa pressure level.
(b) Stations higher than 2300 GPM but not exceeding 3700 GPM shall report in GPM, the geo-potentials of 700 hPa pressure level.
(c) Stations whose elevation exceeds 3700GPM shall report in GPM the geo-potentials of 500hPa pressure level.
An example of reporting the pressure of a station whose elevation is in the range of 800 – 2300 GPM is shown below:
Height: 896.7 M
Barometer reading (Corrected for Index, temperature and gravity) = 905.6 hPa
Dry bulb reading: = 64.6°F
From table III values read
For 904 = 1421
For 906 = 1439
The difference of 2 hPa = 18 GPM
The difference for 1.6 hPa ( 905.6-904) = (18/2) x (16/10)
∴ QFF = 1421 +14.4
= 1435.4 GPM
(Note: Geo Potential Meter (GPM) is approximately numerically equal to the altitude in meters)
Importance of QFE and QNH in Aviation
QFE is set on altimeter while in the vicinity of the airfield. This is the current value of the pressure reduced to the level of the airfield. An Altimeter set to QFE will read zero while on ground, where as while flying in the vicinity of the air field, Altimeter set to QFE indicate reasonably true height above the airfield level.
An ICAN altimeter whose subscale is set to the current value of QNH will read while on ground the elevation of the airfield above sea level, while flying in the immediate neighbourhood of the airfield, the Altimeter will indicate reasonably true altitude. The current value of QNH is supplied to aircraft in flight by ATC in hPa, inches or millimetres.
Datum level is the elevation of the station in meters to which the barometric pressure refers for the purpose of maintaining continuity in pressure records, though the barometer is shifted subsequently for any reason. For this purpose the height of the barometer cistern as on 1st January 1960 is taken as datum level for station functioning on that date. For stations started after 01st January 1960, the height of the barometer cistern at the time of starting the station is taken as the datum level and this height is maintained there after irrespective of subsequent shifts of the barometer.
Cistern level is the height of the cistern of barometer installed at a station from aerodrome reference point (ARP).
Aerodrome Reference Point
The aerodrome reference point is selected at each aerodrome by the survey of India in consultation with the chief operation officer of the aerodrome during the initial survey of the aerodrome. The point is as nearer to the geometric centre of the landing area as practicable taking future development / expansion of the aerodrome into account.
Aneroid Barometer (Principle and Construction)
Aneroid Barometer is a portable Barometer in which no fluid is used for measurement of atmospheric pressure. If a thin flexible metal is held at the edges, it will be deformed if the pressure on one side is greater than on the other. The Aneroid Barometer consists of two such membrane made into a hermetically sealed chamber with a vacuum inside. One membrane fixed at its centre while outer membrane is attached to two metal posts. The Metal posts pass through holes made in a lefty metallic spring. One end of the spring fixed to block while the other end is connected to a lever. When the atmospheric pressure increases, the outer membrane is compressed and the spring is pulled together. Similarly, when the pressure decreases the vacuum expands and the layer relaxes. The movement is magnified by system of levers and is indicated on the dial calibrated to read the pressure directly.
The spring prevents the chamber from collapsing. The aneroid barometer gives the QFE directly. Before it is put to use the reading should be compared with a standard Barometer and if necessary small adjustments can be made by manipulating a screw at the bottom.
Method of Use
The barometer is to be checked as often as possible against mercury barometer; any necessary adjustment can be done at least once a month and preferable once fortnightly. When reading the instrument, the barometer face should first be taped gently once or twice and the position of the eye adjusted so that the line joining it and the end of the pointer is perpendicular to the face of the instrument. Readings are only made to the nearest whole hPa unless the change in pressure for a short period is required.