Altimeter, Axioms of Altimeter
BAROMETRIC HEIGHT MEASUREMENT
Pressure altimeters are instruments which indicate aircraft height above a selected pressure datum. They operate on the principle that air pressure decreases with height, and they are in fact aneroid barometers graduated to indicate height rather than pressure i.e. they measure the pressure of the atmosphere at the level where the altimeter is located and presents an altitude location in feets or meters. The pressure altimeter uses ambient atmospheric pressure also known as Static Pressure as its source of operation. In order to be calibrated, certain assumptions must be made concerning the manner in which air pressure decreases with height and this has given rise to a number of model atmospheres.
Pressure Lapse Rate
As height increases, pressure decreases, but this decrease is not proportional to the increase in height because the density of air varies with height. A practical approximation for the lower levels of the atmosphere is that a decrease in pressure of one millibar equates to an increase in height of 30 feet.
Temperature Lapse Rate
Temperature does not remain constant but varies with height in a complex manner. The temperature lapse rate depends on the humidity of the air, and is itself a function of height. This variation greatly affects the relationship between pressure and height. To calibrate an altimeter to indicate barometric height it is necessary to make some assumptions as to the temperature structure of the atmosphere. The relationship can be expressed in mathematical form for each of the various layers of the atmosphere and the instrument can then be calibrated accordingly.
P.S. A barometrically derived height must therefore be used with extreme caution as a basis for terrain clearance. However, provided that all aircraft use the same datum and the same assumptions in the calibration of their altimeters, safe vertical separation between aircraft can be achieved.
Principle of Operation of a Simple Altimeter
Fig below is a schematic diagram of a simple altimeter. The instrument consists of a thin corrugated metal capsule which is partially evacuated, sealed, and prevented from collapsing completely by means of a leaf spring, or in some cases by its own rigidity. The capsule is mounted inside a case which is fed with static pressure from the aircraft’s static tube or vent. As the aircraft climbs the static pressure in the case decreases allowing the spring to pull the capsule faces apart. Conversely a decrease in height compresses the capsule faces. This linear movement of the capsule face is magnified and transmitted via a system of gears and linkages to a pointer moving over a scale graduated in feet according to one of the standard atmospheres.
- Expands when ambient pressure decreases, i.e. when ac climbs
- Contracts when ambient pressure increases, i.e. when ac descends
P.S. In simple altimeter there is only one capsule so sluggish indication of altitude change. In Sensitive altimeter, there is a stack of capsules, therefore, very responsive indication of altitude changes. The sensitive altimeter is designed for more accurate height measurement than the simple altimeter although the principle of operation is the same. Typically there will be three pointers, one rotating every 1,000 ft, one every 10,000 ft and a third every 100,000 ft.
A sensitive altimeter has a millibar scale so that it is possible to set whatever datum pressure is desired, above which height is to be measured. Thus if airfield level pressure (QFE) is set, the altimeter will read zero on the ground and height above airfield once airborne. If sea-level pressure (QNH) is set, the altimeter will indicate height above sea-level (i.e., airfield elevation on the ground). The millibar setting can be altered in the air to reflect changes of pressure with time, location or required datum level.
The servo-assisted altimeter is designed to relieve the capsule of the work required to drive the mechanical linkage. Changes of barometric pressure are still sensed by the contraction or expansion of evacuated capsules, but the mechanical transmission is replaced by a position control servo system, i.e., the movement of the capsule is transferred to the pointers by means of amplified electrical signals.
In addition to increased accuracy and sensitivity, the arrangement has the advantage that the altitude information can be easily transmitted to other systems, e.g. IFF / SSR. In current altimeters the three needle display is replaced by a digital display, and an auxiliary pointer moving over a scale graduated in 50 ft increments from 0 – 1,000 ft.
Pressure Altimeter Errors
- Instrument Error.
- – Due to Mechanical Linkages which causes friction and creates lag between capsule expansion and pointer movement. This also includes other mechanical errors during construction.
- Lag Error. It is perceptible in rapid climb and descent. Since the response of the capsule and linkage is not instantaneous, the altimeter needle lags whenever height is changed rapidly causing an under-read on climbs and an over-read on descents. Basically altimeter indication not keeping in pace with the ac. The amount of lag varies with the rate of change of height. Time lag is virtually eliminated in servo-assisted altimeters and may be reduced in others by the fitting of a vibration mechanism.
- In rapid descents altimeter overreads due to lag error
- In rapid climb the altimeter underreads due to lag error
- Static Balancing, i.e. symmetrical laying out on fuselage to overcome effects of yawing, side-slip, cross winds.
- Basically overcome by accurate design testing and location of static plates
- Pressure Error. Basically error in sensing correct static pressure. Pressure error occurs when the true external static pressure is not accurately transmitted to the instrument. A false static pressure can be created by the effect of the air flow passing over the static vent. Although the error is generally negligible at low speeds and altitudes, it can become significant at high speeds, or when services such as flaps, airbrakes, or gear are operated. Avoidance or reduction of the effect is accomplished by careful probe or vent design and location. Residual error is calibrated for each aircraft type and detailed in the Aircrew Manual or ODM, or automatically in an air data computer or pressure error corrector unit (PECU).
- Hysteresis Error. (Inelastic response of the capsule). A capsule under stress has imperfect elastic properties and will settle to give a different reading after levelling from a climb compared to that obtained after levelling from a descent. i.e. doesn’t regain original shape after levelling out at a original altitude. It is more pronounced over a period of time and can be overcome by using better quality material for capsule.
- Non Standard Environmental Conditions
- Pressure and temperature not being the same as ISA. The ISA conditions invariably does not exist. The Lapse rate of temperature and pressure do not hold as per assumption used for calibration. The capsule is subjected to pressure values which are different from calibration values.
Non Standard pressure
High- Low – High. Means when flying from a area of high pressure to an area of low pressure the altimeter over reads.
Low – High – Low Means when flying from an area of low pressure to an area of high pressure the altimeter under reads.
Example. Strong Starboard drift in Northern Hemisphere, the altimeter Over reads, it means ( Going from high pressure to Low pressure)
P.S. The same rule follows in case of temperature
Servo Assisted Altimeters
- Uses E and I bar mechanism
- Flux between E and I bar set up as per reference datum setting
- Error signal flows in servo system until E and I bar is reset to neutral position.
Accuracy of Altimeters
- For altimeters designed to operate upto 30,000 ft it is + 2 Hpa
- For altimeters designed to operate upto 50,000 ft it is + 3 Hpa
The datum setting window in the altimeter is called Kollsman Widow. In this window one can set the pressure datum setting as per the requirement of the flight in Hpa,or inches depending on the type of altimeter.
The datum setting on the Kollsman Window that will cause the altimeter to read zero on ground. With QFE set in the altimeter will read the height above ground level when in flight.
It is mean sea level pressure corrected for temperature, adjusted for a specific site or datum like an airfield, being the most obvious example. When this is set on your altimeter, it will read your Height not altitude. It will read zero at airfield elevation and after take off will read your HEIGHT above that specific airfield. If you fly to another airfield of different elevation and/or different QFE pressure, you will have to ensure you reset that particular airfields QFE if you want your altimeter to read zero on touchdown.
The datum setting in the Kollsman Window that will make the altimeter read aerodrome elevation on ground. With QNH set the altimeter will read the ht above mean sea level when ac is in air.
The pressure is measured at station (QFE) then reduced down to mean sea level pressure. When set on your altimeter it will read your Altitude. Set on the tarmac at your airfield the altimeter will display the airfields elevation above mean sea level.
This is the most commonly used pressure setting in the commercial world. Its probably the most useful setting to have, as nearly all aviation references to elevation are in relation to mean sea level. The mountain peaks on a map, airfield elevation, target elevation, minimum safe altitudes enroute etc. Incidentally, QNH is given as a regional pressure setting and should be updated with new ones if you leave its area of reference into a new QNH pressure region. The QNH is the LOWEST FORECAST pressure at mean sea level for a given day to ensure that safe terrain separation is maintained regardless of the days variation in pressure.
QNH is QFE corrected for elevation of aerodrome. Correction is applied as per ISA lapse rate of pressure
QNH = QFE + Elevation/ Pressure Lapse Rate ( in Millibars)
For aerodromes above MSL QNH > QFE. Lapse rate taken is 1 Hpa/28 ft or as specified (30 ft). 28 ft lapse rate is actually valid upto 5000 ft.
QNH and QFE values are always rounded down and reported in whole numbers. Example..1013.25 Hpa will be reported as 1013.
Example. Elevation = 920 ft, QFE = 975 HPa, Find QNH?
Ans. QNH = QFE + elev/30
= 975 + 920/30 = 1005.6 HPa which will be reported as 1005 HPa
QNH > QFE implies aerodrome is above MSL as is here in this case.
This is standard setting value of 1013.25 HPa (When setting will be set as 1013). With this value set in, in flight the altimeter will give a reading which is referred as Flight Levels. It is used predominantly for enroute flying. Above Transition Altitude, all ac maintain this setting of 1013 HPa or 29.92 inches.
QNE = The International Standard Atmosphere (ISA). It is the average mean sea level pressure around the globe. It is planet earths mean atmospheric pressure at sea level basically. This pressure setting is refered to as STANDARD in aviation. STANDARD is set from QNH when climbing up through the “Transition Level”. Your altimeter will then read your FLIGHT LEVEL. A reading of 25,000ft is FL250. 5,000ft = FL050. 13,500ft = FL135
QNH = Altitude (AMSL)
QNE = Flight Level
QFE = Height (AGL)
QFF = Not used for altimeter settings
Transition Altitude (TA)
A transition altitude is that altitude below which the ac report their vertical position as altitude. QNH is set in the datum setting.
It is defined for all aerodromes and is published in AIP and aerodrome documentation. It is calculated keeping in mind the obstructions within a distance of 25 nm around the aerodrome.
In India the transition altitude is 4000 ft (Minimum). ICAO recommends minimum of 3000 ft.
It is defined as a layer of nominal depth of 1000 ft or 1500 ft (1499 ft as per Flip), which provides transition zone for climbing/ descending ac to reset altimeter setting.
Above Transition altitude while climbing ac maintains QNE and the vertical position is reported as Flight Level. Transition Level is the lowest Flight Level available for use. When descending the ac reports its vertical position as altitude in transition layer and changes datum setting to QNH at transition level.
The altitude indicated on the altimeter is called indicated altitude. It is independent of pressure setting in the window. i.e. whatever be the pressure setting the indicated value on the altimeter is called indicated altitude.
Pressure Altitude is the indicated altitude on the altimeter when the Kollsman window is set to QNE. This is also reffered to as FL. ( indicated altitude = Pressure altitude when setting is 1013).
If on a given day QNH at an airfield >< 1013.525 HPa then PA> Elevation of the airfield
If QNH > 1013.25 HPa then PA < Elevation of the airfield
Datum change Vs Altimeter Indication
If datum setting knob is rotated clockwise that means is increased, the altimeter indication also rotates clockwise that is increases and vice- versa.
Vertical distance of an ac above the terrain is called absolute altitude or ht above ground level (AGL) This reading is obtained from Radio altimeters. a close approximation occurs when QFE is set on pressure altimeter.
The true vertical distance of an ac above sea level, that means the actual altitude is called true altitude. The difference in indicated and true altitude occurs due to the effect of non standard atmospheric conditions. The same value of PA, gives a lower value of TA and therefor lesser clearance from terrain, when the temperature is lower than ISA values.
If ISA deviation is zero then PA = TA
If ISA deviation > zero (means positive) then TA > PA
If ISA deviation < zero (means negative) then TA < PA
TA + PA + (4 x ISA Dev x PA in thousands of feet)
Example. PA = 15000 ft
Actual Temp = -2ºC
ISA Temp = 15 – 15 x 2 = -15ºC
ISA Dev = Actual – ISA temp = -2 – (-15) = 13ºC
TA = 15000 + (4 x 13 x 15)
= 15780 ft
Density Altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of observation. In other words,density altitude is air density given as a height above mean sea level.
This altitude is PA corrected for Non standard temperature variations. When conditions are standard, PA = DA.
If temperature > standard then DA > PA and vice versa.
This is important as it directly relates to aircraft performance.
DA = PA + (120 x ISA Dev) ( Actually 119.6)
Example. PA = 13000 ft, Temp = 2ºC, DA = ?
Std Temperature at 13000 ft = 15 – (13 x2) = -11ºC
ISA Dev = actual – Std = 2 – (-11) = 13ºC
DA = 13000 + (120 x 13) = 14560
DA and PA will coincide when ISA dev is zero.
If temperature is warmer Then ISA Dev is higher then DA is higher than PA
If temperature is colder than ISA Dev is less so DA is less than PA
In altimeters a higher pressure datum happens at physically lower level.
A lower pressure datum happens at physically higher level