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EHM-1997 Alexander:
1. How does an altimeter work?

Written by Ricardo Plácido  
Thursday, 06 November 2008 22:39
Source: IVAO Academy

What is it for?

Unlike a car or a boat, operating an aircraft takes place in the 3 dimensions. In addition to the horizontal plane, an aircraft also requires height to operate between its point of departure and point of arrival.
Vertical navigation is important for 3 reasons:


* Terrain Clearance so you don't hit obstacles on the ground;
* Traffic Separation to make sure you don't hit other aircraft above or below you;
* To calculate the performance capabilities of an aircraft for safe and efficient operation.    
The Altimeter is the instrument used to measure the vertical distance of an aircraft above a specified reference point or level. In its simplest form, an altimeter can be compared to a barometer measuring atmospheric pressure.

An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry.

How does it work?

Air pressure decreases with an increase of altitude — about one millibar (0.03 inches of mercury) per 27 feet (8.23 m) near sea level.

A pressure altimeter (also called barometric altimeter) is the traditional altimeter found in most aircraft. Inside, an aneroid capsule measures the air pressure from a static port (a small hole) outside the aircraft and makes the needles in your altimeter move.

Explained in a simple way, the altimeter contains a little metallic vacuum box called an aneroid barometer (aneroid means NO AIR) that reacts on pressure differences. Since the higher you go, the lower the pressure will be, there is an easy reference between pressure and altitude.

There is a small needle connected to the metallic box and that needle is put in front of an altitude scale.

To correct the altitude for the pressure level, there is an adjustable knob to insert the correction. That correction is either to the standard pressure (1013 Hpa or 29.92 inches) or to the local pressure.

EHM-1997 Alexander:
2. Height, Altitude or Flight Level?

Written by Ricardo Plácido  
Monday, 09 June 2008 01:02
Source: IVAO Academy

As far as aviation is concerned, there are several ways of indicating the vertical position of an aircraft. Unlike most people think, ALTITUDE and FLIGHT LEVEL are not equivalent. Let's see the differences.

First of all, a word about the units. Usually, vertical positions are expressed in FEET (ft). However, for gliders, some helicopters and some Russian made aircraft, vertical positions can also be expressed in METERS (m).

Note: ft (feet) is sometimes abbreviated by ' (for instance 1000' = 1000 ft).


DEFINITIONS

HEIGHT and QFE:
A height is the vertical position of an aircraft above the the SURFACE (whatever earth or water, a lake for instance). Such a position is expressed in feet AGL (Above Ground Level) or feet ASFC (Above Surface).

An altimeter set on the QFE setting indicates the HEIGHT (above the ground level of the airport giving the QFE). When on the ground at the airport set at the QFE, the altimeter shows 0 (zero).
A radio-altimeter also indicates a HEIGHT.
The QFE (rarely used in civil aviation) is the atmospheric pressure measured at the airport. The higher is the airport altitude, the lower is the QFE.

ALTITUDE and QNH (local altimeter setting):
An altitude is the vertical position of an aircraft above the MEAN SEA LEVEL (MSL). Such a position is expressed in feet AMSL (Above Mean Sea Level).An altimeter set on the QNH setting indicates an ALTITUDE. When on the ground, at the airport, set at the QNH, the altimeter shows the airport altitude. The QNH or local altimeter setting is the result of a calculation according to the airfield altitude and the QFE. It gives the atmospheric pressure that would be measured if the airport was at the sea level.

Commonly, QNH or local altimeter setting is used worldwide below the transition altitude (TA). See further.

FLIGHT LEVEL (or LEVEL) and STANDARD pressure:
A Flight Level (FL) is the vertical position of an aircraft above the ISOBARIC SURFACE 1013,25 hPa (or 29.92 inHg). This pressure being called the STANDARD altimeter setting. Such a position is expressed in FL (Flight Level) and in hundreds of feet. (e.g. FL 330 = 33000 ft above the isobaric surface 1013,25 hPa/29,92 inHg).When flying IFR, the Flight Level (FL) always ends by 0 (40-50-60-...-180-190-200-210-220-etc...). Amongst other In the USA, FLs start at 180 because the transition altitude (TA) is 18000 ft. See further.When flying VFR, the Flight Level (FL) ends by 5 (45-55-65-etc...).An altimeter set on the STANDARD altimeter setting indicates a FLIGHT LEVEL. Commonly, the STANDARD altimeter setting is used worldwide above the transition level (TRL). See further.

Note: Most altimeters in hPa don't show decimals and don't enable to select 1013,25. In that case, select 1013.
Flight Levels (FLs) are used instead of QNH because 1013 or 29.92 is a standard setting all over the world unlike the QNH, which could be different from one point to another. For long flights, pilots would have to ask for the QNH regularly. There is no need to do so with the standard setting. Thus, everybody is using the same reference when en-route. When departing, or when landing, they remain in a small area and can use the local altimeter setting. See further.

TRANSITION ALTITUDE (TA):
The Transition Altitude (TA) is the altitude AT OR BELOW which pilots have to use the QNH setting (or the local altimeter setting). That means pilots are flying at ALTITUDES. In the USA, the TA is always 18000 ft. In other countries, the TA may vary; 5000 ft (when possible) often is a standard, but lots of different values are used according to the airfield surroundings.

TRANSITION LEVEL or TRANSITION LAYER (TL):
The Transition Level (TRL) is the flight level ABOVE which pilots have to use the STANDARD altimeter setting 1013 hPa or 29.92 inHg. That means pilots are flying at FLIGHT LEVELS. The Transition Level (TRL) is the first FL ending by 0 available above the transition altitude (TA); the Transition Level (TRL) is calculated according to the transition altitude (TA). The Transition Layer is the gap (when any) between the TA and the TRL (its minimum size is 0, its maximum size is 999 ft).The TRL can't be assigned to an aircraft for flying. The minimum usable FL is TRL+10 in order to keep a safe separation with an aircraft flying at the TA.Some additional explanations are necessary at this point: The higher you are, the lower is the atmospheric pressure. Each time you climb 28 feet, you lose 1 hPa. Say the pressure at altitude = 0 is 1013 hPa. At altitude = 600 ft, the pressure is 1013 - (600/28) = 1013- 21.4 = 991,6 hPa.

This also means that for a difference in altitude of 600 ft, the difference in pressure is 21.4 hPa.

Let's have a look at the examples in the picture below:

Far left, QNH is 1034 hPa. The difference with the standard setting 1013 is 21 hPa, representing a difference in altitude of about 600 ft (as calculated above) between the 1013 and 1034 hPa isobaric surfaces.The pressure altitude is 4400 ft, that is to say FL 44. The first FL ending by 0 available above 44 is 50, involving TRL=50 and a transition layer of 600 ft. First flyable flightlevel is FL60!

In the middle, QNH = 1013 hPa. There is no difference between the QNH and the standard setting, so no difference between 5000 ft QNH and FL 50.50 is already an FL ending by 0. No changes have to be made. TRL=50 and there is no transition layer. First flyable flightlevel is FL60!

Far right, QNH is 991 hPa. The difference with the standard setting is 22 hPa, representing again a difference in altitude of about 600 ft between the 991 and 1013 isobaric surfaces.

The pressure altitude is 5600 ft, that is to say FL 56. The first FL ending by 0 available above 56 is 60, involving TRL=60 and a transition layer of 400 ft. First flyable flightlevel is FL70!



The Transition Level (TRL) is chosen according to the QNH, whatever the initial transition altitude is (5000 or 18000 ft, the calculation mode is always the same and enable to create the following):

Transition AltitudeQNH 977 hPa and below or 28.87 inHg and belowQNH 978 to 1012 hPa or 28.88 to 29.91 inHgQNH 1013 to 1048 hPa or 29.92 to 30.95 inHgQNH 1049 and above or 30.96 inHg and above5000 ft706050406000 ft807060507000 ft908070608000 ft100908070etc...............18000 ft200190180170
Note: In some countries (Middle East particularly), transition levels are defined and not calculated.

USE OF ALTIMETER SETTINGS

SEMI-CIRCULAR CRUISING LEVEL SYSTEM:

In order to ensure safe separation between aircraft above the transition level, it has been decided to allocate FLs to aircraft according their track(!). This is called the semi-circular cruising level system also known as NEODD-SWEVEN rule (north-east is odd, south-west is even). It works as follows:

Track between 000° and 179°Track between 180° and 359°ODD LEVELS EVEN LEVELS FL30 or 3,000 ft

FL50 or 5,000 ft

FL70 or 7,000 ft

FL90 or 9,000 ft

FL110 or 11,000 ft

FL130 or 13,000 ft

FL150 or 15,000 ft

FL170 or 17,000 ft

--USA-CAN-----------------------------

FL190

FL210

FL230

FL250

FL270

FL290

--------------------------------

FL330

FL370

FL410

...

FL40 or 4,000 ft

FL60 or 6,000 ft

FL80 or 8,000 ft

FL100 or 10,000 ft

FL120 or 12,000 ft

FL140 or 14,000 ft

FL160 or 16,000 ft

--USA-CAN-----------------------------

FL180

FL200

FL220

FL240

FL260

FL280

--------------------------------

FL310

FL350

FL390

...
Remark 1:
Since Flight Levels are used above the transition level only, the USA and Canada use altitudes below FL 180 since the TA is 18000 ft.

Remark 2:
Below FL 290, standard spacing between aircraft is 1000 ft, so FLs are defined 10 by 10. Above FL 290, standard spacing is 2000 ft and FL are defined 20 by 20. So FL 300 is not available. The next is FL 310, now considered as even. Then 330 is odd and 350 is considered as even again etc... However, this rule is modified in certain areas, especially over the Atlantic, the Pacific and Europe, where spacing remains 1000 ft between FL 290 and 410. This modification is known as RVSM (reduced vertical separation minima) and should be applied everywhere in the future.

Remark 3:
Some countries may have another cruising level system to match their main traffic flows. In France for instance, odd levels are used for tracks 090°=>269°, and even levels for tracks 270°=>089°. This kind of difference is usually notified on navigation maps (on French maps, a little arrow applied next to airways names shows the way for odd levels). It is still called semi-circular system, but North-South configuration.

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