Altimetry, height, altitude or level?

Objectives:

• to explain the difference between height, altitude and flightlevel
• to explain the difference between and the purpose of QNH, QFE and standard setting
• to point out the importance of correct altimeter settings

1.DEFINITIONS :

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

Remark : The following explanations are somewhat simplified; there is a much more scientific way of explaining altimetry but it would remain a little bit esotheric.

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). See L5-Conversions for advanced details.

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

1.1 - 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.

1.2 - 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.

1.3 - FLIGHT LEVEL or LEVEL and STANDARD pressure :

A Flightlevel (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 flightlevel (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 flightlevel (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.

Flightlevels (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.

1.4 - 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.

1.5 - TRANSITION LEVEL (TRL) and TRANSITION LAYER :

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 :

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) :

Note : In some countries (Middle East particularly), transition levels are defined and not calculated.

2 - USE OF ALTIMETER SETTINGS :

3 - 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 :

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. See RVSM-MNPS for details.

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. See RVSM-MNPS for details.

In general, when nothing is indicated, the standard semi-circular system (East-West) has to be used. Due to these differences, according to the country you are flying in, you may be instructed to use levels different from those normally used in the standard semi-circular rule.

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