Height, altitude or flight 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

Remark : The following explanations are very simplified and kept practical; there is a much more scientific and technical (but for many people also boring ...) way of explaining altimetry. A lot of professional in-dept material can be found on the internet.

1.DEFINITIONS :

There are several ways to indicate the vertical position of aircraft and/or obstacles. HEIGHT, ALTITUDE, ELEVATION and FLIGHT LEVEL are used. Each has another meaning and is used in a particular situation.

1.0 Units

In general, vertical positions are expressed in FEET (ft).

In some parts of the World, such as in the Russian Federation and China, vertical positions are expressed in METERS (m).

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

Depending on the aircraft equipment, the pressure altimeter will only accept a sub-setting in hectoPascal (hPa) or inches of Mercury (in Hg). Some will show both hPa and Inches.

Note1: Conversion from FEET to METERS and hPa to InHg (and vice versa) can be found in the Conversion Tables.

Note 2: Some pressure altimeters show a Millibar (MB) setting instead of hPa. This is not a problem since 1 MB = 1 hPa!

Note 3 : Most altimeters in hPa don't show decimals and don't enable to select 1013,25. In that case, select 1013.

1.1 - HEIGHT and QFE

In general, HEIGHT is the vertical distance of an aircraft above whatever SURFACE (buildings, mountains, a lake, etc.). HEIGHT is expressed in feet AGL (Above Ground Level).

By setting the QFE value of an airport, the pressure altimeter will show the HEIGHT above that airport. On the ground, the altimeter will show 0 (zero).

The QFE is the atmospheric pressure measured at the airport. The higher the airport elevation, the lower the QFE.

A radio-altimeter also indicates a HEIGHT, but above whatever you are flying at that moment, not just the airport!

Disadvantages:

• because an airfield is often located lower or higher than the surrounding ones, the QFE for each airfield will be quite different. Even if they are a few miles apart.
• each time a pilot flies over another airfield, he has to change his altimeter sub-setting to the QFE of that airfield. Not very practical!

QFE is NOT used TO SEPARATE aircraft vertically!

1.2 - ALTITUDE and QNH - ELEVATION

An ALTITUDE (ALT) is the vertical distance of an aircraft above the MEAN SEA LEVEL (MSL). For objects and obstacles on the surface of the earth, the word ELEVATION (ELEV) is used instead of altitude. ALTITUDES and ELEVATIONS are expressed in feet AMSL (Above Mean Sea Level).

When a QNH is set on the aircraft's altimeter, the word ALTITUDE is used. On the ground at an airport, the altimeter will (approximately) show that airport ELEVATION when the aerodrome QNH is set on the aircraft's altimeter.

Air Traffic Control will pass the QNH to pilots

• on clearing them to descend to an altitude below the Transition Level (TRL),
• during initial approach clearance
• when cleared to enter the traffic circuit,
• as part of a taxi clearance to departing aircraft,
• on request of the pilot, or
• when the QNH changes.

Worldwide, QNH is used for separating aircraft and preventing collisions with obstacles at and below the transition altitude (TA). See further.

Disadvantage:

• when the weather is unstable, the QNH between airfields across the country can vary significantly. Cruising at M.080 with a high performance aircraft, you'd be changing the altimeter setting to the QNH of each airfield you cross overhead every few minutes, from London to Kuala Lumpur. Not practical for cruising at high altitudes and speeds!

1.3 - TRANSITION ALTITUDE (TA)

The transition altitude (TA) is the altitude AT OR BELOW which pilots have to use the QNH setting. That means you are flying at ALTITUDES.

A common TA may be determined for an entire FIR or a specific one for an airfield in a mountaneous area. In Microsoft Flight Simulator, the TA is always 18000 ft by default.

1.4 - TRANSITION LEVEL (TRL) and TRANSITION LAYER :

The TRansition Level (TRL) is the FIRST FLIGHT LEVEL that may be used ABOVE TA. From here, pilots have to use the STANDARD altimeter setting 1013 hPa or 29.92 inHg.

The TRansition Level (TRL) is calculated according to the QNH. That is why when the QNH is very high, TRL can be eg. FL70 while TA is 7500 ft.

The Transition layer is the gap between the TA and the TRL.

When the Transition layer is less than 1000 ft, traffic at both the TRL and TA need horizontal separation.

1.5 - FLIGHT LEVEL and STANDARD pressure

A Flight Level (FL) is the vertical distance of an aircraft above the ISOBARIC SURFACE of 1013,25 hPa (hectopascal) or 29.92 in Hg (inches of Mercury). An "ISOBARIC SURFACE" is the "invisible landscape" that connects all points with the same atmospheric pressure. In aviation, 1013,25 hPa (hectopascal) / 29.92 in Hg (inches of Mercury) are referred to as the STANDARD altimeter setting.

Flying above the Transition Level with the STANDARD setting 1013 hPa/29.92 inHg on the altimeter, this will indicate Flight Levels (FL) instead of Altitudes e.g. 33000 ft with STANDARD altimeter setting = FL 330.

Simply expressed: Flight Level is the altitude (at standard QNH) in feet, without the two zero's at the end (20.000 feet becomes FL200).

Commonly, the STANDARD altimeter setting is used worldwide at or above the transition level (TRL). See further.

2 - USE OF ALTIMETER SETTINGS :

When cruising at or above Transition Level (TRL), use the Standard Altimeter setting 1013,25 hPa or 29.92 In Hg (Red part on the drawing).

During descent through Transition Level (TL), select QNH.

Cruising at or below Transition Altitude (TA), use QNH (Blue part on the drawing).

When climbing through the Transition Altitude (TA), the Standard Altimeter is set to 1013,25 hPa or 29.92 In Hg.

(Sources: Annex 2 - Rules of the Air / Doc 8168 - Aircraft Operations - Vol. 1 / Doc 4444 - Air Traffic Management)

Module Manager e-mail: acm@ivao.aero and acam@ivao.aero

Contributing Author(s) / Support:

• Mike Kara
• ...................................

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