Altimetry

Altimetry (Metric, QFE/QNH, and Other Variations)
Altimeter Settings

Definitions
The definitions are clear but the source material is not. For a look at the definitions with the source material, see Altimetry / Altimeter Settings.

QFE
.“Field Elevation” - Based on setting a locally provided altimeter setting which is determined by adjusting an altimeter on the ground until it reads zero. QFE allows us to read height above the runway.

QNH.
“Height” - Based on setting a locally provided altimeter setting which is determined by adjusting an altimeter on the ground until it reads the station's correct elevation above the sea level reference datum. QNH allows us to read field elevation on landing.

QNE
​.“En Route” - Based on setting 29.92” or 1013.2 hPa, gives height above a theoretical datum which is not adjusted for atmospheric conditions. QNE provides the basis for flight levels.

​ICAO Document 4444

Transition altitude
The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes.
Transition Layer
The airspace between the transition altitude and the transition level.
Transition Level
The lowest flight level available for use above the transition altitude.

The key takeaway here is the transition altitude expresses the highest possible altitude, above that they don't exist. So when you are climbing, once you've past the transition altitude, they no longer exist so you might as well go to 29.92/1013. When you are descending, the lowest possible flight level is at the Transition Level and once you have past it, you might as well go back to QNH. Confused? Yeah, me too.

Figure: Altimetry Chart Differences

The Jeppesen Airway Manuals do not explain charting differences and in some cases the differences are not consistent. You are left to compare one chart with another and look for someone with recent experience to determine what procedures are really being used at a particular airport. You can learn a few things by comparing three types of charts, and from that draw three conclusions:

• Millimeters vs. Hectopascals. If the approach briefing information says "Alt Set: MM (hPa on req)" then you set millimeters, if it says "Alt Set: hPa" you are setting Hectopascals.


• QFE vs QNH. If you are expected to fly QFE, the heights inside the parentheses will be bolded, i.e., DA(H) 820' (200'). You should also see "QNH on req (QFE)" in the briefing information.


• Metric vs. Feet. If you see feet to meters conversion tables, you will probably be vectored in meters.

The number of airports using QFE or metric has declined dramatically over the last two decades but you need to be cautious when flying anywhere that used to be a associated with the former USSR. It pays to talk to a pilot with recent experience.​

ICAO Procedures

ICAO Document 4444 §4.1

4.10.1.1 For flights in the vicinity of aerodromes and within terminal control areas the vertical position of aircraft shall, except as provided for in 4.10.1.2, be expressed in terms of altitudes at or below the transition altitude and in terms of flight levels at or above the transition level. While passing through the transition layer, vertical position shall be expressed in terms of flight levels when climbing and in terms of altitudes when descending.
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4.10.1.2 When an aircraft which has been given clearance to land is completing its approach using atmospheric pressure at aerodrome elevation (QFE), the vertical position of the aircraft shall be expressed in terms of height above aerodrome elevation during that portion of its flight for which QFE may be used, except that it shall be expressed in terms of height above runway threshold elevation:

1. for instrument runways, if the threshold is 2 metres (7 feet) or more below the aerodrome elevation, and


2. for precision approach runways.

4.10.1.3 For flights en route the vertical position of aircraft shall be expressed in terms of:

1. flight levels at or above the lowest usable flight level;


2. altitudes below the lowest usable flight level;

except where, on the basis of regional air navigation agreements, a transition altitude has been established for a specified area, in which case the provisions of 4.10.1.1 shall apply.

​These ICAO procedures are almost universally used. There are exceptions by country (e.g., North Korea) and within countries (e.g., China). You must check the applicable AIP or Jeppesen Airway Manual, Air Traffic Control, State Rules and Procedures to be sure.

​Chart Identification is left up to you, but as a general rule if the chart says "Alt Set: hPa" and there are no metric conversion tables, you are probably dealing with an ICAO standard chart requiring you to fly altitudes in feet and the altimeter set to QNH.

Figure: Example ICAO Standard Approach Chart
Singapore ILS DME Rwy 02L, WSSS, page 11-1, 25 Nov 11

Metric Procedures

1. If the approach will be flown using QNH procedures, you should plan on setting primary and secondary altimeters to QNH once descending through the transition layer.


2. Vector heights will be given in meters. The approach plate will contain a conversion box for these altitudes. Read back the height in meters but fly the height in feet.


3. Approach procedure heights are shown in feet. On most QNH approach plates, DA or MDA altitudes are bolded: DA(H) 308' (200').

Chart Identification is left up to you, but as a general rule if the chart says "Alt Set: hPa" you will set your altimeter to Hectopascals, if the heights outside the parentheses are bolded you will be flying QNH, and if the chart has feet to meter conversion tables, you can expect to be vectored in meters.

Figure: Example Metric/QNH Approach Chart
Beijing ILS DME Rwy 18R, ZBAA, page 11-5, 25 Jan 13

QFE Procedures

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1. On most QFE approach plates, the approach plate altimeter setting block will say (QFE) in bold print and the DA or MDA heights are shown in bold: DA(H) 820' (200'). There are exceptions and the only way to be sure is to check the appropriate Jeppesen Airway Manual ATC pages. If the approach will be flown using QFE procedures, you should plan on setting primary and secondary altimeters to QFE once descending through the transition layer.


2. Vector heights will be given in meters. The approach plate will contain a conversion box for these altitudes. Read back the height in meters but fly the height in feet.


3. Approach procedure heights are shown in feet. DA(H) or MDA(H) is given with the altitude based on QNH outside the parenthesis and the height based on QFE inside the parenthesis. It is critically important to remember whenever you have set QFE as your altimeter setting, you are flying to the “H” - the height inside the parenthesis. Reminder: When you set a QFE altimeter setting, your altimeter will read zero after you have landed.

Chart Identification is left up to you, but as a general rule if the chart says "Alt Set: MM" you will set your altimeter to millimeters, if the heights inside the parentheses are bolded you are probably dealing with a QFE chart, and the presence of feet-to-meters tables means you will be vectored with altitudes in meters.

Figure: Example Metric/QFE Approach Chart
Moscow Sheremetyevo ILS or PAR Rwy 07L, UUEE, page 21-1, 1 Nov 13

Hybrid Procedures

Some countries have additional procedures at airports where no transition levels/layers are provided or where the elevation makes setting QFE impossible. Some countries have several procedures within their own borders, China for example:

Jeppesen Airway Manual, Air Traffic Control, State Rules and Procedures, China, Rules and Procedures, 11 Oct 2013] At aerodromes of high elevation: When the aircraft altimeter subscale cannot be set to the atmospheric pressure at the aerodrome elevation, it will then be set to 1013.2hPa before take-off, with the indicated altitude interpreted as zero altitude. When the aircraft altimeter subscale cannot be set to the atmospheric pressure at the aerodrome elevation, landing is to be made with the assumed zero altitude notified by the air traffic controller before landing.
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I think this approach used to be QFE and required you to use an altimeter setting with an altitude pad provided by the tower, since you would not be able make your altimeter read zero on the ground at such a high elevation. Tower would give you an altimeter setting and tell you that runway elevation will indicate, for example, 2000 meters on touchdown. It appears most of these high elevation airports, like this one, have converted to QNH. But the high altitude note still appears in the ATC pages.

Figure: Example Hybrid Approach Chart
DME Rwy 02, ZPLJ, page 11-1, 27 Dec 13

An Example from Not Too Long Ago

Lets have a look at this approach into the old Soviet Union, which looks more complicated than it really is. But back then, with no GPS and FMS to help us, it was a handful. These days as the aircraft get smarter they seem to be cramming more and more stuff on the approach plate which makes it that much more important to study the chart and figure out what you need to know when all the electrons go on strike.

Figure: Old USSR Approach
Yerevan Cat II ILS Rwy 09, UBBB, Page 11-1A, 14 Feb 92

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