Altimetry: QNH, QFE and the standard setting

An altimeter is just a barometer with a height scale, so the height it shows is only as good as the pressure you tell it to expect, which is why pilots are forever setting and resetting it.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

Three settings, three meanings

The altimeter measures the pressure outside and converts it to a height. Move the subscale to a different reference pressure and the same air gives a different reading. Three settings matter, and the UK CAA and ICAO Doc 8168 (PANS-OPS) describe how each is used:

• QNH is set so the altimeter reads altitude above mean sea level. On the ground it shows the aerodrome's elevation. This is what you use for terrain and obstacle clearance at lower levels.
• QFE is set so the altimeter reads height above a chosen datum, usually the aerodrome or the runway threshold. On the ground it reads about zero. It is convenient in the circuit, because the needle shows height above the field.
• The standard setting, sometimes called QNE, is 1013.25 hPa (1013.2 hPa rounded, or 29.92 inHg). An altimeter set to it reads pressure altitude, the basis of flight levels. Everyone in the upper airspace uses it, so all aircraft share one reference and their indicated separation is real even if the actual sea-level pressure varies along the route.

Why a common reference matters up high

If every aircraft in the cruise used its own local QNH, two aircraft over different pressure regions could read the same altitude while being at different true heights, or different altitudes while level with each other. The fix is to have everyone above a certain level set the same number. That number is the standard setting, and the heights measured against it are called flight levels: FL350 means the altimeter reads 35000 ft on 1013 hPa, regardless of the real sea-level pressure below.

The handover happens at two related boundaries. The transition altitude is where you switch from QNH to the standard setting while climbing. The transition level is the lowest usable flight level above it, where you switch back to QNH while descending. Between them lies a thin transition layer you do not cruise in.

Where EASA and the FAA differ

Two conventions differ by jurisdiction, and mixing them up is a real hazard.

First, the transition altitude itself. The FAA fixes it across the United States at 18000 ft. In Europe and the United Kingdom the transition altitude is variable and often much lower, published for each area, with the UK historically using values such as 3000 to 6000 ft (one reason European authorities have worked towards more harmonised, higher values). Always read the transition altitude for where you actually are.

It is tempting to finish the American half of that sentence with "so flight levels begin at FL180 everywhere", and it is not quite true. 14 CFR 91.121(b) makes FL180 the lowest usable flight level only when the local altimeter setting is 29.92 inHg or higher. As the pressure falls, the rule's table steps the lowest usable flight level up with it in 500 ft increments, through FL185, FL190, FL195, FL200 and FL205, reaching FL210 in the lowest band the table covers (an altimeter setting of 27.41 to 26.92 inHg). On a deep low over the United States, in other words, FL180 is simply not available, and the bottom of the flight-level band climbs to keep a real gap above the fixed 18,000 ft transition altitude. The transition altitude is fixed; the lowest usable level above it is not.

Second, the units in the report, which our guide to reading a METAR covers in detail. The ICAO and EASA convention reports the pressure setting in whole hectopascals with a leading Q, for example Q1013. The United States reports the altimeter setting in inches of mercury with a leading A, for example A2992, as the FAA AIM sets out. The standard setting is the same physical pressure either way, but set the wrong number in the wrong units and your altimeter will be badly out.

A worked sequence

Imagine a European departure from an aerodrome at 200 ft elevation with a transition altitude of 5000 ft and a local QNH of 1005:

• On the ground you set 1005, and the altimeter reads about 200 ft, the field elevation.
• You climb on QNH. Passing 5000 ft (the transition altitude) you set 1013, and from there you read flight levels.
• You cruise at, say, FL120 on the standard setting.
• Descending later, ATC passes the transition level, FL65. Passing it you reset to the current QNH (which may now be different from your departure value).
• Below the transition level you fly altitudes on QNH for the approach, and might use QFE in the circuit if that is the local practice.

Where FL65 comes from is worth working through, because it is the one number in the sequence a pilot cannot simply read off. ICAO Doc 7030 requires the transition layer in the European region to be at least 1000 ft deep, so the transition level has to sit a clear 1000 ft above the 5000 ft transition altitude, at 6000 ft above mean sea level or higher. But the transition level is measured on 1013, not on 1005. With the QNH 8 hPa below standard, and about 27 ft to the hectopascal, an altimeter set to 1013 over-reads by roughly 8 x 27 = 216 ft. So a true altitude of 6000 ft AMSL shows up as about 6216 ft on the standard setting, which is FL62 and a bit: not a usable level.

Try the levels either side. FL60 sits at about 6000 - 216 = 5784 ft AMSL, leaving a layer of only 784 ft, which is too thin. FL65 sits at about 6500 - 216 = 6284 ft AMSL, leaving 1284 ft above the transition altitude, which satisfies the rule. FL65 is therefore the lowest usable level, and that is the one ATC passes you.

Common pitfalls

• Forgetting to change the setting at the transition altitude or level is one of the most common altimetry errors, and it puts your indicated height at odds with everyone else's.
• Assuming FL180 is always available in the US. The transition altitude is fixed at 18,000 ft, but under 14 CFR 91.121(b) FL180 is the lowest usable flight level only when the altimeter setting is 29.92 inHg or higher; below that it steps up.
• Guessing the transition level. It is not simply "the next level above the TA". In Europe the layer must be at least 1000 ft, and the level is measured on 1013 while the TA is measured on QNH, so the answer moves with the pressure. Take the level ATC gives you.
• Confusing QNH and QFE can leave you flying a circuit or approach a full field-elevation lower or higher than you intended.
• Cold weather makes the altimeter read high, so in very cold air your true altitude is lower than the indicated altitude, and a cold-temperature correction may be required on an approach. See ICAO Doc 8168 for the procedure.
• Old pressure on a long flight. Reset QNH to the current value for where you are, not where you took off.

In Pilot EFB

Pilot EFB shows the decoded METAR alongside the raw text, so the QNH (or the altimeter setting in inches of mercury, depending on the region) is spelled out for you in plain language with the original group preserved. It does not set your altimeter or replace the pressure ATC passes you, and it is not a certified Electronic Flight Bag. Use it as a study and planning aid, and set your altimeter from the value given by ATC or the official report. Saved weather stays readable offline; pulling a fresh observation needs a connection.