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, so flight levels begin at FL180 everywhere. 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.

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 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, perhaps FL55. 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.

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.
• 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.