QFE, QNH, QFF and QNE

Altitude, height or elevation: which is which?

Altitude, height, elevation

One picture for all the words. Tap Next.

1. Mean sea level Mean sea level (MSL) is the average level of the sea between high and low tide. It is the zero line that altitudes, elevations and charts are measured from, so every pilot and every chart starts from the same place.
2. Elevation Elevation is how high a point on the ground, such as an airfield or a peak, is above MSL.
3. Altitude and height Altitude is how far the aircraft is above MSL. Height is how far it is above a chosen datum, for example the airfield.
4. Transition altitude The transition altitude (TA) is the highest altitude at which aircraft always fly on the local QNH. At or below it, an aircraft's vertical position is given as an altitude.
5. Transition level and layer The transition level (TL) is the lowest flight level at which aircraft always fly on the standard setting, 1013.2 hPa. The airspace between the TA and the TL is the transition layer.
6. Flight levels and pressure altitude Flight levels are surfaces of constant pressure, counted from 1013.25 hPa and 500 ft of pressure apart: FL50 = 5000 ft, FL300 = 30 000 ft. With 1013.2 set, the altimeter shows pressure altitude, and no one needs to keep updating QNH. Pressure altitude 19 000 is written FL190: drop the last two zeros.

Takeaway: Altitude is measured from mean sea level (MSL). Height is measured from a chosen datum, often the airfield. Elevation is how high the ground is above MSL. The transition altitude (TA) is the highest altitude flown on QNH; the transition level (TL) is the lowest flight level flown on 1013.2. Between them lies the transition layer.

What does each pressure setting make the altimeter read?

QFE, QNH, QFF, QNE

An airfield 1500 ft above the sea.

1. QFE: zero on the ground The pressure at the aerodrome reference point (the highest point of the runway). With QFE set, the altimeter reads zero on the ground (or its own height above the ARP) and height above the field in the air. Also called the zero setting.
2. QNH: elevation on the ground The station pressure reduced to MSL assuming ISA conditions. With QNH set the altimeter reads the airfield elevation on the ground and altitude in the air. Used for separation from terrain and from other aircraft.
3. QFF: for the weather map The station pressure reduced to MSL using the actual temperature of the air column. It is the value plotted on synoptic charts to draw isobars. Hotter than ISA: QFF is less than QNH. At sea level QFF = QNH = QFE.
4. QNE and regional QNH QNE is the altitude the altimeter shows with 1013.25 hPa (29.92 in) set, used at high-altitude airfields. Regional QNH is the lowest QNH forecast for an altimeter setting region in the next hour: issued every hour, valid one hour, and safe for terrain clearance.

Memory hook: QFE zero, QNH elevation, QFF charts, QNE 1013.

Takeaway: QFE: field pressure, reads zero on the ground. QNH: reduced to MSL with ISA, reads the elevation. QFF: reduced with the actual temperature, for charts. QNE: the reading with 1013.2 set.

Which Q code does what?

QFE, QNH, QFF, QNE

Each pair looks alike. The rule underneath tells them apart.

• QFE: The pressure at the aerodrome (the highest point of the runway). Set it and the altimeter reads 0 on the field, and height above the field in the air. QNH: The aerodrome pressure brought down to MSL using ISA. Set it and the altimeter reads the field elevation on the ground, and altitude above MSL in the air. QFE: height above the field. QNH: altitude above the sea.
• QNH: Reduced to MSL with the ISA temperature and lapse rate. Used for flying: separation from terrain and other aircraft. QFF: Reduced to MSL with the actual temperature of the place. Used only by meteorologists to draw isobars on synoptic charts. QNH flies. QFF draws charts. At a sea-level field they are equal; in air warmer than ISA, QFF < QNH.
• QNE (1013.25): The standard setting, 1013.25 hPa = 29.92 in. Reads pressure altitude; at or above the transition level that is a flight level (FL190 = 19,000 ft pressure altitude). Regional QNH: The lowest pressure forecast in an altimeter setting region, issued every hour and valid for one hour. Keeps you clear of terrain. Above the TL: 1013. Below the TA: QNH.

Memory hook: QFE = Field Elevation zero. QNH = Nautical Height (MSL). QNE = Standard 1013.

What will your altimeter read?

Fly the altimeter

An altimeter is an aneroid barometer marked in feet. Turn the subscale, move the aircraft, change today's pressure and temperature, and watch the needles. The red aircraft is where you really are; the dotted one is where the altimeter says you are.

• Sea-level field
• Field at 600 ft
• Delhi 778 ft
• Leh 10,700 ft
• On the ground: QFE
• The teacher's example: field elevation 600 ft, QFE 990 hPa. With QFE set, the altimeter reads 0 on the field (the 'zero setting').
• On the ground: QNH
• QNH = QFE + 600 ÷ 30 = 990 + 20 = 1010 hPa. With QNH set, the altimeter reads the field elevation, 600 ft.
• On the ground: 1013
• With 1013 set it reads the pressure altitude of the field: (1013 - 990) × 30 = 690 ft, not the elevation.
• High to low: look out below
• You set 1013 and fly into an area where the pressure has fallen to 1000 hPa. At the same true altitude the altimeter now reads about 390 ft too high. Keep the reading constant and you will descend: High to Low, look out below.
• Low to high
• Flying from low to high pressure without resetting, the altimeter under-reads: you are higher than it says.
• A cold day
• 20 °C colder than ISA at 10,000 ft: the altimeter over-reads by about 7% (1% for every 3 °C). Warm to cold, look out below.
• A hot day
• 15 °C warmer than ISA: the altimeter under-reads by about 5%. You are higher than it says.
• Leh: QFE off the scale
• At a very high field the QFE (about 656 hPa here) is below the subscale's range, so it cannot be set. That is why QNE is used at high-altitude airfields.

Memory hook: QFE: zero on the field. QNH: field elevation on the field. 1013: flight levels. High to low (or warm to cold), look out below.

Takeaway: The altimeter only measures pressure and turns it into feet above whatever pressure is set on the subscale. Set QFE and it reads height above the field; set QNH and it reads altitude above MSL; set 1013.25 and it reads pressure altitude. If the pressure falls, or the air is colder than ISA, it over-reads: you are lower than it says.

Can you work QNH, QFE and QNE out in your head?

Worked sums

Each card is one worked example. Use 30 ft per hPa unless told otherwise.

1. QFE 950, QNH 1000 Elevation = (1000 − 950) × 30 = 1500 ft. With 1013 set on the ground the altimeter reads (1013 − 950) × 30 = 1890 ft: more than the elevation, because QNH is below 1013.
2. QFE 970, QNH 1020 Elevation = (1020 − 970) × 30 = 1500 ft. With 1013 set: (1013 − 970) × 30 = 1290 ft: less than the elevation, because QNH is above 1013. Set pressure below QNH: reads low. Set pressure above QNH: reads high.
3. QFE 1000, elevation 270 ft 270 ÷ 27 = 10 hPa, so QNH = 1000 + 10 = 1010 hPa.
4. Took off at A (400 ft, QNH 1010)... ...and landed at B (600 ft) without resetting. The altimeter reads 700 ft. It reads 100 ft high, which is about 4 hPa at 27 ft per hPa. Reads high means the pressure fell from A to B (high to low, over-read). QNH at B ≈ 1010 − 4 = 1006 hPa.
5. Alticor and D value The correction from indicated to true altitude, used for mountain flying. Alticor = indicated − true. D value = true − indicated; it decreases flying from high to low. Rough rules: add 30 ft per hPa that MSL pressure is above 1013.2 (subtract below). Add 1% of the indicated altitude for each 3 °C warmer than ISA (subtract colder). Add the two together.
6. Indicated 10 000 ft, MSL 1003 hPa, ISA − 6 °C Pressure: 10 hPa below 1013, so −300 ft. Temperature: 6 °C colder, 2% of 10 000, so −200 ft. True altitude ≈ 10 000 − 500 = 9500 ft.

Takeaway: Elevation ≈ (QNH − QFE) × 30 ft. With 1013 set on the ground the altimeter reads (1013 − QFE) × 30 ft. Alticor = indicated − true; D value = true − indicated.

Will the altimeter over-read or under-read?

Over or under?

Tap a situation, then what the altimeter does.

• Flying from high to low pressure
• Flying from low to high pressure
• Air colder than ISA
• Air warmer than ISA
• Wind from port (NH)
• Wind from starboard (NH)
• Subscale set higher than the real QNH
• Subscale set lower than the real QNH

Memory hook: High to Low, or Hot to cold: look out below (over-reads).

Takeaway: Pressure falling, or air colder than ISA: over-reads, you are lower than shown. Pressure rising, or air warmer than ISA: under-reads, you are higher than shown.