1 — What Do QNH, QFE and Standard (1013) Mean?
An altimeter is just a barometer with a height scale. What it reads depends entirely on the reference pressure you dial into the subscale.
- QNH — set it and the altimeter reads altitude above mean sea level; on the ground it shows aerodrome elevation. This is your normal VFR setting.
- QFE — reads height above the aerodrome; it reads zero on that runway. Useful for circuits at some fields.
- QNE / standard (1013.2 hPa, 29.92 inHg) — the common datum everyone shares up high, giving pressure altitude, read as a flight level (Lesson 1).
2 — Transition: Altitudes Below, Flight Levels Above
So that everyone at altitude shares one datum, aviation splits the vertical world in two at the transition altitude/level.
TA is published per State/aerodrome — e.g. Germany 5 000 ft, Austria 10 000 ft.
Below the transition altitude (TA) you fly altitudes on QNH. Above the transition level (TL) you fly flight levels on 1013. Between them sits the transition layer. The switch is one knob: climbing, set 1013 as you pass the TA; descending, reset QNH as you pass the TL.
3 — How Does the Semicircular Cruising-Level Rule Work?
In level cruise above 3 000 ft above the ground, you don’t pick any height you like — you fly a level appropriate to your magnetic track, so opposite-direction traffic is always separated by 1 000 ft. Drag the track and see:
Applies in level cruise above 3 000 ft AGL. VFR levels are the IFR level + 500 ft.
Eastbound (track 000–179°): odd thousands + 500 ft → 3 500, 5 500, 7 500, 9 500 ft (FL35, 55, 75, 95). Westbound (track 180–359°): even thousands + 500 ft → 4 500, 6 500, 8 500 ft (FL45, 65, 85).
4 — “High to Low, Look Out Below”
Your altimeter only tells the truth if its setting matches the air around you. Fly from high pressure into lower pressure (or warm into colder air) without resetting, and the altimeter over-reads — you are lower than it shows.
5 — Worked Example & the Checks
You’re tracking 250° magnetic, below the TA, and want to cruise around 6 000 ft VFR:
- Setting: below the TA → QNH (you fly an altitude, not a flight level).
- Track band: 250° is 180–359° → westbound → even + 500.
- Level: the even-plus-500 option nearest 6 000 is 6 500 ft. (Eastbound, you’d have flown 5 500 or 7 500.)
6 — The Lowest Usable Flight Level
Two effects from §4 — low QNH and cold air — both make a flight level sit lower than it reads. So when you climb onto 1013 above the transition altitude, the lowest flight level you may use isn’t fixed: it rises as the pressure or temperature falls. Two everyday calculations come out of this.
(a) Lowest FL above the transition altitude, from QNH. A flight level is only usable if its true altitude still sits above the transition altitude (TA). Each hPa of QNH below 1013 lowers a level’s true altitude by about 30 ft (27 ft is the precise figure).
Worked example — TA 5 000 ft, QNH 988 hPa (25 below 1013). The pressure shortfall is 25 × 30 = 750 ft, so a level’s true altitude is 750 ft below its nominal height. The lowest level whose nominal altitude clears 5 000 + 750 = 5 750 ft is FL60. (At QNH 1013 you could have used FL50.)
(b) Lowest FL for obstacle clearance, with QNH and temperature. Over high ground you need a minimum true altitude — the obstacle plus the minimum obstacle clearance (MOC), 1 000 ft en route (2 000 ft over mountains). Then correct that height up for cold air and low QNH to find the flight level that delivers it. Set the numbers and watch each correction stack:
MOC = minimum obstacle clearance (1000 ft en route; 2000 ft over mountains). Cold air and low QNH both make a flight level sit lower than it reads, so each pushes the lowest usable level up. Warmer-than-ISA air only adds margin, so it is ignored here. The answer is rounded up to the next 500-ft level.
