Basic requirements; procedures applicable to operators and pilots (except tables), incl. ICAO Doc 7030 regional supplementary procedures.
This topic is ICAO-framework material — the content below is a study aid, not cited regulation. Always verify against the current ICAO documents and national AIP.
The altimeter subscale is set to a reference pressure so that the instrument's indication has a defined meaning. Three settings are in standard use.
QNH causes the altimeter to read the aerodrome elevation when the aircraft is on the ground. In flight, QNH gives altitude above mean sea level (AMSL). Pilots use QNH for terrain and obstacle clearance below the transition altitude.
QFE is the pressure at the aerodrome reference point. With QFE set, the altimeter reads zero on the ground and indicates height above that aerodrome in flight. QFE is less common in modern ICAO operations but may appear in national procedures or military practice.
Standard pressure (1013.25 hPa / 29.92 inHg) is used at and above the transition level. The resulting indication is a flight level (FL). All traffic at cruise levels references standard pressure, eliminating altimeter errors that would otherwise accumulate across regions with different QNH values.
These terms are not interchangeable. Altitude is vertical distance measured from AMSL (QNH reference). Height is vertical distance measured from a specified datum, usually aerodrome elevation (QFE reference). A flight level is a surface of constant atmospheric pressure referenced to 1013.25 hPa, expressed in hundreds of feet — FL095 is nominally 9 500 ft on standard pressure.
The transition altitude (TA) is the altitude at or below which pilots control vertical position by reference to altitudes on QNH. It is published in national AIPs and on instrument approach charts. Values vary considerably by state — commonly 5 000 or 6 000 ft AMSL across much of Europe, but ranging from 3 000 ft to 18 000 ft in other regions.
The transition level (TL) is the lowest flight level available for cruising above the transition altitude. It is determined by ATC based on the prevailing QNH and communicated to crews via ATIS or clearance delivery. A lower QNH means the TL must be raised to maintain adequate separation from the TA.
The transition layer is the airspace between the TA and the TL. Climbing aircraft cross it upward — they quote altitude until passing the TA, then switch to 1013.25 hPa and quote flight levels. Descending aircraft cross it downward — they fly flight levels until reaching the TL, then set the current QNH and revert to quoting altitude.
Before departure, the crew sets the local QNH and confirms the altimeter reads within the permitted tolerance of the aerodrome elevation (typically ±60 ft for a serviceable instrument). En route, ATC may pass updated QNH; the crew sets it promptly.
On approach, the ATIS or approach controller provides the current QNH. The crew sets it, cross-checks both altimeters, and uses the resulting altitude indications for the approach. No approach should be commenced without a current, verified QNH set.
Operators are responsible for ensuring crews are briefed on the applicable TA, for including altimeter-setting checks in normal and approach checklists, and for ensuring awareness of the TL in use.
ICAO regional supplementary procedures adapt the basic framework to local conditions. European airspace, for example, specifies how frequently ATC must update and pass QNH, minimum usable flight levels above mountainous terrain, and coordination procedures at FIR boundaries where the TA differs. Pilots flying internationally must consult the relevant regional supplement and the destination state AIP; assuming the home-state TA or procedures apply elsewhere is a common and potentially serious error.
This block connects to your school's own Operations Manual (OM-A/B/C/D). In the full product it shows, cited to your manual, how YOUR organisation implements the regulation above — private to your school. (Demo placeholder.)
The cases below are labeled illustrative context for this training module, not regulatory text. They represent recent published material included to connect the subject matter to operational consequence.
Note: all three cases concern radio altimeters — radar-ranging devices that measure actual height above terrain using reflected radio energy — not pressure altimeters or the altimeter-setting procedures covered in block 1. They are included here because unreliable altitude data at any point in the measurement chain carries the same category of safety consequence during approach and landing.
5G C-Band interference with radio altimeters — Canadian airspace
The FAA issued airworthiness directives requiring revised crew procedures and system operating limitations for Boeing 737 series CASE-1, 747 series CASE-2, and a broad range of legacy Boeing and McDonnell Douglas types including 707, 717, 727, DC-8, DC-9, DC-10, MD-80, MD-11, and MD-90 variants CASE-3 when operating in Canadian airspace. The cause in each case was wireless broadband operations in the 3.7–3.98 GHz band (5G Lower C-Band) rendering radio altimeter outputs unreliable.
Radio altimeters feed critical approach-phase automation: autopilot, autothrottle, and flight director systems use RA inputs to manage flare, go-around arming, and wind-shear alerting. When RA outputs are corrupted, these systems can behave unpredictably, increasing crew workload at the highest-workload phase of flight and — in the worst case — reducing the crew's ability to maintain safe approach, landing, or go-around.
The PPL-level takeaway: reliable altitude information is a foundation of safe operations at every level of aviation. Whether the failure mode is an incorrectly set QNH on a pressure altimeter or electromagnetic interference corrupting a radio altimeter, the consequence is the same — the crew is flying without a trustworthy height reference. Verification, cross-checking, and currency of altimeter settings are not procedural formalities; they are the first line of defence against controlled flight into terrain.
1. What does a pilot's altimeter indicate in flight when QNH is set on the subscale?
The lesson states that in flight, QNH gives altitude above mean sea level (AMSL), used for terrain and obstacle clearance below the transition altitude.
2. What must a descending aircraft do when it reaches the transition level?
The lesson specifies that descending aircraft fly flight levels until reaching the TL, then set the current QNH and revert to quoting altitude.
3. What effect does a lower prevailing QNH have on the transition level?
The lesson states that a lower QNH means the TL must be raised to maintain adequate separation from the TA.
4. What is the typical permitted tolerance when verifying the altimeter against aerodrome elevation before departure?
The lesson states the crew confirms the altimeter reads within the permitted tolerance of the aerodrome elevation, typically ±60 ft for a serviceable instrument.
5. Who determines the transition level, and how is it communicated to flight crews?
The lesson states the transition level is determined by ATC based on the prevailing QNH and communicated to crews via ATIS or clearance delivery.