Procedures for extreme QNH-values outside the altimeter setting scale

Some aircraft have limitations to the QNH-values that can be set into their altimeter.
This applies to older altimeters with mechanical restrictions making it impossible to enter the QNH above or below certain values.
Typically, some older altimeters are limited to use QNH-values between 942 and 1049 HPA.
Values above or below these are not physically possible to enter into the altimeter.
Most of the aircraft in commercial use in Norway today use modern electronic altimeters without these limitations.

Neither ICAO nor EU/EASA have published recommendations on how to handle QNH-values above or below values which are physically possible to enter into the altimeter.

Norwegian ANSPs must check their own systems to verify that they are compatible with extreme QNH-values or ensure that they have mitigations in place.

Operators of aircraft with altimeters that have limited QNH-value range are expected to establish procedures and quick reference tables for pilots that compensate for the QNH-values that exceed system limits.

The pilot is expected to compensate and correct any inhibited weakness in the altimeter system without involving the air traffic services.

During periods of extreme QNH-values the pilot in command shall adhere to the following procedure:
a) Set the correct QNH in the altimeter
b) If a) is not physical possible, use the operators procedure and tables for corrections
c) If neither a) nor b) is possible: Don't fly in controlled airspace or in uncontrolled airspace with compulsory two-way radio communication requirements (TIZ, TIA and certain HTZ)

In order to standardise the method of which pilots correct their altitudes, the operator shall promulgate tables, for instance in their QRH (Quick Reference Handbook) or QRC (Quick Reference Card), for easy access. The tables should reflect the following:

• In the ICAO Standard atmosphere, 1 HPA equals approximately 30 feet at levels below 5000 feet. At higher altitudes, 1 HPA will be a physical larger difference, but this is negligible as this procedure will mostly be used below 7000 feet. Corrections for temperature on approach procedures and minima must be done as normal in addition to this QNH-correction.
• For correction of pressure that is higher than what can be set into the altimeter the pilot should subtract the altitude that appears from the table.
  Example: 1050 HPA is the maximum value that can be set into the altimeter and the actual pressure is 1053 HPA. The difference is 3 HPA, which equals 90 feet, which is rounded off to the nearest 100 feet. If cleared altitude is 5000 feet, the pilot should set QNH 1050 and fly at 4900 feet indicated on the altimeter (cleared altitude 5000  feet minus 100 feet correction).
• For correction of pressure that is lower than what can be set into the altimeter the pilot should add the altitude that appears from the table.
  Example: 942 HPA is the lowest value that can be set into the altimeter and the actual pressure is 939 HPA. The difference is 3 HPA, which equals 90 feet, which is rounded off to the nearest 100 feet. If cleared altitude is 3000 feet, the pilot set QNH 942 and fly at 3100 feet indicated on the altimeter.