Airspace and Altimeters

Jan 8, 2022
3 min read

Updated: Jul 30

Understanding the Importance of Correct Pressure Settings

A pilot’s route takes them through controlled airspace. As they approach the airspace boundary, they decide to fly under the airspace because the radio frequency is congested, and the controller seems busy. After consulting the chart, they note the controlled airspace begins at 3,000 ft, and they are currently flying at 2,800 ft. Confident in their altitude awareness, they proceed, believing they are safely below the airspace.

But which altimeter setting should be used in this situation?

The Right Altimeter Setting Matters

It's vital to use the correct pressure setting when flying near the boundaries of controlled airspace. The required setting depends on whether the base of the airspace is defined by altitude or by Flight Level (FL).

If the base is expressed as an altitude, the Local QNH of the controlling aerodrome must be used.
If the base is expressed as a Flight Level, the Standard Pressure Setting (1013 hPa) is required.

Examples of Controlled Airspace

The Solent CTA (Class D) airspace shown on an aeronautical chart

The Solent Control Area is Class D airspace and is expressed as an altitude (3000'-5500'), so the Local QNH must be used.

The Borders CTA airspace shown on an aeronautical chart

The Borders CTA is Class D airspace and is expressed as a Flight Level (FL75-FL125), so the Standard Pressure Setting (1013hPa) should be used.

The Role of the Regional Pressure Setting (RPS)

Outside controlled airspace, pilots often rely on the Regional Pressure Setting (RPS). This setting represents the lowest forecast pressure within the relevant Altimeter Setting Region and is designed to aid terrain clearance in areas where:

There is no available Local QNH
Radio communication is limited or unavailable

However, the RPS is not an actual pressure, it’s a forecast. This means the altimeter will indicate height above the lowest forecast pressure, which can vary significantly from the Local QNH.

If the pilot is using the RPS, they may well infringe controlled airspace believing they were below it by 200 ft.

A Common Pitfall

Using the RPS near controlled airspace can lead to serious misjudgements in altitude.

For example:

Controlled airspace base: 3,000 ft (altitude)
Pilot using Local QNH: Altimeter shows 2,800 ft → Clear of airspace
Pilot using RPS: Altimeter also shows 2,800 ft, but the aircraft is actually at 3,100 ft relative to Local QNH → Airspace infringement

This occurs because a 10 hPa difference between Local QNH and RPS equates to approximately 300 ft (with each hPa corresponding to ~30 ft). That’s enough to cause an unintended entry into controlled airspace.

Illustration showing controlled airspace infringement by the use of the incorrect altimeter setting — In the example above, the airspace is defined as an altitude (3000-5500). The pilot on the left is using the Local QNH and remains clear of controlled airspace. The pilot on the right is using the Regional Pressure Setting and infringes controlled airspace, though his altimeter reads 2,800ft.

In the example above, the airspace is defined as an altitude (3000-5500). The pilot on the left is using the Local QNH and remains clear of controlled airspace. The pilot on the right is using the Regional Pressure Setting and infringes controlled airspace, though their altimeter reads 2,800 ft.