A pilot’s route takes him through controlled airspace. As he approaches the airspace boundary, he decides to fly under the airspace because the radio frequency is congested and the controller seems busy. After referencing his chart, he notes that the airspace begins at 3,000ft and he is at 2,800ft. As long as he watches his altitude carefully, he is sure that he won’t infringe the airspace.
Which altimeter setting should the pilot use? It is vital that pilots remember to use the correct pressure setting when flying above or below controlled airspace.
If the base of the airspace is defined as an altitude, then the Local QNH of the controlling aerodrome within the controlled airspace must be used. However, if it is defined as a Flight Level, then the Standard Pressure Setting (1013hPa) must be used.
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 is Class D airspace and is expressed as a Flight Level (FL75-FL125) so the Standard Pressure Setting (1013hPa) should be used.
Pilots often use the Regional Pressure Setting (RPS) whilst operating outside of controlled airspace. The RPS is the lowest forecast pressure within the Altimeter Setting Region. It is not an actual pressure value, only a forecast, so when this setting is in use the altimeter will indicate height above the lowest forecast pressure. The Regional Pressure Setting’s purpose is to aid terrain clearance in large areas where there is no local QNH or where radio communication is poor. It can be significantly different to the Local QNH and its use near to controlled airspace is the leading cause of vertical infringements.
If the pilot is using the RPS, he may well infringe controlled airspace believing he was below it by 200ft.
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.
There is a difference of 10hPa between the Local QNH and the Regional Pressure Setting. As a change of one hectopascal equates to approximately 30ft, the difference between the indicated altitude of the aircraft using the RPS and its altitude relative to the Local QNH is 300ft.
How can this situation be avoided? It is vital to remember that the correct pressure setting must be used when operating close to controlled airspace. The Local QNH can be obtained from the ATIS, VOLMET, or listening to Air Traffic Control. If there is any doubt, you should speak to Air Traffic Control.
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