We all know the difference (I hope) between QFE, QNH and QNE. Common practice in the UK is for PPL’s to fly the circuit on QFE, and use QNH for cross-country flights, although CPL’s operate entirely on QNH. There is scope to get things wrong, whether using QFE or QNH:
- Setting QFE instead of QNH when you depart on a cross country flight means your altitude (referenced to sea-level) is several hundred feet higher than your altimeter shows. At Andrewsfield, which is close to Stansted, this means there is a very real risk of infringing controlled airspace. How to avoid this mistake? After setting your altimeter on the ground, check it against the airfield elevation, and understand any local restrictions on altitude while departing.
- Military airfields often ask you to fly on their QFE for a MATZ penetration. Forgetting to re-set QNH on leaving the MATZ means you could be several hundred feet higher than the altimeter shows.
- And finally regional QNH, or Regional Pressure Setting, as it is more correctly known. RPS is not a QNH measurement, it is the lowest forecast pressure in the entire pressure setting region, over the next hour. Why the lowest pressure? Because that guarantees that your MSA (Minimum Safe Altitude) will give you adequate obstacle clearance. The RPS you are using could
be quite a bit lower than the local QNH. And the relevance of this is?…. If you are trying to avoid controlled airspace above, you may be higher than your altimeter suggests. How to avoid this mistake? Always fly on the local QNH near CAS. You can get this by tuning the ATIS of the airport within the CAS.
To illustrate this, consider a flight northwards between the airspace of Manchester and Leeds Bradford. CAS starts at 3000ft and there is an obstacle with an altitude of 1925 feet. You decide to fly at an MSA of 2900 ft using the Barnsley RPS of 1002 hPa. You think you are just below the base of CAS, but in fact, if the local Leeds QNH is 1013, you are actually at 2900 + 330 = 3230 ft and inside CAS!