x-country flying - QNH or RPS?

[umbongo]

I understand that the RPS is the lowest pressure which will be experienced within the altimeter setting region over a certain period (1hr?), whereas the QNH is the 'actual' equivalent sea level pressure at a given weather station, corrected for an ISA atmosphere.

The RPS is clearly conservative for terrain clearance (therefore good for low, military style flying etc), but it is presumably bad for ensuring clearance from controlled airspace above the aircraft, for the same reason. Is that likely to lead to airspace incursions by light aircraft or are the differences between the RPS and the 'local' QNH too small to worry about? - Presumably not, or why bother having an RPS??

In a nutshell, which setting should one use and when?

[Chilli Monster]

Fly under CAS without a relevant QNH and you will end up commiting an airspace bust.

I'd like to see RPS abolished, replaced with sufficient airfield ATIS (there are plenty in England, just northern Scotland and Wales that's the problem) that you can fly on an accurate, relevant pressure setting. There's not just ATIS of course - anywhere here think of using Volmet broadcasts?

The simple answer to your question is, if you have access to an accurate, current QNH fly on that. If not then RPS for terrain awareness / avoidance. However - I'd treat it as a last resort.

[London Mil]

CM, your campaign agianst RPS is well documented. However, you have just discounted a significant part of the UK land mass with your Northern Scotland and Wales statement. Furtermore, you neglect to notice that the UK FIRs also encompass a volume of sea that is greater than the total land mass. Now, being a military type, I really don't want to be floating around above the sea with no horizon some 200+ miles from the 'local' airfield on their QNH.

The bottom line is that there is a need for the RPS, it just doesn't fit with the majority of civil ops. I agree - if youy are close to an airfield and can obtain the QNH, then that is what you should do (I think there is merit in the ATIS idea). However, I don't see why the RPS should be treat with such disdain. If you use the CAS bust argument, see my reply on the other thread.

[umbongo]

thanks for your prompt answers!

i saw the QFE vs QNH thread but thought my question might get lost in the bun fight...

So if i understand correctly - when i go flying under the london TMA, i need the London QNH, not the Chatham RPS. Fine.

[London Mil]

Absolutely. Similarly, if you are flying close to an airfield (for argument's sake let us say 30-40nm), use that airfield's QNH. If you cannot get a QNH, use the RPS. London/Scottish info can help.

[chevvron]

When flying under ANY terminal control area you MUST use the QNH of an airfield under that TMA; that includes military pilots too. I've seen military aircraft transit through MID indicating 2700ft with Gatwick inbounds descending to 3000ft, all because the mil pilot exercised his prerogative not to contact ATC and used the RPS instead of observed QNH. I've had one or two civilians recently who routed roughly CPT - GWC saying they were at a flight level when the QNH was 1024; had a hell of a job convincing them they MUST fly on QNH 'cos they were under a TMA and the TA is 6000ft

[Chilli Monster]

If you cannot get a QNH, use the RPS. London/Scottish info can help.

And the last line of my post said............................? ;)