[Dave W]

...probably just to trip up some pilots ...

That's just unfounded and illogical paranoia. (To which I am not always immune myself, nowadays...)

Cui bono?

There's no benefit in doing that; it seems evident that ASRs are still there due niche usage in part but more to bureaucratic inertia and a general unwillingness to change.

[Flying_john]

Makes me wonder whether ATC are using this RPS to correct Flight Levels to a Height above sea level before deciding if someone is flying in the bottom of a TMA perhaps.

Would it work this way round?

If your transponder is reporting its level at 1013 hPa and somehow the RPS is say 3 or 4 hPa different from the area you are flying in, would it push your "corrected" Altitude to make you appear inside the bottom layer of controlled airspace.
Genuine question ?

[James Chan]

paranoia

Doh! I left out my wink earlier.

But I agree with your point on bureaucratic inertia and a general unwillingness to change.

[mick w]

The SD profile at the bottom keeps you safe

[2Donkeys]

Makes me wonder whether ATC are using this RPS to correct Flight Levels to a Height above sea level before deciding if someone is flying in the bottom of a TMA perhaps.

Would it work this way round?

If your transponder is reporting its level at 1013 hPa and somehow the RPS is say 3 or 4 hPa different from the area you are flying in, would it push your "corrected" Altitude to make you appear inside the bottom layer of controlled airspace.
Genuine question ?

I doubt it, unless things have changed muchly.

The controller of a chunk of controlled airspace will have the governing QNH set on his display (Southend will have their QNH set for example). Aircraft below transition will be shown relative to that QNH and those above will be shown as a Flight Level. Regional Pressure will not feature.

It will be immediately apparent if a bust occurs and the yardstick for the bust will be that local QNH or 1013 dependent on the airspace’s vertical extents.

[Marvin]

From the UK AIP ENR 1.7 Altimeter Setting Procedures.

3.10 When flying in Airspace below TMAs and CTAs detailed above, pilots should use the QNH of an adjacent aerodrome when flying at or below the Transition Altitude. It may be assumed that for aerodromes located beneath such Areas, the differences in the QNH values are insignificant. When flying beneath Airways whose base levels are expressed as Altitudes pilots are recommended to use the QNH of an adjacent aerodrome in order to avoid penetrating the base of Controlled Airspace.

And

5.2.2.2 Outside of Controlled Airspace: VFR. When flying under VFR, aircraft in level cruising flight and operated in accordance with the visual flight rules above 3000 ft amsl are not required to conform with the cruising levels at ENR 1.7 paragraph 6.1 unless flying in conformity with the conditions specified by the appropriate air traffic control unit. The latest and most appropriate lowest forecast Regional Pressure Setting value should be used for checking terrain clearance.

There is also this little Gem:

3.1 The Transition Altitude within the UK is 3000 FT except in, or beneath, that Airspace specified at paragraph 4.1.

There are in fact three transition Levels in the UK; 3000ft, 5000ft and 6000ft and these are defined in the AIP.

There was a CAA project to raise transition level above 10,000 ft but that is stalled for various reasons.

So a right B>?<*&S muddle it would seem.

[Dave W]

So a right B>?<*&S muddle it would seem.

Yet, with a fairly simple solution staring everybody (except the rest of the World) in the face.

[Ibra]

Makes me wonder whether ATC are using this RPS to correct Flight Levels to a Height above sea level before deciding if someone is flying in the bottom of a TMA perhaps.

Would it work this way round?

If your transponder is reporting its level at 1013 hPa and somehow the RPS is say 3 or 4 hPa different from the area you are flying in, would it push your "corrected" Altitude to make you appear inside the bottom layer of controlled airspace.
Genuine question ?

Controllers only care about separating traffic, anyone flying bellow procedural airways MEA or radar MVA or off-airways directs will be asked to own the ground and keep it in sight if he is using his eyes or use MSA if he is in clouds, I doubt RPS or ASR is what saves the day...

Controlled airspace CTR/CTA/TMA (not ATZ) is usually defined with STD or QNH altimeter settings, it may get tricky for flying IFR or transiting VFR near military placea on QFE but if you can get QNH from ATC if you ask for it or do own conversion

[As I CFIT]

Let's ditch ASRs/RPS and have our LARS/ATC/Information units pass the QNH applicable to your nearest area of controlled airspace instead of RPS, in instances where they would currently do the latter. Each unit will only need a couple of up-to-date QNHs to hand and a rough idea of where you are in the case of London/Scottish/[insert preferred AFIS airfield here].

There are in fact three transition Levels in the UK; 3000ft, 5000ft and 6000ft

These are transition altitudes. Only highlighting this because there's a very small chance that it might help someone and I've got time on my hands.

[Mike Tango]

Makes me wonder whether ATC are using this RPS to correct Flight Levels to a Height above sea level before deciding if someone is flying in the bottom of a TMA perhaps.

Would it work this way round?

If your transponder is reporting its level at 1013 hPa and somehow the RPS is say 3 or 4 hPa different from the area you are flying in, would it push your "corrected" Altitude to make you appear inside the bottom layer of controlled airspace.
Genuine question ?

No.

The radar processors use an appropriate local QNH to convert the transmitted flight level to altitude where that’s required.

I learnt about Regional Pressure Settings on my controller training course, then never used them throughout my controlling career.

[Flying_john]

The radar processors use an appropriate local QNH to convert the transmitted flight level to altitude where that’s required.

Any idea what the allowable tolerance would be ? - 1 hPA, 2 perhaps ?
Just trying to get a feel for the potential errors in the whole system from the transponder sending its pressure altitude (+/-) and the allowed altimetry error and the allowable local QNH difference etc etc.
What might it all add up to (+ or -), are we talking 10's of feet or 100 feet ?

[2Donkeys]

Not all busts are vertical. Many are lateral.

[Mike Tango]

Any idea what the allowable tolerance would be ? - 1 hPA, 2 perhaps ?
Just trying to get a feel for the potential errors in the whole system from the transponder sending its pressure altitude (+/-) and the allowed altimetry error and the allowable local QNH difference etc etc.
What might it all add up to (+ or -), are we talking 10's of feet or 100 feet ?

Well, if you take the London area, it’s the London (ie Heathrow) QNH that is used to drive the radar processors that convert and display the level data as appropriate on the radar screens within the Terminal Control operation. However the different airfields around the TMA will still use their local QNH for their own traffic and the occasional difference of maybe one or two hectopascals is not considered significant.

Otherwise, as I think is already appreciated here, the level reading from the aircraft’s transponder is considered within tolerance if it’s seen to be +/-200ft from what is reported.

For real time infringement purposes though that is for a verified readout. In other words an aircraft talking to ATC such that the discrepancy is known and the actual level confirmed.

If traffic not talking to ATC is showing as 100ft vertically inside CAS, ATC is not allowed to assume the actual level may be 100ft below CAS and the level being reported by the transponder is (just) within tolerance. Or at least not until it is subsequently verified.

Depending on the ATC unit and the aircraft equipment level, some units can now see the actual pressure setting set in the aircraft via Mode S, and will even alert when a significantly incorrect altimeter setting is being used.

[TheDealer]

I’m interested to know what you use it for?

Nice to have if flying in open FIR without any local airports around....over Wales or the Irish seas for example..... whatever the general view about them, they exist so it would seem useful to know which one is applicable.....much like the FIS frequency being shown.....For clarity, Hannah has confirmed that SD doesn't show them and there are no plans to incorporate the feature.

[Iceman]

For the record, neither does ForeFlight show the UK RPS regions.

Iceman