Not sure if this has been discussed before. The QNH provided from time to time by ATC often results into a barometric altitude which is about 200ft different (tipically lower) from GPS (ie real) altitude. This means that flying 50-100ft below the lower bound of a CAS may result in a violation detected by ATC radar. Who is to blame in such a case?
mancival wrote:
Not sure if this has been discussed before. The QNH provided from time to time by ATC often results into a barometric altitude which is about 200ft different (tipically lower) from GPS (ie real) altitude. This means that flying 50-100ft below the lower bound of a CAS may result in a violation detected by ATC radar. Who is to blame in such a case?
Vertical airspace boundaries are determined by barometric altitude, not real altitude. Likewise, ATC radar gets barometric altitude from the aircraft transponder. So in this respect the difference between barometric and GPS altitude is a non-issue. Unless, of course, you are using GPS altitude as altitude reference, but you should never do that except possibly to determine terrain clearance.
It is a much bigger problem that legal altimeter tolerances and QNH roundoff (or round down, rather) can easily put you inside CAS even if your altimeter shows you to be 50-100 ft below.
Yes.
However setting the notorious RPS will get you busted and the CAA won’t care that ATC issued it.
Not sure I fully understand this sentence from Airborne_Again “ATC radar gets barometric altitude from the aircraft transponder”.
1) what if transponder is set in a mode without altitude?
2) what if the pilot, by mistake, inputs the wrong QNH, so that his barometric altitude appears “legal” to him, but he is in fact inside Class A CAS? Does the transponder broadcast to ATC radar also the QNH along with the barometric altitude?
mancival wrote:
Not sure I fully understand this sentence from Airborne_Again “ATC radar gets barometric altitude from the aircraft transponder”.
1) what if transponder is set in a mode without altitude?
In that case ATC radar can’t tell what altitude you’re at, so it can’t detect any violations.
2) what if the pilot, by mistake, inputs the wrong QNH, so that his barometric altitude appears “legal” to him, but he is in fact inside Class A CAS? Does the transponder broadcast to ATC radar also the QNH along with the barometric altitude?
The transponder transmits standard setting altitude (flight levels) regardless of the QNH you’ve set. If you’re flying below the transition altitude then the ATC radar system converts the transmitted altitude using the current QNH setting for the airspace you’re in.
Very clear, thanks. I thought around big airports (eg Gatwick) they had radars to see anything (with transponder or not)
The civilian primary radars (the ones which don’t need transponder) only do 2D.
mancival wrote:
Very clear, thanks. I thought around big airports (eg Gatwick) they had radars to see anything (with transponder or not)
Indeed, in some places there is primary radar that is not dependent on transponders, but regular primary radar can’t determine the altitude of an aircraft.
Under a radar or traffic service, radar will ask for your altitude to check the Mode C output is correct. Occasionally an aircraft leaving White Waltham with a faulty transponder might make Heathrow miss a heartbeat as it reports 3,000 feet AMSL while in the circuit.
In the UK most radar is primary plus secondary.
The primary range tends to be shorter. Also some units are procedurally prevented from offering a given level of service if you are primary only.
Today in the UK nearly all primary traffic, about 50% of total GA, is the result of the crazy CAA no-prisoners policy. One man’s work 
There are no Mode A only transponders really.
