Transponder Mode C
 

Mode C altitude transmissions are independent of the barometric altimeter sub-scale setting. The transponder can get its information from one of two sources: an encoding altimeter, which transmits a pressure altitude reading to the transponder or (more commonly) a blind encoder permanently set to 29.92 (pressure altitude). In either case, the altimeter setting does not affect the information sent which is always based on 29.92. ATC’s computers apply the current altimeter setting converting it to altitude (which will only match your indicated altitude if you have set the correct QNH).
FlySafe
PJ88

Yes that is what an encoder does, however a pilot flies via the information displayed on the Altimeter, so if that is in error so will his maintained height. However if the aircraft is flying level this will show as an erroneous altitude being flown to ATC out by the amount the altimeter is in error by. The passing altitudes on first contact on climb will only highlight a gross error say more than 300 ft from what the screen shows and the pilots reports. Considering an Cat 1 minima is 200ft then 300ft the wrong way could put you in the ground. Hence why lower approaches have altimetry check points. Cat 2/3 rely more on radio altimetry so its not as critical.

Don't need to wear that hat LB as you seem to be perfectly capable of embarrassing yourself over your obsession with reading back the QNH.

If you don't understand the readback requirements of section g of AIP GEN 3.4 then no one can help you. If you don't comprehend why you aren't given the QNH on departure when you are required to state the altitude you are passing then no one can help you. If you can't discern why the QNH on an ATIS is the only bit of information that can kill you if you have written the incorrect numbers then no one here can help you. Like I said don't go flying in NZ if you want to keep your sanity. Incorrect QNH settings continue to be a problem on two crew airliners when the ATIS is printed out so why do you think that you will never make a similar mistake as a single pilot?

I dunno, still feels like storm in teacup to me.

"sixthousand AlphaBravoCharlie"

or

"sixthousand QNH1015 AlphaBravoCharlie"

How much extra does that add? about a second? two maybe? Some people's "ahhhh" placed between "squawk" and "1234" is way longer than that!

Ah the irony.

The thread was started by someone pointing out (correctly) that it was unnecessary to read back a particular word. A single f*cking word.

I point out that it is objectively unnecessary to read back QNH in a particular set of circumstances, and the answer is it must be read back because that’s what the rule book says.

It all make perfect sense. It must be a consequence of my poor airmanship and a lawyerly obsession with strict compliance with rules.

LB for me there is a difference between reading back "standby" (the controller is using it to specifically ask you to not reply) and reading back the QNH which has a safety implication if this is not correct.

I personally find confirming to the controller that you will not reply, (when they have just told you not to reply), mildly annoying, whilst reading back QNH has at least a safety check value so is not a biggie (for me).

I know some people get annoyed over calling WACs "WAC Charts" and ATMs "ATM machines" and this issue feels the same - I reckon there are other things that really do deserve me twisting my knickers over - eg people who are wrong on the internet...

Indeed the irony

Yet you are strenuously objecting to having to read back something that is correctly required. Just because you think its unnecessary is completely irrelevant. Its just a casual observation but there seems to come a point with posters that have quite high post counts where they get upset with anyone who has a contrary view and end up in a vortex of vitriolic keyboard bashing. I would respectfully suggest that if this is causing you such angst then give yourself a break and put the keyboard away for a while.

43Inches. You will see that my post was at time 7.48 - exactly the same time as yours. So we were posting in parallel and I was not disagreeing with the post that you were writing at the time but hadn't appeared. I agree with you. The purpose of my post was to correct the info on transponder altitude information posted by LB at 7.26.
Fly Safe
PJ

Is it full moon again?

I didn't post any incorrect information about transponder altitude information. I know how transponders work. Down to every component on their circuit boards in the aircraft and at the other end in the SSR.

It's precisely because of how transponders and SSRs work that renders the repetition of QNH unnecessary after a pilot has reported being at an altitude and in receipt of ATIS with a QNH in an SSR environment. That SSR data either confirms the accuracy of what the pilot reported or raises doubts about its accuracy...

Either the pilot has received the ATIS QNH and properly set it and is flying the reported altitude, or s/he's f*cked it up or is making it up. And, if s/he*s f*cked it up or is making it up, that will show on the transponder reported (and SSR corrected) altitude and that's when there's an actual need for words on the issue to be exchanged between ATC and the pilot.

I know there are circumstances in which there is no SSR coverage. I know there are circumstances in which aircraft are climbing or descending. I was only talking about circumstances in which a pilot in an aircraft in SSR coverage reports at an altitude with an ATIS code.

Jeeezus, this weirdness must be a symptom of long Covid (or perhaps chemtrails).

The SSR data will only be comparable if the aircraft is in level flight, the climb report to departures is +-300ft and really can't be any tighter as there are delays in the system. An arriving IFR aircraft from the flight levels may never fly level long enough for a SSR to verify it has the correct QNH set prior to an approach. So with regard to large aircraft operations it is a critical check to ensure correct QNH is passed on. However even though a light VFR may not see the issue, if ATS get picky with which situation they provide it inevitably they will forget to pass it to an IFR aircraft that then proceeds to bust a minima and blame ATS for not updating them on QNH. Rules like this are usually reflective of arse covering as well as safety benefits.

There is only one person howling LB and that is you. The only thing that is weird is your obsession with having to a readback of a required item. The thread was started by an ATC'er just asking pilots to not readback the word standby when they are asked to "standby". No one agrees with you and your quest to have readback of the QNH an optional item. Incorrect setting of the QNH is a problem and can lead to accidents thats why it is a readback requirement worldwide and not just on approach into Canberra. If you are aiming for 5000 posts then fill your boots but don't think that everyone else is the problem or suffering from some sort of malaise.

Dare we refer back to this thread on the front page showing what happens when you get the QNH wrong??

Granted I think the English/French usage of the ATCO played a significant part, but simply relying on "It's part of the ATIS which you've already told me you've got" removes a layer of safety in the same manner that not confirming your CPDLC-issued departure clearance would. "Well, we got it printed from 'the magic box', so why do we need to confirm it?"

*sigh*

It's amazing how many pilots - I assume they're pilots - have so much difficulty with written comprehension.

I'm not on a "quest to have readback of the QNH an optional item".

Reading back a number proves only one thing: The pilot heard the number and can read it back.

Well, given pilots are highly-trained individuals (of a higher class than your average trained monkey, one would like to think) it's not unreasonable to think that they, on the whole, most of the time, might be able to set their altimeter(s) QNH to the same figure as the one they just read back to ATC, however... altitude encoder errors do happen. eg. the Mode C encoder in the flight-school aircraft I recently did my test in was out by nearly 200' and resulted in some interesting conversations between me, my examiner and ATC. Stressful, to say the least!

AIUI that's one reason ADS-B is mandated in all IFR-certified aircraft - because GPS Altitude (not Altimeter altitude) is broadcast so ATC (and anyone else nearby with ADS-B In!) can read it direct - rather than rely solely on some highly-trained pilot-monkey entering a number.

Anyway, this thread seems to have drifted slightly off-topic. Some recent thermal activity perhaps?