Where I am the QNH can often be 3mb different in the adjoining FIS sectors on a beautiful VFR day.
Yes; a quick look at some MSLP charts suggests this is quite possible.
It doesn’t affect traffic within CAS. It just means that flying at a fixed altitude will actually be flying a curved surface of true altitude. That is what happens when you fly a FL – you fly a curved surface. But it can potentially affect whether traffic OCAS is depicted as infringing, if it is using an altimeter setting which is not yours.
skydriller wrote:
Additionally, I usually set the airfield height on the Altimeter at my home non-controlled aerodrome before take off, and this can be 1-2mb different to the given FIS QNH
That can easily be from your altimeter’s calibration. That’s 1-2mb is 30-60ft, within tolerances.
alioth wrote:
If there’s more than 1mb difference in some CTA somewhere, no one will be flying (at least VFR)
Hmmm….not so sure about this…
Where I am the QNH can often be 3mb different in the adjoining FIS sectors on a beautiful VFR day.
Additionally, I usually set the airfield height on the Altimeter at my home non-controlled aerodrome before take off, and this can be 1-2mb different to the given FIS QNH – now bear in mind we have class C at 2000ft above the airfield.
Regards, SD..
If there’s more than 1mb difference in some CTA somewhere, no one will be flying (at least VFR)
Could that be proven in court beyond reasonable doubt?
We know, for instance, that from LFLI to LFHM (27 miles) QNH can and does vary by anything up to ten millibars with little or no wind. We also know that regional and local QNH can be markedly different here in the UK.
In the same vein, is a Distriict Judge really going to buy the idea that the CAS floors shown on NATS/CAA charts are “pressure altitude transmitted by the accused’s own cheap unverified electromechanical instruments to a distant computer, electronically processed/interpolated according to some algorithm and indicated on a NATS screen” rather than “actual” feet above mean sea level? English jugdes are pretty good at making prosecutors prove that sort of thing.
That makes sense.
Dimme wrote:
But a Mode-S transponder still radiates altitude using Mode-C, right? If not, older secondary radar installations would not be able to receive altitudes from “pure” Mode-S transponders.
If a mode S transponder gets a mode C query, then it will give a mode C response, but if it gets a mode S query, it will give a mode S response…
Peter wrote:
It would be a really bad idea to have two altitude encoders, when your encoding altimeter is implementing the autopilot’s altitude preselect.
Why? If the transponder is entirely independent of the autopilot, it shouldn’t matter that much.
Peter wrote:
11 wires?
The GAE12 weighs about 2 grams and measures about 5mm x 12mm x 5mm and is fitted directly on the back of the transponder’s tray. It’s considerably smaller than the actual static air line barb you screw into the back of it, it’s worlds apart from the old Ameri-King shoeboxes. If you’re installing a GTX3x5, you have to either install the configuration module or the GAE12 there, so there isn’t even an opportunity cost in wiring it up (4 short wires into the main D-sub connector), since you have to put either it or the config module there. The GTX3x5 can also run off multiple pressure altitude sources in a priority order, so if one packs up you’ve got another.
In the US, controllers will request that transponders showing more than 200 feet off of the indication shown by the pilot to stop squawk mode C. However, that is a different issue than busting airspace. It is not clear if there is a buffer for this purpose. I would think that if it triggered an investigation, there would be more involved. VFR aircraft are not required to have the altimeter checked, so over time the indication might change and the pilot not be aware of a growing difference with the encoder..
But surely that doesn’t affect what encoder the transponder can use?
It would be a really bad idea to have two altitude encoders, when your encoding altimeter is implementing the autopilot’s altitude preselect.
the wiring from the KEA130A to the transponder will probably cost more and weigh more than a GAE12
11 wires?
When my old one sent an erroneous altitude output last year, there was no follow-up at all from anyone, despite it apparently being 2000’ off according to Manchester (Ronaldsway reported no problems on subsequent flights when I asked them to check what it was transmitting).
That’s probably because you live in the middle of nowhere 
Peter wrote:
Sure, though not all users have that option. For example King autopilot users are STC-bound to use the KEA130A encoding altimeter (or whatever).
But surely that doesn’t affect what encoder the transponder can use? (unless you want to do all the fancy extra stuff such as transmit your autopilot’s set altitude, which isn’t of that much benefit in a light aircraft) And the GAE12 is inexpensive enough (and small enough) to warrant using – the wiring from the KEA130A to the transponder will probably cost more and weigh more than a GAE12.
Peter wrote:
but the cases where somebody got away with a bust with an off spec transponder involved a formal Part M company altimeter test and a report to the CAA by a (gosh) “licensed engineer”, so I reckon 500 quid…
When my old one sent an erroneous altitude output last year, there was no follow-up at all from anyone, despite it apparently being 2000’ off according to Manchester (Ronaldsway reported no problems on subsequent flights when I asked them to check what it was transmitting).
