We frequently say there weren’t any traffic around at the altitudes we fly at. Now that I frequently hang out around FL200 on my longer flights I do see quite a bit of the big boys pass me with 1000ft difference in altitude.
I’ve also was made aware of the importance of a correctly working encoding altimeter for the transponder. When traveling at eg. FL180 mine likes to encode up to 200ft too much and that does upset the Boeing and Airbus crews even when passing more than 4 NM away.
The encoder works fine at lower altitudes but up high the encoding fluctuates between 100 and 200 too much. Good to have two other altimeters to be able to ensure ATC I’m traveling at the right level. There was even a question about having a second transponder. Now I know why some have that. :-)
I use the up/down buttons on the KFC225 autopilot to adjust the actual altitude + / – 25ft until the FL readout on the transponder reads exactly right. Do you have such a FL display somewhere, Stephan?
Not many jets fly enroute at FL200 but I recall being on one Gatwick to Brussells A320 which flew FL190 the whole way (the pilot told me afterwards).
I have a Garmin GTX330 and yes I’ve been looking for the feature to adjust the encoding but didn’t find it.
Do you have a reason to believe your transponder is encoding incorrectly?
On an N-reg you have a 2-yearly transponder check and it has to meet a spec all the way up to the aircraft certified ceiling (or something like that).
I think the reason for having a second transponder is in case yours fails and then you will AFAIK be denied access to the Eurocontrol system.
Stephan_Schwab wrote:
I’ve also was made aware of the importance of a correctly working encoding altimeter for the transponder.
I accidentally noticed an error of my SDDR 31 encoder at nearly the same time. The reading on the Trig-Transponder was 200 ft too low at QNH 1013, which is out of tolerance. Finding the encoder was not easy, but I did. The avionics shop can test and calibrate the encoder again for 120 Euro.
I have never noticed another aircraft between FL 140-200. But I remember a flight with BA from London Heathrow to Tegel at FL 170 the entire route.
A lot of the turboprops between UK and Belgium/Netherlands fly high teens/low 20s. And I have also seem a number of CAT kept down low when the upper sectors are busy. Schiphol to LCY for example they aren’t in cruise for long.
Hi Stephan, it seems that there’s a discrepancy between the pressure altitude indicated by your transponder and your altimeter?
What is your transponder showing when flying at FL200? Is it showing FL200 or FL202 ? On the function display of the GTX 330 you can switch to pressure alt:
When flying at the service ceiling with my aircraft (DA40) there is a 200ft difference between the primary (G1000) altimeter and the standby altimeter.
On the ground they indicate the same value. I always fly the altitude which is displayed on the G1000, as that is what is radiated by the transponder.
Peter wrote:
I use the up/down buttons on the KFC225 autopilot to adjust the actual altitude + / – 25ft until the FL readout on the transponder reads exactly right. Do you have such a FL display somewhere, Stephan?
Of course if the altitude encode is wrong then you are flying at precisely the wrong altitude.
Getting the transponder to tell ATC that you are at exactly their assigned level will keep them happy, but it doesn’t do anything for safety.
The fact that altimeters show differing readings demonstrates exactly why RVSM certification is not straightforward, and things like static port positioning is so important.
Neil wrote:
things like static port positioning is so important
While I agree this is important, it’s unlikely the issue, I’d think that all the encoders in an SR22 are fed by the same static port lines, and what this here is is a discrepancy between the different altitude encoders / instruments.
To me it looks like Stephans plane is out of spec, Part 43 Appendix E (c) states that the altitude displayed in the altimeter is no more than 125ft different than the one reported by the Mode C transponder.
This requirement is very hard if you use different encoders for the altimeter and the transponder – the absolute error allowed exceeds this above FL200.
No, the altitude on the transponder is corrected on the radar with the actual local QNH. It sends out in 1013 Hpa. Don’t mess with that.
