[DanH]

Wrapping up various loose ends on the new -8, including a transponder/Mode C check.

91.217 requires data correspondence (within 125 feet) between transmitted altitude and indicated altitude, which is certainly a sensible requirement given separate, unconnected devices (altitude encoder and mechanical altimeter). However, times change and so does the equipment. My GRT EFIS incorporates an internal pressure sensing device which supplies an electronic output to both the pilot's display and the GTX327 transponder. There is only one pressure measuring device, not two. Is it possible to transmit an altitude different than the altitude displayed on the screen?

 

[Ironflight]

Is it possible to transmit an altitude different than the altitude displayed on the screen?

It's digital, so if the transponder is getting anything other than garbage, it is going to see exactly what the EFIS puts out Dan - I suspect that you already suspected this....

I could postulate in insanely smart failure that might fool it, but it would be well below my "worry line"....

Paul

 

[Rick_A]

Dan,You may have already done this but put the 327 in the function mode that displays Pressure Altitude and set the EFIS to 29.92. If things are working correctly, the PA displayed on the 327 will match the altitude displayed on the EFIS.

With the GRT as the encoder, these 2 number should always match. There may be a slight difference because the 327 rounds to 25' increments if I remember correctly.

 

[Walt]

The altitude data will be the same, so assuming the data is getting to the transponder uncorrupted, they should be the same. The check confirms the xpdr is actually transmitting the correct coded altitude, this test is both a transponder check and encoder check.

 

[Walt]

With the GRT as the encoder, these 2 number should always match. There may be a slight difference because the 327 rounds to 25' increments if I remember correctly.

The 327 rounds to 100' increments in mode 'C'.

 

[SteinAir]

Maybe Carlos can chime in about their system, but as a whole what you see is what is transmitted. There are variations in how the EFIS guys do temperature compensations, etc.. but on the average the aformentioned statement is true.

That being said, if you have access one of the newer generation test boxes (we use an IFR6000), you can plug the efis or encoder directly into the test box or in between the efis and the transponder, then read the raw data either serial or parallel to confirm any issues or accuracy of the data.

As other mentioned, if you set the EFIS to 29.92 it should as a whole equal what is displayed on the TXPDR and is being sent to it.

My 2 cents as usual.

Cheers,
Stein

 

[CarlosF@grtavionics]

Standard vs. Current Barometric Pressure

Is it possible to transmit an altitude different than the altitude displayed on the screen?

The GRT EFIS transmits standard barometric pressure (29.92 in hg or 1013.25 millibars). This is what the transponder uses and displays.

The GRT EFIS altimeter (any sensitive altimeter) is to be set (by the pilot) to current reported barometric pressure below 18,000 feet. At 18,000 feet (FL180) and above the altimeter is to be set by the pilot to standard pressure.

The transponder and altimeter will only indicate the same altitude when the altimeter is set to standard barometric pressure (29.92 in hg or 1013.25 millibars).

I hope this helps and I understood the question correctly.

Best regards,

Carlos Fernandez
GRT Avionics

 

[DanH]

Thanks guys.

New question (or confirmation) please. For VFR operations I need:

(1) a transponder check, and
(2) a check of encoded altitude

...which can be done with test equipment connected to (or receiving from) the transponder's antenna output. I gather there is no reason to remove any equipment from the aircraft, unless of course it fails the test?

Because the EFIS serves as the altitude encoder, I assume an accurate calibration of its altitude function (with pressure test equipment) is also required?

Anything else?

 

[Walt]

Thanks guys.

New question (or confirmation) please. For VFR operations I need:

(1) a transponder check, and
(2) a check of encoded altitude

...which can be done with test equipment connected to (or receiving from) the transponder's antenna output. I gather there is no reason to remove any equipment from the aircraft, unless of course it fails the test?

Because the EFIS serves as the altitude encoder, I assume an accurate calibration of its altitude function (with pressure test equipment) is also required?

(1) True, 43 Appendix F
(2) True, 91.217 correspondence check; ref 43 Appendix E, Para (C)

Seems wierd but for VFR you only need to verify that the xpdr is putting out altitude that is within 125 ft of your altimeter, so really your altimeter can be off and you can still pass the test. Not recommended if you fly in or around class B especially.

 

[B Cunningham]

The GRT EFIS transmits standard barometric pressure (29.92 in hg or 1013.25 millibars). This is what the transponder uses and displays.

The GRT EFIS altimeter (any sensitive altimeter) is to be set (by the pilot) to current reported barometric pressure below 18,000 feet. At 18,000 feet (FL180) and above the altimeter is to be set by the pilot to standard pressure.

The transponder and altimeter will only indicate the same altitude when the altimeter is set to standard barometric pressure (29.92 in hg or 1013.25 millibars).

I hope this helps and I understood the question correctly.

Best regards,

Carlos Fernandez
GRT Avionics

Wondering if anyone with the GRT EFIS and the 327 transponder has experienced intermittent mode "C" output? I had mine go inoperative for about 30 minutes at 8500', then come back during the descent to land... My original Alt Encoder from GRT had to be replaced -- it never worked at all. Now I'm getting this.... Ideas?