Mode C is reading high....

[Dico]

Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

Thanks,

Dico

[Mark Hansen]

On 03/11/07 07:49, Dico wrote:
> Hello,
>
> I've noticed during a flight the other day to Columbus that our Mode C
> was reading high... by about 250 feet. The altimeter seems to be
> indicating fine as on the ground, with the correct altimeter setting,
> it shows the correct altitude. As well, flying the ILS, i make a habit
> to check the altitude when crossing the FAF and it is correct too.
>
> Any thoughts on what the issue is?

The transponder uses its own altitude encoder, not the plane's altimeter.

I think there's a regulation which states that the transponder must be
within 100 feet of actual pressure altitude, so you probably should have
it looked at.

>
> Thanks,
>
> Dico
>



--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

[RST Engineering]

Not necessarily. There are optical encoding altimeters. They are not
common, but they do exist.

Jim



"Mark Hansen" > wrote in message
...


>> Any thoughts on what the issue is?
>
> The transponder uses its own altitude encoder, not the plane's altimeter.
>

[Michelle P]

Dico wrote:
> Hello,
>
> I've noticed during a flight the other day to Columbus that our Mode C
> was reading high... by about 250 feet. The altimeter seems to be
> indicating fine as on the ground, with the correct altimeter setting,
> it shows the correct altitude. As well, flying the ILS, i make a habit
> to check the altitude when crossing the FAF and it is correct too.
>
> Any thoughts on what the issue is?
>
> Thanks,
>
> Dico
>
You could have water in the line. This usually causes strange readings.
If it is past the Altimeter then it will not affect it.

Michelle P

[Dave Butler]

Dico wrote:
> Hello,
>
> I've noticed during a flight the other day to Columbus that our Mode C
> was reading high... by about 250 feet. The altimeter seems to be
> indicating fine as on the ground, with the correct altimeter setting,
> it shows the correct altitude. As well, flying the ILS, i make a habit
> to check the altitude when crossing the FAF and it is correct too.
>
> Any thoughts on what the issue is?

The altitude your mode C reports comes from a separate pressure
transducer, not your flight instruments, so the accuracy/precision of
your mode C altitude is not related to the accuracy/precision of your
flight instrument.

The mode C reported altitude is pressure altitude, so needs to be
corrected for barometric pressure for comparison with your flight
instrument. The altitude shown on ATC's display is corrected to the
local altimeter setting, but the raw data from your transponder is not.

Mode C altitude typically has a resolution of 100 feet, which gives rise
to possible sizable rounding errors. Newer pressure transducers used
with TSO146 GPSs have a resolution of 10 feet.

[Robert M. Gary]

On Mar 11, 7:49 am, "Dico" > wrote:
> Hello,
>
> I've noticed during a flight the other day to Columbus that our Mode C
> was reading high... by about 250 feet. The altimeter seems to be
> indicating fine as on the ground, with the correct altimeter setting,
> it shows the correct altitude. As well, flying the ILS, i make a habit
> to check the altitude when crossing the FAF and it is correct too.
>
> Any thoughts on what the issue is?

1) the attitude from your transponder is pressure altitude, ATC
corrects for pressure.
2) I learned long ago not to try to debug issues with transponders,
they seem to be well out of the range of non-radio,electronics people.
However, generally adjustments are very inexpensive. In fact, most
places that quote fixed price IFR certs include basic adjustments to
the transponder. Every time I bring my plane in for IFR cert they have
to do some tuning of the unit.

-Robert

[dlevy]

......somebody in rec.aviation.ifr would just *love* this one.....

"Robert M. Gary" > wrote in message
oups.com...
> 1) the attitude from your transponder is pressure altitude, ATC
> corrects for pressure.
><snip>
> -Robert
>

[Newps]

Why?



dlevy wrote:
> .....somebody in rec.aviation.ifr would just *love* this one.....
>
> "Robert M. Gary" > wrote in message
> oups.com...
>> 1) the attitude from your transponder is pressure altitude, ATC
>> corrects for pressure.
>> <snip>
>> -Robert
>>
>
>

[dlevy]

Transponder attitude is not corrected by ATC. Never has and never will be.
I, as the pilot in command, control transponder attitude. I'll shut the
damn thing off.

"Newps" > wrote in message
. ..
> Why?
>
>
>
> dlevy wrote:
>> .....somebody in rec.aviation.ifr would just *love* this one.....
>>
>> "Robert M. Gary" > wrote in message
>> oups.com...
>>> 1) the attitude from your transponder is pressure altitude, ATC
>>> corrects for pressure.
>>> <snip>
>>> -Robert
>>>
>>

[Robert M. Gary]

On Mar 12, 3:38 pm, "dlevy" > wrote:
> Transponder attitude is not corrected by ATC. Never has and never will be.
> I, as the pilot in command, control transponder attitude. I'll shut the
> damn thing off.

How do you control the altitude the transponder puts out?
-Robert, CFII

[Steven P. McNicoll]

"dlevy" > wrote in message
...
>
> Transponder attitude is not corrected by ATC. Never has and never will
> be. I, as the pilot in command, control transponder attitude. I'll shut
> the damn thing off.
>

That'll show 'em. Transponders with attitude. That's all we need.

[RST Engineering]

Read it again, Gary. You don't control the altitude, but you can control
the ATTITUDE, which is the grammar error the OP made.

Jim




"Robert M. Gary" > wrote in message
ups.com...
> On Mar 12, 3:38 pm, "dlevy" > wrote:
>> Transponder attitude is not corrected by ATC. Never has and never will
>> be.
>> I, as the pilot in command, control transponder attitude. I'll shut the
>> damn thing off.
>
> How do you control the altitude the transponder puts out?
> -Robert, CFII
>

[John Godwin[_2_]]

"Steven P. McNicoll" > wrote in
link.net:

> That'll show 'em. Transponders with attitude. That's all we need.
>

Boy, nothing like a ****ed off transponder.