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by0ung(at)brigham.net
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 Posted: Sun Jan 15, 2006 5:11 pm Post subject: transponder cert |
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Try this: land at an airport with an approach control, and go to a point
on the runway or taxiway where the altitude MSL is known exactly, (it's
on the approach plate) then set your altimeter to the barometric
pressure on the current ATIS, and then ask ATC what your Mode C readout
is. (ground control can read it off the tower BRITE scope) Now you know
how accurate your Mode C is, and also know how accurate your altimeter
is. If either one is off, you need to go to an avionics shop.
xxxxxxxxxxxxxxxxx
if i remember corectly the blind encoders are set to 29.92 inches of mercury..... and will only match exactly if your altimiter is set for 29.92 inches..... also the blind encoders only change altitude every 100 ft. and i would have to do some research to remember if they switch at an even 100 ft or if they switch at the 50 ft intervals. ie 45050 4650 4750 vs 4500 4600 4700
if i have mis understood this information please someone set me straight.
boyd young
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Richard Pike
Joined: 09 Jan 2006
Posts: 1669
Location: Blountville, Tennessee
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| Posted: Sun Jan 15, 2006 6:30 pm Post subject: transponder cert |
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This may not answer your question, but here is how ATC does it:
ATC is interested in if your mode C is within 300 feet of what you claim
your altitude is. For instance, "Tri City Approach, Kolb Experimental
N420P is 20 SW of TRI, flying NE along the Interstate to VJI at 3,000,
squawking 1200." They give you a discreet code, ID you, and as long as
your mode C readout is within 300' or less of what you claim it is, they
are happy. If not, they will tell you that your mode C differs by more
than 300', stop altitude squawk, and then they will add your N # to the
monthly gripe sheet that goes to FSDO. Then you will get a query from
FSDO as to why your mode C is off, and they will advise you that they
are now anxiously awaiting a copy of the receipt you need to get from
your friendly local avionics shop, verifying that you have had your mode
C recalibrated and it is now within tolerance. (Whee!)
To avoid such expensive unpleasantries, that is why I suggested what you
copied below, because here is how it works in practice: To ensure that
your mode C is accurate, ATC sets the altitude filter limits of their
radar to the field elevation of their primary airport, so they can see
your mode C as you are going down the runway, and verify that it is the
same as the airport elevation. They know the airport elevation, so they
know what your mode C ought to be reading. Or if you call them while
airborne, they ask you "Say altitude." And you need to be within 300' or
less of what your mode C readout is.
As far as how the ATC interrogator compensates for the aircraft's mode C
always being on 29.92, and compensating for the variation between the
barometric pressure and the fixed setting - I called out to the tower
and none of my old buds knew how it worked, just that it did. (Sort of
like automatic transmissions?)
Grey Baron? Input? You da geek on this one-
Irrespective of the technical details, that is how the system works, and
that is what they look for. And you want your altimeter to agree with
what your mode C is squawking. Setting your altimeter to the current
ATIS and field elevation while you are sitting on a taxiway of known
elevation at that primary airport and then cross checking with ATC to
see if everything agrees will work every time, trust me.
Richard Pike
30 years ATC, Tri City Regional Airport (retired)
b young wrote:
| Quote: |
Try this: land at an airport with an approach control, and go to a point
on the runway or taxiway where the altitude MSL is known exactly, (it's
on the approach plate) then set your altimeter to the barometric
pressure on the current ATIS, and then ask ATC what your Mode C readout
is. (ground control can read it off the tower BRITE scope) Now you know
how accurate your Mode C is, and also know how accurate your altimeter
is. If either one is off, you need to go to an avionics shop.
xxxxxxxxxxxxxxxxx
if i remember corectly the blind encoders are set to 29.92 inches of mercury..... and will only match exactly if your altimiter is set for 29.92 inches..... also the blind encoders only change altitude every 100 ft. and i would have to do some research to remember if they switch at an even 100 ft or if they switch at the 50 ft intervals. ie 45050 4650 4750 vs 4500 4600 4700
if i have mis understood this information please someone set me straight.
boyd young
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_________________
Richard Pike
Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0
Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing. |
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jlbaker(at)telepath.com
Guest
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| Posted: Sun Jan 15, 2006 7:57 pm Post subject: transponder cert |
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| Quote: | stop altitude squawk, and then they will
add your N # to the monthly gripe sheet that goes to FSDO.
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Dang. Us ex Mil-ATC types just let it go at stopping the squawk......
| Quote: | As far as how the ATC interrogator compensates for the aircraft's
mode
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| Quote: | C always being on 29.92, and compensating for the variation
between
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| Quote: | the barometric pressure and the fixed setting
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http://tinyurl.com/92jev
A not too technical expalnation...disregard the 200 foot issue
on discrepancy, it's 300 ft as mentioned.
Jim Baker
580.788.2779
'71 SV, 492TC
Elmore City, OK
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a58r(at)verizon.net
Guest
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| Posted: Mon Jan 16, 2006 8:00 pm Post subject: transponder cert |
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It was alleged by Rev Pike that I should know the method used at the
ATCBS interrogator site in setting the "zero" altitude. Perhaps his
memory is more solid-state then mine, because it has been some 55
years since I cogitated about ATCRS. It's like someone asking an OF
a question about modern computer memory, when his experiences ended
with ATARI.
The station setting refers to 29.92 in Hg. Digital circuitry
automatically alters (add or subtracts) the site pressure setting for
display on the controllers' scopes. The plane's Mode C altitude
sensor is based (hardwired) on 29.82, however the pilot's altitude
readout displays his altitude as referenced to baro setting at the
departure point (or enroute when given a different alt. set.).
There are two classes of alt encoders: mechanical (a bellows drives a
code wheel) or the pressure-chip design (an integrated circuit chip
with pressure sensitive component). These are called Blind encoders
because there is no outward indication. The pilot's readout can have
hands driven by the mechanical type, with a setting nob that only
affects his display.
And yes, the hundreds may click over at 50', considering the short
term accuracy of the alt. encoders and their vibrating mounting.
Once I had a stuck D4 pulse in my altitude reply, causing O'Hare to
think I was at some 30,000' inbound in a Cardinal...my alt said 4,000.
Bob N.
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