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#11
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Transponder problems with ATC COMS - Video
The only time blind encoders will accurately match the altimeter is when
the altimeter is dialed in at 29.92. Also remember that encoders only report in 100 ft increments. You are allowed to deviate between the altimeter and the encoder by 125 feet. If this is off then either the altimeter is off or the encoder is off. If the altimeter reads correct altitude while parked at the airport with a current baro setting, then I suspect the encoder is off. If the static line had a leak or is disconnected from any instrument in the aircraft then both the altimeter and the encoder should be off the same amount since they are using cabin altitude and should match altitudes. That 125 foot difference can be important. you may be indicating 2990 feet while your transponder is reporting 3100 feet. If the class B airspace starts at 3000, then you may be validation of busting class B and be totally legal. BT wrote: Blind encoders are just that.. Blind, they do not care what the current proper "altimeter setting" is. ATC decoders know the altimeter settings for a given area and automatically apply the correction before displaying. I've watched entire digital radar displays report everyone 300ft off instantaneously as the altimeter setting table was loaded into the system, a momentary blink when there was no corrections, and then the updates as the new setting corrections were applied to the ModeC data received. ATC will issue the altimeter setting to the pilot in the event the "displayed" altitude does not agree with the pilot reported altitude just in case the pilot has miss set or not set the altimeter for the correct pressure. In such cases normally about a 200-300ft difference, the pilot dials in the correct pressure and then either reports a new altitude that may agree with the reported altitude, or descents/climbs to the reported altitude and the ModeC now agrees. It appears that the Garmin 3xx series transponder may have an error in it's sensor, a leak in the static line to the sensor or something creating a vacuum near the sensor as it was reading about 700-800ft to high. You state that the altimeter agreed with the ASOS setting on the ground. At ground level and not moving (ground speed = zero) does the reported FL altitude display agree? If they agree, then I would suspect that the static line might be off the back of the encoder transponder and that in-flight the encoder is sensing cabin pressure and not outside static pressure. In flight in C-172s, the inside cabin is slightly lower pressure than outside due to vacuum effects of a leaky fuselage. I'm somewhat surprised, if the pilot is part owner and flies this aircraft regularly, she does not know the operation of her transponder. She can select STBY, ALT and put in the code, but she does not understand it's operation. A nice thing about those transponders.. the VFR button will automatically load 1200. BT "Viperdoc" wrote in message ... I have flown with a couple of Garmin 327's and now have two 330's. The pressure altitude readout on the transponder comes (as I recall) from the blind encoder. It is obviously not connected to the altimeter setting. So, the encoder could be culprit for the mis reported altitude. I suppose there could be a difference in altitudes of 700 feet if the day was very non-standard (such as very cold, high pressure, etc). |
#12
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Transponder problems with ATC COMS - Video
On Nov 24, 7:15*pm, "Viperdoc" wrote:
I have flown with a couple of Garmin 327's and now have two 330's. The pressure altitude readout on the transponder comes (as I recall) from the blind encoder. It is obviously not connected to the altimeter setting. So, the encoder could be culprit for the mis reported altitude. I suppose there could be a difference in altitudes of 700 feet if the day was very non-standard (such as very cold, high pressure, etc). That is for sure. The altitude used by the transponder for all us flying non-digial, non-flat screen bug smashers is not the same necessarily as what you see on the panel. However, in my experience when ATC is getting the wrong altitude its almost always either a calibration issue or a wiring (i.e. dirty connectors, etc) issue. Its usually something a good avionics shop can fix in minutes. -robert |
#13
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Transponder problems with ATC COMS - Video
A Lieberma wrote:
On Nov 24, 7:54 pm, "Robert M. Gary" wrote: My guess is that its a new C-172 with the G1000 with the transponder just built in. There is a button along the bottom of the PFD that brings up transponder options. The transponder is no longer a separate unit. -Robert Robert, One thing I can say, it wasn't the G1000 I put in my reply to BT it was a Garmin product looking at pics on Google. I hope instructors learn from this to take the time to teach how to stop the mode C. In my training, I never got such training. My transponder on my plane is the king analogue flaver, just off, stby, alt and test and turn knobs to squawk. Nothing to think about on my transponder but when you start getting digitalized with more options, maybe some basic transponder 101 with explanations on why you do what should be in order for flight training??? Yes, instructors should cover things like this, but the pilots should also read the avionics manuals! Most people I know have never read the manual for their car let alone their avionics. There is simply no excuse for this. Matt |
#14
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Transponder problems with ATC COMS - Video
On Nov 24, 11:38*pm, "BT" wrote:
I'm somewhat surprised, if the pilot is part owner and flies this aircraft regularly, she does not know the operation of her transponder. She can select STBY, ALT and put in the code, but she does not understand it's operation. A nice thing about those transponders.. the VFR button will automatically load 1200. Thanks BT for the informative posts. My plane doesn't have all this fancy gagetry, so I was literally winging it on a prayer and things just didn't add up in my head, thus shutting the Xponder off.. With all the nuances of learning how to fly, learning how to own a plane, it does not surprise me she didn't know the operation of her transponder. Her "lesson" was set it and forget it OR let it do everything automatically. If you think about it, at least for me, even in my flight lessons, I never got any instructions on how to operate the transponder other then coding in the numbers or doing "one button operations". Never once was I ever shown how to stop Mode C operations. |
#15
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Transponder problems with ATC COMS - Video
On Nov 25, 12:41*am, Dave wrote:
That 125 foot difference can be important. you may be indicating 2990 feet while your transponder is reporting 3100 feet. If the class B airspace starts at 3000, then you may be validation of busting class B and be totally legal. Dave, this was my precise reason for shutting the Xponder down especially thinking ahead for the return trip home. KMBO is under the floor of KJAN charlie airspace and I was concerned that if we flew at 1500, ATC would see us at 2200. In my mind, no information is better then wrong information with what cards were dealt to me even if I wasn't PIC. |
#16
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Transponder problems with ATC COMS - Video
On Nov 25, 7:23*am, Voyager wrote:
Yes, instructors should cover things like this, but the pilots should also read the avionics manuals! *Most people I know have never read the manual for their car let alone their avionics. *There is simply no excuse for this. Agree Matt for airplane owners, but I guess some forgiveness should be given for "renters" for practicality purposes. I don't know of any airplane that has all the user manuals for all the avionics in the plane. I would suspect checkout should include instructions on Xponder operations and during training as well. I never got such training. |
#17
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Transponder problems with ATC COMS - Video
"Dave" wrote in message
... You are allowed to deviate between the altimeter and the encoder by 125 feet. Says who? |
#18
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Transponder problems with ATC COMS - Video
On Nov 25, 9:05*am, "Mike" nospam @ aol.com wrote:
"Dave" wrote in message ... You are allowed to deviate between the altimeter and the encoder by 125 feet. Says who? Part 91.217 para b, and that can be accurate 95% of the time. |
#19
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Transponder problems with ATC COMS - Video
"noname" wrote in message
... On Nov 25, 9:05 am, "Mike" nospam @ aol.com wrote: "Dave" wrote in message ... You are allowed to deviate between the altimeter and the encoder by 125 feet. Says who? Part 91.217 para b, and that can be accurate 95% of the time. Thanks. I was confusing this with the 75' thumb rule for the altimeter. |
#20
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Transponder problems with ATC COMS - Video
On Nov 25, 5:42*am, A Lieberma wrote:
Agree Matt for airplane owners, but I guess some forgiveness should be given for "renters" for practicality purposes. I don't know of any airplane that has all the user manuals for all the avionics in the plane. *I would suspect checkout should include instructions on Xponder operations and during training as well. I never got such training. Ok, so if you come in and want to rent a C-172 but have never flown with a 430 should I require you to do a 5 hour training course? I thought most pilots wanted simplier checkouts, not longer. I think the rated pilot renter is responsible for figuring out what he doesn't know (and needs to know) and figuring the answer out, either by looking up on-line manuals or asking a CFI. Its not fair to require 5 to 10 hour checkouts in every 172 just because some pilots will not otherwise learn the avionics specific to the aircraft. -Robert, CFII |
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