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Mode C Altitude Reporting And ATC  
User currently offlineModesto2 From United States of America, joined Jul 2000, 2819 posts, RR: 5
Posted (8 years 10 months 2 weeks 19 hours ago) and read 9787 times:

My understanding of mode C transponders is that they send the controller pressure altitude. Of course, this often differs from indicated altitude. Once reaching the controller's computer, that computer then adjusts the pressure altitude for the current barometric pressure. Therefore, the indicated altitude should equal the altitude displayed on the controller's screen. Is this correct? Earlier today, my mode C was apparently adding 300 feet to the indicated altitude so controllers kept asking me to verify my altitude (i.e. indicated = 6000 but controller's screen showed 6300).

On a side note, the more I fly, the more I want to know about air traffic control and ATC procedures. Are there any good websites or books re: air traffic control procedures. Understanding the "other" side should hopefully make me a better and well-informed pilot. Thanks.

16 replies: All unread, jump to last
 
User currently offlineBigJimFX From United States of America, joined Jun 2005, 321 posts, RR: 0
Reply 1, posted (8 years 10 months 2 weeks 19 hours ago) and read 9786 times:

As far as I know that is true. All the planes I've flown with Mode C xponders show a press. alt. on it. ATC will probably ask to verify altitude if it differs by 100-300 ft. Part of my checklist in a C172 was to make sure that the altimeter, after adjusting for Baro. press. indicated feild elevation within 7 ft.

If you have any questions about ATC procedures, check out the FAA website. All of the regs on there.

Happy Flying airplane 



I'd like to thank me for flying Me Airways...
User currently offlineMir From United States of America, joined Jan 2004, 21882 posts, RR: 55
Reply 2, posted (8 years 10 months 2 weeks 15 hours ago) and read 9741 times:

Quoting BigJimFX (Reply 1):
Part of my checklist in a C172 was to make sure that the altimeter, after adjusting for Baro. press. indicated feild elevation within 7 ft.

Did you perhaps miss a 5 in there? I've been taught 75 feet.

Quoting Modesto2 (Thread starter):
On a side note, the more I fly, the more I want to know about air traffic control and ATC procedures. Are there any good websites or books re: air traffic control procedures. Understanding the "other" side should hopefully make me a better and well-informed pilot. Thanks.

Listen to some live feeds (many available at www.liveatc.net ), and give it a shot, either on FS multiplayer or with a dedicated ATC sim (ATC Simulator 2 is out, at www.atcsimulator.com ). I've learned a lot from the feeds, and doing some ATC in multiplayer FS sessions has really allowed me to put what I hear into perspective.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineBigJimFX From United States of America, joined Jun 2005, 321 posts, RR: 0
Reply 3, posted (8 years 10 months 2 weeks 13 hours ago) and read 9721 times:

Quoting Mir (Reply 2):
Did you perhaps miss a 5 in there? I've been taught 75 feet

Yeah... Sorry about that, I was in a hurry to get out of the house. But yeah, I ment 75ft from field elevation.



I'd like to thank me for flying Me Airways...
User currently offlineJspitfire From Canada, joined Feb 2005, 308 posts, RR: 2
Reply 4, posted (8 years 10 months 2 weeks 1 hour ago) and read 9693 times:

Quoting BigJimFX (Reply 1):
ATC will probably ask to verify altitude if it differs by 100-300 ft.

I decided to look this up in the Canadian Regulations, and now I'm rather confused. It says:

VERIFY YOUR ALTITUDE - This phraseology may be used when it is necessary to validate altitude readouts by comparing the readouts value with an altitude reported by the aircraft. An altitude readout is considered valid if the readout value does not differ from the aircraft-reported altitude by more than 200 ft, and invalid if the difference is 300 ft or more.

So what if the difference is 250 ft? Is that valid, or not?

Jason


User currently offlineRomeoMike From Canada, joined Nov 2005, 36 posts, RR: 0
Reply 5, posted (8 years 10 months 1 week 6 days 22 hours ago) and read 9672 times:

ATC radar reports altitude in hundreds of feet, and pilots report it in hundreds of feet, so I doubt 250 would come up that often  Smile

User currently offlineMir From United States of America, joined Jan 2004, 21882 posts, RR: 55
Reply 6, posted (8 years 10 months 1 week 6 days 22 hours ago) and read 9670 times:

Quoting Jspitfire (Reply 4):
So what if the difference is 250 ft? Is that valid, or not?

A transponder only reports altitude in hundreds of feet, so it can't send a difference of 250ft. It's either 200 or 300.

ATC reads your altitude in hundreds of feet as well. (5,000ft is 050, 19,000ft is 190, etc.)

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinePilotpip From United States of America, joined Sep 2003, 3152 posts, RR: 10
Reply 7, posted (8 years 10 months 1 week 6 days 15 hours ago) and read 9624 times:

If you want to learn more about ATC, tour a Tracon facility. I've toured STL tracon twice now and it's really neat to see things from their side. The controllers I've met have been very friendly and very informative. You'll be a better pilot after being in the room and watching a controller vector an aircraft around for a few minutes and follow the view they see. They are paid to help you out, and understanding why they do what they do goes a long way.

I'm also reassured that the voices I've heard in my head for the last 5 years are real people  Smile Putting faces and names with the voices was really a highlight. 35 minutes after getting vectored all over town I got to ask the controller why she did it and it just so happened that a similiar scenario was playing out with some bizjets and a couple little guys shooting approaches so I got a real time example as it developed.



DMI
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 8, posted (8 years 10 months 1 week 5 days 17 hours ago) and read 9566 times:

By the way, Mode C transponders only report pressure altitude calibrated to 29.92". The ATC decoding system which receives the Mode C reply automatically applies the appropriate correction for local barometric pressure.


Position and hold
User currently offlineDw747400 From United States of America, joined Aug 2001, 1265 posts, RR: 1
Reply 9, posted (8 years 10 months 1 week 5 days 16 hours ago) and read 9555 times:

It could be a problem with the calibration of your transponder. My transponder went haywire once while under the BWI bravo rings and announced I was at 14,000 feet. According to ATC, we managed to scare a WN flight enough that he took evasive action.

Needless to say, ATC figured out pretty fast that our 172 hadn't made it to 14,000 within three minutes of takeoff, but not quite fast enough for the 737 that was apparently passing through 14,000!



CFI--Certfied Freakin Idiot
User currently offlineIAHFLYR From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 10, posted (8 years 10 months 1 week 5 days 5 hours ago) and read 9530 times:

The altitude verification can and is used often when a controller is looking to validate that you are in fact level at your assigned altitude should there be an issue where the altitude displayed was different for a few radar sweeps and it set off the error alter that centers have and no terminal facilites are getting.

Mode C altitude can be off up to 300'! At the 300' point the pilot is asked "to verify altitude" and if it is in fact 300' or more off from the altitude the pilot says then the next thing is "stop altitude sqwakkkkkk, altitude differs by more than 300'" and then you get the "ya gonna get that fixed anytime soon"!  Smile



Any views shared are strictly my own and do not a represent those of any former employer.
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 11, posted (8 years 10 months 1 week 5 days 5 hours ago) and read 9526 times:

Quoting IAHFLYR (Reply 10):
stop altitude sqwakkkkkk

Funny to hear on the radio, too. Most encoding altimeters in light aircraft are blind (they provide no visual readout of their indicated altidue.) "Seeing" encoding altimeters are available, though, and that's why it's a good idea to spring for one. They can also serve as a backup to the primary altimeter in case of a failure, provided you can do a little math to compensate for non-standard atmospheric pressure.



Position and hold
User currently offlineMir From United States of America, joined Jan 2004, 21882 posts, RR: 55
Reply 12, posted (8 years 10 months 1 week 5 days 4 hours ago) and read 9522 times:

Quoting Bri2k1 (Reply 11):
They can also serve as a backup to the primary altimeter in case of a failure, provided you can do a little math to compensate for non-standard atmospheric pressure.

Don't they work off of the regular altimeter? And thus, wouldn't they be useless if that were to fail?

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineDw747400 From United States of America, joined Aug 2001, 1265 posts, RR: 1
Reply 13, posted (8 years 10 months 1 week 5 days 3 hours ago) and read 9516 times:

Quoting Mir (Reply 12):

Don't they work off of the regular altimeter? And thus, wouldn't they be useless if that were to fail?

At least in the 152s and 172s I fly, they are connected to the pitot static system. Nonetheless, if the altimeter itself fails through no fault of the pitot-static system, it could, in theory, be used as a backup altimeter.



CFI--Certfied Freakin Idiot
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 14, posted (8 years 10 months 1 week 4 days 19 hours ago) and read 9478 times:

Most advanced planes will have redundant pitot-static systems, too. Of course, they will also have multiple altimeters to go along with them.


Position and hold
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Reply 15, posted (8 years 10 months 1 week 2 days 23 hours ago) and read 9447 times:

Hot Air Balloon flying in controlled airspace you need a transponder and ideally a C sqwaker in order to be seen. Some airports allow you to fly in their airspace giving updates of your height and position without a transponder, but that has gone wrong and the consequences are messy....

User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 16, posted (8 years 10 months 1 week 2 days 19 hours ago) and read 9436 times:

In the US, a transponder with mode C capabilities is required in class A, B, and C airspace, and within 30 NM of the primary class B airport, but not in class D, E, or G. Class D and E are considered controlled airspace, and mode C is not required.

The transponder reply appears on the controller's screen next to the radar return, but a radar echo can be detected without a transponder. I believe manned free balloons fall into that category.

That being said, there are "legal but stupid" minimums. For example, flying in class G airspace VFR is legal during the day with 1 SM visibility and staying clear of clouds, but it's not smart to be flying around at 120 knots, 600 feet AGL, maneuvering around clouds, with less than 30 seconds of forward visibility. Operating a balloon in busy airspace without mode C may be legal, but not smart.



Position and hold
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