Do you monitor mode C ?

[Acker Demick]

I am a PPL who routinely bimbles about the South of England in a C150. I have recently invested a minor fortune to install transponder + alt encoder so that (i) I show up on TCAS and (ii) the local LARS units know what altitude I'm at (e.g. above/below their zones, not barging through them) even if I havn't called up for a Flight Information Service. However, in the few weeks I have been squawking, I have got the strong impression that most controllers have Mode C turned off on their radar screens, and just rely on verbal altitude reports from pilots. For example: on two occasions I have asked controllers to confirm mode C reception (during quite RT periods), and each time the controller needed time before responding (changing display modes?). Also I have heard ATC warning othere a/c of traffic, which I'm sure was me, with "no altitude information". Am I right, or have I got the wrong end of the stick? Is there a problem of screen clutter that drives you to disable mode C returns? If so, is there any point in making Mode C compulsory for light aircraft, as I understood will happen soon? Curious to hear the ATC view.

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If God had meant us to fly he would have given us more money

[This message has been edited by Acker Demick (edited 20 May 2000).]

[This message has been edited by Acker Demick (edited 20 May 2000).]

[This message has been edited by Acker Demick (edited 20 May 2000).]

[Spotter]

Assuming that you were squawking 7000, then it is possible that the ATCO may have had his radar set up to filter out 7000 squawks. (Why he would do this in a LARS or airfield environment i don't know, but i believe that it is used by area controllers to avoid loads of unwanted returns below CAS from obscuring airways traffic).

If you're squawking 7000 then as you have not been identified and had your mode C verified the controller cannot rely on its readout, but would normally say something like "unidentified traffic in your 12 o'clock tracking left-right indicating 3000 ft not verified".

You may have too low for the secondary radar to get a good signal. Do you know where you were in relation to the radar? It may not have been at the airfield you were talking to,but could be some distance away. I expect you know that the farther away from the radar aerial you are the poorer the low level coverage is. Eg Birmingham. secondary radar is on Clee Hill (about 30nm west of the airport) so if you are 30nm east of Brum....

Don't know if any of that helps...have you thoroughly checked out the performance of you x-ponder?

[APP Radar]

1. In Portugal Mode C is mandatory for all aircrafts (including light a/c) flying within controlled airspace.
2. Controllers do monitor Mode C information for the aircrafts under their responsability (identified aircrafts).
3. When traffic information is given about one aircraft not identified altitude is not given because ATCo didn't had the chance to verify the information about Mode C.
4. If you we're squawking 2000 or 7000 the ATCo might have filtered you off or, even possible, the system is unable to show your Mode C due to the presence of other aircrafts with the same squawk.
5. Try squawking 7001 (non discrete code) and ask the controller if he's seing you on position XXXX and what is your Mode C indication.

[NudgingSteel]

As Spotter says, area controllers often filter 7000 squawks out since they're almost all at low level, hence of no interest to their airways traffic. Approach controllers, however, will often be keeping an eye on 7000 codes since they often call us up asking for a FIS or zone entry, the little monkeys! (only kidding!). In order to identify you, and if you've got a transponder, you'd be issued a discrete squawk. Depending on the speed of the secondary radar head, it can take a few seconds for the display to update, hence the delay you referred to.

Also bear in mind that a 7000 squawk on a VFR aircraft is about as vague as you can get - it just says to the radar and TCAS receivers "A plane's here". In order to positively identify you, or provide any service other than FIS, or to check your Mode C, I'd definitely be wanting to identify you with one of my own squawks, and watch it appear as I peer into the screen....be cautious of people giving you information based on an assumed ident!

Final point....at my (northern) airport we only filter out upper airway level stuff, hence we see everything at lower levels - I'd say Mode C is extremely useful, since even though you might be passing our zone without speaking to us, and hence we can't verify the accuracy of your Mode C, it still gives us a rough idea for traffic information / avoiding action purposes.
Hope this makes sense!
PS Don't use code 7001, cos in the UK, it's allocated to something - and it's not you and your Cessna!!!