Mode C 30 mile circle around Class B on charts ?

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Hi Phil
What is the 30 mile circle called Mode C drawn around Boston I see
on the chart/map ?
Is it basically mean all aircraft (even single engine) MUST have
a Mode C (sends out altitude) transponder if enter within 30 miles
of Boston/Logan ?

If your transponder does not have altitude reporting - you can't go
in there then ?
Will FAA ATC know if you don't have it and tell you to get out ?

I dont remember seeing this line drawn on the charts around Boston
back 20-30 years ago -
Did this line on charts begin around after 9/11/01 ?

I think I remember there was some strict condition back 20-30 yrs
ago still about having to have Mode C transponders to go into
Boston/Logan airport.
But dont think there was anything drawn on charts and it did not go
out as far as 30 miles.
Am I right about that ?

It looks like now that Hanscom is under/within this 30 mile circle
- so then do ALL aircraft have to have this Mode C transponder
onboard if they want to takeoff from ground at Hanscom ?
This is pretty restrictive isn't it ? And costly ?
Mode C transponder cost more $$ than basic transponders, don't they ?

-- jim kenn, March 6, 2010


To answer your question about whether Mode C (altitude encoding) transponders are costly... Given that Mode A transponders aren't manufactured anymore and could probably be obtained for free off eBay, I think it is unarguable that Mode C transponders are more expensive. Keep in mind that you need to get the transponder recertified as accurate every two years, which costs about $250.

To operate from Hanscom you do need a Mode C transponder in order to be legal (since we are within the Mode C veil).

If you don't have a Mode C transponder, will ATC "tell you to get out"? How could they? They have no idea what your altitude might be. They have no idea what frequency you're monitoring, if any.

I'm not sure how old the Mode C veil idea is. You're the first person who has ever asked me about it. Given that Mode C transponders are obsolete currently and that the newer fancier Mode S transponders were developed in the 1970s, I suspect that the Mode C veil is probably more than 20 years old.

-- Philip Greenspun, March 7, 2010

To answer the follow-up question below: "If ATC sees a plane (blip) in their 30 mile zone and no altitude encoding - then won't they just radio the pilot and tell him to get out ?" ...

ATC does not have the technical capability, as far as I know, to transmit on all VHF communication frequencies simultaneously. If you popped up near Minuteman Airport in Stow, in theory they could assume that you're monitoring the 122.8 CTAF frequency for 6B6 and try to tune one of their transmitters to 122.8 (I'm not even sure how easy it is for them to do that). But if you haven't been flying since 1980 and therefore didn't realize that Mode C was now required, it seems unlikely that you'd know that Boston Approach in that area tends to operate on 124.4. So there would be no point in them wasting radio time calling an unknown aircraft on that frequency.

-- Philip Greenspun, March 7, 2010

It's called a Mode C veil. I don't know when it was introduced, but I think it has been around since long before 2001.

You can read about the transponder requirement in FAR 91.215. (You can either buy a paper copy of the FAR/AIM or get it as a PDF from the FAA's web site.) The only exception for the Mode C veil is for aircraft that were certified without an engine-driven electrical system.

I think it is a reasonable regulation, especially when you consider that airport surveillance radar (ASR) can't determine the altitude of targets, only their horizontal position (azimuth and range). The Mode C transponder, which transmits your altitude, displays your altitude on the controller's display and significantly contributes to more accurate separation.


-- Todd Ramming, March 6, 2010

Thanks - I just dont remember back 25-30 yrs ago ever seeing this solid line on the charts and going 30 miles (?) out tagging it as Mode C But I do remember CFI saying back then a plane needs to have altitude encoding transponder to go in there. But I dont think back then it extended all the way out to 30 miles. Has it always been 30 miles out ? Back then I think Boston was called a TRSA and Hanscom type airports were TCA's(terminal control area) - was no Class B or D etc back then (pretty sure)

I didnt know the other type Mode A is now obsolete. Yeh I'm glad things are progressing and there is this Mode C altitude thing on all airplanes to hopefully make things better for us and ATC and hopefully safer overall.

-- jim kenn, March 7, 2010

This ASF Aopa link below - answers some of my questions. I guess after Aug '86 this Mode C reqm't started about.

But to ask you again Phil - If ATC sees a plane (blip) in their 30 mile zone and no altitude encoding - then won't they just radio the pilot and tell him to get out ? They will know he aint Mode C because there wont be any altitude showing.

-- jim kenn, March 7, 2010

From Experience: If you contact an ATC controller providing radar service, and you're not broadcasting your altitude with your radar return, they will tell you so. Depending on the load, they may ask you to try power cycling your xpdr because it greatly increases their workload.

If you're travelling xc and it's a busy center, they will tell you loud and clear during the handoff that you're not transmitting altitude information.

If you're within an altitude reporting area, VFR, without flight following, there's no way they will know how to get ahold of you. You'll basically be "in the way" and they will have to clear a vertical tube around you to maintain separation. They'll call you out to their radio contacts so they can avoid you.

From 14 CFR 91.215 (repeated in AIM 3-2-3)

If you have no transponder because the plane was certified without an electrical system, you can fly, but you have to stay under A, under 10,000MSL, outside of A, B & C, and below the CEILING of B & C.

ATC can, upon request, grant you a Mode C exception upon request if your transponder has no mode-C, or if your transponder has failed en- route to your final destination or to a repair depot.

Otherwise, if you have no transponder at all, you'll need to call ahead at least an hour.

-- Josh Davis, March 21, 2010

If ATC gets a primary return ("skin paint") from your fuselage/engine and is feeling snarky, they can track you all the way to touchdown and have someone meet you on the ramp...not cool.

-- Robert Gardner, November 2, 2011

The requirement for mode C transponders came about from a mid-air collision near Cerritios, CA in 1986.

-- Steve Craigle, September 10, 2014

The reason for the Mode C veil has everything to do with safety and keeping the NAS efficient. Veils are there for a reason and the reason is volume of aircraft near busy airports. Controllers are required to call traffic to aircraft. When there is a primary target (one without a transponder) it is displayed as a small blip or a blip with a triangle on top. There is no altitude displayed and it is not verified. This increases the workload on the controller. The Controller is required to call traffic to all the airliners for that primary target that is possibly in conflict with the flight paths. The primary target could be an aircraft at 200' or be at 10000'. We don't know and that is the problem. Now if the aircraft has a transponder and it I verified, there is no need for a traffic call to be given to a regional jet at 6000' that is converging horizontally with a Cessna that is vfr at 3500'. As far as aircraft who are not required to have a working mode C in the mode C veil, I am sure they are few and far between. It is a violation of FAR's to fly in the mode C veil with your mode C turned off or not operating. Even with the correct documentation for a non working unit, you will be required to have it placarded as Non-Working in the aircraft as well as a log book entry taking the unit out of service and still will be in violation. So the bottom line is. In a Mode C veil, you are required to have a certified and operable transponder with Mode C and it needs to be on.

-- Chip Dugan, September 25, 2014