jeff trikeAerotrike Cobra 912, with a Rival-X wing.
1800 hrs as of January 2019
Class C inner ring SURFACE?
A pilot friend of mine pointed out an interesting "loophole."
The inner ring of the class C goes from the "surface" of the airport to approximately 4000AGL (rounded up to nearest 100 ft). The Airport is up on a mesa, next to a river valley. The river passes through the Class C inner ring, but it over 400 ft below the airport. Is it legal to fly in this swath of airspace, below the Class C inner ring without contacting the tower? The river valley is in a 1/3 mile swath of empty flood plain, with no buildings.
This came up because I saw a plane flying very low over the river last weekend (low for a GA, about 200AGL) just outside the inner ring, but headed in that direction. We were trying to figure out what he was up to.
Comments
Here is an excerpt from AIM 3-2-4. Class C Airspace:
Although
the configuration of each Class C airspace area is
individually tailored, the airspace usually consists of
a 5 NM radius core surface area that extends from the
surface up to 4,000 feet above the airport elevation,
and a 10 NM radius shelf area that extends no lower
than 1,200 feet up to 4,000 feet above the airport
elevation.
I think the key wording here is: ... from the surface up to 4,000 feet above the airport elevation
I'm guessing this would disallow any flights from the surface (regardless if that is below the airport elevation) up to the top of the airspace without prior ATC contact. There may be exceptions but I'd check to confirm that with the specific airport.
Another thing to note is that a Mode C Transponder is required to overfly Class C airspace.
Sounds like a stretch. The AIM says:
The top of the airspace and the bottom of the shelf area are referenced to the airport elevation, but "surface" is not.
An elevation is defined relative to something... In this case the top or shelf of the airspace is defined relative to the airport elevation. The word "surface" is used in many contexts that define airspace limits and this, to my understanding, always refers to ground level, not some relative elevation.
Interesting that a transponder is needed to overfly a class C, but not to underfly one, so if the local airport says its out of their box (I agree a stretch) then anyone could fly there.
What airport is this specificly? The sectional is very specific. Post the sectional and a location and I will be happy to assist with this question.
Cool question / situation Jeff. Maybe you can underfly a Class C inner ring without a transponder. However another way to look at the definite rule that you need a transponder to fly within or above the inner circle is that they basically state that the inner circle is an unlimited column of airspace which requires a transponder and is not limited by their ceiling (so why would it be limited by their floor?).
Maybe the GA pilot you saw was in contact with the tower, who cleared him without transponder to stay below their surface and report on the other side (probably limited radio / radar coverage to give position reports down there as they go). I know Santa Barbara's class C radar can't see us when we are below ~200 feet in the outer ring, and ask us to give position reports if we want to be down on the beach in that area. Or maybe he didn't have permission but figured they couldn't see him down there?
Actually, now that you bring this up, Santa Barbara Airport is a somewhat similar situation- it's on a cliff up above the beach. So, technically the beach is below their surface. The sectional lists their airport surface as 13.5 feet, but for most of the coast there the cliffs are pretty tall, 100-200' tall.
KLAX is 125 feet above the beach / ocean surface level ! ;)
We have a twice per year pilot / ATC get together at the airport where pilots and ATC share their thoughts and concerns and then a Q&A session where you can ask ATC guys anything. At those meetings, I found out that for a large part ATC will typically let you slide if you're within 300 feet of your correct altitude, but if you are outside that range, they will see it as a deviation / may need to report it (this is my opinion of what they said, YMMV). Given that, I clear airspace ceilings by at least 300' (ie to over fly a Class D ceiling of 2500' some pilots I've talked to think that 2501' is OK (as long as their altimeter is set and working correctly)). I try to clear it at >2800 (2500+ >300) instead to give that minimal margin of error. With a canyon 400' below surface, you might actually be able to give that 300' margin of error with their "surface," but I agree it's probably a stretch-- the LAX sectional, even though the airport surface is 125' up seems to strongly imply ;) to me that the ocean out west is surface as well.
This is a little off the subject, but good to remember. Airspace violations and deviations are based on what the airport's radar shows your location as, regardless of what your GPS track log says. Even though your GPS is usually more accurate, and radar distances fudge a little, the radar distance record is the legal definition of where you are. So, while you might give 300 or 500 feet (ie, 1/10th of a mile or less ) as a minimal altitude clearance, you should give 1-5 miles of clearance laterally. This FAA Safety email I got just yesterday, reiterates that is still their position:
FAA Safety Team | Safer Skies Through Education
Avoiding Pilot Deviations
Notice Number: NOTC4802
AVOID The PD
Airborne pilot deviations (PDs) are common - especially in high-traffic metropolitan areas - and they are relatively easy to avoid. It's really as simple as 1, 2, 3:
Share with a friend! https://www.faasafety.gov/files/notices/2013/Jun/PDA_Flyer.pdf
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Hey, here's another question, how many of you would fly under the 1500' outer ring of Santa Barbara (where you don't technically need a transponder or to even speak with ATC / airspace endorsement to fly down the coast)? :)
No matter what the altitude of the airport, the surface of B, C and D is defined as the the "lateral boundries" so if you are trying to fly below the surface you would be underground, below the surface - not good.
The airport I am talking about is ABQ (Albuquerque).