Which frequency to contact when arriving at a Class C airport when the tower is open, and which frequency when it is closed?

I came across this question on my PPL exam and I'm not able to properly interpret some pieces.

The question is:

When approaching Lincoln Municipal from the west at noon for the purpose of landing, initial communications should be with:
A) Lincoln Approach Control on 124.0 MHz.
B) Minneapolis Center on 128.75 MHz.
C) Lincoln Tower on 118.5 MHz.

A related question is:

What is the recommended communications procedure for landing at Lincoln Municipal during the hours when the tower is not in operation?

And the answer to this is: "Monitor airport traffic and announce your position and intentions on 118.5 MHz."

Note: I was not provided the sectional chart, only the AFD.

I've copied the relevant lines from the airport chart supplement:

LINCOLN MUNI (LNK) 4 NW UTC-6(-5DT) N40°50.05' W96°45.55' 1218 B S4
FUEL 100LL, JET A TPA--2218(1000) ARFF Index B

COMMUNICATIONS:
CTAF 118.5
ATIS 118.05
UNICOM 122.95
COLUMBUS FSS (OLU) TF 1-800-WX-BRIEF, NOTAM FILE LNK.
RCO 122.65 (COLUMBUS FSS)
APP/DEP CON 124.0 (170°-349°) 124.8 (350°-169°)(1130-0630Z‡)
MINNEAPOLIS CENTER APP/DEP CON 128.75 (0630-1130Z‡)
TOWER 118.5 125.7 (1130-0630Z‡)
GND CON 121.9
CLNC DEL 120.7

AIRSPACE:
CLASS C svc 1130-0630Z‡ ctc APP CON other times CLASS E.

My calculations are as follows: The question states that we're approaching at "noon" and the chart says Lincoln Muni is 6 hours behind UTC. So I subtract the offset (-6) to noon. This gives me $$1200 - (-0600) = 1800\text{Z}$$.

It was my understanding that when the airport is in towered operation, we contact the control tower (CTAF) as we do in a lot of class D airspaces. Especially since class C requires 2-way communication before entering. The CTAF for Lincoln Muni is in operation between 1130Z to 0630Z. Since 1800Z falls in this interval, I figured CTAF should be contacted and hence my answer was C. Moreover, if we are outside hours of operation, then I thought the standard was to contact UNICOM, which would be frequency 122.95.

But based on the answers to the above question it seems I've understood the complete opposite of the right operations.

My confusion:

1. Am I miscalculating my zulu time?
2. Exactly when do I use ctaf vs. app/dep con vs. UNICOM in class C/E airspace? (Sometimes I've seen class E airports have CTAF and unicom frequencies be the same thing, and sometimes they are different.)
3. When do I use RCO?
4. If I'm operating between 0630Z and 1830Z, do I only use Minneapolis app/dep con for all communication and avoid CTAF since it is not in operation? Or, is CTAF related requests channeled to a different frequency?
• @randomhead, I'm not sure I agree with your recent edit to the title question... My interpretation of the OP's confusion is in not understanding who controls what airspace. You did a good job of explaining it in your answer, but I think the emphasis should be on unlinking the idea that a tower has anything to do with class C. (although your wording doesn't clash with the angle I took in my answer, in fact it actually complements it somewhat, so maybe thanks?!) Dec 31, 2021 at 18:40
• @Michael as someone who works at a Class C up/down I am surprised that you think think "a tower has [nothing] to do with Class C." Having an operational tower is an integral component of Class C airspace; Approach provides Class C services in the outer ring and Tower provides those services in the surface area. When the tower is operational the Class C airspace ceases to exist, even if the overlying Approach is still operational; see the current sectional and note that the Class C airspace effective times overlap precisely with the hours the Tower is open. Dec 31, 2021 at 18:51
• @randomhead, well maybe not "nothing", but can you state with absolute certainty that no tower controlled airport within C ever shuts down while approach remains open? Because if the Twr/App team is truly integral 100% of the time then why bother listing operating hours separately? Regardless of the answer, the OP seemed to think that tower controls the outer ring of the C. Either that or he didn't remember which ring was the biggest. I'm just trying to discern the root confusion and clarify the separation of responsibility. Dec 31, 2021 at 19:12
• And please clarify what you mean by "When the tower is operational the Class C airspace ceases to exist..." Dec 31, 2021 at 19:15
• @Michael: Hours are listed separately for clarity. The tower can shut down while Approach remains open, for sure; that happens nightly at LNK. But when the tower is shut down Class C services are not provided; one of the main services provided is "sequencing of all aircraft to the primary airport"—which is not possible if the airport is non-towered and pilots self-sequence on the CTAF. Clarification: my typo, of course I meant "when the tower is not operational the Class C airspace ceases to exist." Dec 31, 2021 at 19:19

It was my understanding that when the airport is in towered operation, we contact the control tower (CTAF) as we do in a lot of class D airspaces.

You have the wrong understanding.

At a Class D airport, the overlying radar facility (which may be a TRACON or a Center) is responsible for sequencing IFR aircraft only. VFR aircraft may call the Tower controller directly and will be sequenced by the Tower controller as necessary, given the feed of IFR aircraft they are receiving from the overlying approach control.

At a Class C (and Class B) airport, the overlying radar facility is responsible for sequencing all aircraft to the airport, both VFR and IFR (absent standing or case-by-case coordination to the contrary). This means you must initiate contact with the radar facility and follow their instructions; if they tell you to enter a left base, you should enter a left base, and not proceed to standard 45º downwind entry, for example!

If the tower is open and Class B/C airspace is in effect, the radar controller will tell you exactly when you should contact the Tower controller (and the frequency in use, if there is any possibility for confusion). You should not change frequencies without being instructed to. (The same goes for a Class D airport, if you happen to be receiving radar services from the overlying radar facility.)

1. Am I miscalculating my zulu time?

No, you have it correct. 1200L = 1800Z; both the Tower and the Approach are open, and Class C airspace is in effect.

1. Exactly when do I use ctaf vs. app/dep con vs. UNICOM in class C/E airspace? (Sometimes I've seen class E airports have CTAF and unicom frequencies be the same thing, and sometimes they are different.)

See this answer to the question "What is the difference between CTAF, UNICOM, and MULTICOM?" Note that your question can very likely be answered by reference to the Pilot/Controller Glossary, and you should always check the Glossary first if you come across a term you don't know. But as a primer:

• UNICOM is a non-governmental service generally run by an FBO at an airport. The UNICOM operator can receive your requests for services (fuel, snacks, lavatory, etc). You can use this frequency at a towered or non-towered airport to request such services.
At a non-towered airport, the UNICOM operator can also relay the current weather, suggested runway, and known traffic operating in the vicinity of the airport—but they are not a controlling agency and do not issue clearances or instructions. Full authority and responsibility remains with the PIC to choose a suitable runway, make suitable traffic calls on CTAF, and see-and-avoid other traffic.

• The Common Traffic Advisory Frequency (CTAF) is used by pilots when the Tower is closed, to self-announce position and intentions and, by listening to other pilots' announcements, build a traffic picture in their mind in the absence of a Tower controller. At airports without a tower, it is common for CTAF and UNICOM to be the same frequency; at airports with a part-time tower it is common for CTAF to be what would be the Tower frequency if the Tower were open, and UNICOM to be separate.
Pilot-operated airport and approach lighting is often controllable by clicking the mic three, five, or seven times on one of these frequencies, usually the CTAF.

When you are approaching an airport which has Class C (or B) airspace in effect, you must contact the radar facility listed in the Chart Supplement and shown in the magenta box on the sectional. If the Tower is closed and Class E airspace is in effect, you may contact the radar facility listed in the Chart Supplement to receive radar services, but this is not a requirement. See my answer to your fourth question below. If you are talking to the radar facility and the Tower is open, they will tell you when to contact Tower; if you are talking to the radar facility and the Tower is closed, they will tell you when you are "approved" to change to the advisory frequency (CTAF).

1. When do I use RCO?

The RCO (Remote Communications Outlet) is a radio transceiver which provides on-the-ground communications capability at a non-towered airport to an ATC or FSS unit. It is used for relaying IFR clearances to departing IFR aircraft, and relaying IFR cancellations from arriving IFR aircraft. As a VFR pilot you would be unlikely to use an ATC RCO, but you might use an FSS RCO to open or close a VFR flight plan.

1. If I'm operating between 0630Z and 1830Z, do I only use Minneapolis app/dep con for all communication and avoid CTAF since it is not in operation? Or, is CTAF related requests channeled to a different frequency?

Careful... if you are operating between 0630Z and 1130Z you would use the Minneapolis Center frequency for any purpose that you would normally use an ATC frequency when operating in Class E airspace, such as requesting traffic advisories or vectors around hazardous weather. There is no requirement to contact Minneapolis Center on that frequency, however, because the tower is closed and the Class C airspace does not exist.

You will continue to use the CTAF to announce your position and intentions and coordinate with other pilots, just the same as you would use the CTAF at any other non-towered airport, regardless of whether you had been receiving radar services from Minneapolis Center or not. If you had been, at some point (~10NM out) they would have issued you "Squawk VFR, change to advisory frequency approved" at which point your radar services would be automatically terminated and you would change to the CTAF.

NOTE that the current Chart Supplement (accessible from the FAA's digital Chart Supplement search page) is somewhat different from the Chart Supplement information provided in your question; the Class C effective times have changed and ZMP is no longer mentioned as the approach facility when the tower is closed. But having worked through the logic given the question's information, you can similarly look at the current information and determine that everything still holds—with the one exception that Omaha Approach on 124.0 is now the "Class E services" frequency even when the Tower is closed.

• Couple clarifying questions. I always fly out of a class D and they require all planes to first contact ground to taxi to the runway. Then they say contact "tower." So then I switch frequencies and contact "tower." When I say "tower" is that the same thing as what you're referring to as "radar facility?" You mentioned they are only required to sequence in IFR. So does that mean it's possible that if they don't have to, they won't sequence me in as a VFR for landing? If so, whats the right procedure to land for VFR? Jan 2, 2022 at 21:57
• Also, you used some terms that I'm not exactly sure how to interpret. This is the airport I use: aeronav.faa.gov/afd/02dec2021/sw_216_02DEC2021.pdf . The frequency listed as "CTAF" is the frequency we use to contact "tower" when we are at the runway ready for takeoff. This frequency takes care of departing and landing flights (VFR and IFR). In your answer, you mention "CTAF to be what would be the Tower frequency " indicating that it's possible CTAF and Tower are different. I'm a bit confused by this. Jan 2, 2022 at 22:04
• Along the same lines, in your answer you mentioned "tower frequency," "CTAF," and "ATC frequency." Using KMYF as an example, I'd say all three frequencies are the same. Am I wrong? Jan 2, 2022 at 22:08
• In my answer "Tower" usually means "the controller who issues T/O and landing clearances" (which we in ATC call the "Local" controller, who works alongside the "Ground" and "Clearance Delivery" controllers) but sometimes it means "the tower cab facility where the LC, GC, and CD controllers work." The radar control position is a very separate function; at both LNK and MYF the radar facility is physically separate from the tower cab facility. At other places (like where I work) the two functions are co-located in an "up/down" facility. But the distinction is usually irrelevant to pilots. Jan 2, 2022 at 22:42
• At MYF the main Tower (LC) frequency is 119.2. During certain hours a second LC position dedicated to controlling RY 28R is opened and that LC position uses 125.7. At night when the controllers go home pilots can self-announce on the CTAF which happens to be the same frequency as the main LC frequency, 119.2 (this is a very common setup but not guaranteed to be true everywhere). The CTAF does not exist when the tower is operational; CTAF is an aid to pilots at non-towered airports only. Jan 2, 2022 at 22:49
1. Am I miscalculating my zulu time?

Your 1800Z is correct, assuming DST is not in effect. And the “‡” symbol means the times move with DST anyway, so even if the correct time is 1700Z, it wont change the answer.

1. If I'm operating between 0630Z and 1830Z, do I only use Minneapolis app/dep con for all communication and avoid CTAF since it is not in operation? Or, is CTAF related requests channeled to a different frequency?

I’m going to answer this first since it will lay the foundation for #2.

Both TOWER and APP/DEP CON are shown as open “1130-0630Z‡” per the A/FD, and you are arriving at 1800Z, which falls squarely inside that period. That is what you’d expect at noon local time.

MINNEAPOLIS CENTER is shown as taking over APP/DEP CON overnight at “0630-1130Z‡”, which does not apply to your arrival at 1800Z.

So, the question is whether your initial contact should be with TOWER or APP/DEP CON. The answer is right there under AIRSPACE: when APP/DEP is open, call them first.

You are approaching from the “west” (roughly 270°), which is in the range “170°-349°”, so the frequency to use is 124.0.

1. Exactly when do I use ctaf vs. app/dep con vs. UNICOM in class C/E airspace? (Sometimes I've seen class E airports have CTAF and unicom frequencies be the same thing, and sometimes they are different.)

CTAF (Common Traffic Advisory Frequency) is what you would use when the tower is closed, making it an uncontrolled field.

UNICOM is for talking to the FBO, such as to advise them you’ll be arriving, request instructions for parking on their ramp, etc.

CTAF and UNICOM are often the same frequency at non-towered fields, but not at towered fields, even when the tower is closed.

1. When do I use RCO?

You use an RCO to contact FSS for filing/opening/closing flight plans or getting weather updates.

Just because the tower is closed, doesn't mean that approach control is closed. In fact, Lincoln Municipal doesn't have its own approach controller; 124.0 is the frequency for Omaha approach.

If you'll notice on the sectional, the instructions are to "Contact Omaha Approach within 20 NM on 124.0".

One other thing I noticed about your question, CTAF and tower frequency are not necessarily the same thing. Most airports use the same frequency for both, but there's no guarantee of that.

1. Your UTC calculations are fine.
2. If the airport has an approach frequency, you always use it when you're approaching the airport unless specifically instructed otherwise by another ATC controller, chart supplement, etc. UNICOM is used to communicate with non-ATC facilities, such as ordering fuel from the local FBO.
3. RCO's are just another frequency to contact the local Flight Service Station, used for filing flight plans, getting weather briefings, etc.
4. If there is an approach frequency available, you should generally use when approaching the airport. They will advise you to contact tower (if the tower's open) or switch to the CTAF (if the tower's closed) at the appropriate time. Otherwise, go directly to the tower/CTAF.
• Ahh I see. But for the question, we were not provided the sectional chart. Dec 30, 2021 at 22:01
• @Jonathan I was editing the question to add some more info when you posted your comment. But, in short, you should always contact approach first unless it's closed, and the hours of operation for approach are not necessarily the same as for the tower. If there are no hours in the supplement, you should assume that it's open 24 hours. Dec 30, 2021 at 22:12
• The sectional is not controlling; the Chart Supplement is. R90 is a 24/7 facility but I see no evidence that the sector providing Class C services at LNK is itself operational 24/7. There's a reason the Chart Supplement lists ZMP as the app/dep facility from 0630-1130. Dec 30, 2021 at 23:31
• @randomhead When did I say that the sectional is controlling? Dec 31, 2021 at 2:15
• You said that, because R90 is open 24/7, you should "always use the approach frequency when approaching the airport" (as indicated on the sectional). As far as I can see from the CS, that is bad advice because, from 0030-0530 local time, that frequency is unmonitored and ZMP owns the airspace. Just because some positions at the TRACON are continuously staffed does not mean that they all are, which is precisely why you must look at the supplement. Dec 31, 2021 at 3:24

Contact the controlling authority for the airspace you would like to enter.

That might sound oversimplified, but let me explain. From your answer it is apparent that you are very thorough, but I think you may have gotten buried in the extraneous details and zulu math of the question. It might help if you instead look at this spatially, and/or chronologically.

What I mean by that is look at the chart and ask yourself which airspace you will encounter first. (I realize one wasn’t provided in this question, but at some point you should be able convert your digital rote memory of the airspace diameters into a mental picture) If you are approaching the airport from say 20 miles away, and there is a 10 mile range ring of Class C airspace you will encounter, you had better talk to the owner of that airspace BEFORE you get to the owner of the 5 mile ring for the Class D airspace, right? Otherwise you will have trundled through at least 5 miles of radar controlled airspace without talking to anyone! (I trust you remember that two way communication is required to enter Class C…)

Think of the approach controllers as the first gatekeeper, their radar can see further than the binoculars of the people in the tower, so their range ring extends further and it only makes sense that you should call them first. Approach controls Charlie, Tower controls Delta. In your example the tower and approach hours are the same, so it isn't an either/or question. You will talk to both - approach first, and they will hand you off to tower as you get closer.

Having said that, 10 miles is the minimum and isn’t really a lot if the airport is busy with IFR fast movers, so most approach facilities really like to be called at around 20 miles. (the “undepicted” outer part of the Class C)

Now, there are a couple other small points in your question that you didn’t ask directly about, but I think bear mentioning.

First, “under” class C generally implies a satellite airport beneath the outer/upper shelf that is 5-10 miles from the center and does NOT extend to the surface. In other words, not in class C. Since LNK is the main airport serviced by the C, and since C goes to the surface in the middle, I edited your title question to substitute “in” for “under”.

Second, you are using the acronym CTAF in the context of the Control Tower. [e.g. “we contact the control tower (CTAF)…”]

CTAF stands for Common Traffic Advisory Frequency, it is not the control tower or any controlling entity. It is just a frequency. Although in many cases like this the frequency is the same, the purpose is actually almost opposite; at least in terms of who does the controlling:

• If the tower is open, you contact the tower on tower frequency, and they control the traffic.
• If the tower is closed, you contact other traffic on the common traffic advisory frequency, and work things out amongst yourselves. (i.e. you broadcast your position and intentions to enhance the awareness of other pilots in the pattern just as you would at a non-towered airport.)