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

Question

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.

The correct answer is A.

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?

Answer 1

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.)

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Your questions:

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.

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.)

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).

3. 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.

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?

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.

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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.

Answer 2

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.

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?

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.

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.)

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.

3. When do I use RCO?

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

Answer 3

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.

So, to answer your specific questions:

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.

Answer 4

Short answer:
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.)