ATC Messages
    Information
    Clearances
    Instructions
Clearance Format
    Table
    Clearance Limit
    SID
    Route
    Altitude
    Mach
    Departure, En Route, Holding or Approach Instructions
    Special Instructions
    Traffic Information
Examples

ATC Messages

In response to many questions from pilots regarding what to do with certain clearances received from ATC, I have prepared a mini-series on IFR Flight. Over time, IFR clearances will be discussed, bit by bit, including tidbits specific to each phase of flight, like departure, arrival, holds and so on. Even if you think you know all there is about one particular part that I cover, it may be worth a read. I have learned a few things while reading the AIP myself. That's the one thing about aviation: you can never seem to learn it all. To begin this series, we should have a look at some basics, including definitions for ATC communications.

ATC Information: This is simply what it seems like. ATC will, when workload permits, provide a pilot with information believed to be pertinent to the flight. This could include information about other traffic, weather conditions along the route of flight, or other known hazards to air navigation. Other ATC messages that might appear in this category are Suggestions (offering information on courses of action to avoid known weather, traffic, etc) and Requests (when asking for pilot's intended routing, altitude, etc).

ATC Clearance: The AIM defines an ATC Clearance as "authorization by an air traffic control unit for an aircraft to proceed within controlled airspace under specified conditions". The conditions referred to here include altitude, speed, route of flight and a time period. Clearances are based primarily on traffic, but also consider other things such as restricted airspace. Of the items mentioned above, ATC will normally issue a clearance for the pilot to fly the route requested in his flight plan, and attempt to provide the pilot with an altitude that's as near to the requested altitude or flight level as practical. The speed element mentioned above is also taken off the flight plan even though it is not always directly specified in the clearance, allowinga minimal tolerance either side. If the time period isn't directly specified, such as "do not depart until ..." or "clearance canceled if not airborne by ...", the controller will be monitoring the progress of the flight to ensure that it is moving along as planned, either by radar or by position reports. Once received and read back, the clearance is deemed by ATC to have been accepted, and therefore must be complied with. Failure to comply with an accepted clearance will result in less than the appropriate separation, which will endanger your flight, and perhaps someone else's. In the real world, it also will generally mean some paper work for both sides. If a clearance is unacceptable, ATC should be notified as soon as possible so that alternative actions may be taken. If a clearance has been accepted and it turns out that conditions have changed and you can no longer comply with it, you should notify ATC as soon as possible. Clearances are easily discerned since they normally include words or phrases like "ATC clears..." or "Cleared on course".

The AIM states that an ATC Clearance shall be obtained before take-off from any point within controlled airspace or before entering controlled airspace for flight under Instrument Flight Rules (IFR) or during Instrument Meteorological Conditions (IMC). IFR Clearances must be read back in full, word-for-word, except in certain circumstances. When an IFR clearance is obtained on the ground prior to departing from a controlled airport and a SID is part of the clearance with no modifications to the SID or other parts of the flight plan, the pilot is only required to read back the aircraft's callsign and transponder code assigned. Note for VATSIM pilots: this does not apply to an uncontrolled airport, so if a Tower is not in operation (or there is no _TWR online and _CTR or _APP controller is not assuming those duties) the clearance must be read back in full, even if a SID is issued. SIDs are looked at briefly below, and explained more fully in a future topic.

ATC Instruction: Instructions differ slightly from clearances. First of, ATC instructions will rarely include the term "instructs", though it may be used to add greater emphasis. They will normally include words or phrases that are directives, such as "turn left" or "climb now", as opposed to permissive words like "cleared on course", and that sort of thing. Instructions are meant to be carried out with minimum delay. Yes, it is possible to refuse instructions, but you had better have a good reason. Acknowledging the instruction means to ATC that you will follow the direction issued and will do so promptly. Instructions require readbacks if they will affect the clearance you are currently operating on. For example, "Reduce speed 180 knots or less" is an instruction that should be read back for verification of the speed and complied with if it's acceptable. "Descend to 4,000, leave FL200 now" also should be read back since it involves a change in operating altitude. An example that doesn't really require a read back would be a case where a pilot has been given descent at "pilot's discretion" and now ATC requires the pilot to begin descent and says, "Commence descent now". Acknowledgment alone will suffice in such a case as that altitude should have already been read back with the initial descent clearance. Once again, if you can't comply with the direction, notify ATC as soon as possible so they can make another plan. It may be drastic if you can't, so be prepared.

Now that we have some definitions set out, let's look at an ATC clearance in a little more detail.

Clearance Format

To help keep ATC clearances understandable, ATC are trained to use standard phraseology and a normal sequence to the parts. We'll look first at the order, then discuss the individual parts. This order comes from the ATC Manual of Air Traffic Services (called MATS).

Not all of these items exist in every clearance. For example, you would not issue a SID with an approach clearance, and many clearances issued will only be modifications of certain portions. For example, a descent clearance that doesn't modify the routing will not require the route to be re-issued with the new altitude. Let's look at each section of the table above, now.

Prefixes are pretty simple, so I'll skip that one.

Aircraft dentification must be included in any clearance or instruction is issued, even if conversation has already been taking place with a pilot. This is to ensure the correct aircraft receives the clearance.

The Clearance Limit is just that: the limit to which an aircraft is authorized to proceed. So what does one do if one reaches one's clearance limit and hasn't received anything from ATC? The AIM, RAC Paragraph 8.9, says the pilot is supposed to hold at the clearance limit, maintain the last assigned altitude and request further clearance from ATC. If contact can't be established, the pilot is to follow standard communication failure procedures. I detail holds in in a separate article, since they are a whole topic in itself. The point here is to give ATC and the pilot an idea of where the aircraft is heading and what it should do when it gets there. ATC should issue a holding clearance, further clearance or an approach clearance at least 5 minutes before an aircraft reaches its clearance limit to prevent such an occurrence. If an IFR flight plan for a "round robin" is filed (a flight which departs and arrives at the same airport with approaches at other airports while enroute), ATC should issue clearance to the destination airport.

The AIM says a Standard Instrument Departure (SID) is a "Planned IFR air traffic control departure procedure", and it is meant to ease communication and planning. SIDs are frequently confused. I've seen real world aircraft fly them when they haven't been assigned, fly part of the SID (for example, climb to the published altitude of 5,000 feet, but turn on course instead of sticking to runway heading), or, most commonly, proceed on course to flight planned altitude instead of flying the SID in the first place, even after acknowledging the SID in the clearance. If a SID is issued as part of the clearance, it should be flown as described on the chart. Most vector SIDs allow ATC the flexibility to assign a new altitude, a new heading, or both. This gives the pilot the benefit of the Comm Failure procedures described on the SID chart and ATC the benefit of better choices of altitudes and headings for separation. As part of the readback of an IFR clearance with a SID, ensure you read back any modifications to the SID. Unlike a STAR for the arrival portion of your flight plan, SIDs should not be filed as part of your flight planned route. A SID, as defined above, is an ATC procedure. An RNAV STAR, on the other hand, is geared toward operations for aircraft using FMS, and therefore meant to be filed in flight plans by pilots wishing to use them. Pilots should only fly a SID if it was issued as part of the IFR clearance. If ATC wants it, or wants to allow you to fly it, they'll state it specifically in the clearance.

There are several ways of issuing a routing in an IFR clearance. The most drawn out way is, of course, to issue a clearance with the full flight planned route. This means that the pilot will have to read back the full flight planned route. Such a process can take a lot of time. The most commonly used phrase for this portion is "via flight planned route". This means that ATC has reviewed the flight planned route and, the route as filed is acceptable for flight and separation -- at least, as far as the current controller's airspace is concerned. In many cases, ATC will specify the route to be flown to the first fix or waypoint and then use the phrase, "via flight planned route". Another way to assign a route is if ATC receives a detailed request directly from the pilot. Perhaps a request for a change to the destination due to weather, for example. In this case, ATC may issue a clearance, "via requested route". Since the pilot has already given the requested route, it does not need to be read back again. These are the most common ways of issuing routes in IFR clearances, whether pre-departure or while enroute.

The Altitude portion of an IFR clearance is pretty straight forward. Maintain the altitude specified. Every attempt will be made by ATC to assign an operationally suitable altitude for the aircraft type and route to be flown. For this purpose, "operationally suitable" means an altitude within 4,000 feet of the flight planned altitude. ATC won't, for example, clear an aircraft operating between Vancouver and Calgary an altitude of 5,000 feet since the Rocky Mountains would have something to say about it. Also, jet aircraft can operate more efficiently at higher altitudes, so someone crossing the North Atlantic would have ample reason not to want to fly 3,000 NM at 15,000. In many cases the flight would be impossible due to the fuel load required to make such a trip. If it is not possible to initially assign an operationally suitable altitude, ATC should inform the aircraft of the time or location to expect one. For example, "Maintain 4,000, expect flight level 200 ten minutes after departure." With the altitude instruction, there are some other possibilities, too. For example, the pilot may be cleared to climb or descend with the phrase, "when ready". This means the climb/descent may commence when the pilot sees fit, but must climb or descend at a normal rate for the aircraft type once he starts. Also temporarily levelling off at interim altitudes is not permitted. If climbing, the pilot may not descend to an altitude already vacated, and if descending, the pilot may not return to a higher altitude vacated earlier. Also, a pilot cleared on a transition for an approach, be it an RNAV STAR, DME arc, or other published transition, may not descend to altitudes on the transition simply because they are published. The pilot must be in receipt of a descent clearance or approach clearance that allows the descent. If the cleared altitude is lower than a published restriction, the restriction must be met unless specifically cancelled by the controller. One last thing about altitude changes is that ATC may, on request from the pilot, approve a "VFR climb/descent". Included with this will be the altitudes where the VFR restrictions are applied. For example, "make climb from 4,000 to 6,000 VFR". ATC will not grant such a request in airspace where VFR flight is not permitted. This includes Class A airspace, and also any airspace where ATC must provide IFR separation between IFR and VFR such as Class B airspace.

Mach Number isn't normally specified for flights in domestic airspace, nor is it an item for aircraft not flying on Mach numbers, such as Dash 8's -- it only applies to jet aircraft, and then only at higher altitudes where the transition from Indicated Airspeed (IAS) is made to the Mach meter for speed. It can be stated for separation purposes if required. Aircraft assigned Mach numbers are to maintain a narrow tolerance of 0.02 Mach from the value issued and acknowledged in an IFR clearance. Aircraft not operating on Mach numbers may be issued a True Airspeed (TAS) to be flown, typical of non-radar environments, and this speed is often expressed in this position in the IFR clearance.

Departure/En Route/Holding/Approach instructions are placed here and include any restrictions to be adhered to in the clearance or transitions from one phase of flight to another. If there are no other aircraft operating around the departure airport, ATC may very well simply issue an IFR clearance to destination with a route (likely the flight planned route) and altitude (again, likely the flight planned altitude) and a transponder code to squawk. This means the pilot may depart any runway (unless one is specified directly in the clearance) and climb out while proceeding on course. A pilot will still have to make allowances for VFR traffic, especially at uncontrolled airports, but there is no IFR restriction in to the flight. If ATC has traffic to work around, a variety of departure instructions may be issued to keep aircraft apart. For example, the controller may state, "depart runway 24, climb runway heading to 4,000 before proceeding on course." An altitude will have been issued in a previous portion of the clearance so the pilot may climb to that altitude, but the initial part of the climb, up through 4,000, shall be accomplished on runway heading in this example. Other restrictions may be received, such as "not above 6,000 til 20 DME west". This sort of restriction may be used in a radar environment, but is more commonly found in a non-radar, or procedural, environment. Stay tuned for my later part in this series on how to fly in just such an environment. With a holding clearance, a pilot may be given detailed holding instructions which state where to hold in relation to the clearance limit, what the inbound track will be, any modifiers to the standard holding pattern, or DME values to use as inbound and outbound fixes. For approach clearances, transition instructions may be stated such as which RNAV initial approach segment to fly, or a straight-in approach via an Intermediate Fix/Intermediate Waypoint (IF/IWP) or an RNAV STAR.

In the Special Instructions and Information section, the most common items issued are the transponder code to squawk and instructions to contact a particular ATC unit at a particular time, location or altitude. Also, instructions like when to expect higher and what altitude, or a routing to expect when further clearance is received, if cleared to hold short of destination, may be issued here. In the event of a holding clearance, a pilot will also be given an Expect Further Clearance (EFC) time or Expect Approach Clearance (EAC) time. These times are to be respected by ATC and pilots in the event of radio failure. If a pilot is holding at a fix awaiting an approach and and the EAC time issued as part of the holding clearance is reached, an attempt to contact ATC for further instructions should be made. In fact, a pilot should be trying to do so about 5 minutes prior to this time to give time to ascertain that communications with ATC are still alright. If ATC can't be reached for further instructions at this time, the pilot should commence an approach, or commence shuttling down to the procedure turn altitude if above it, and then start the approach. ATC should also be paying attention to the time given and provide the pilot with further clearance or an approach clearance not later than 5 minutes prior to the previously-issued time. This section is is really a "catch-all" for anything that doesn't fit elsewhere, and it's not limited to what I've said here, either.

Aircraft departing airports in uncontrolled airspace will often be issued information on known traffic operating in the area. Also, aircraft issued VFR restrictions with their clearance should be issued traffic information to assist them in maintaining visual separation. In either of these cases, and others not mentioned, Traffic Information will be issued at this spot in the clearance. There are other types of clearances as well that may require traffic information, such as holding with minimum vertical separation between aircraft, AKA a "stack".

Examples

Let's look at some examples of clearances. I've captured a flight plan to work with as it appeared in Euroscope for the VATSIM network.

In this case, a departing aircraft calls the TWR at CYHZ for IFR clearance prior to start-up. The controller will check for a valid route filed, an altitude appropriate to direction of flight (FL180 for westbound, shown at the bottom of box 4 from the left), and so forth. The dialog might sound like this:

"Good evening, Halifax, UKV2175, ready to copy IFR to Teterboro"
"UKV2175, Halifax Tower, Good evening. ATC clears UKV2175 to the Teterboro airport via the Halifax 4 departure, flight planned route, squawk 3101"

In this example, I've color coded the items to match the table above. I know it's tough to read and compare colors, but the idea is more to show the different parts that exist in the example than to have you scrolling up and down. Not all elements from the table exist in this clearance above. Let's say the traffic is light and the IFR controller just wanted to let the aircraft proceed on course immediately after take-off rather restricting him to fly the SID. The clearance may sound like this:

"ATC Clears UKV2175 to the Teterboro airport via direct ACADN, flight planned route. Maintain FL180. Depart runway 23, fly heading 235 til able direct ACADN and proceed on course. Squawk 3101"

While the SID is no longer in the clearance, you can see that a couple more items are: altitude and departure instructions. It is also possible to issue the same clearance without including the departure instructions (the words in dark blue). This would allow the flexibility for the pilot to choose the runway for departure, and change it, all without requesting amendments to the IFR clearance (note that at a controlled airport the pilot must still request any new runway with the tower -- the tower need not, in this case, seek approval from the IFR unit for a change in runway, since none was specified, though professional courtesy suggests this should be coordinated anyway).

Let's say the route isn't acceptable, perhaps because of a line of thunderstorms is moving in near ACADN, so the pilot is asking for a slight change to the route. The new clearance might sound like this: 

"ATC Clears UKV2175 to the Teterboro airport via direct YQI, direct ENE, flight planned route. Maintain FL180. Depart runway 23, turn right, fly heading 265 til able direct YQI and proceed on course. Squawk 3101"

Notice the different wording in the route description. Here, the controller has removed ACADN from the flight planned route and inserted a new fix, the YQI VOR. In this example, the aircraft is to bypass ACADN in favor of the new fix, then rejoin the flight planned route at ENE. Accordingly, the clearance must specify the new routing ("direct YQI") and a transition back to the flight planned route where the controller wants the pilot to rejoin it ("direct ENE, flight planned route"). If the entire route were changed, the term flight planned route would be omitted altogether.

After the aircraft gets airborne, the pilot calls up and requests a new route, perhaps again due to changing weather patterns. The pilot now wants to go further north initially, then rejoin the original flight planned route:

"Moncton, UKV2175 requesting direct EBONY direct ENE and then rejoin our flight plan route."
"UKV2175, cleared via requested routing."

Notice how little is in this clearance: just identification of the aircraft and the routing.

How about some examples of aircraft receiving holding clearances:

"CVA822 is cleared to the Moncton NDB via direct, maintain 5,000. Hold east, inbound on the localizer, expect approach clearance at 1435"
"CVA116 is cleared to the Halifax 292 radial at 50 DME, maintain 15,000. Hold northwest, inbound on the 292 radial, expect further clearance via direct MUVOX and the FUNDY5 Arrival at 2015"

In the first example, the aircraft is being set up to hold over a NAVAID, perhaps to wait and see if the weather improves. An Expect Approach Clearance time is the proper term in this case. In the second example, the aircraft is being cleared short of destination -- it's common for aircraft approaching a busier terminal area to receive holding clearances which keep them outside the terminal area due to congestion already within it. In such cases, the aircraft will be issued the holding instructions and, if the routing desired by the controller doesn't match the flight plan route of the aircraft, the clearance must contain a route in the EFC portion.

Another example would be an aircraft requesting an altitude change, but it can't be approved because of IFR traffic nearby. As indicated above, if the aircraft is in such conditions to allow an aircraft's request for a VFR climb or descent, once requested, the clearance might look like this:

"SAC317 descend to 4,000. Make descent from 7,000 to 5,000 VFR. Traffic at your twelve o'clock and 2 miles, westbound Navajo at 6,000."

Lastly, an example of an oceanic clearance so you can see Mach number placement along with a restriction. This example assumes the aircraft is in flight in the Moncton FIR, heading eastbound at FL290.

"CVA822 is cleared to London Heathrow via direct VIXUN, NAT WHISKEY. Climb to FL330. Mach 0.82. Cross VIXUN level at FL330."

There is an awful lot said here that I haven't covered in any real detail. More will follow in the next parts as the weeks drag on. This was just meant to give you a look at clearances and how they're meant to be formed.

* The term "when ready" is an ICAO term that replaced "at pilot's discretion" in conjunction with altitude clearances in Canada. Our American brethren still use the term "pilot's discretion" when the pilot is permitted to commence descent at a time convenient for the flight. The terminology is slightly different from "when ready" in that the term "at pilot's discretion" allows a pilot to level off at an interim altitude for a period of time and then continue the descent when the pilot deems it suitable. With "when ready", the pilot may commence the altitude change any time after receipt of the clearance, but must descend at an rate commensurate with normal aircraft performance and may not level off above the cleared altitude. In VATSIM, American pilots and Canadian pilots that get their information from American resources may not understand the difference in these two terms.

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