Clearances and Instructions Commentary

General Information for Clearances and Instructions

When an air traffic controller gives you an instruction or a clearance you must reply, either accepting or refusing the clearance. You accept a clearance simply by saying your callsign. In Canada VFR pilots are not required to "read back" (repeat) instructions and clearances. To refuse the clearance, you say "unable" and then give a very brief reason. Once you have accepted a clearance, you are expected to to comply with it. If at any time it becomes unsafe to comply with part of a clearance, even if you have already accepted it, immediately do whatever is necessary to ensure safety, and then, as soon as possible, tell the air traffic controller what you are doing. Don't deviate from a clearance more than you have to. Note that it's not a big deal to refuse a clearance. Nobody makes a report on it or gets annoyed. It is a huge deal to do something without a clearance or to disregard a clearance you have accepted.

Your safety is always your responsibility. No matter what a controller tells you to do or tells you not to do, you must make the final decision what is safe to do in your aircraft. Air traffic controllers are human beings and sometimes say left instead of right.  They may not know where the clouds are, and may not be aware of all traffic in your vicinity.  There are even student air traffic controllers, like in this joke.

Almost all the questions in this section can be answered just by understanding the previous two paragraphs. It's all covered in CARs 602.31. Here are some examples to make them clearer.

Example 1

Tower: Echo Alfa Charlie, maintain two thousand.
The pilot is being told to fly at two thousand feet asl. She is cleared to climb or descend to that altitude.
Pilot: Echo Alfa Charlie unable VFR at two thousand. Request one thousand five hundred.
The pilot sees clouds ahead at 2000' so she refuses the clearance, explaining the reason, and giving an alternate suggestion.
Tower: Echo Alfa Charlie, one thousand five hundred is approved.
The tower approves the pilot's idea.
Pilot: Echo Alfa Charlie
The pilot reads back her callsign, signifying that she understands, and will comply. The tower now expects her to climb or descend to 1500' and maintain that altitude.

Example 2

Tower: X-ray India India turn left two seven zero.
The tower wants the pilot to make a left turn to a heading of 270 degrees.
Pilot: X-ray India India unable, traffic.
The pilot looked left in preparation for the turn and saw a twin-engine aircraft coming up on his left. Yikes! He refuses the clearance, with the one-word explanation "traffic."
Tower: X-ray India India, pass behind the Aztec when able, fly two seven zero.
The controller gives the pilot new instructions.
Pilot: X-ray India India
The pilot accepts the new instructions.

Example 3

Tower: Victor Romeo Victor, cleared to land two five, hold short three zero.
The controller clears the pilot to land on runway 25, telling him to stop and wait before crossing runway 30, which intersects runway 25.
VRV Pilot: Victor Romeo Victor, hold short three zero.
The pilot accepts the clearance, reading back the hold short instruction.
Tower: Zulu Victor Papa, cleared to land three zero, aircraft landing 25 will hold short.
The tower clears someone else to land on an intersecting runway.
ZVP Pilot: Zulu Victor Papa
The other pilot acknowledges his landing clearance.
Meanwhile, the pilot of VRV touches down a little long, a little fast, and applies the brakes ... but nothing happens.
VRV Pilot: Tower, Victor Romeo Victor unable hold short, brake failure.
The pilot immediately informs the tower that he can no longer comply.
Tower: Zulu Victor Papa, pull up and go around.
The tower keeps the other aircraft off the intersecting runway.
ZVP Pilot: Zulu Victor Papa
The ZVP pilot puts in the power immediately, and then acknowledges, so the controller knows he is complying.

The student pilots in these jokes are given instructions which they must obey.

Question-by-Question Explanation of Clearances and Instructions

10.01 & 10.02
An ATC instruction is an order. If you can safely comply, you must.  The instruction becomes effective as soon as you receive it, but you are expected to acknowledge it. A clearance has some form of the word "clear" in it. If you accept it, you must comply with it. The clearance becomes effective as soon as you acknowledge it. Do not "read back" clearances or instructions if you are VFR. It takes too much time on the frequency.

The difference between a clearance and an instruction is the word clear and that a clearance is effective when accepted, while an instruction is effective when received. The difference is subtle and as either can be refused for safety reasons, the only time the difference really matters much is when writing the PSTAR.

If ATC wishes to give you advice requiring no reply, they will say so.  They sometimes do this when they want to give wind information to an aircraft on short final, or let a number of aircraft know about some situation.  
"Bravo Oscar Papa, no need to reply, wind is now two nine zero at twenty-five gusting thirty."
"All banner tow airplanes, no need to reply, the F16 flyby will take place in thirty minutes."

(1) He cannot disregard it: he accepted it.
(2) If he complies with only the suitable part, he is still disregarding part of the clearance.
(3) Any time you must deviate from a clearance or instruction, you must inform ATC.
(4) Do what you have to, and say what you're doing.

Notice that this question is almost identical to the previous one?  It's because it's a very important concept.   In controlled airspace, (a) ensure you have permission before doing anything, (b) do what you are told as long as it can be done safely, and (c) if the instructions you are given are unsafe, or become unsafe, take whatever action is necessary to ensure safety, and inform the controller what you are doing as soon as possible.

(1) This choice is wrong because although YOU are responsible for your traffic separation, ATC still has to provide spacing for IFR aircraft.
(2) This choice is wrong because responsibility is not divided.  it is always one party that gets the blame.
(3) Watch that little word not.  It's easy to miss when reading an answer choice.  I've lost a mark or two over the years to hidden nots.
(4) The only time the pilot is relieved of responsibility for separation from other traffic is when he is on an IFR flight plan, in IMC (in cloud).

(1) If it's not the right clearance, turn it down, and ask for what you want.
(2) If you don't tell the controller what is wrong with the clearance, how will you get a better one?
(3) You're not required to read back any of the clearance, so if you acknowledge it at all, you have accepted the whole clearance.
(4) Typically you are cleared to take-off, to land, or to do something that you requested.  Here are some examples of refused clearances:

Echo Juliet X-ray, cleared take-off runway one two
Pilot: Echo Juliet X-ray, unable, request runway two five for crosswind

Tower: Bravo Juliet Quebec cleared to land runway two six right
Pilot: Bravo Juliet Quebec is going around for a coyote on the runway.

Tower: Skywatch Ten, cleared to orbit over the downtown core as requested.
Pilot: Tower, Skywatch Ten done here ... now requesting southeast to Silver Bridge.

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This page written 8 October 2002 by Robyn Stewart.  Last revised 29 December 2002.

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