and Instructions Commentary
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.
General Information for Clearances and Instructions
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.
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
Tower: Echo Alfa Charlie, one thousand five hundred is
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.
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.
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
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,
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
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
"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
(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
(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:
Tower: 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
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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