I took a quick trip with a friend in his Cessna 182 the other day to a local airport for lunch. Leaving our home airport, there were several aircraft in the circuit practicing take-offs and landings. There were so many aircraft in the circuit, the tower was giving instructions to keep the pattern organized like "FABC, extend your downwind, I'll call the base turn" or "GXYZ, keep the turn square to final, traffic ahead to depart". These types of instructions are quite common at busy training airports and the pilots in the circuit vary greatly in experience, some are dual flights with an instructor on board, some are solo flights, some are commercial students, and some could be on their first or second solo flight. With the amount of traffic and the wide gap in experience and skill, it's really important our communications are concise; both the controllers and the other pilots need to know if an aircraft given an instruction will comply and they need to know quickly. One of the responses pilots were giving to these instruction was, "affirmative." As an example,
Tower: "FABC, do a right 360 for spacing and rejoin the downwind"
FABC: "Affirmative, FABC"
"Affirmative" doesn't really work as a reponse to an instruction. It's a great answer to a question (it means "yes") but, as a response to an instruction, not so much. If a teacher told a student to have a report in by Wednesday, you wouldn't expect the response to be "yes." You would expect the student to say, "OK" or "I'll have it for you Wednesday" or "I'm not going to be here Wednesday, how about Thursday". When tower gives an instruction they need to know two things, 1) the pilot understands the instruction and 2) they will comply with the instruction.
Here are a few responses that will work,
WILCO - Wilco is the short form of will comply. If the tower asks you extend the downwind leg and the pilot responds with WIlCO, everyone knows the pilot understands the instruction and will comply with it.
Tower: "FABC, fly runway heading on departure, I'll call the crosswind turn."
FABC: "Wilco, FABC"
Read the instruction back - Reading the instruction back in part or in full is more common when flying IFR but it leaves no doubt that the pilot understands the instruction even if it ties up the frequency a little longer. Consider using this method when the instruction contains a limitation or restriction of some kind (e.g., hold short, not below, not above, maintain).
Tower: "FABC, cleared left base runway 13, not below 1500' until advised"
FABC: "cleared left base runway 13, not below 1500' until advised, FABC"
FABC: "not below 1500', FABC"
Registration - Technically, VFR aircraft are only required to respond with their identification markings (e.g., FABC) when given an instruction by ATC unless the controller asks the pilot to read back the whole clearance (CAR 602.31(1)(b)(ii)).
Tower: "FABC, fly heading 010 for traffic, I'll advise the base turn."
UNABLE - It's really important to remember, just because ATC gives a pilot an instruction that doesn't mean the pilot can safely comply with it. A pilot can always respond to an instruction with UNABLE if the pilot believes complying with the instruction will be unsafe. The key is to let the controller know as soon as possible and the controller will work something else out. Don't be shy using this.
Tower: "FABC, proceed direct the threshold, turn final over the numbers"
Next time ATC provides an instruction, try and use one of the responses above and save "affirmative" for answering questions. For more information on ATC communications in Canada see NAV Canada's VFR Phraseology Guide and the COM section of the Aeronautical Information Manual.