'Surcharges' originate from Category 12 of the fare rules for the particular fare components on the ticket. They show up in the fare calculation ladder with a letter 'Q'. They can be charged for many reasons including but not limited to: fuel, insurance, navigation, peak travel, sector, seasons, security, equipment type or category, airport/terminal, RBD, validating carrier, waiver of other fare rules such as advance purchase, min/max stay etc, and of course: miscellaneous.
Because 'Q' surcharges are part of the fare, they can earn commission on commission-eligible fares, and interline to other carriers. To make changes to 'Q' surcharges requires the fare itself to be republished.
'Carrier Surcharges' (or imposed fees) are the fancy name given to YQ and YR. They are imposed for a more limited set of items - namely fuel, insurance, and miscellaneous/other. They also show up as part of the 'tax' calculation on the ticket.
Unlike 'Q' surcharges which are imposed by the fare owner, YQ and YR are imposed by the marketing carrier based on various conditions including sectors, journey/trip/destination, fare type, RBD, and government regulations in various countries limiting the amount of these types of surcharges.
YQ and YR amounts and rules are filed separately from fares which allows them to be updated without having to republish fares. The rules for calculating YQ and YR can be very complex especially when multiple marketing carriers are involved. As well, when flights are marketed by carrier A and ticketed by carrier B, carrier A and B need to have a bilateral agreement for carrier B to collect the YQ/YR for carrier A, and such agreements may not cover all situations. This can result in vastly different YQ/YR being charged on a mixed-carrier itinerary depending on who issues the ticket, or whether flights are booked as codeshares or not.
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Regarding your specific questions on HKG trips, this is fairly easy to explain.
The $7-8 CAD surcharge is the 45 HKD sector surcharge collected on the HKG-Canada segment that AC (or all airlines, really) pays to HKG for security. This amount varies on all of the tickets because of currency fluctuations.
The extra $100 for your July 2016 ticket is likely a peak travel surcharge. I don't know your exact fare but this did seem to exist in some AC fares last year:
Code:
V FARE BASIS BK FARE TRAVEL-TICKET AP MINMAX RTG
34 ZXCSTT Z R 3814.00 T31MY -/‡ -/12M PA01
PASSENGER TYPE-ADT AUTO PRICE-YES
FROM-YTO TO-HKG CXR-AC TVL-15JUL16 RULE-BAC1 IPRP/3
FARE BASIS-ZXCSTT SPECIAL FARE DIS-E VENDOR-ATP
FARE TYPE-BX RT-BUSINESS CLASS EXCURSION
CAD 3814.00 0301 E30APR16 D-INFINITY FC-ZXCSTT FN-2B
SYSTEM DATES - CREATED 29APR16/1521 EXPIRES 13JUN16/1122
12.SURCHARGES
IF INFANT WITHOUT A SEAT PSGR UNDER 2.
THERE IS NO MISCELLANEOUS/OTHER SURCHARGE PER
DIRECTION PER ANY PASSENGER.
IF CONTRACT BULK INFANT PSGR UNDER 2.
THERE IS NO MISCELLANEOUS/OTHER SURCHARGE PER
DIRECTION PER ANY PASSENGER.
IF INCLUSIVE TOUR INFANT WITHOUT A SEAT PSGR UNDER 2.
THERE IS NO MISCELLANEOUS/OTHER SURCHARGE PER
DIRECTION PER ANY PASSENGER. ‡
MD«
ORIGINATING CANADA INBOUND - ‡
SEASONAL SURCHARGE OF CAD 300.00 WILL BE ADDED TO
THE APPLICABLE FARE PER ANY PASSENGER FROM 01JAN
THROUGH 10JAN.
AND - SEASONAL SURCHARGE OF CAD 100.00 WILL BE ADDED
TO THE APPLICABLE FARE PER ANY PASSENGER FROM
27APR THROUGH 29APR.
AND - SEASONAL SURCHARGE OF CAD 100.00 WILL BE ADDED
TO THE APPLICABLE FARE PER ANY PASSENGER FROM
01JUL THROUGH 31JUL.
AND - SEASONAL SURCHARGE OF CAD 300.00 WILL BE ADDED
TO THE APPLICABLE FARE PER ANY PASSENGER FROM
01AUG THROUGH 17AUG.
AND - SEASONAL SURCHARGE OF CAD 450.00 WILL BE ADDED
TO THE APPLICABLE FARE PER ANY PASSENGER FROM
18AUG THROUGH 24AUG.
AND - SEASONAL SURCHARGE OF CAD 600.00 WILL BE ADDED
TO THE APPLICABLE FARE PER ANY PASSENGER FROM
25AUG THROUGH 05SEP. ‡
MD«
AND - SEASONAL SURCHARGE OF CAD 300.00 WILL BE ADDED ‡
TO THE APPLICABLE FARE PER ANY PASSENGER FROM
06SEP THROUGH 11SEP.
FROM HKG TO YYZ, SECTOR SURCHARGE OF HKD 45 APPLIES.
For the Carrier Surcharge (YQ), Hong Kong regulates the amounts that can be charged. For a good portion of 2016, the amount that could be charged on Canada-HK trips was 0. They only came back in late 2016.