How do you go from S9 to Y0 in a day?
 

So I was looking at a flight
CX 100 SYD - HKG for departure tomorrow

yesterday it was S9. Today it is Y0!
Similar with all the other ex-SYD departures on the day.

How does that happen? They sold a whole bunch of tickets in the interim? Sounds possible I guess.

I was banking on the flight not being so full... but looks like we are in for a full flight now

Not 100% sure but my here is my theory:

CX have two different 777 versions they typically send to Sydney, one has 268 Economy seats (77G 3-3-3 config) and the other has 296 economy seats (77K 3-4-3 config). They often a day or two change the aircraft in the system depending on load - so whilst you might have had the 296 seats loaded in (despite still seeing the 3-3-3 seat map), they have changed this to the 3-3-3 aircraft reducing the number of seats by 28.

Lately, they've been running the 77G with the 3-3-3 config on CX100/101 but they could swap that out last minute with the larger one at the last minute but in a worst case scenario they've got 28 overbookings which they could either upgrade (if space left in higher cabins) or move to a different CX, VA or QF service.

Another possibility is they have snagged a load of heavy time critical cargo and how do not have the weight for passengers.

I suspect this could be related to the CI strike. Rerouting via HKG is a viable option for passengers travelling from SYD to TPE.

or that snowstorm in japan which caused rebookings and reroute... . who knows

Most likely reason is a problem with an other flight that causes knock on effect. As posted above doesn't even have to be Cathay, if an other airline has to cancel they will take all available seats.

Maybe a group bought 9 tickets?

I concur. 2 pairs of CI's TPE-SYD-TPE are cancelled on 12FEB and 13FEB per https://www.china-airlines.com/uk/en...e/announcement. BR doesn't serve SYD and no one else flies direct, so CX becomes the prime option. In fact, I would say CX and CI have a very good relationship when it comes to accommodating each other's stranded pax.

Perhaps a tour group, perhaps rerouting by CX or other airlines, or perhaps equipment change.

fwiw i see a CI plane parked at the gate here now in SYD

Here's my two cents. With my borderline-obsessive monitoring of EF, I've noticed this many times before, especially on the week of departure. FWIW, these have all been short-haul routes with high frequencies, and whenever this happens, the flights are mostly overbooked (resulting in op-ups from Y).

I've taken it to be the CX revenue management algorithms in play. They'll try to sell whatever they can - and have capacity for - a few days prior to departure (Y may be already overbooked, but W/J may have seats to op-up people to). This just happened to me over the weekend. I booked a flight on Friday (at the time S9) and when I checked EF on Sunday, it was Y0. And I think I must have witnessed this at least a dozen times before.

Of course, with OP's case, the underlying reasons might be different (CI disruptions etc.). However, I see this as something that CX does regularly (at least on some routes) rather than a special situation.

Pretty sure this is the right answer. CX revenue management is far more "loose" with their algorithm than most carriers. This sudden zeroing out happens ~90% of the time on SIN-HKG routes and I have a significant sample size on this (+/- 50 flights over the last 2-3 years, and I monitor every one with ExpertFlyer).

Is there no equipment change on those flights?