I'm flying Tokyo to London Heathrow with a transfer in Beijing next month. It's a single booking with Air China.

Air China have just changed our connecting flight at PEK, so we will only have 50 minutes to transfer, not the 3 hours we expected.

Is this even possible?!

I'll give you 25% odds, but since it's on a single ticket, I'd expect them to take decent care of you during the misconnect.

I thought Air China minimum connection time at PEK for Intl-to-intl transit is 60 minutes. Which would make this a non-legal connection that needs to be fixed before your journey starts. Call CA and point this out, as it would be better to get rebooked on a different sequence now. If you want to risk these particular flights, assume you'll misconnect and the rebooking might cause you to get to London a day later than you planned....so have some flexibility on your UK arrival end.

I have done a connection in PEK recently and think you have virtually no chance of making it. The airport is huge, the lines everywhere are long, the agents processing/verifying the visas and passport or boarding passes do thorough verifications and take ages to clear each pax (they just stare at you without uttering a word) ... so i would be surprised if you succeed in making it. Good luck, and do report back on how it went.

I wouldn't be that worry about 50 min. as both flights are with Air China.
It should be possible.
My gf did it within 30 min. in august as her flight from Nanning had a huge delay in Beijing and she just had 50 min. left before her departure to Germany.
One ticket on CA.
She got pick up by CA staff at the aircraft and guided to the international gate through "not official ways" to the gate (incl. security check and immigration).
CA is aware about the tight connection and it would wonder me if they can't bring you on the London flight.

As said, ONLY if there's a CA rep waiting at the gate with a big sign reading "Mr & Mrs Gamj90."

If you have to do this yourself, stop at the CA transit desk first -- even though you have your boarding pass -- and ask for assistance getting through security quickly.

Your gf did not have intl to intl though

Int-Int is actually easier than int-dom or vv if you're on CA.

It can be quick too. I did Montreal-Beijing-Nanjing. I never exited to the main hall as I went straight to my domestic gate right after immigration and my luggage was checked through.

I'd say, with the right "rush" attitude, a 50min int-int connection will work out (even if it's illegal) if the following happens:
-On-time arrival, ie doors open no later than 40min before departure (leaving you about 30min till boarding is closed) - quite good chance for this if arriving on a long-haul flight. Or during non-rush hours/months.
-Boarding passes for connecting flight in hand
-Knowing where to get upon arrival (left side of immigration, the transfer immigration document check counter, followed by security downstairs)
-Immediately making yourself known to the staff that you've a tight connection. At security, they actually got a monitor with a stuff next to it showing "tight connections" that are allowed to jump in front of the queue
-Obviously walking fast to and from transfer security

Post #10 by YuropFlyer is spot on.

Other problem for the OP is that the flight to London is always a long way away from the transfer security area.
Correct “rush” attitude also includes having correct clothes and footwear on and being prepared to RUN.

In April I did a 1 hour connection on Air China. Came up from Hong Kong, connecting to New York/JFK. So the HK flight arrived about 15 minutes early. Fantastic, right? Except.....we parked at a bus gate far away from the terminal. Damn, I am screwed! Got on the bus, got to the terminal. At the top of the escalator an Air China rep was waiting for us. He whisked us the front of the security line. It still took some time to get through but getting to the front of the line made all the difference. We made the connection, actually with some time to spare. HOWEVER - our bags did NOT make the connection. And no Air China rep was inside of the customs area to inform us. We waited for all of the bags to come off and did not see ours. So we went through customs and to the baggage office. The rep there was well aware, he had a list of the bags that missed. The delivered the bags the next day. I wish they would have informed us, at least by e-mail, so we did not have to waste 40 minutes waiting for bags that were never coming.

But yes, it is possible to make a tight connection. The key is getting an airline rep to get you to the front of the security line.

I've never seen an airport where there was pro-active information that bags didn't made it right at the luggage belt, unless like half of the aircraft was missing theirs. Probably there are some airports that have some kind of setting of individual pre-information, but I've yet to experience it.

Also, usually lost&found baggage office is right next to the luggage belts. Isn't that the case in JFK? Looks like a weird setup if you've to get landside for the lost&found office.

When someone else took my bag in PEK, the proper CA lost & found staff was very friendly and helped me to reunite me with my bag in an efficient manner (and the other pax with his) - not much Air China can do at some random outstation to improve service there (it's often not even airline staff at outstations, but rather 3rd party personal)

Unfortunately the baggage office for CA was outside of customs at JFK.