A follow-up statement from AC corporate communications as USA Today picks up the story:
"After takeoff the pilot got an indicator that one of the cargo hold heaters had malfunctioned and that the temperature was falling, as it is very cold at the high altitude where our planes fly," Air Canada spokesman John Reber said in a statement to Today in the Sky. "While a heater is not normally a critical component, on this particular flight there was a live animal in the hold."
"Air Canada's pilots are professionals who are responsible for the entire flight," the airline added in its statement. "As soon as the crew became aware of the temperature issue, the Captain grew rightfully concerned for the dog's comfort and well-being. With the altitude it can become very uncomfortable, and possibly the situation could have been life threatening if the flight had continued."
Air Canada pilot lauded for diverting trans-Atlantic flight to save dog
The dog makes a good news story, but I'd guess there's other cargo that might not take well to being unexpectedly frozen. Besides, how much insulation is there between the cargo area and the passenger cabin?
Don't know about those new-fangled 787's. But most recent aircraft have enough bleed air from the high bypass turbofans to keep the cabin comfortable even with a fault in the cargo holds. Or so I've been told in ground school.
The earlier jet airliners were short on excess bleed air to run the packs, particularly at low power settings.
The B-727 had a cargo heat switch on the flight engineer's panel. It's been a long time, was this switch off for takeoff? Or, was it one that you closed at top of descent before you pulled the lever to silence the gear horn as the throttles came back?
I do remember being told by my PFE instructor that improper manipulation of the B-727 cargo heat switch could endanger live animals in the hold.