R :
I've had a quick scan through the accident report - see my post above. The problem was first noticed at about FL230. The wrong engine was shut down, due to various factors, and only failed at about 900' on the approach. That was insufficent time to get the now shut down but apparently wholesome donk going again.
From recollections of jumpseat rides, it's in the order of 30 seconds from the moment the first action is taken until the engine has stabilised at idle. Note this is IDLE - not producing power enough to sustain flight - so you might need to add some more time on, but it's possible that the forward airspeed will help reduce start up time because the mass flow of air required is at least partially present.
There are Jumbo drivers here, they'll be able to tell you how long it takes to get an engine that size going. Maybe Eric Moody might enlighten us ?
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I got bored with "WhoNeedsRunways"