Originally Posted by
pilotmike
It is a speed brake, manually deployed by the pilot, at will, and it is available to be used at any speed. It is usually selected some 100 - 200' above threshold before touchdown. It is not a lift spoiler, and it is not generally used in normal flight.
It's different on the approach to London City, with the 5.5 degree approach. There the speed brakes are deployed as soon as the aircraft is stabilised on final descent. If you are in Central London and LCY is on runway 10, you can look up and see them open up with about 5 miles to go. And in the cabin you can feel it .....
Some may remember that when Prince Andrew/Fergie got married they left for their honeymon in an RAF 146. As it turned away from the terminal for departure, and in front of all the TV cameras, the crew opened up the speed brakes, and they had a "Just Married" banner hanging from the inside of the mechanism. In idle moments I wondered if they got airborne with it still there.