I've briefed many a new copilot on possible speed loss, when we get it they ask how did you know we'd get speed loss I answer experience and now you gained that too.
.Real story. 737-200 on dark night ILS to Runway 6L Guam Agana in the Marianna Islands. Cloud base about 1500 ft There are two runways at Guam. 6L and 6R. 6L has ILS. 6R has no aids. We received Instructions from ATC to break right when visual for sidestep right to 6R. As mentioned, there were no aids on 6R, so once you break right from the localiser 6L, it is a matter of glide path judgement. A taxiway with holding point joins 6L and 6R at their landing thresholds and separates both runways by about 300 metres. On this occasion, a PANAM 747 was holding on the taxiway intending to depart 6L (10,000 ft long). At 4 miles on final and breaking clear of cloud, we announced side-stepping right to land 6R. ATC then clears the 747 for take off 6L. So far, so good. With no glide slope guidance available for 6R, and glide slope 6L unusable for 6R, we drifted lightly high as we crossed threshold 6R. Wind calm.
Without any warning, turbulence hit the 737 which rapidly banked 20 degrees left and right and sank like stone. At the same time a stunned co-pilot called "Christ! Bug minus 20." . As PF I firewalled the thrust levers and hauled back for a raw data go-around on instruments. Fortunately this may prevented what would have been an extremely heavy and uncontrolled touch-down. We went into cloud while on the go-around and were then radar vectored for an ILS and landed safely. ATC queried the reason for the go-around and was told by the PM "severe windshear."
In fact, it was not windshear as such; but break-away jet blast from the 747 still on the taxiway at 90 degrees to us. Despite being cleared to take off on 6L some three minutes earlier, he had delayed lining up for some unknown reason. He did not tell ATC. From the left seat on short final I was aware of a shape with flashing lights on the dark taxiway adjoining both runways but didn't twig it could have been the 747. We had then passed close behind him as we crossed the threshold of 6R just as he opened up to break-away power to taxi for 6L. His jet blast hit us just before we flared. What saved us was the fact we were slightly high on slope over 6R threshold.
. All the briefings in the world cannot cover the one-in-thousand unexpected speed loss.