"Mary Shafer" wrote...

(Must wreak havoc with arrival times in London! I suppose pilots must
get permission to ride a 300 mph jet stream? That could shave two
hours off a flight BOS-LON.)

No permission that I know of, although it shows up in the flight
planning. When the jet stream and the flight track coincide, the
ground speeds really soar. However, the jet stream doesn't follow
great circle routes, so sometimes it's a crosswind instead of a tail
wind going east[1].

Seldom is the jet stream 300 mph -- nominal maximum in the winter time is
200 knots or so, though I've seen 220 knots a couple times...

Airline dispatchers routinely adjust routes to take advantage of favorable
jetstream winds, and to avoid horrendous headwinds when possible. Various
ATC agencies do their part as well, adjusting oceanic tracks with the
migration of the jetstreams.