Donder10 From Canada, joined Oct 2001, 6659 posts, RR: 23 Posted (11 years 11 months 1 week 4 days 1 hour ago) and read 1073 times:
I was looking at the site www.worldairroutes.com and I found one route from South African Airways.On a flight from Johannesburg to Cape Town the 747SP was crusing at .871 mach@43,000 feet.IS this normal for the 747 SP?If so it must be the fastest crusing commerical jet in service surely?Thanks,Alex
Sonic99 From , joined Dec 1969, posts, RR: Reply 4, posted (11 years 11 months 1 week 3 days 23 hours ago) and read 961 times:
Ya that's pretty quick, but is that ground speed or air speed. Second, if the pilot caught the jetstream then that plane will be cruising for sure. BTW, I was on an AF flight from YUL to CDG and the speed indicator on the cabin screen showed 1,083 kph (relative to the ground).
That was in a 747-400.
Sonic99 From , joined Dec 1969, posts, RR: Reply 11, posted (11 years 11 months 1 week 2 days 15 hours ago) and read 792 times:
Yeah I hear you. Look, the 744 I was on obviously caught the jetstream and that added to the ground-relative speed. I'm sure the pilot max-ed the throttles to whatever the top speed is - we left YUL nearly 2 hours late but landed in CDG early! That was a quick trip over the pond for me... actually the quickest yet, just a shade under 5.5 hrs.
Cfalk From , joined Dec 1969, posts, RR: Reply 17, posted (11 years 11 months 1 week 2 days ago) and read 718 times:
Mach is measured by airspeed, not groundspeed, so whether the plane had caught a favorable jetstream or not is entirely irrelavant.
The SP, with much less weight and the same wing as the 747-200 and -300, could fly significantly higher and faster - the limiting factor (depending on the latitudes) was that flying as high as it could (49,000 feet was not unheard of) led to fuel icing problems, as the temperature of the fuel in the tanks dropped below -30 degrees (or is it -40) which is the freezing temperature of Jet A fuel. Normally, exterior temperature is colder than that, -50 or -60 or so, but friction of the air over the wing will warm it up slightly, bringing the fuel temperature back to operating levels.
I read an article written by a former PanAm SP pilot who described such flights quite nicely - I'll try to find the link.
Flying higher also allows you to fly faster, as there is less drag resistance.
The new Polar flights (like JFK-Hong Kong) apparently also have this fuel freezing issue to look out for, and requires extra attention during the flight over the cold polar regions.