SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 1, posted (7 years 5 months 2 weeks 1 day 2 hours ago) and read 1737 times:
It depends on which yardstick is more relevant. BTW it is most likely that we cross over between knots and mach number in the upper twenties, 270 or 280.
At altitude the speed given is in mach number because that is what we are all looking at. If we are at the same flight level, or will be when a conflict arises, we are at the same temperature therefore Mach 1 is the same value for each plane and likewise, any decimal of that is also going to be the same for both airplanes. If I'm doing .81 and overtaking another, they can slow me down or speed them up by .01 or .02 and know it will work out - without knowing exactly how many knots (or furlongs-per-fortnight) that represents.
Down close to the ground when we are all referencing knots it is just more intuitive. Controllers can just plan to have everyone match their speeds at the point where it becomes necessary. Example, I am out of sixteen thousand on the approach doing 320 so I have a ferocious overtake going on that guy ahead who is just out of ten and at 250. It looks like I'm going to fly right up his tailpipe but the savvy controller knows that I will be slowing to 250 when I get to ten and it will work out.
Similar thing when winds aloft are quite different between altitudes or flight levels. LAX is landing west, into the wind but out here forty miles east and 12-13 thousand feet there is a big east wind blowing. Gives me an exaggerated overtake rate on the plane ahead and below because he has already flown into the west winds at lower altitudes. Initially this really concerns the controllers but they see one plane after another rocket down to some point on the arrival and suddenly slow way down because of the headwinds at lower levels.
Controllers are really smart guys. They figure this stuff out quickly.
Jamesbuk From United Kingdom, joined May 2005, 3968 posts, RR: 5 Reply 3, posted (7 years 5 months 2 weeks 23 hours ago) and read 1644 times:
ok basically my Q has nothing to do with towers or heights or anything just basically when you above the transition altitude between knots and mach why dont you talk to the center reporting mach not knots
lol rgds james
You cant have your cake and eat it... What the hells the point in having it then!!!
Grbld From Netherlands, joined Dec 2005, 353 posts, RR: 3 Reply 5, posted (7 years 5 months 2 weeks 22 hours ago) and read 1612 times:
Quoting Loggat (Reply 4): above the transition alt. controllers ask for Mach number, and below they ask for airspeed.
In the US maybe (where transition altitude is 18,000 ft) but here at AMS for instance, TA is just 3,000 ft. No relation there. It's more like when at cruise level, you stick to Mach, when in the departure or arrival stage, ATC wants airspeed for sequencing. Sometimes they'll tell you to fly 300 kts or greater when you're still descending and you're only doing .80/270 and it's not physically possible to fly faster than 300 kts, because you're still flying mach number. That's okay though, it's how they set up the arrival sequence.
We change over to constant mach climb (from IAS climb) when we reach the desired mach number (unless done automatically - like in the Airbus). The changeover altitude is solely based on the previous IAS. For example climbing with 340 knots to M.79 requires the change over in FL230. IAS of 300 gives you a changeover to M.79 at FL300. Turbulence penetration speed from IAS 275 to M.76 occures in FL310. Remember that the speed of sound only depends on temperature and it is decreasing with decreasing temperature. Above FL360 under ISA it stays constant and the indicated airspeed depends on pressure and other factors. Above the change over altitude you will climb with a constant percentage (around 80%) of the speed of sound which is decreasing as the temperature decreases. In FL390 you will have an IAS of 240 kts while cruising with M.78
Quoting SlamClick (Reply 1): Controllers are really smart guys. They figure this stuff out quickly
Loggat From United States of America, joined Feb 2000, 660 posts, RR: 0 Reply 7, posted (7 years 5 months 2 weeks 20 hours ago) and read 1565 times:
Quoting Grbld (Reply 5): In the US maybe (where transition altitude is 18,000 ft) but here at AMS for instance, TA is just 3,000 ft.
I wasn't referring to the transition altitude of 18,000ft (in the US) where standard altimeter settings are used above. I meant the transition altitude where you use Mach vs. Airspeed (as SlamClick said, about FL270-FL280 typically). I probably should not have used the phrase "transition altitude"
There are 3 types of people in this world, those that can count, and those that can't.
Grbld From Netherlands, joined Dec 2005, 353 posts, RR: 3 Reply 8, posted (7 years 5 months 2 weeks 9 hours ago) and read 1465 times:
Quoting Loggat (Reply 7): I wasn't referring to the transition altitude of 18,000ft (in the US) where standard altimeter settings are used above. I meant the transition altitude where you use Mach vs. Airspeed (as SlamClick said, about FL270-FL280 typically). I probably should not have used the phrase "transition altitude"