smartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0 Posted (3 years 1 month 1 week 4 days ago) and read 3711 times:
In a lot of our company approach briefings, there is mention of sticking to standard approach speeds. i.e. 220 at the IAF, 180 on base/final approach track and 160 from 8nm -4nm, I have not been able to find any reference to where these speeds have come from other than they seem to fall within the range of speeds depicted in the Pan Ops Doc 8168 Vol 1, Table I-4-1-2 for various parts of the approach.
In regards to the FMC the default speed that comes up for the decent which is 240/100, from reading up on it, the FMC takes the airport restriction speed but with minus 10 knots, what is the reason for deducting 10 knots off it?
tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 2, posted (3 years 1 month 1 week 3 days 23 hours ago) and read 3693 times:
Quoting smartt1982 (Thread starter): In regards to the FMC the default speed that comes up for the decent which is 240/100, from reading up on it, the FMC takes the airport restriction speed but with minus 10 knots, what is the reason for deducting 10 knots off it?
Same reason that the actual certified ceiling is usually 100' above an even flight level...you want margin to move without busting the limit.
If you fly at exactly 250, a 1 knot overshoot renders you illegal. Nobody will probably care but it's not good practice to put yourself in a situation where you can't comply with the requirements.
Quoting Mir (Reply 1): Probably because the autothrottle isn't capable of holding 250 exactly - there's going to be some margin for error.
Autothrottle is good to +/- 5 knots, typically (hence Vref+5 for approach speed on Boeings), but even if you're a human pilot you can't hold speed exactly. Flying at 250 basically guarantees that you'll bust the limit from time to time.