Flying the Plane
Monday, 07 January 2008 20:22
Hello Capt Lim!
Very informative, extensive and thoroughly enjoyable site!
I have 3 issues lingering in my mind.
1. When aircraft are assigned altitudes during peak periods, are they also assigned cruise speeds? Or, are these factors already taken into account
for the various aircraft types by the controllers when the flight plans are being filed?
2. During the cruise, is the Mach Number preset by the auto-throttle or is it gradually achieved by manually setting the throttle levers?
3. And finally; what are the cruising speeds of the various airliners?
Thank you very much in advance and take care of yourself and the passengers.
Thank you for visiting my site. Here are the answers to your questions:-
1. Every aircraft has an optimum cruising speed at a particular level. For the Boeing 777, it
is Mach 0.84 at around 35,000 feet or Mach 0.86 for the Boeing 747-400. When filing a Flight Plan, the cruising speed must be given for every types of aircraft. During peak periods, the ATC or Air Traffic Control may assign a common speed for spacing to ensure safety. So a Boeing 747 pilot may not be able to fly his desired speed of Mach 0.86 and fly at Mach 0.84 instead if she is behind a Boeing 777.
2. On the Boeing 777, the cruising speed at any level is calculated by the computer of the aircraft. For example, at 28,000 feet, the computer (FMC) will command a speed of Mach 0.81. When the computer is coupled or connected to the auto-pilot and the auto-throttle, the power setting is achieved automatically. There is no need for the pilot to manually set the
throttle settings as he would have done in older aircraft.
3. Here are the optimum cruising speeds for some of the more modern Airliners flying today.
Airliners Optimum cruising Speeds (Mach)
Boeing 747-400 0.86
Boeing 787 0.85
Boeing 777 0.84
Boeing 767 0.80
Boeing 757 0.80
Boeing 737 -800 0.78
Boeing 737 - 300/400/500 0.74
Airbus 380 0.85
Airbus 340 -300/600 0.82
Airbus 330 0.82
Airbus 320 0.78
Airbus 310 0.78
McDonnell MD-11 0.85
For your information, Mach 1.0 is equivalent to the speed of sound. It is 758 miles per hour or 660 knots at sea level. This speed varies with altitude and temperature. Mach 0.84 means the speed of the Boeing 777 is cruising at 84 % the speed of sound at 35,000 feet.
Bye bye and safe flying!