— -- Question: Hello Captain Cox, I would like to know the most efficient rate and speed of climb of a commercial aircraft such as a Boeing 737-700 or Airbus A321? I have two flight-simulator games on my computer and to hit cruise altitude quickly (about 33,000 feet) I start my climb at about 3,500 feet per minute going just under 250 knots below 10,000 ft. After that I adjust speed to about 280 knots inching towards 300 knots, averaging about 2,500 fpm. The problem is I've exhausted almost 10% fuel and my forward speed is just about 200 knots before reaching cruise altitude, which shortens my range. What is a more realistic climb scenario for a real aircraft, so that you can achieve maximum distance?
— submitted via email
Answer: Normally, climb speed is set by the flight-management computer based on weight, temperature and cruise altitude. As a general rule, the 737s climb at a lower speed, but faster rate than the Airbus. The difference is due to the algorithms in the computers.
Most jets climb at 250 knots up to 10,000 feet due to FAA regulations. Above 10,000 feet, 280 to 300 knots with a transition to Mach .7 around 24,000 feet are average for the 737. The Airbus will climb 250 knots up to 10,000 feet, then accelerate to 300 knots or a little more, then transition to Mach .8 around 24,000 feet. The Airbus is more of a cruise climb, where the higher speed is near cruise speed but the rate of climb is less than the 737. The A321 at high weights does not climb rapidly.
The amount of fuel used during takeoff and climb is much higher than cruise. If you are trying to get absolute maximum range, then you will want to slow down the cruise speed to "long range cruise" values. You probably can find these values online; an example would be normal cruise of Mach .8, where long range cruise could be Mach .68. Flight management computers provide the exact values based on weight and temperature. Using "long range cruise" speeds is rare in normal operations.
One other observation about your computer scenario: The true airspeed is greater than indicated airspeed. Consequently, an indicated airspeed of 200 knots at 33,000 feet with standard temperature would result in a true airspeed over 330 knots. This is a slightly slower than normal. We generally see the true airspeed around 420 knots.
Q: How many RPM's does a jet engine turn during takeoff? I have heard it can be as high as 35,000 RPM's. Also, once at cruising speed, do you cut back engine power?
— Gary A. Money
A: The RPM of engines vary with individual types of engines. No, they do not spin at 35,000, but more in the range of 10,000 RPM. Within most jet engines, there are two or three different shafts, each of which turn at a different rate. Yes, at cruise the RPM is less than at takeoff; the amount varies by engine type.
John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.