Flight speeds - Knots / Mach : Speeds / Altitude. 737 or similar?

Question

Hi.

These questions are mainly regarding 737 or similar.

1: I was wondering if any one knows why planes have speed settings in 'Knots' and 'Mach', and what is the difference?

2: Also, is there different cruising speeds related to the planes altitude?
What i mean is, if plane is at 18,000ft (Climbing), would it fly at different speed than if it was at 28,000ft? (Climbing)

3: I was also wondering what the normal climb and descent rate is? (FPM) Does this get adjusted as altitude is gained / decreased? I think it would depend on the weight of passengers / luggage etc. But is there a rough average?

4: Also wondering the average cruising speed at 30,000 or 32,000ft?

5: Is there a speed limit under 10,000ft?

Thanks in advance.

Answer 1

Answers are for the MD80



Knots are "nautical miles per hour", while Mach is a percentage of the speed of sound. A nautical mile is 6076 feet, and is one minute of longitude at the earth's equator. Knots are a carryover from nautical terminology, much of which is used in aircraft.



Below 26000 ft, we use airspeed, while above 26000 feet, we switch from airspeed to Mach number for our speed reference.



Normal climb for the is 320 knots to 26000 ft, then .72 Mach above that. Rate of climb varies, while airspeed or Mach are maintained constant. Upon reaching cruise, the airplane levels off and speeds up to .76 Mach.



Normal cruise speed for all altitudes is Mach .76, or about 485 knots, which varies slightly with altitude



Climb rate depends on weight, but will normally run 2000 - 3000 feet

per minute.



Max speed below 10,000 is 250 knots

Answer 2

1. Speeds expressed in knots are important at low speeds and altitudes. Speeds expressed as a ratio to the speed of sound (Mach numbers) are more convenient at high speeds and altitudes. Since airliners can fly in both regimes, they have instruments that can display speeds in both ways.



2. Cruising speeds do not vary enormously, but they vary somewhat, based not only on cruise altitude but other factors such as weight, winds encountered, and desired fuel burn and performance.



3. There isn't a normal vertical speed, but there is a range of common speeds. Climb rates can be very substantial: a lightly loaded airliner can exceed 6000 fpm in a climb without too much trouble, at low altitudes. Similar descent rates are possible but not seen as often: it's harder to make an airliner descend than it is to make it climb, because you can just push the throttles forward to climb, but in a descent you must find a way to shed kinetic energy, and you can only do that so fast (outside of emergencies). Most vertical speeds are considerably more modest than 6000 fpm; 2000-3000 fpm would be a more common range, and the longer the change in altitude, the more gentle the rate is likely to be.



4. Typical speeds might be between 420 and 500 knots true airspeed, depending on many factors. The speed over the ground might be very different, depending on winds aloft.



5. Yes, in the USA and many other countries, speed is limited to 250 KIAS below 10,000 feet, unless a higher minimum is required for safety. In the USA, at least, this limit cannot be waived by ATC, contrary to a widely-held misconception.

Answer 3

1. Already answered.



2. Climbing through 18,000' MSL you would be flying an indicated airspeed, or maybe a vertical rate such as 1000 fpm. Climbing through 28,000' MSL you would probably have crossed oveer to flying a mach number. Once your indicated airspeed brings you to the mach number you plan to use in cruise, just climb at that mach number.



3. No normal climb or descent rate.



4. Mach .80.



5. 250 KIAS (in US controlled airspace). "Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots." "If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed." [US FAR 91.117 (a) & (d)]