Ingineer said:
For instance, A 747 can cruise at 507 knots (KTAS) when at 35,000 feet, but below 10,000 feet you are not permitted to exceed even half that! (250 knots)
-Phil
The 250kt/10,000feet speed is a USA regulation (that virtually none of the rest of the world uses). It is the result of this:
16 December 1960 -
A United DC-8 (four engine jet) was inbound to JFK (then known as Idlewild) from LAX. A TWA Super Connie (four engine radial) was inbound to La Guardia from Columbus. The DC-8 began descent rather late and hence came down fast. Also it only had one VOR operational ( long before glass cockpits and sophisticated nav systems). While trying to get two radials from a single VOR the DC-8 crew overshoot their waypoint at nearly 400kts and collided with the Super Connie over Brooklyn. 128 + 6 on ground killed. After that, the US introduced two requirements: speed limit of 250Kts below 10,000 feet and all large airliners must also be equipped with DME (distance measuring equipment) as well as VOR (Very high frequency Omni-direction Radio).
The ground speed at altitude is a great example of thin air. The only way to push the airplane through the air faster is to have lower aerodynamic drag than the loss of power from diminished oxygen to the powerplants (jets don't have turbo/superchargers to compensate for reduced oxygen density).
You can play around with this to see the effects of altitude / air density on speed:
http://www.hochwarth.com/misc/AviationCalculator.html#CASMachTAS" onclick="window.open(this.href);return false;
So, the 507 knot Boeing 747 at 35,000 feet will have:
True Air Speed:
507 knots (this is not ground speed, because we don't know the winds)
Mach: 0.879 (ratio of speed of sound to actual speed... way too fast for normal cruising... wastes fuel)
Airspeed
302 knots (this is a measure of compressed air through a small "pitot" tube)
At 45,000 feet, it looks more like this:
True Air Speed:
507kt (this is normally called TAS)
Mach: 0.884
Airspeed
241 knots (this is normally called IAS... Indicated Air Speed)
Notice how much lower the air density is, and hence the IAS for the same TAS ?
At 10,000 feet, and no speed limit
True Air Speed:
507kt (this is normally called TAS)
Mach: 0.794
Airspeed
445 knots
See how much closer TAS is to IAS the closer we get to earth and denser air? Now, let's get down to:
Sea level:
True Air Speed:
507kt (this is normally called TAS)
Mach: 0.766
Airspeed
507 knots
IAS now equals TAS. A normal jet would not be able to go 507 knots IAS, therefore 507 knots at sea level wouldn't be possible. But it's common at 35,000 feet.
Every plane has published speed limits, and they vary with altitude (and some other factors). So, excluding regulations like the 250 kt USA speed limit, a typical jet might have an absolute "redline" at 330 knots IAS, meaning that plane will only go 330 knots TAS at sea level.
But, at altitude like 35,000 feet, there isn't enough power to move the plane anywhere close to 330 knots IAS, but as we can see, 240-300 knots IAS does a great job at going 400-500 knots TAS. Also, with a 330 kt IAS redline, the same plane won't do 507 kts TAS at 10,000 feet either; more like:
True Air Speed: 379kt TAS
Mach: 0.594
Airspeed 330 knots IAS (this is limiting)
Above about 30,000 feet, we no longer consider IAS, but instead rely on Mach speed limits. Let's say our typical jet has a Mach limit of 0.88, which again will prevent the plane from going above 507 kts TAS.
But, these are all limits. The typical actual profiles would be:
200 kts IAS max in the airport traffic area [about 200 kts TAS]
250 kts IAS max below 10,000 feet (USA) [about 250-280 TAS]
280 - 320 IAS climb speed [about 310 - 450 TAS]
M0.76 - 0.84 in cruise [about 420 - 480 TAS]
280 - 320 IAS in descent above 10,000 feet (USA) [about 310 - 450 TAS]
