Confusion on airspeed.

[TheNoobestNoob]

Sorry as i may seem as a dumb. But i was wondering, i was on a flight and they said its airspeed was 900 kmh roughly 485 knots. But one thing i know is that the planes reach overspeed when travelling over 340knots+ for some planes. Can some please explain what i am missing out. Thanks. And yes this question may seem out of the blue, but i cant find any available aviation forum.

[wifesaysno]

Ah airspeed confusion. Happens more often than you think! Throw in how does one accurately measure it during flight tests regardless of weather and you have a lot more fun.

In aviation there are 3 basic speeds: GROUND speed, TRUE airspeed, and INDICATED airspeed. Ground speeds are affected by wind conditions and true airspeed (so a tail wind adds to your ground speed). TRUE airspeed is the speed of the aircraft relative to the AIR MASS it is flying through (you can get higher true airspeeds when you go higher and this is the 'standard' for how fast a plane goes). INDICATED airspeed is how fast you are relative to an airmass at sealevel at standard conditions.

Now the funny bit is on things like the U-2, the TRUE airspeed (which dictates loads on the aircraft) can be maxed out with the INDICATED (which dictates stall) at the minimum! Your true will increase as your indicated decreases as you go up high.

True airspeed is constant....only your ability to reach them changes with weather/atmosphere.
An aircraft will stall at the same indicated airspeed regardless of weather/atmosphere.

At low level with no wind, TRUE airspeed and GROUND speed should match. At low level with ISO standard atmosphere TRUE and INDICATED will match. With no wind, all three will match.

The original gold standard for measuring airspeed for record breaking required the aircraft to fly no higher than 200ft above ground level dead straight for 3km 3 times!! This was done all the way up to Darryl Greenamyer in an F-104 at something like 968mph!!! This low level was done because of the above. Now-a-days there are other tricks

Fly low, go fast
Tailwinds and high GROUND speeds!

[garymbuska]

Now that is what I call nailing it on the head.
You can really see a difference when flying east to west or west to east as depending on the season you will have a head wind one way and a tail wind the other and these winds can really blow. It is not unusual to see what are calleds winds aloft at and over 100 MPH. This is why you always check the weather on you chosen route. Failure to do so could be disastorious.. If you are flying into any kind of a head wind it will mean it will take more fuel for the trip. But do not for get that one important factor known as MTOW (MAX TAKE OFF WEIGHT) You do not want to go over this number at all.

[Azframer]

Yea I ran across some of those 100+ not head winds the other day going east to west. Really slows down a flight, love those tail winds though, they speed things up for you. I've been spending some time in that Just Flight 757-200 and was flying into Dallas and got caught up in some wind shear or wind shift. trying to line up a approach. I survived it.
We had a bad storm in Flagstaff Az and took off with 757 using ATC and got smacked with head wind at 99 knots after I was asked to turn 180 and then was hit with a tail wind while foolishly trying to get above the storm too quickly. Ended up in a flat spin from 20,000ft. Ground level or area is 7,000ft so did not have enough to recover from that mess, I was getting the nose to drop when I hit the ground.


Rick

[Flacke]

Sorry as i may seem as a dumb. But i was wondering, i was on a flight and they said its airspeed was 900 kmh roughly 485 knots. But one thing i know is that the planes reach overspeed when travelling over 340knots+ for some planes. Can some please explain what i am missing out. Thanks. And yes this question may seem out of the blue, but i cant find any available aviation forum.

Hello the Noob, I am assuming that you were on an actual Airline flight. If so, the 485 knot true airspeed they quoted would be perfectly normal. All airplanes have a maximum allowable airspeed that should not be exceeded or "overspeed" results. Overspeeds on airliners, LearJets etc. occur well above 485 knots. One particular airplane may overspeed at 340 knots, and some faster and some slower. It depends upon the airplane. So to answer your question, 485 knots was probably well within the safe flight envelope of the aircraft that you were on.

[wifesaysno]

I should add some examples, on a recent flight one of our company Lancair Evolutions had a real incredible speed ground speed:
The Evolution is redlined (Vne) at 256KIAS or INDICATED airspeed and usually cruises near 285 KTAS (true airspeed)....but the recent flight saw ground speeds of over 356 knots! Wheew that is fast

....and boy we love fast at Lancair!

Our "little" 2 seater, the Legacy, can hit up to 390mph on the Reno race course....but that is neither indicated, true, or ground speed....what that is is distance of track/time for 1 lap. The distance taken as the minimum it takes to fly the course, in other words, they are a weee bit faster than that on the course....not to bad considering stock P-51s race around 350mph again track speed not ground, indicated, or true.

A Legacy last spring set 4 new world records for its weight class with one speed run hitting 388mph. This was done at 150ft Above Ground Level in bumpy hot air...so that is GROUND speed....

[TheNoobestNoob]

Thanks really appreciate it.

[Brian Z]

While we're at it, we can throw in one more confusing number related to airspeed-- mach number. This one especially confuses sim pilots who start exploring small business jets. As you climb up to cruising altitude, you'll probably notice the redline on your airspeed indicator creeping down. So, in a Citation X, which is supposed to be capable of something like 540 kts, the redline is all the way down to 280. What's going on? Well, as the air gets thinner, sound travels more slowly, so Mach 1 happens at a slower speed. And as previous replies have explained, true airspeed is much higher than indicated at these altitudes. So, no your model isn't messed up, that's the way it actually works!