# air speed inconsistant with the speed indicator

nottobe Captain

on Learjet (its the only jet I have ever flown in the sim), the speed indicator only shows half of the actual speed. say, when i fly 0.75 Mach, which is about 510 knots displayed on the GPS, but the speed indicator only shows 227. what is this figure? actually, 510 is also displayed in pink color below the indicator. So i guess 227 is another measurement that i dont know yet.......

Matthew Shope (mypilot) Chief Captain

Your airspeed indicator shows how fast you are traveling through the air. (not taking wind into consideration) On your GPS it shows you how fast you are traveling over the ground. Do you understand? I could get some more detailed instruction if you need it.

Matthew Shope (mypilot) Chief Captain

Here. Just put this into "My Pictures" and zoom in. I don't have much time. That should help you.

nottobe Captain

are you saying that the tail wind is blowing 233 knots?
must be hurricane on Mars......
seriously, this speed difference is basically consistant through all altitudes.
when i first flew it the over speed buzzer sounded while i was only flying a little over 300 on the indicator

Guest Ed Guest

mypilot is essentially correct, but a better way to put it would be to say that the airspeed indicator shows a relative measure of pressure on the front of the plane due to moving through the air. The airspeed indicator shows you the INDICATED airspeed (KIAS = knots indicated air speed). As you go higher, the air gets thinner, so the airspeed indicator is not as accurate and will always read LOWER than your true speed through the air.

The GPS is showing you the speed over the ground. Indicated airpseed will always be lower than ground speed, although at very low altitudes they will be closer than at high altitudes.

The Lear Jet has two maximum operating speeds, one expressed in KIAS and one expressed as a Mach Number. You can't exceed either one without getting the overspeed warning, but the relationship between the two numbers depends on altitude (as you have already discovered).

If I recall correctly, the Lear has maximums of 330 KIAS, or Mach .82-- whichever is less. At low altitudes, .82 is a lower speed than 330 KIAS, and that is why you got the warning at low altitude while flying at 300 KIAS. As you go higher and the air gets thinner, the Mach number is a better measure of your speed through the air, so you can fly at Mach .82 and only be showing about 230 KIAS (and a ground speed around 500 knots).

In the Lear, I usually switch from using KIAS to using Mach no. (on the Autothrottle) at about 25,000 feet. After I get above 10,000 feet, I set the A/T to 300 KIAS, then when I pass through 25,000 feet I switch the A/T over to Mach and set it to .82. And remember to make the same change when you're descending, or you'll get an overspeed warning coming down.

Ed

nottobe Captain

but i still dont understand: when I fly Mooney, the KIAS is pretty close to the GPS speed, even in a windy condition, the difference is not more than 40-50 knots, this makes sense to me, 50 kt wind is very strong. I compare them at the same speed and same altitude, say 200 knots at 10000 feet. but LearJet makes 230 knot difference which doesnt make sense to me.

Jonathan (99jolegg) Chief Captain

I'm not too sure what you're looking at. I just tried travelling at 200 KIAS at 10000 feet in a Learjet and a Mooney. It is extremely hard to fly at 200 KIAS in the Mooney at this height but at 180 KIAS the ground speed was around 206 knots and in the Learjet at 200 KIAS the ground speed was around 236 knots. Basically, they would be near enough the same if the Mooney actually went to 200 KIAS, so I'm not really sure where the problem is I suppose it could be due to different pressures in the air and different ground levels i.e. somewhere like Denver is around 5000 feet above sea level. Try it yourself in the same conditions and see what you come up with, otherwise I'm a bit stuck 😞 . Hope it helps 😀

Greg (FL050) First Officer

Airspeed is always changing and is always different at different altitudes. It is different at altitudes because of temperature, density altitude, pressure, etc... At these different altitudes, your plane is always reacting these changes. For example, if the density altitude is high, your planes performance will degrade (This is worse at higher altitudes) because there isn't as much particles in the air to go through your engines as there is at 5,000 feet.

There are 5 types of airspeeds, of which 2 are what you are talking about:

Ground Speed: This is the speed that is read off of your GPS. This is your airspeed affect by winds; obviously this is your speed over the ground.

Indicated Airspeed: This is the speed at which is indicated on your airspeed indicator not corrected for winds.

Say you are doing 300knts indicated airspeed, your groundspeed may be 400. This does not mean you have a 100 knt tailwind, it just means you have a tailwind that is pushing you and you are actually doing 400 knts over the ground.

Let me know if you need more help.