Airspeed ind. problem

[flyguy44]

Reading nearly 20k higher airspeed at cruise than actual on my RV-8. Need help determining where problem may be and what to do. Thx, Bill

 

[Mel]

Check your static system

Almost all indicated airspeed errors are caused by a static system problem.

 

[flyeyes]

Reading nearly 20k higher airspeed at cruise than actual on my RV-8. Need help determining where problem may be and what to do. Thx, Bill

Kevin Horton's site has tons of info which will help here.

Is this a new problem?

I would think the place to start is to put a manometer on the pitot tube in the hangar, which should tell you if the airspeed indicator itself is OK. If that checks out, then look for static leaks (which the manometer will help with as well).

here is another link

 

[Kevin Horton]

Have a look at my info on testing for static source position error.

The first things to check are static system leaks and ASI instrument error. A static system leak in the cockpit could lead to a higher ASI reading (cockpit pressure often a bit lower than ambient), but probably not 20 kt worth. But a combination of static system leak and ASI instrument error could do it.

 

[RV7AV8R]

Had a similar problem in my RV7. A static leak will also cause the altimeter to read high compared to your GPS altitude. I removed the nylon fittings into the back of my instruments and replaced with brass and fixed the problem. After 4 years all 5 fittings had loosened. I have been told not all nylon fittings go bad, but mine sure did.
John Adams

 

[elippse]

Had a similar problem in my RV7. A static leak will also cause the altimeter to read high compared to your GPS altitude. John Adams

You can't compare GPS altitude with baro altitude. Baro altitude is based upon atmospheric variables which means it doesn't necessarily coincide with absolute altitude above the ground. GPS altitude is based upon height above a mathematical model of the shape of the earth, and will always be the same for any location.

 

[airguy]

But assuming your Kollsman window has been adjusted for local pressure, they should be very close.

 

[n5lp]

But assuming your Kollsman window has been adjusted for local pressure, they should be very close.

They should indeed be close on the ground at the location the altimeter setting is for. That is one reason we use altimeter settings. The other big reason is so that aircraft in the same area will be flying at corresponding, not true, altitudes. In other words, everyone that shows 8,500 feet, in the same area, will be at about the same altitude.

Since the temperature profile aloft is always different though, with everything working properly, the GPS altitude and altimeter altitude can be very far apart aloft. When this is the case, the GPS altitude would be expected to be closer to actual height above mean sea level.

At one time I did part time work at a high altitude flight service station (Bryce Canyon, Utah). We were not allowed to give the altimeter setting to overflying aircraft because that caused such huge altitude errors. For landing the altimeter setting did what it was supposed to. It caused the aircraft altimeter to read near the actual elevation at touchdown.

When things are close, do not rely on a barometric altimeter for terrain clearance!

 

[RV7AV8R]

These comments are all true of course, but if you have a static leak and your cabin is slightly negative air pressure like my RV7 is, the altimeter will be almost always reading high when compared to the gps while you are moving. Another tell tail sign is that you can hold a constant GPS altitude and vary your speed. The altimeter shouldn't change but if you have a static leak it will as the suction in the cabin will vary. If I recall correctly the altimeter goes higher with speed.
John Adams

 

[flyguy44]

Thx for advice guys...will ck. out w/ manometer in next week or so. if its static, guess I have a little work to do.