Winds Aloft Error?

[kalewis]

Have about 20 hrs behind skyview and now on some XC flights I'm noticing the winds aloft info is erroneous.  The compass is calibrated... GPS is spot on.  Just today the info gave winds 289 @ 30 / XW factor 27.  Forecast for 3000' was 320 @ 13 / 6000' 310 @ 12. On course heading was 009.  Always seems to give very high wind speeds at low altitude. 

Any ideas ?!?

Thanks. 

 

[bkthomps]

i've seen this as well, not sure what is causing it-

 

[dynonsupport]

The math in SkyView is probably not wrong. We have way to many of these in the field to imagine a bug there, but anything is possible.

Anytime you think the wind is wrong, here's what you should write down:

Airspeed (IAS)
OAT
Altitude
Heading
GPS track
GPS ground speed

On the ground, you can do the math yourself with your E6B and make sure it was right. I'm pretty sure you will find that it is.

In 75% of the cases, the issue is that your heading is wrong. If you are 100% sure it's right, then the next thing is usually an airspeed error, causing your IAS and thus TAS to be off. It doesn't take a lot, and we've seen tons of airplanes with poor static systems that cause airspeed and altitude errors. There are some simple ways to verify your static system if you search around a bit.

In the picture above, we have IAS of 94.5 knots, OAT of 35F, and altitude of 7750'. This calculates to a TAS of 106.48, matching the screen at 106 knots TAS.

Then, we have 96 knots ground speed, track of 010, and mag heading of 351.

Well, you can see your ground track and mag heading are way off, almost 20 degrees. It takes a good crosswind to do this. Also, your ground speed and TAS are 10 knots off, so you need a 10 knot headwind to do that. To have a crosswind that blows you 20 degrees off while also slowing you down 10 knots is a good wind.

Sure enough, my E6B shows a wind speed of 33 knots at 287 degrees. Pretty close to the 30/289 that we are showing (our winds are filtered a bit so the exact numbers on the screen aren't directly used for the wind shown at that moment).

So, either your heading or TAS is off. Note that in your situation, having the compass off even one degree changes the wind speed by 2 knots!

 

[kalewis]

Thanks for the info. When you say the heading is off by a degree or two, that implies the heading calibration?

 

[bkthomps]

or some type of interference causing the mag heading to be different than it actually is- since it will use your mag heading with your GPS track to determine what the wind is doing, if the mag heading is wrong, then so will the xw factor

 

[dynonsupport]

That means the heading is wrong for some reason. Either it's being interfered with by some other thing in the airplane, or hasn't been calibrated, or isn't well-calibrated.

 

[kalewis]

I'm sure there is nothing wrong with the heading (interference), the avionics package was all pre- engineered in our RV-12 if you are familiar. The calibration seems fine, however I guess it is possible that a more precise calibration is possible. I used the compass feature on my iPhone and had a helper position it on the tail as we moved it across each heading. The GPS has been accurate on the XC flights. As far as pitot static, it was checked to be w/in tolerances and airspeed and altitude was verified with another aircraft in flight. Not saying that is 100% accurate either but we were within 3-5 kts and 20'. He was flying a glass panel RV-10.

 

[lolachampcar]

I ran into a similar problem and, as Dynon points out, solved it by correctly calibrating heading and fixing my static issues.  The first required I pick the correct alternator charge output so that my in flight conditions matched my calibration conditions.  That was not too hard.  However, properly fixing my static issues was non-trivial.  I used data logging above 2400 baud (4800 worked well for me) then worked on a long runway to lift off and maintain a set height above the runway while the plane accellerated.  I then went back and checked my density altitude.  As installed, I lost about 200 feet accelerating from 55 to 155 knots!  My static is part of my pitot (built into the Dynon AoA) so I tried the standard o-ring just before the static fix. That had me gaining 200 feet.  I finally nailed it down using shrink tube and, at last check, had about 5 feet worth of change which is well within my ability to fly that close to the runway.

Lower speeds would seem to cause less error so this may not be a problem for you.  It was a big issue for me.  My ground and TAS now agree within a few knots on a clam day with TAS at 190 knots.

 

[Janekom]

I tried the standard o-ring just before the static fix

Can someone explain this to me please?

 

[skysailor]

If your static port does not stand proud of the surface and "trip" the air going past, there are often times errors in the static pressure. Small errors in static pressure can affect altitude and indicated airspeed. Both of these are used to compute TAS as shown above so an error is static pressure has big consequences.

Some people with a static port that is simply a hole drilled through the skin have found putting even a small barrier in front of the port (an O ding in his case since he modified the Dynon pitot to add a static port there) enough to cause the microturbulence necessary to allow for accurate readings. This might even be as small as a 1/2" long piece of .032" aluminum just in front of the static port.