Do you always believe your ground speed?

[Joe Larussa]

So I’m flying home from SanDiego this morning and I start analyzing my speeds. I look at my ground speed and I’m like, no way. 140 mph airspeed and about 180 mph ground speed. 40 mph tailwind on a pretty calm morning at 12,000.  I check Flightaware when I get home and it checks out. Forecast winds were next to nothing. What gives?

[bcg]

I haven't seen many anemometers mounted on 12,000' poles when I've been out flying so I'm going to say the winds aloft forecasts are educated guesses at best. The best way to actually measure it at altitude is probably to compare a plane's IAS to its GS. I don't know how else they could get real data.

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[M20F]

I can’t tell from your gauges what OAT was but at 20C your TAS would be 177 MPH based on the gauges I can see.  So 180 MPH GS with little to no wind seems correct. 

[Joe Larussa]

Yeah, I ran the true airspeed at 170 mph. Not sure why my brain didn’t make that connection. 

[Ragsf15e]

21 minutes ago, Joe Larussa said:

So I’m flying home from SanDiego this morning and I start analyzing my speeds. I look at my ground speed and I’m like, no way. 140 mph airspeed and about 180 mph ground speed. 40 mph tailwind on a pretty calm morning at 12,000.  I check Flightaware when I get home and it checks out. Forecast winds were next to nothing. What gives?

As @M20F said, that’s totally legit.  Here’s the deal - ICE-T-G Indicated, Calibrated, Equivalent, True, Ground. You’re looking at 140mph indicated, but youve got to adjust to cal, equiv, true and finally to ground.  Do all of that and 180mph makes sense. Temp and alt pressure are the big players.

BL - at higher altitudes, thinner air makes the difference between indicated and true much bigger.  True will be faster than indicated.  Jets fly at like 300kias indicated, but 450kts true.  12,000’ gave you about 40mph difference.

[PT20J]

It appears that your G5 is lacking a a GTP 59 OAT probe and/or a GAD 13 OAT/TAS adapter. Those would give you a TAS readout above the IAS tape and would be useful since you don’t have an analog true airspeed indicator. 

Skip

[201er]

2 hours ago, Joe Larussa said:

Yeah, I ran the true airspeed at 170 mph. Not sure why my brain didn’t make that connection. 

Might need to invest in an oxygen system and pulse oximeter… 

[larryb]

I find forecast winds quite inaccurate. I always subtract 20kt from whatever GP says and am happy if it’s not that bad. If they say expect 10 kt tailwind I’m happy if I have better than a 10 kt headwind. So I’d be very happy with an unexpected 40 mph tailwind!

[ArtVandelay]

I find forecast winds quite inaccurate. I always subtract 20kt from whatever GP says and am happy if it’s not that bad. If they say expect 10 kt tailwind I’m happy if I have better than a 10 kt headwind. So I’d be very happy with an unexpected 40 mph tailwind!

Windy forecasts are reasonably accurate.
GP doesn’t sum up the total effect of the winds very well, maybe just use the closest forecast? Might help to select altitudes the forecast for (5000,6400,10000….), don’t know what it would do if you select 8000.

[Ragsf15e]

10 hours ago, larryb said:

I find forecast winds quite inaccurate. I always subtract 20kt from whatever GP says and am happy if it’s not that bad. If they say expect 10 kt tailwind I’m happy if I have better than a 10 kt headwind. So I’d be very happy with an unexpected 40 mph tailwind!

Interesting, I find foreflight winds to be pretty close.  They give you the total for the flight and then the forecast at points (vors) along the way at your cruise altitudes.  I check them a lot and they typically seem reasonable.  The overall single number “flight total” winds are somehow an average over the whole route, so possibly not accurate at each point.

One other thing to keep in mind… the winds we are “computing” in our airplane are only as accurate as our indicated airspeed and magnetic heading.  I find if you don’t adjust for calibrated airspeed and for any magnetic errors on your airplane, the wind computation is garbage in/garbage out.  The other components are likely solid (oat, track, and groundspeed), but IAS and hdg are problematic.

[carusoam]

Let’s see…

If your ground speed is provided by your WAAS enabled GPS….

It is updating very quickly, with accuracy measured in a few feet…

And….

If I don’t trust one source… I can fall back to plan B… a second WAAS source….

If you visually would like to see how accurate your ground speeds are… with a graphic overlay to know where you were at what time…. You can use a nice app called CloudAhoy….  

Best regards,

-a-

[Joe Larussa]

21 hours ago, 201er said:

Might need to invest in an oxygen system and pulse oximeter… 

Lol! Unfortunately I have both! 

[FlyingDude]

Do you remember your fuel burn that day?
Thanks.
(map+rpm by any chance?)