Determining actual TAS

flyingcheesehead

flyingcheesehead

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So I've heard here in the past that the way to find your real TAS is to fly N, S, E, W headings and then average the ground speeds. There was a discussion about this on an email list I'm on where someone said it had to be ground tracks, and the argument ensued...

However, it seems that *neither* will work. If you have any winds aloft, averaging GPS groundspeed on 4 cardinal tracks/courses will give you a result that's too slow (the higher the winds, the slower the result), but averaging the GPS groundspeed on 4 cardinal headings will give you a result that's too fast (the higher the winds, the faster the result)!

So, what's a real, accurate method?
 
for some reason, I thought they recommended 3 headings 120 degrees apart.

btw, how accurate are the airspeed indicators that incorporate temp and altitude for TAS?
 
The attached files may help. I used this on my Capstone Project for my masters.


NOTE: For whatever reason, I can't upload an XLS file, so I added the .txt in order to fool the system. You will need to remove the .txt when you save it on your computer.
 

Attachments

  • GPS PEC Method.doc
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  • GPS PEC.XLS.txt
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rbridges said:
for some reason, I thought they recommended 3 headings 120 degrees apart.
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I did N, E, and W headings and typed them into an online calculator (I think it was this one). The result seemed accurate...it also calculated the wind and it was within 1 knot and a couple degrees of the forecast winds a loft.

rbridges said:
btw, how accurate are the airspeed indicators that incorporate temp and altitude for TAS?
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Those basically are just an airspeed indicator with an E6B superimposed on it. So...as accurate as the airspeed indicator. An ASI in an Arrow I flew was about 7 knots too high.
 
Could always look at your PFD. It's displayed just right of 'TAS'.

:)
 
Captain said:
Could always look at your PFD. It's displayed just right of 'TAS'.

:)
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That's what I use. Of course if the pitot static and the OAT are off, there might be some issues. :)
 
Captain said:
Could always look at your PFD. It's displayed just right of 'TAS'.

:)
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I have to go to the true airspeed calculator page on the 430 to do mine. It's usually pretty close to just taking IAS * (1+ (alt in 1000s x 2)/100). But I'm also usually below 10,000ft.
 
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EdFred said:
I have to go to the true airspeed calculator page on the 430 to do mine. It's usually pretty close to just taking IAS * (1+ (alt in 1000s x 2)/100). But I'm also usually below 10,000ft.
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That's fine as long as your airspeed indicator is accurate, since one input to that 430 calculator is calibrated airspeed based on indicated airspeed from the airspeed indicator +/- position error from your POH. However, it's a garbage-in/garbage-out system, and doesn't help if your installed airspeed indicator isn't accurate.

The purpose of using those on-line TAS calculators based on GPS ground speed is to confirm the accuracy of your airspeed indicator. You fly the three legs recording GPS GS on each leg along with the IAS/PA/Temp. Next, convert IAS to CAS using your POH, and then convert CAS to "indicated" TAS using the 430 or other flight computer system. Then crank the 3-leg GPS GS data into the REA Comp (or similar GS->TAS calculation system) to get "actual" TAS. Finally, compare the "actual" TAS based on GPS GS (a pretty accurate system) to the "indicated" TAS to see how accurate your airspeed indicator really is.

Et voila!

In the case of my old Cougar, I was wondering why I had consistently lower TAS based on IAS than expected based on power setting and the POH cruise performance tables. Further, plugging the CAS into the calculator always showed lower headwinds and greater tailwinds than expected. I did the REA Comp trick, and lo and behold, my airspeed indicator was reading 8 knots low in cruise.
 
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I had the same thing on a previous airspeed indicator, coupled with the RPMs reading high. I swore I had the worlds slowest Cherokee. New ASI plus tach and I gained over 10kts. My current ASI checks out with the the course measurement. Of course if you fly straight into the wind, and straight with it, all you have to do is average them. And with GPS that's easy enough to figure out.
 
As I posted in the email discussion, this link graphically shows a 3-track method by plotting a circle with a radius of TAS, using the three ground speed vectors and deduces the wind speed.

http://www.dragonnorth.com/tas_fnl3.pdf

It is accurate, way more so than any simple averaging method. The RMS method that Paul S posted is also very accurate, but relies on flying 3 tracks 120 deg apart.
 
Inverted said:
Sounds super cumbersome to find out a seemingly meaningless answer.
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Yup.

And quite a number of aircraft have TAS gauges. Alternatively, use the IAS and add 2% for every 1000 feet indicated altitude. That presumes the IAS/CAS conversion is small, but it usually is in cruise.

Just how precisely do you need this anyway?

The only thing you really need TAS for is to report it to ATC if it differs from what you filed in IFR, or perhaps to amend a VFR flight plan (that's better done with time and ground speed).
 
MAKG1 said:
And quite a number of aircraft have TAS gauges.
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...but those are subject to many of the same errors as would affect the accuracy of the "regular" airspeed indicator.

Alternatively, use the IAS and add 2% for every 1000 feet indicated altitude. That presumes the IAS/CAS conversion is small, but it usually is in cruise.
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Again, this will not help determine the accuracy of the airspeed indicator.
 
Ron Levy said:
Again, this will not help determine the accuracy of the airspeed indicator.
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If you suspect the ASI isn't accurate wouldn't it be easier to take it to an avionics shop? If you find out from all your testing that it is wrong, you'll need to take it there anyway.

And if you don't really care that it is wrong what is the purpose of finding out the true TAS?
 
Ron Levy said:
...but those are subject to many of the same errors as would affect the accuracy of the "regular" airspeed indicator.

Again, this will not help determine the accuracy of the airspeed indicator.
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If that's the point, sure. But how did you get that the point was to determine accuracy of the airspeed indicator? The OP didn't say.

A problem with flying three legs is that the wind has to be the same on all three legs or the measurement is wrong.

A real accurate way to do it is to wait for calm wind (say, late at night) and time a measured mile.

You can do it with two measured ground speeds and track angles, but the trig is kinda nasty.
 
MAKG1 said:
If that's the point, sure. But how did you get that the point was to determine accuracy of the airspeed indicator? The OP didn't say.
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'Cuz what other point would there be?

A problem with flying three legs is that the wind has to be the same on all three legs or the measurement is wrong.
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That's true of any number of legs. Luckily, winds don't tend to change that much that fast...

A real accurate way to do it is to wait for calm wind (say, late at night) and time a measured mile.
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'cept for the fact that there aren't really any wind socks at 3000 feet to let you know that the winds aloft are calm.
 
Enter slow flight. (I found slow flight more susceptible to GS changes). Keep turning until your GS is at it's slowest. That's the direction the wind is from. Pour on the coals to desired power. Note GS. Do a 180. Note GS. Average them.

Holy crap you people make things difficult.
 
An easier way to do it would be to enter a constant bank 360 over a known point. At the end of the 360 note where you are in relation to the point.
 
EdFred said:
Enter slow flight. (I found slow flight more susceptible to GS changes). Keep turning until your GS is at it's slowest. That's the direction the wind is from. Pour on the coals to desired power. Note GS. Do a 180. Note GS. Average them.

Holy crap you people make things difficult.
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Even simpler. Just do it at one speed. Preferably desired power.

Slow flight is much more sensitive to GS changes because the wind is the same and the TAS is lower. But since you're looking for the TAS, that cancels.

If you're anywhere near a windsock (or a good substitute like rising smoke), that can give you a clue.
 
MAKG1 said:
Even simpler. Just do it at one speed. Preferably desired power.

Slow flight is much more sensitive to GS changes because the wind is the same and the TAS is lower. But since you're looking for the TAS, that cancels.

If you're anywhere near a windsock (or a good substitute like rising smoke), that can give you a clue.
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Except at 7000AGL the winds aren't always what they are at the surface. Right now even between 3000 and 6000 the winds changed by about 60 degrees. How's that smoke or windsock going to help?
 
Ron Levy said:
I'm not seeing any sort of Part 91 exception to that requirement. Have I missed something?
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should vs shall.

5-3-3. Additional Reports
a. The following reports should be made to ATC or FSS facilities without a specific ATC request:
 
EdFred said:
should vs shall.

5-3-3. Additional Reports
a. The following reports should be made to ATC or FSS facilities without a specific ATC request:
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Also, 5-3-3 is the AIM, not the CFR. The AIM does not impose any regulations.
 
acrophile said:
Also, 5-3-3 is the AIM, not the CFR. The AIM does not impose any regulations.
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True, but the question was why one would want a true airspeed measurement aside from a rough, labor intensive, and largely pointless ASI cslibration. Making that report -- whether it's required or not -- is one example.

If you're talking about required things, the calibration falls under 14 CFR 91.411 for the static side and 14 CFR 91.205 for all the pitot/static instruments. You can't fly with it if you suspect it isn't working right.
 
MAKG1 said:
True, but the question was why one would want a true airspeed measurement aside from a rough, labor intensive, and largely pointless ASI cslibration. Making that report -- whether it's required or not -- is one example.

If you're talking about required things, the calibration falls under 14 CFR 91.411 for the static side and 14 CFR 91.205 for all the pitot/static instruments. You can't fly with it if you suspect it isn't working right.
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Wrong answer. 91.411 is for the static side, not the pitot side. In fact the term "pitot" or "airspeed" doesn't even appear in subpart E.

Man, you are just batting 1.000 in this thread aren't ya?
 
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EdFred said:
Wrong answer. 91.411 is for the static side, not the pitot side. In fact the term "pitot" or "airspeed" doesn't even appear in subpart E.

Man, you are just batting 1.000 in this thread aren't ya?
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Umm, if you read what you responded to, you'll find it says exactly what you "corrected" it to.

You'll also find I never claimed that an airspeed report was required, though you clearly assumed it. It's a reason someone might need TAS, whether it's required or not.

I think you need some practice reading.
 
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The implication was clear, and I was not the only one who thought so.
 
EdFred said:
The implication was clear
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I see no such implication in Makg1's remarks, and you've said nothing about why you think so.

and I was not the only one who thought so.
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Really? I don't see any posts here besides yours attributing that implication to Makg1.
 
EdFred said:
Enter slow flight. (I found slow flight more susceptible to GS changes). Keep turning until your GS is at it's slowest. That's the direction the wind is from. Pour on the coals to desired power. Note GS. Do a 180. Note GS. Average them.

Holy crap you people make things difficult.
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:yes:

Bingo, we have a winner. This method is most accurate because it eliminates the crosswind crab factor. :yes::yes:
 
MAKG1 said:
Then you weren't the only one who was wrong.
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As I was told in every writing class I had, the burden is on the author to get his point across clearly.
 
EdFred said:
As I was told in every writing class I had, the burden is on the author to get his point across clearly.
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Well, at least one poster didn't misread it. I said exactly what I meant, and it was literally true without obfuscation. There is only so much an author can do with a reader who doesn't actually read.

I don't care to make you admit a mistake. That's not necessary, and I'm aware it's not in your nature. But to say it's mine? Out of bounds.
 
So mentioning a calibrated ASI has exactly what to do with 91.411? Oh yeah, nothing.

And where are those 6,000' AGL windsocks I can find in order to align myself with the wind? I'm having trouble finding them.
 
EdFred said:
So mentioning a calibrated ASI has exactly what to do with 91.411? Oh yeah, nothing.
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Look again. Look at the WHOLE thing. Even the part you didn't highlight.

EdFred said:
And where are those 6,000' AGL windsocks I can find in order to align myself with the wind? I'm having trouble finding them.
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OK, you presumably have no experience with wildfires, so I'll just assume you don't know that a column of smoke can rise to the tropopause under some conditions.

I've seen quite a lot of smoke over 10,000 MSL. Just because you haven't doesn't mean it doesn't exist.

And the 7000 AGL thing was YOUR addition. Some of us fly at lower altitudes sometimes, and no such general solution was requested by the OP.

Though, even at 10000 MSL, it's not THAT hard to eyeball the WCA. You can't tell a headwind from a tailwind that way, but you sure can tell a significant crosswind.

Oh wait, now I suppose you'll tell me that my solution doesn't work on Venus. I'll give you a preemptive pass. You would be right. It won't work on Venus.

Dude, I know you have an axe to grind. But don't make up situations just to create an opportunity to call me an idiot. ESPECIALLY if you aren't going to get it 100% right. Feel free to tell me when I'm actually wrong, but if you're going there, it has to be reciprocal.
 
EdFred said:
As I was told in every writing class I had, the burden is on the author to get his point across clearly.
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Typically, yes. But when a reader is sufficiently pedantic and careless, eager to spot errors but oblivious to his own, then even the clearest writer cannot avoid being misunderstood.
 
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