How accurate is the TAS calculator on a Garmin GPS?

k9medic

k9medic

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During my monthly 2.5 hour flight to the Bahamas yesterday, I got to thinking about the accuracy of the TAS calculator on the Garmin GPS.

In the past, I used my G430W to calculate my TAS which almost always came out at 131kts. During my past annual, we did some slight rigging adjustments and removed the "courtesy step" and now I am seeing TAS calculations of around 139kts.

Can this be accurate? Did I really gain 8 kts. by removing a step and making a slight rigging adjustment?
 
k9medic said:
During my monthly 2.5 hour flight to the Bahamas yesterday, I got to thinking about the accuracy of the TAS calculator on the Garmin GPS.

In the past, I used my G430W to calculate my TAS which almost always came out at 131kts. During my past annual, we did some slight rigging adjustments and removed the "courtesy step" and now I am seeing TAS calculations of around 139kts.

Can this be accurate? Did I really gain 8 kts. by removing a step and making a slight rigging adjustment?
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What type aircraft are you talking about? I know an out of rig aircraft can make a major difference.
 
Temp has to be accurate for TAS calculation. If you had some sun hitting the probe it could easily make that much of a difference.
 
I believe TAS in the Garmin is determined by a formula based on IAS, angle of heading vs ground track, and gps based ground speed. If the IAS is off, TAS will be off as well.
 
k9medic said:
Can this be accurate? Did I really gain 8 kts. by removing a step and making a slight rigging adjustment?
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I just had my flight controls rigged which were “way out of spec” and picked up about 7 knots. Also the tail beacon was replaced with LED (lower profile). I’ve verified it several flights. Congrats on your new found speed!
 
I have heard of several cases of speed gains after painting an airplane, with these gains being attributed to correcting mis-rigging while re-hanging the control surfaces. Something like up to 10-12 knots, roughly. So, not unheard of. The step is probably good for about 1-2 knots.
 
EdFred said:
Even easier: turn to a heading where you have max(or min) ground speed. Take down speed. Fly the reciprocal. Average them.
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That does not work for a variety of reasons. The above is far more accurate.
 
Jeff767 said:
That does not work for a variety of reasons. The above is far more accurate.
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It absolutely works. You are straight into the wind and then straight with it. It's not a random heading.

But I'm more than interested to hear these variety of reasons why it doesn't work.
 
EdFred said:
It absolutely works. You are straight into the wind and then straight with it. It's not a random heading.

But I'm more than interested to hear these variety of reasons why it doesn't work.
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:popcorn: Me too.
 
EdFred said:
But I'm more than interested to hear these variety of reasons why it doesn't work.
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Ditto.

I cannot think of any, other than deceleration during the 180 turn, which would require waiting a few minutes to pick up speed again before recording the groundspeed. But that also affects the method that Jeff likes, the double 90-degree turn scheme in the webpage calculator.
 
asicer said:
CAS, actually. That plus DALT.

No. Those are for computing winds.

Yes, if IAS is used as the basis for CAS.
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Ah yes, I stand corrected. Let me know if this is right:

IAS is used as part of the formula to determine TAS
TAS and ground track/heading and GS is used to determine winds

If your IAS is off, then TAS and winds are wrong.
 
EdFred said:
It absolutely works. You are straight into the wind and then straight with it. It's not a random heading.

But I'm more than interested to hear these variety of reasons why it doesn't work.
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It’s difficult to find the exact heading to be perfectly into the wind vice a GPS track where wind direction is irrelevant. Any errors are magnified in the turn back. Most GA aircraft will exhibit precision in the heading while turning. I have never known anyone flight testing to use a two leg method.
 
Cluemeister said:
Ah yes, I stand corrected. Let me know if this is right:

IAS is used as part of the formula to determine TAS
TAS and ground track/heading and GS is used to determine winds

If your IAS is off, then TAS and winds are wrong.
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Yup
 
Jeff767 said:
Just follow the instructions here and fly a couple of headings and you will have your exact TAS. No need to guess.

http://www.csgnetwork.com/tasgpscalc.html
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This is the first step in what is one of many ways to calculate a source error correction to go from IAS to CAS. Once you have CAS, TAS is only a matter of a temperature correction at the airpeeds most of us fly. If you believe the corrections in your POH it's unnecessary. If you want to do it, it only needs to be done once (per indicated airspeed). Doing it every time you want true airspeed is a waste of time and effort.

Nauga,
who thinks the tower flyby method is more fun
 
I fully expect that the Garmin TAS calculation is quite accurate. Problem is garbage in, garbage out. For it to be correct it must be fed correct data.
 
k9medic said:
Can this be accurate? Did I really gain 8 kts. by removing a step and making a slight rigging adjustment?
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I'm not inclined to believe a slight rigging adjustment and the removal of the step netted you that increase in a 6-260. A large rigging adjustment might, but that presupposed your airplane was flying wicked slipped beforehand. 139 KTAS is pretty fast for a stock -260 with the pre-78 wheel pants, especially if flown at anything less than full out revving and burning 14+.

Error can be induced by reading those analog airspeed indicators and inputting that value in the calculator. Best bet would be to go with a nominal GPS ground speed multi-track pattern at the power setting and altitude of your usual choice, and see what kind of speed that math nets ya. If it is significant (I consider 5+ significant if no changes are being made to the power selection and weight profile you otherwise fly before), then perhaps the rigging was that far off.

The ultimate measure of improvement is your block times vs fuel costs anyways. Cheers!
 
hindsight2020 said:
I'm not inclined to believe a slight rigging adjustment and the removal of the step netted you that increase in a 6-260. A large rigging adjustment might, but that presupposed your airplane was flying wicked slipped beforehand. 139 KTAS is pretty fast for a stock -260 with the pre-78 wheel pants, especially if flown at anything less than full out revving and burning 14+.

Error can be induced by reading those analog airspeed indicators and inputting that value in the calculator. Best bet would be to go with a nominal GPS ground speed multi-track pattern at the power setting and altitude of your usual choice, and see what kind of speed that math nets ya. If it is significant (I consider 5+ significant if no changes are being made to the power selection and weight profile you otherwise fly before), then perhaps the rigging was that far off.

The ultimate measure of improvement is your block times vs fuel costs anyways. Cheers!
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Yep, that’s what I do. Go up to a typical cruising altitude and power setting. I hit the 4 cardinal headings, write down the GS on each, add them up and divide by 4. Done it several times thru the years and within a couple knots every time.
 
Jeff767 said:
It’s difficult to find the exact heading to be perfectly into the wind vice a GPS track where wind direction is irrelevant. Any errors are magnified in the turn back. Most GA aircraft will exhibit precision in the heading while turning. I have never known anyone flight testing to use a two leg method.
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You don't need to find the exact heading, given the +/- .5 kt (or mph or kmh) uncertainty introduced by the G430 display only giving you 3 digits (unless you know how to get more resolution out of the 430 display). So for a 10 knot head/tailwind, +/- 18 degrees of the exact wind direction will display the same ground speed. To see for yourself, fly a constant speed level 360, and watch the ground speed on your GPS, and note how many degrees of heading you roll through at the max and min ground speeds.

My comments are in context of the OP's application; i.e. did my GA plane pick up a few knots? I'll grant that for commercial, civil, and military applications the methodology and instrumentation becomes much more complicated to eliminate as much uncertainty as possible when qualification and production tests are performed to verify an aircraft meets a specification.

Unfortunately for the OP, if he never tried to measure his TAS with one of the methods above before the mods, doing the test now won't answer his question.
 
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Jeff767 said:
It’s difficult to find the exact heading to be perfectly into the wind vice a GPS track where wind direction is irrelevant. Any errors are magnified in the turn back. Most GA aircraft will exhibit precision in the heading while turning. I have never known anyone flight testing to use a two leg method.
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If you are within 5 degrees you are 99.8% of what you're supposed to be ground speed wise for True AirSpeed on an average. as far as the magnifying errors on the turn back you let everything settle out wait for a minute and then read the ground speed. So I don't know where you're getting this. I mean I get that it's the internet and somebody pointed out a different way to do something so you have to defend it to the nth degree but you're going to be within one knot doing it my way.
 
Piperonca said:
I've always heard that CAS is IAS corrected for installation error. AOPA has a decent description here: https://www.aopa.org/news-and-media...er/flight-training-magazine/truth-in-airspeed
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'Source error correction' is a shorter way of saying "static source position error correction," or errors resulting from the static source picking up some dynamic pressure due to the installation never being a perfect place to sense static pressure. In other words, 'installation error.' The AOPA article incorrectly attributes this solely to pitot errors, while most decent flight test textbooks address the static errors and the relative insensitivity of pitot probes to off-axis airflow.

Nauga,
pneumatically
 
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Don’t forget position error with CAS. In can be significant especially in helos. At standard conditions (sea level / 15 C) you can still have errors in indicated and true even with no instrument error involved. If the pitot tube isn’t going thru the air parallel to the oncoming air, it’ll indicated lower than true. Sometimes (ram air effect) it can indicate higher than true during standard conditions. High power climbs in helos present their own problems as well.
 
IIRC the Black Hawk helicopter can have up to 15 kt error dependent on certain climb rates and airspeed ranges. Yes, helicopters are strange things, always trying to beat the air into submission.
 
nauga said:
'Source error correction' is a shorter way of saying "static source position error correction," or errors resulting from the static source picking up some dynamic pressure due to the installation never being a perfect place to sense static pressure. In other words, 'installation error.' The AOPA article incorrectly attributes this solely to pitot errors, while most decent flight test textbooks address the static errors and the relative insensitivity of pitot probes to off-axis airflow.

Nauga,
pneumatically
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So, for us simple minded ones, can we lump it all into "pitot-static system"? Then our Mark I eyeballs read IAS and the tables or correction plots for the system (if available) give us appropriate CAS, thereupon which TAS may be obtained?
 
Piperonca said:
IIRC the Black Hawk helicopter can have up to 15 kt error dependent on certain climb rates and airspeed ranges. Yes, helicopters are strange things, always trying to beat the air into submission.
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You can actually get a 30 kt change in indicated during a slow speed high power climb. You can even get copilot readings 30 kts less than the pilot side. Leave the pitot cover on one side and the stab fails also...seen that first hand.:D

All of that is understandable considering the effect of rotor wash. What can be confusing is straight and level flight. At standard conditions we’re taught indicated and true are equal. Not so much. They’re equal based on a set airspeed that equates to the pitot tubes meeting the air in parallel. In the Black Hawk thats 100 kts. At 50 KIAS, your true is 58. That’s due to the pitot tubes going they the air at an angle (nose up). What’s strange is, at say 150 KIAS, your true is less (147). That’s because the pitot tubes are getting some ram air effect as it flows around the “green house” and speeds up. Indeed strange stuff in helos in IAS and CAS.
 
nauga said:
'Source error correction' is a shorter way of saying "static source position error correction," or errors resulting from the static source picking up some dynamic pressure due to the installation never being a perfect place to sense static pressure. In other words, 'installation error.' The AOPA article incorrectly attributes this solely to pitot errors, while most decent flight test textbooks address the static errors and the relative insensitivity of pitot probes to off-axis airflow.
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Curious, does that mean planes with a static port on each side are a little more accurate than those with only 1 port on one side?
 
Piperonca said:
So, for us simple minded ones, can we lump it all into "pitot-static system"? Then our Mark I eyeballs read IAS and the tables or correction plots for the system (if available) give us appropriate CAS, thereupon which TAS may be obtained?
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If you're applying a correction that's already been determined, yes. The terminology is a little different, in that indicated the calibrated to indicated correction does not include instrument error (that's in 'observed' to indicated) but the end result is the same. It's only if you're trying to measure the error that the distinctions become important, as test techniques and data reduction may change depending on the real error source.

As an example of this, source error correction testing is frequently done with a trailing cone with a static source on it to get a near-true static reading with significantly less error than the installed source. If the error were really all on the pitot side this method would not work...but it does work quite well if done correctly.

Nauga,
with cones, bombs, pacers and theodolites
 
asicer said:
Curious, does that mean planes with a static port on each side are a little more accurate than those with only 1 port on one side?
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They can be, if the source errors cancel in sideslip or other dynamic conditions. If both positions have similar error in balanced flight, however, two will be just as inaccurate as a single one.

Nauga,
unbalanced
 
nauga said:
If you're applying a correction that's already been determined, yes. The terminology is a little different, in that indicated the calibrated to indicated correction does not include instrument error (that's in 'observed' to indicated) but the end result is the same. It's only if you're trying to measure the error that the distinctions become important, as test techniques and data reduction may change depending on the real error source.

As an example of this, source error correction testing is frequently done with a trailing cone with a static source on it to get a near-true static reading with significantly less error than the installed source. If the error were really all on the pitot side this method would not work...but it does work quite well if done correctly.

Nauga,
with cones, bombs, pacers and theodolites
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Gotcha. You're up there measuring stuff, holding a heading and airspeed to a gnat's azz, and reading/interpreting telemetry, all to write a rather impressive flight test report. I'm the old guy worn out from teaching students why CAS is important, and later in life telling developers to use the right correction curves with their performance prediction models. I tried not to fog their minds with anything more complex than that.
 
Piperonca said:
I'm the old guy worn out from teaching students why CAS is important, and later in life telling developers to use the right correction curves with their performance prediction models. I tried not to fog their minds with anything more complex than that.
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If you believe the corrections in the POH (and to a large extent they are believable, with some reservations) then all the triangle courses and up/downwind legs, etc, are a waste of time and gas. If you don't believe them the error sources and how to test them become more important.

Nauga,
who does them quick and dirty as a first pass, but not using any of the methods discussed here ;)
 
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