OAT requirement?

MauleSkinner said:
All true, except where you used the word “planning”. If you don’t know how your TAS compares to what you think it’s going to be, we call it “guessing”.
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True, but here's the reality - it really doesn't change much unless you are having an engine issue.

When I was a sub-500 hour pilot I used to calculate my TAS all the time on cross countries. I did it for practice, proficiency on basic pilot skills and because it gave me something to do on long cross countries.

But what I noticed is that...it really didn't change from flight to flight assuming I used the same power settings. So now I just do it when I'm learning a new airplane OR I suspect I'm losing TAS due to an engine issue.
 
Lindberg said:
How do you measure winds aloft by knowing TAS?
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Delta between TAS and GS for headwind component, then add in wind correction angle for the other vector.
 
Realistically, it’s pointless to know TAS with pinpoint accuracy since you’ll never know wind very accurately anyway, and other error sources will make a few knots one way or the other irrelevant. Just using the rule of thumb that TAS is approximately CAS + 2% for every thousand feet MSL is good enough for most practical purposes, given typical GA altitudes and airspeeds.

If you have an analog ASI with a TAS calculator built in, you can dial in the temp and altitude to read TAS. You’ll notice that the temp scale is tiny and 5 or 10 degrees C makes only a small difference in the calculation.
 
Fearless Tower said:
True, but here's the reality - it really doesn't change much unless you are having an engine issue.

When I was a sub-500 hour pilot I used to calculate my TAS all the time on cross countries. I did it for practice, proficiency on basic pilot skills and because it gave me something to do on long cross countries.

But what I noticed is that...it really didn't change from flight to flight assuming I used the same power settings. So now I just do it when I'm learning a new airplane OR I suspect I'm losing TAS due to an engine issue.
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But if you NEVER calculate TAS, per @Lindberg ’s statement, you’re not going to be able to learn how fast the new airplane flies (so you’re “guessing” instead of “planning”), as well as guessing at your performance changes.
 
Half Fast said:
If you have an analog ASI with a TAS calculator built in, you can dial in the temp and altitude to read TAS. You’ll notice that the temp scale is tiny and 5 or 10 degrees C makes only a small difference in the calculation.
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Assuming you have a temp gauge. ;)
 
Half Fast said:
Realistically, it’s pointless to know TAS with pinpoint accuracy since you’ll never know wind very accurately anyway,
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Interestingly, the more accurate my TAS calculations and heading indications got, the more accurate the winds aloft forecasts became.
 
MauleSkinner said:
But if you NEVER calculate TAS, per @Lindberg ’s statement, you’re not going to be able to learn how fast the new airplane flies (so you’re “guessing” instead of “planning”), as well as guessing at your performance changes.
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Of course, as a student I learned how to do TAS calculations with an E6B, and later taught it. To be honest, thinking back to flying in the “early days” it was not something I did often, but did check periodically.

Now, however, it’s my main speed/performance parameter in cruise flight. Both the Avidyne in my old Cirrus and the Dynon in my Sky Arrow show TAS, and I’m constantly checking it to verify performance. In cruise flight it replaces IAS for my go to speed check. But since a lot of experienced pilots here never bother with it, I’ll assume it’s just my preference and not a requirement.
 
All I know is that my fancy G5 will tell me the direction the wind is coming from and velocity along with true airspeed and ground speed using an under wing temperature sensor. I plan on ditching the unicorn temp gauge when I replace my windshield.
 
MauleSkinner said:
Assuming you have a temp gauge. ;)
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So at best, OAT is useful for imprecisely calculating TAS once when you get a new plane. But it's still possible to calculate TAS without it, so helpful, but not necessary. Have we solved the problem?
 
Half Fast said:
How can you tell wind speed from ground speed alone? You need both ground speed and TAS to determine the wind, as well as crab angle if you want to know more than the headwind (or tailwind) component.
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a few years back I played around with a gps in a raspberry pi and determined that it is possible to figure TAS using just gps data. However, it has several limitation and assumptions such as a course change and no significant speed or altitude changes for some period of time. The algorithm was essentially doing triangulation averages. The method based on GPS alone isn’t reliable enough, but is achievable.

If you could get other inputs such as airspeed and heading plus the gps, you could accurately do all kinds of things.
 
Lindberg said:
So at best, OAT is useful for imprecisely calculating TAS once when you get a new plane. But it's still possible to calculate TAS without it, so helpful, but not necessary. Have we solved the problem?
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Depends. What problem are we trying to solve?

and why is it imprecise?
 
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Lindberg said:
But it's still possible to calculate TAS without it, so helpful, but not necessary. Have we solved the problem?
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I must have missed it. Can you calculate TAS using other parameters without knowing the temperature? On my 496 OAT is still one of the required fields to calculate TAS, even with many of the other parameters known.

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Pretty sure my Cirrus also needed an OAT probe for proper TAS calculation.

But I'm open to the possibility it may not be needed, so please enlighten me.
 
FastEddieB said:
OK.

But isn’t the fly in that ointment not knowing the actual wind velocity in flight?
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I don't know? All I know is that so far the only reason I've seen for needing to have an OAT probe when VFR is to calculate TAS for flight-planning purposes when you get a new plane.
 
Lindberg said:
All I know is that so far the only reason I've seen for needing to have an OAT probe when VFR is to calculate TAS for flight-planning purposes when you get a new plane.
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FYI: As best as I can recall OAT indicator requirements were a certification requirement for certain aircraft. For example, turbine aircraft require one. I don't believe any CAR 3 light recip aircraft had a requirement but when Part 23 was initiated there was a requirement on the recip side. I don't have access at present to confirm or not.
 
Lindberg said:
I don't know? All I know is that so far the only reason I've seen for needing to have an OAT probe when VFR is to calculate TAS for flight-planning purposes when you get a new plane.
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Stick to the middle of the sky…it’s safer there.
 
Someone may have said this above, but if so, I missed it.

I believe the other use of an OAT is when flying in temperatures near 32 F. I have experienced going from above to below 32 F while flying in light rain. Remarkable what the difference was when the OAT dropped a degree. Lights of houses and cars magically disappeared. The windshield turned white, with wet snow sliding up it.

The OAT had been in the 40's an hour before, at the same altitude, and I wished that I had been monitoring the OAT, and descended before the transition to snow. I learned a lot that night, and watching OAT was one of them.

TAS is useful, but freezing temperatures coming to your altitude can be much more important. I have had that experience 4 times, in a Cessna 172.
 
Amusingly, I ended up with two OATs. I've got one on the MVP50 and one on the JPI830. Somewhere, rolling around in a box is my old junk bin OAT that I used to have in the plane stuck through a random hole in the canopy bow.
 
I dont think the gt50 is certified
 
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