I´m confused the TAS ring is not set to 41F at 2.700FT, looks more like 5.000FT?
The fixed part of the TAS calculator has been put back in the wrong place at some time in the past?
If you imagine moving the TAS calculator to sea level and about 60F IAS should equal TAS but you can see that it doesn’t
Well, as Achim said – with a mechanical AI it depends how you set it, and i set it on the line.
In the Cirrus it shows less than 2 degrees and of course it’s just as “nose heavy” as it will always be when you’re alone.
And of course it will vary with speed. I am oretty sure it’s close to 0 on the PFD when then plane reaches it’s level top speed, but have to check again!
TAS should be 105 Kts. The scale has most likely been turned beyond the stop.
the outside temperature (°F) ring is wrong in comparison to the inner ring. You can’t have 40°F at 3000ft and 90° at sea level. No standard atmosphere.
The airspeed indicator was changed not so the outer scale.
Almost, Norbert (I am almost 100% sure it is the other way ’round).
@Tumbleweed: if it had been turned “beyond the stop”, then for the “wrong” TAS indication, there would be corresponding “wrong” pressure altitude and temperature numbers on top…
@Peter: the true airspeed bezel is clearly marked as “KTS”.
For comparison, here is the same instrument on a different C172:
See the difference?
Taking the “stub” (used for rotating the bezel) as a reference, if you go 20 degrees counterclockwise from there, you see the marking for “80”, then “100” and so on.
Now look at the “wrong” one. 20 degrees to the left of the “stub”, there is..: “100”!
So…, the “true aispeed bezel” is clearly the wrong one for this type of ASI.
Probably (my guess), this bezel belongs to the ASI of a 182 or so. (When the needle points straight down, this C172’s ASI reads about 94 KIAS; in a 182, it will read roughly 110!).
My guess is that either:
1. The aircraft was delivered with the wrong bezel (very, very improbable… 
)
or
2. At some point in time, the bezel got lost or broke up in pieces. The mechanic in charge of replacing the bezel either ordered the wrong part number (and hoped nobody would notice ) or he had this one in his stock and didn’t realize they don’t work to well for these C172 ASIs.
Go figure!
Hmmm… how many years has it been like that, and who normally flies it?
I have no idea. Again, it’s a club aircraft…
Now here comes the funny part of the story:
Immagine boscomantico, enjoying his flight to France, with the autopilot on.
At a certain point in time, I started to play with that bezel.
I accurately set up the OAT (41 degrees F) over the current PA (3000 feet).
122 knots TAS!!!
Now I knew this couldn’t be true at that PA and OAT…
Of course, the idea that there might be something wrong with the instrument didn’t cross my mind at the time…
So I re-checked the adjustment again and again, thinking “WTF is going on???” …. “am I really too dumb to set this damn bezel correctly???” 
Can you immagine that?
I finally gave up and resorted to just enjoy the flight.
However, I must confess that when I was home, doubts crossed my mind and I fired up the old E6B just to make sure that those 122 knots couldn’t be…
Sure enough, the correct TAS for the 102KIAS turned out to be 106 knots or so.
After that, it slowly dawned on me, but it was only when I looked closely at that other photo that I got the idea…
Funny experience it was…
Good night!
call me an ignorant, but are you guys regularly checking TAS using the the plastic ring ? I have GPS ground speed, I have IAS/CAS, I have fuel flow (if I am happy, if not I need stopwatch and fuel gauge). Setting up the temperature on the plastic ring is not that precise and there is a mental math with that. So I am using the ring from time time, but usually there are more interesting things to play with ;-)
I agree; knowing the TAS is not going to make you conduct the flight any differently (on an aircraft which is performance limited anywhere above about 8000ft) but I sometimes have a look at the TAS for information for my trip writeups
This is why I have not spent money on an airdata computer box. It would be about 2k by the time I am done with it and I have better things to do which really improve the aircraft. Perhaps if I install the GTN750 I would put one in.
I don’t see why the TAS ring should be particularly inaccurate, however. The IAS-TAS increment, at “our” speeds and altitudes, is only slightly sensitive to altitude and OAT, so the sliding scale works OK. Obviously one needs an accurate OAT probe anyway for knowing when to expect icing, and I would not accept an error greater than 1C on that (have a PT100 thermometer, accurate to 0.1C, for checking both of my two OAT probes on the ground). The IAS reading should also be accurate so one can tell if the engine is making its rated performance, and this is easy to check using the “three GPS speeds” method.
