Wrong answer on TAS dial

Jaybird180

Jaybird180

Final Approach
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Jaybird180
What am I doing wrong to get TAS calculated on my ASI?
Photo 1 here. Photo 2 on next post
 

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Photo 2.
 

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Looks accurate to me. If it was a standard lapse rate from 15 C you'd be truing out at 133. So 138 with 11 c at 7200 PA doesn't seem too far off. Even the rule of thumb 2 % per each 1,000 ft comes close to what your ASI is showing.
 
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On the ASI, you set pressure altitude (7,200) opposite outside air temperature. Positive temperatures are on the left.

So, in those pictures, you've configured it for 7,200 PA and +16C or so. If your OAT gauge is reading 16C, you've set it up correctly, but I'm guessing that your OAT wasn't +16C.
 
bqmassey said:
On the ASI, you set pressure altitude (7,200) opposite outside air temperature. Positive temperatures are on the left.

So, in those pictures, you've configured it for 7,200 PA and +16C or so. If your OAT gauge is reading 16C, you've set it up correctly, but I'm guessing that your OAT wasn't +16C.
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He is showing his OAT electronically as 11C (which corrected for compressibility is 9.5C), but the ballpark remains -- 136-138kt TAS
 
Always use the highest number when describing your airplanes performance! :D
 
Jaybird180 said:
Photo 2.
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I wondering if the OAT is accurate though. I know you were in ATL a couple days ago. You hang a right and go down to FL? I was out flying in north GA 2 days ago and was showing 4-5 C at 2,000 MSL. Had pitot and eng anti-ice running. It's even colder out today minus the drizzle.
 
Velocity173 said:
I wondering if the OAT is accurate though. I know you were in ATL a couple days ago. You hang a right and go down to FL? I was out flying in north GA 2 days ago and was showing 4-5 C at 2,000 MSL. Had pitot and eng anti-ice running. It's even colder out today minus the drizzle.
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Me too. I disregarded the result for lack of believability. I was going 135GS according to both GPSs, and I doubt there was zero wind. Performance charts says my C-172S does 124KTAS at 8000, another reason to disbelieve the result.
 
oldShar said:
He is showing his OAT electronically as 11C (which corrected for compressibility is 9.5C), but the ballpark remains -- 136-138kt TAS
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Compressibility? Okay, teach me something....
 
Jaybird180 said:
Me too. I disregarded the result for lack of believability. I was going 135GS according to both GPSs, and I doubt there was zero wind. Performance charts says my C-172S does 124KTAS at 8000, another reason to disbelieve the result.
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Your performance charts also presume maximum gross weight. You can also get much higher speeds flying in rising air. I've done a Vno climb in a 172 before, in probable mountain wave conditions.

115 KIAS out of a 172 is also unusual. Maybe solo at full rental power. But at 8000 feet? Not likely without some help from either gravity or thermals or both.
 
oldShar said:
He is showing his OAT electronically as 11C (which corrected for compressibility is 9.5C), but the ballpark remains -- 136-138kt TAS
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Gwen

At our speeds and altitudes, compressibility is negligible to the point of ignoring it. It's when you get to speeds over 300 mph, really when you get to (or are trying to get to) the trans-sonic speeds. Jet fighters worry about this, not us.

Ref: Aerodynamics for Naval Aviators.
 
I gather you were at 8000 feet with +11C OAT and 120 KIAS. That computes to 138.3 KTAS, which is pretty close to what your TAS ring shows. What makes you think that isn't right? What power setting were you using? Were you in mountain wave?
 
Ron Levy said:
I gather you were at 8000 feet with +11C OAT and 120 KIAS. That computes to 138.3 KTAS, which is pretty close to what your TAS ring shows. What makes you think that isn't right? What power setting were you using? Were you in mountain wave?
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7500MSL power around 2650ish, close to 75%. Smooth ride
 
Looks to me that you've set 7,600' at 11°C. That's close but setting the 7,200' more accurately would have improved the result slightly.
 
RotorAndWing said:
Huh??:eek:

Some of the worst turbulence I've ever flown through was mountain wave. During the winter certain routes over the rockies have to be rerouted due to mountain wave activity.

Mountain Wave
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Some of the strongest mountain waves I've flown through have been dead smooth -- with autopilot altitude hold, the only way you could tell was the oscillating changes in pitch/speed. Perhaps I should have said "A smooth ride is not uncharacteristic of mountain wave."
 
I don't have altitude hold on the AP :-(

I did have to make pitch and trim changes often. Not sure if it was irregular as I don't often set the AP and just sit back and chill. The "oscillations" were very mild, enough that the thought occurred to me to just let it vary altitude, but I got annoyed at anything more than 200', despised 300' and adjusted to arrest.
 
Ron Levy said:
Some of the strongest mountain waves I've flown through have been dead smooth -- with autopilot altitude hold, the only way you could tell was the oscillating changes in pitch/speed. Perhaps I should have said "A smooth ride is not uncharacteristic of mountain wave."
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The mountain waves I flew through were anywhere from moderate to severe turbulence. The last mountain wave activity I flew through was going south down the east side of Taiwan with a strong westerly flow, we had to slow down then change altitude a couple of times. Once south of Taiwan and away from the mountains it smoothed out.

In my experience 90% of mountain wave activity was associated with turbulence.
 
RotorAndWing said:
The mountain waves I flew through were anywhere from moderate to severe turbulence. The last mountain wave activity I flew through was going south down the east side of Taiwan with a strong westerly flow, we had to slow down then change altitude a couple of times. Once south of Taiwan and away from the mountains it smoothed out.

In my experience 90% of mountain wave activity was associated with turbulence.
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My experience is pretty much the opposite. YMMV. In any event, the lack of turbulence does not mean you're not in mountain wave, and being in the upswell of a mountain wave would easily explain a 140-knot TAS in a C-172S on the lee side of the Appalachian Mountains at 8000 MSL, where I do a whole lot of flying. Of course, a few minutes later, you're going to be 20-30 knots slower to maintain altitude, but we only have that one snapshot to go on. The alternative is there's an inaccuracy somewhere in his airspeed measurement system, because that airplane ain't goin' that fast without help or an error.
 
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Ron Levy said:
My experience is pretty much the opposite. YMMV. In any event, the lack of turbulence does not mean you're not in mountain wave, and being in the upswell of a mountain wave would easily explain a 140-knot TAS in a C-172S on the lee side of the Appalachian Mountains at 8000 MSL, where I do a whole lot of flying. Of course, a few minutes later, you're going to be 20-30 knots slower to maintain altitude, but we only have that one snapshot to go on. The alternative is there's an inaccuracy somewhere in his airspeed measurement system, because that airplane ain't goin' that fast without help or an error.
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Well, something's amiss. I didn't see 20kt slowdowns and my GS was pretty consistent. Maybe that Serial Number is one of the Cessna Sleepers. Couple that with a suspect tachometer.... Altimeter was 30.10 so that's not a lot of High pressure to throw things off.
 
Jaybird180 said:
Well, something's amiss. I didn't see 20kt slowdowns and my GS was pretty consistent. Maybe that Serial Number is one of the Cessna Sleepers. Couple that with a suspect tachometer.... Altimeter was 30.10 so that's not a lot of High pressure to throw things off.
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A tach reading way low could be involved, except you just can't get enough HP to go that fast in a 172S at 8000 MSL even at full throttle unless someone sneaked a turbocharger under the hood while you weren't looking. My money's on the airspeed indicator being off due to some undetermined problem. You should try doing a 3-GPS-leg TAS check as described here.
 
Ron Levy said:
A tach reading way low could be involved, except you just can't get enough HP to go that fast in a 172S at 8000 MSL even at full throttle unless someone sneaked a turbocharger under the hood while you weren't looking. My money's on the airspeed indicator being off due to some undetermined problem. You should try doing a 3-GPS-leg TAS check as described here.
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I read the readme on that site and it doesn't make sense to choose that over doing the average of 4-Cardinal headings.
 
Jaybird180 said:
Well, something's amiss. I didn't see 20kt slowdowns and my GS was pretty consistent. Maybe that Serial Number is one of the Cessna Sleepers. Couple that with a suspect tachometer.... Altimeter was 30.10 so that's not a lot of High pressure to throw things off.
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High pressure won't affect the ASI. It measures pressure differences.

If you were flying parallel to a ridge on the upwind side (say, 'cause it's pretty and you're sightseeing -- seems likely east in autumn), you could have seen continuous ridge lift. It can be significant, and can be very smooth in stable conditions.
 
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