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 Post subject: IAS versus TAS and Vne
PostPosted: Fri Jan 14, 2011 8:06 am 
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Joined: Fri Jan 05, 2007 7:46 pm
Posts: 8
Location: Stockton, Ca.
In short I have questions about the relationship of TAS (True Air Speed) and Vne (Never exceed speed).

The setup:
I'm a hobby pilot in the San Francisco Bay Area, fly out of Livermore. Basically do the $100 burger weekend routine, usually cruise around between 3,000 and 5,500 feet. No reason to get very high usually, I rent a Cessna 172SP for most flying.

I mess around with the TAS adjustment on the airspeed indicator. The TAS versus IAS is rarely of any interest at the altitudes I fly. I use GPS for navigation and am very familiar with the terrain so have little interest in TAS for navigation. Probably sounds bad but that's the way it is.

I recently flew to Los Angeles and the trip back required an altitude of 12,500 to get over clouds at the San Gabriel mountains, basically the Interstate 5 pass that's called the grapevine. At this altitude the TAS was a significant difference. So much so that if the TAS were applied to the IAS the indicator would have been well within the yellow arc. Clearly a descent from that altitude would appear to exceed the Vne speed, was my gut feeling but I did not understand exactly what I was observing.

The CFI:
I talked to a CFI at the FBO where I rent the plane. His response was fairly textbook. The IAS is a measure of the air molecules that enter the pitot tube. Those same molecules are hitting the control surfaces of the plane. The IAS is the speed of interest and there is nothing to worry about. TAS is only for navigation.

Another Source:
So I'm reading the latest issue of Aviation Safety (vol XXXI, number1). There is an article titled "Slow Down, You Move Too Fast" by Steven Gibb. This article is dealing exactly with this issue. To quote the part that caught my attention "Vne is NOT an indicated airspeed. Instead, Vne is a true airspeed". In short the airspeed indicator displays IAS with the one exception of the red line, that is TAS. The author went on to confirm what my worry was, that is that a small airplane at high altitude can easily exceed the Vne during descent.

The author did not state but I presume that when the TAS is effectively a yellow arc IAS that you should be sensitive about turbulence also.

So, I would sure like to hear what others have to say about this. I know it's a long post and appreciate the attention and replies.


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 Post subject:
PostPosted: Wed Jan 19, 2011 7:01 am 
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Joined: Sun Feb 26, 2006 1:48 pm
Posts: 132
Location: San Francisco
It was a bit of a surprise for me to hear that Vne would be based on TAS rather than IAS. I would have said what the other CFI told you ... that your Vspeeds are about the molecules of air hitting the airframe and therefore should be based on IAS and not TAS. However, after a bit of research I believe that the above is true for all Vspeed except Vne and it may have something to do with the difficulty in establishing a Vne speed due to the several different types of limitations that may be factors when determining that number. For an interesting discussion on this have a look at this builders forum post -

http://www.bluemountainavionics.com/tal ... .php?t=629

Anybody else want to chime in here?

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"To Live is To Fly" - Townes VanZandt
www.thefinerpoints.net
jason@thefinerpoints.net


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PostPosted: Tue Mar 15, 2011 10:03 am 
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Joined: Fri Jan 05, 2007 7:46 pm
Posts: 8
Location: Stockton, Ca.
Jason, thanks for the reply and research. I read that link, seems like Vne is a bit like Va, the answer has many dependencies.

I was surprised at first discovery of this concept as I had never heard anything of this sort in training. Maybe there aren't enough small planes exposed to this issue to warrant further burdening of a student. Maybe the few pilots who are affected are either very lucky or know it already.

I was also surprised by the lack of chatter about this subject in other forums. Must not affect that many pilots. Although the fact that Aviation Safety chose to publish it makes me wonder if they think there's some very lucky or ignorant pilots who never thought about this.

I'll just play it safe (like my initial gut feeling) and treat TAS as IAS when at extreme altitude (for a Cesna 172) for descents and rough air.

I will haul this issue out once in awhile when I get to play "stump the CFI" though.


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