@etn maybe your speed indicator(s) show a too high value? I’ve seen factory new planes with factory new ASI that showed a whopping 15% too high of a value. Microlight. In Cruise speed indicated 230 kph instead of 200 kph. Speed sells …
There could even be a technical issue, e.g. a leaking static pressure that can lead to a deviation in the order of magnitude.
However, in the beauty of the theory your initial understanding above is correct and TAS should match GS in zero wind conditions.
Peter wrote:
To convert IAS to TAS, add 1.5% to 2% per 1000ft. So at 5000ft, an IAS of 100kt will be a TAS of about 107-110kt. If whatever instrument you have is telling you otherwise, it is probably dud – unless the temperature is way off standard.
All the observations I did were indeed with temperatures much higher than standard. I should probably wait a few months for temperatures closer to standard and have a look at it again. Another example below. Wind was calm that evening.
I have 2 OAT probes, one connected to the AV-30, the other one to the engine monitor. The difference is usually within 1-2°C of each other (max observed: 3°C), “good enough” I would say. Probably some calibration error, but as I do not know which one to trust I don’t know which one to adjust. Curiously enough, they usually show the same temperature on the ground.
As a side note, a calibration of the AV-30 was performed after installation.
I smelled the subject of TAS would be highly popular the very moment @etn pushed the submit button 
TAS is mostly irrelevant
As a disagree ask any jet or faster flying airplane pilot what speed his Vne is. Most likely Vmo/Mmo, but let’s stay with our little propeller toys for a second… I seem to recall you building an RV-4. Hope you are making good progress on that since building one is a major task.
Now once that RV-4 will be flying you will likely abide to the Vne, 182, which is KTAS.
Van’s pretty soon became aware that there was a problem with their published Vne when a few models started shedding their wings, which is certainly no fun way to end a flight… what had happened was pilots happily cruising the higher RV levels, around or slightly below FL180, starting a descent which quickly accelerated their ship to Vne, which were read on the IAS. This produced some massive overspeeds leading to the problems mentioned above.
An excellent article Flying High and Fast was then published, and Vne on all RVs is now TAS (or IAS whichever the higher).
UdoR wrote:
@etn maybe your speed indicator(s) show a too high value? I’ve seen factory new planes with factory new ASI that showed a whopping 15% too high of a value. Microlight. In Cruise speed indicated 230 kph instead of 200 kph. Speed sells …
Indeed and if you look at Robin’s 1960’s advertisements you will see exactly that! I never saw the advertised 250km/h on my IAS. I compared the TAS shown in the above example (138 KTAS at 5000ft) with the value given in the manual and they are in the same ball park, even slightly above. (there is a thread on this also) I was flying close to the red line to be sure we arrive before the night!
As far as I can tell, though, my ASI are correctly calibrated.
So what is your question? If in some cases the GS is closer to IAS than TAS, then in these cases you simply had a headwind. Or are you saying no matter which direction you go, you always seem to have a headwind?
There is a triangular method of measuring TAS, from which you can calculate the correct IAS, which you could then compare to your IAS.
Dan wrote:
Now once that RV-4 will be flying you will likely abide to the Vne, 182, which is KTAS.
The Vne is always expressed in IAS, not TAS. The Vne limitations are related to the effects of the relative wind on the structure. If air density drops at altitude, so does IAS but also the effect of the relative wind. Therefore, it’s OK to have a TAS higher than the Vne as long as the IAS is lower.
Or am I plain wrong with the above statement???
This obviously applies to single-engine piston flying < 200kt in lower airspace. Obviously the picture is totally different for jet speeds and altitudes.
Vne (various past threads) is set by a number of factors, one of which is flutter. And flutter is related to TAS, not IAS, but in the interests of standardisation of presentation it is specified as IAS.
That also means that if you have a plane with a Vne of say 200kt, and a ceiling of 20k, if you somehow took it to 30k, the 200kt Vne would produce a much higher TAS and would likely take it into the flutter region. This is one reason turboprop conversions like the Jetprop tend to set Vne at a lower value than the piston predecessor.
@pilot_dar is the specialist here.
Back to the question, if your instruments do not show an increasing IAS-TAS gap as you climb, something is badly wrong.
Forget GS. It is nothing to do with your instrument indications. It simply comes from GPS. Yes, it is TAS plus or minus wind, but you can’t measure wind other than by looking at GS-TAS so it is a circular argument. When I fly, once in cruise, I set up the ASI subscale, just for fun, and make a not of the hw/tw component, just for fun 
If I installed an airdata computer box my KLN94 could then tell me TAS, the wind vector, etc, but there is no point.
etn wrote:
The Vne is always expressed in IAS, not TAS. The Vne limitations are related to the effects of the relative wind on the structure. If air density drops at altitude, so does IAS but also the effect of the relative wind. Therefore, it’s OK to have a TAS higher than the Vne as long as the IAS is lower.
I would think it depends what is the limiting factor. TAS is an expression of the kinetic energy in the air mass, while IAS is a expression of the air flow around the hull – “what makes the plane fly”. But if the limiting factor is the main strut, a pull at 120kias would not lead to the same stress if KTAS is 120 or 135 – knowing you don’t have a G-meter. If this is – e.g. the windshield, this is an IAS limitation.
etn wrote:
The Vne is always expressed in IAS, not TAS. The Vne limitations are related to the effects of the relative wind on the structure. If air density drops at altitude, so does IAS but also the effect of the relative wind. Therefore, it’s OK to have a TAS higher than the Vne as long as the IAS is lower.
It depends partially on the aircraft you are flying. In the Virus SW I previously flew, they had a chapter specifically referring to this because a TAS above Vne, whilst IAS indicating in the green arc of the ASI, could induce flutter which could lead (and had led to) tailplane separation. In the handbook they mentioned that above 4000m (13100 feet) Vne limit should be considered TAS, not IAS.
Edited to add:
This is from the Pipistrel Virus SW handbook:
How fast is too fast
Based on two recent unfortunate events, where two pilots lost their newly acquired Sinus and Virus aircraft, the team of Pipistrel’s factory pilots decided to stress the importance of airspeed even more. Do read this passage thoroughly as everything mentioned below affects you as the pilot directly!
The two events
Both the events took place during the first couple of hours pilots flew with their new aircraft. Therefore it is definite they had not become completely familiar with all the flight stages Sinus and Virus offer. The circumstances of both the events were remarkably similar.
Soon after the pilots picked up their new aircraft at the distributor’s, the aircraft were severely damaged aloft. One during the first home-bound cross country flight and the other during the first flights at domestic airfield. Please note the distributor independently tested both mentioned aircraft up to VNE at altitudes reaching 300 to 500 metres (900 to 1500 feet) with great success.
Pilots flew their machines at reasonably high altitudes but at very high speeds. One of them deployed airbrakes (spoilers) at the speed of 285 km/h (155 kts) – where the VNE of the aircraft is 225 km/h (122 kts), the other was flying at 3000 m (10.000 ft) at 270 km/h (145 kts) IAS – where the VNE of the aircraft was 250 km/h (135 kts).
They both encountered severe vibrations caused by flutter. Because of this one aircraft’s fuselage was shredded and broken in half just behind the cabin (the crew were saved thanks to the parachute rescue system), other suffered inferior damage as only the flaperon control tubes went broken. The pilot of the second machine then landed safely using elevator and rudder only. Fortunately both pilots survived the accident without being even slightly injured.
Thanks to the Brauniger ALPHA mfd’s integrated Flight Data Recorder, we were able to reconstruct the flights and reveal what had really happened.
What was the reason for the flutter causing both accidents?
Both pilots greatly exceeded speed which should never be exceeded, the Vne.
With the IAS to TAS correction factor taken into consideration, they were both flying faster than 315 km/h (170 kts)!
You might say: “Why did they not keep their speed within safe limits? How could they be so thought- less to afford themselves exceeding the VNE?” Speaking with the two pilots they both confessed they went over the line unawarely. “All just happened so suddenly!” was what they both said. Therefore it is of vital importance to be familiar to all factors that might influence your flying to the point of unawarely exceeding the VNE.
@etn, you are not “plain wrong”, but it just does not cover the entire spectrum of the equation
Think about those air particles you are flying in… the same zillions of them giving you that miraculous thing called lift, are hitting your dynamic pressure inlet, and are being compared to your static, neutral or reference pressure.
Now depending on the nature of the air itself, dense or less dense, more or less of those particles will be used for reference. The higher one flies, less particles, and the hotter, less particles. This actually is density altitude. And this why TAS becomes (or should) Vne for faster and higher flying SEPs. Certified piston single engine aircraft usually only use IAS as Vne, as their quite draggy airframe is not prone to accelerate as quickly as some high performance homebuilts or else. The margins set by the design engineers of certified aircraft are de facto set way higher, therefore the airframe usually holds together during descent from altitude.
The name True Air Speed says it all. It has not been created for the gallery, but as a measure of the real speed of any craft in relation to it’s medium (air) in a still stage, with the actual density of air particles.
On higher (and faster) flying machines, Vmo/Mmo is used… meaning that as long as the air is thick enough (density of air particles), Vne is TAS (or IAS, the higher of). Once reaching transition, usually around FL200, the air becomes so thin that it becomes difficult to measure/correct precisely, and the Vne used is in Mach (Mmo).
To illustrate this, here’s a shot of my PFD/ND (G5s) whilst cruising 47%… all speeds in knots
Of course, modern instruments such as those, and I presume yours as well, can have the Vne programmed on IAS or TAS. Please refer to whatever the manufacturer of your pride and joy states in the manual(s)… your life might depend on it…
