I (and several of my colleagues) noticed this when we first got our BRNAV fit. I recall chatting about it but never reaching a firm conclusion.

To summarise so far; there are several possibilities.

1. Its a figment of our imagination. An apparerent effect we notice, whilst mentally disregarding countary evidence. To reach a conclusion about this would require a rigorous experiment; as in data gathered in controlled circumstances.

2. Its an indication thing. Something to do with how the nav system performs its calculations. Something like Bulletheads suggestion, maybe related to generally getting less drift when you have a tailwind. Could it be as simple as someone wrote an algorithm for doing the calculation, and just simply got it wrong, and <insert your avionic manufacturer here> has been using it as a library routine ever since? I know it sounds unlikely, but .... wierder stuff has happened in IT before!

If its not one of those 2, then its a real effect, in which case:

3. Something to do with the air mass you're flying through such that a tailwind generally means your true altitude is decreasing, the pay off being an increase in IAS. As per Cows.

4. How about something to do with Earth Curvature? There is a very small performance increasing effect caused by this; in Level flight lift is actually slightly less than weight because the aircraft is not actually in a 'balanced forces' situation because its not actually following a stright line. However the effect is pretty minimal at aircraft speeds. However it does exist, so the faster your groundspeed the less lift you need, which would translate to less induced drag and a higher IAS. However, without doing the calculations, I would be gobsmacked if it could account for more than a fraction of a knot.

4 is a real effect, but just not big enough. 3 needs more investigation.

pb

I've now had a good chance to chat with a meteorologist - the chap in question used to be one of the flying observers on Snoopy and has also been a forecaster instructor at the Met Office college. In short, he knows sheds more than any of us about winds and weather.

His view is that there's no meteorological reason which explains what's being seen.


The more I think about this, the more I think that the phenomenon as seen is a function of the algorithms being used in certain common nav kit, and absolutely nothing to do with aircraft performance or meteorology. Cap'n Pitt Bull's No.2 in other words.

G

Jabawocky - in your original post..
I think tmax was absolutely right to describe this as windshear. But you seem to be talking about something different...
???
1. Sideslip
2. Down/up draught - I saw ±7 kt in my first ever motorglider trip last month (flying level with power constant).

3. Are you sure the power setting is unchanged - how accurately can you measure that?

Hi Tryo

Glad you asked........

Today while doing CAS/Static testing, fling the GPS boxes, I was focussed on other things. There was a 10 knot SE wind blowing, known about and verified by the GPS box tests, and you know what, on the second flight (did not notice the first one) the IAS and TAS and GS in the westerly direction was hard to keep constant. It was while trying to maintain straight and level, that the IAS was 3 knots quiker....... and took a long while to get stable, so I do think its an acceleration and inertai thing now, but again its hard to prove.

It defies the theory, but its real. If only I had time to follow it for 10 min in every direction........I would still be up there:eek:.

The more I talk to people the more they say.....yeah...i have seen that. The Jet drivers only ever notice it when in 120-200 knot machines.

J:confused:

JABA,
sure about thesame weight ?
Daniel

Same weight????

what in about a minute from tur to turn.....say a kg difference?:confused:

I have seen TAS/IAS reduce also after......certain events whatever they may have been.

But well spotted!:ok:

You are quite close, actually.
This was the theory behind pressure pattern navigation used many years ago on long range trans-oceanic flights.
These aircraft were normally equipped with a hi-range radio altimeter, which was used to find the true height, and thereby remain in a least headwind/greater tailwind situation, even though the actual ground track may be more nautical miles...the desired outcome was a minimum time track.

Now, on to position errors....and slightly faster airplanes.
With the original B707, rather large pitot/static errors were noted, so much so that the raw data info could not be reliably used.
Enter KIFIS...Kollsman Intregrated Flight Instrument System, an early sort of air data 'computer', that took in the raw pilot/static inputs, ran it through a series of 'corrections' and out came improved reliable instrument indications.
Usually.:rolleyes:
Then, enter the combined pitot/static probe.
This provided reduced position errors, and its use on some of the first generation wide-body jets (L1011, B747), combined with a true ADC system, provided very accurate airspeed/pressure altitude information.

For light GA aircraft, look for (sometimes) pronounced static errors due in large part to the location of static ports....in relation to the pitot probe.

.

This may be a red herring, but if you are measuring tas, then you must also be measuring air temp and alt to get the right answer. If you aren't using a proper tat probe and just using a simple probe in the airstream, the temp measurement may not be accurate for all conditions of airflow. There may also an effect from the static port w/respect to small variation in measured alt value as the wind changes direction.

Do you get the same effect using a mechanical asi ?...

Chris

Can It be an area with a wind shift and an updraft?
The first would explain the GS increase, the second would explain the IAS/TAS increase, (if level flight and power were maintained during the occurrence).
In addition, if pitch had to be lowered to maintain altitude, an updraft could be considered, as this would mean the IAS increment was actual, not just a wrong indication.
There would be propeller effects, too.
Expert glider pilots can "feel" where the gusts come from, and of course the updrafts and downdrafts.
A gust is transient, but an updraft can be sustained. Normally a head wind decrease will produce a decrease in IAS/TAS followed by an increase in GS until all is back to pre-gust conditions. But a simjultaneous updraft can mask the decrease in IAS or even overcome it and turn it into an IAS/TAS increase.
Of course I am just an amateur who has never been even close to a test pilot school, sorry if bothering you guys.

That is a very good point indeed. you would not really think of "ridge soaring" in a high wing loading piston powered machine!

So it could be that you are surfng and a slight pitch down, and hence the IAS and TAS are affected.

This may not be the wildest thread on pprune, but it is one of the more interesting!

J :ok:

Yes indeed!
There are many interesting threads. I have a lot of questios to make and misteries to solve, I hope those will bee interesting threads too...

For what it's worth...
 

As an FTE and a specialist in air data and navigation systems, I think I may have something to add to this debate.

Firstly, I have to agree with Ghengis; if the airmass is stable (in terms of temperature and density), a change in wind speed/direction WILL NOT change the relationship between CAS and TAS. Windshear could temporarily alter the values indicated in the cockpit due to its effect on the pitot and static systems, but I do not believe windshear is a factor in this discussion.

If the aircraft encountered a sudden change in wind speed/direction, that suggests to me that there may have been a change in other things as well, e.g. QNH, air temperature/density, and perhaps also the vertical component of the relative airflow. If the relationship between pressure height and temperature changes, the air density will change, and thus the relationship between CAS and TAS will also change.

Another thought is that the aircraft in question (being a GA aircraft) probably does not have an air data system that calculates TAS from total temperature; therefore the GPS based navigation system will be trying to derive TAS from the difference between TRK/GS and HDG/CAS using the triangle of velocities and GPS height. This is an error prone method because: 1) the system will have to make assumptions about the atmosphere, particularly the relationship between height and the air density; and 2) it will be trying to do the maths assuming a reasonably constant wind. Also, if the system does not have an airspeed (or heading) data feed, it will also have to make assumptions regarding HDG/CAS. Therefore it is unlikely that the TAS and W/V estimates will be particularly accurate. Typically in these GPS based GA nav systems the information on TAS, W/V, etc is there to help the pilot with situational awareness but is not going to be so accurate that it can be treated as gospel.

To my mind the ONLY way an air data system can get an accurate derivation of TAS is to have accurate and properly calibrated air data sensors (pitot, static and total temperature). If this data is used in conjunction with an accurate source of TRK/GS (e.g. GPS/EGI or INS), and an accurate source of heading (such as an INS or AHRS), then and only then can an accurate instantaneous W/V be calculated.

If someone provides me with exact details of the systems integration on the aircraft in question (particularly the air data and heading sources that feed the navigation system), I may be able to provide a more specific explanation of where the problem is likely to lie.

Hope this helps.

WF