It’s bizarre. GPS groundspeed doesn’t lie, and it’s usually blindingly obvious when the IAS or derived TAS are in error.
In one aircraft I fly the ASI is obviously over-reading by a fair bit. I think it’s because the pitot head is (quite evidently, reads 20mph during power checks) in the propwash, but a co-owner is dismissive of such a notion on the basis that the pitot head is located where the manufacturer intended it.
On another aircraft I fly the 2x IAS indications are pretty much bang on and have an apparently-correct relationship to GPS groundspeed, but the calculated TAS value in the PFD is evidently excessive – I think the OAT input is over-reading.
On the third I’ve never seen an IAS value that appears inconsistent with GPS groundspeed, and it’s a traditional ASI where one sets the pressure altitude and OAT manually to derive TAS, which generally appears to be correct.
Peter wrote:
What has been suggested is instrument error.
And I am fairly sure it is something like that too.
we could all go on a head to head race… during the next fly-in for instance 
Nobody has suggested that. What has been suggested is instrument error.
UdoR wrote:
Well then, looks like I’ll have to invest in some camera equipment to show you guys how to do it
LOL, a good phone will do these days.
Hey, I don’t think anyone wants to say you are talking garbage, certainly not me. I am more concerned that some indications may give you wrong information. And that can end up with unpleasant surprises.
No it’s not
Mooney, you are arguing with a point which nobody has made. I can post the TB20 table too…
It’s like when I once said somewhere that the TB20 has a zero fuel range of 1300nm, somebody pointed out that the zero fuel range is actually zero (which is also correct)
looks like I’ll have to invest in some camera equipment to show you guys how to do it
I think to achieve your numbers, the airframe maker would have to discover some new knowledge of subsonic aerodynamics.
Graham wrote:
is “out of the ballpark”
Well then, looks like I’ll have to invest in some camera equipment to show you guys how to do it
Peter wrote:
It is splitting hairs in this context because the IAS-CAS correction is driven by airframe geometry / pitot tube location, so is a fixed value for a given aircraft design.
No it’s not. This is the correction table of the M20C. Please note that in addition to that, the individual ASI may be off but still in tolerance by 2.2 kt.
Actually, thinking about this: If you got 2 ASI’s and both are off by 2.2 kt, this will, to the naked eye, give a difference of about 5 kts.
Of course it varies with airspeed. At zero airspeed, the correction is zero I don’t have a PhD but I don’t need one to work this out.
But that’s just diffusing the topic. One does this at cruise speed, or whatever speed one has the correction data for. It’s not rocket science.
Then you need to calibrate the fuel totaliser. If you don’t have one, forget about calibrating the fuel flow. And I know that most people who do have a totaliser don’t have a calibrated one. I know this from the TB20 case, where Socata shipped >100 planes with the reading ~20% out, but on EASA-reg there was no legal way to fix this, short of a Major Mod which as far as I could determine nobody had applied for – details. How does this work? Very simple: most people don’t fly further than they need the toilet.
So a bit of work is needed.
Airborne_Again wrote:
In any case, if you want to calibrate your ASI with the 3xGS method you must take the IAS-CAS difference into account.
It’s much easier getting a guy with a test set from the nearest maintenance. Had to do it twice, it’s a 10 minute affair. After that, you’ll know which ASI is correct and which one is not.
