Airspeed Indicator vs HUD Indicated Airspeed

[Cool-Hand]

I was flying an inflight refueling mission with some friends and was putzing around going for the barber pole when I noticed that there was about a 10kt split between the HUD and the airspeed indicator. I'll stick a screen shot on here from this sortie.

 

I was just curious why there would be such a large difference in Indicated speeds?

 

I figured the speed would be calculated by the Flight Data Computer and just displayed in two different places.... or is it a separate FDC or something else driving the HUD information? Seems like it would be strange to have a system like that, especially if you are trying to ride the Vref speed in for landing or something that required precision speed control.

 

Pitot heat was on but no icing conditions were encountered and the same split was present without pitot heat on as well. It was also reported by the guys I was flying with as well.

 

Sorry if it's something obvious and I'm just blind to it or need to dig deeper in the manual :pilotfly:

 

[PFunk1606688187]

I may be wrong, but I think this is the difference between KCAS and KIAS. The HUD is corrected for instrument errors while the steam gauge is not. However what I remember is how it being something to do with errors above sea level and you look pretty close to sea level so... I dunno.

Edited May 22, 2014 by P*Funk

[ralfidude]

is it me or does that look spot on?

[Hobo]

It does look spot on on the big needle, not sure what's going on with the small dial.

Edited May 23, 2014 by Hobo

[cichlidfan]

I think now would be a good time for the OP to tell us what he thinks the gauge reads. :huh:

[Cool-Hand]

Hud says 382 and the gauge says ~394 on my screen. It's a drum type airspeed indicator, same one I use at work. Usually it is run through an FDC (at least ours is unless we revert to the alternate source) and is calibrated for error and other conditions, figured the same for the A10 since it has a AHARS and FDC like the Dash 8's I fly.... the best of late 1980s tech!

 

I was looking back through a lot of my old screen shots and it seems like a varying amount of error between the two. I was just genuinely curious what the explanation for the differences would be and why there would be such a large amount especially on a relatively sophisticated airframe. Why would the analog read differently than the digital read out if they are both going through the FDC? If one failed how would you accurately fly on the other?

 

I apologize for the resolution here are some blown up images

 

 

Edited May 23, 2014 by Cool-Hand

[cichlidfan]

I was sure that was a nine (in the HUD) but now that I look closer...

 

[PFunk1606688187]

Perhaps this is another of those bugs that's never gonna get fixed.

[Cool-Hand]

Hmmm wonder if this should get moved to the bug section then since it doesn't seem like its an over obvious systems difference that should be there

Edited May 24, 2014 by Cool-Hand

[Deagle]

normally the airspeed indicater displays the IAS (Indicated Air Speed)

in the HUD you should have the CAS (Corrected/Calculated Air Speed)

 

the step from IAS to CAS is correction of an error of Angle of Atack and other errors of measurement

[[Knight]]

What about weather? Cold outside? Wind Direction? Pitot Heat on? These are all variables taken into account and may give a false reading. Not saying your wrong but weather can do strange things.

[Cool-Hand]

normally the airspeed indicater displays the IAS (Indicated Air Speed)

in the HUD you should have the CAS (Corrected/Calculated Air Speed)

 

the step from IAS to CAS is correction of an error of Angle of Atack and other errors of measurement

 

Shouldn't the HUD be showing indicated airspeed unless I purposely changed it like switching it to TAS or grounspeed?

At least that's what it says on page 364 of the manual that IAS is the default.

@Murphy87 see the op about settings. If its IAS on both they should read the same regardless.

Edited May 27, 2014 by Cool-Hand

[towsim]

If you open such a drum gauge, you will find an impressive piece of mechanical hardware, but no electronics. Normally it is driven directly by the dynamic pressure from the Pitot tube. There are only some cogwheels, which try to come close to the real value by clever transmissions. If you calculate the airspeed by a computer you have a pressure sensor which gives a value into the Air Data Computer. In difference to the mechanical measurement, the Air Data Compute takes some more values into account. This is mainly the air pressure, air density and air temperature. Furthermore, mentioned in the previous post, there are considerations regarding the position of the Pitot Tube, dimensions of pressure tubes and more, which finally results in the calibrated airspeed. The mechanical gauge does not consider the variable air data. It is calibrated for one standard atmosphere condition in a fixed hardware environment. So it is normal, that there is a difference between the mechanical indication and the computed indication. Whereby the computed speed is the more precise value. The next question would be, why using such a unreliable indicator? It needs no power supply, no computer, only a tube connection to the Pitot tube. This is a real help if you experience an electrical power loss.

[Cool-Hand]

Ah good deal, thanks for the explanation, was just curious. I've never cracked one open nor flown an A10 in real life, but a 12 knot difference seemed like a strangely large split between the two with only a 2 degree nose down pitch and I couldn't find any reference to it in the manual. Either way not like it's a big deal in the sim anyway.