Airspeed indicators in EAS

[RBF]

Quick question that just popped into my mind.

Is there any reason for not having modern airspeed indicators showing EAS instead of IAS?

I mean, after all, dont airplanes really "fly" according to EAS?

[Vessbot]

Pure speculation, but I think there's a good chance that the position, installation, and compressibility error corrections are applied by the computer to the speed before we see it, thus making it actually EAS; but they don't label it that because IAS is the entrenched common term, and you don't want to confuse pilots.

[Goldenrivett]

Originally Posted by Vessbot

Pure speculation, but I think there's a good chance that the position, installation, and compressibility error corrections are applied by the computer to the speed before we see it, thus making it actually EAS; but they don't label it that because IAS is the entrenched common term, and you don't want to confuse pilots.

I don’t think there is any compressibility correction.
Please see Old Smokey thoughts from 15 years ago.

[EXDAC]

Originally Posted by RBF

Is there any reason for not having modern airspeed indicators showing EAS instead of IAS?

How can the airspeed Indicated on an Air Speed Indicator be anything but Indicated Air Speed? Indicated means exactly that - the speed that is indicated by the indicator. No matter what compensation, calibration, or correction is applied it is still Indicated Air Speed.

Shouldn't the question be - why can't IAS be equal to EAS?

[Vessbot]

Originally Posted by EXDAC

How can the airspeed Indicated on an Air Speed Indicator be anything but Indicated Air Speed? Indicated means exactly that - the speed that is indicated by the indicator. No matter what compensation, calibration, or correction is applied it is still Indicated Air Speed.

Shouldn't the question be - why can't IAS be equal to EAS?

Sometimes etymological origin doesn’t map nicely onto technical usage. In this case, after about a hundred years when indicators have become capable of indicating things unthinkable originally - but the need to keep track of some of the speeds before correction has been applied, has remained, leading to some ambiguity.

If you automatically say that whatever is shown is IAS, then you lose the ability to talk about pre-correction speeds. Like most modern jets that indicate TAS as well as IAS - do we get confused and say oh my God, there’s one IAS and another IAS that differs by 50%? Of course not. We know that of the two speeds that are literally indicated, only the one labelled IAS is the aeronautical definition of IAS.

Back to the original question about EAS, now I’m more siding with Goldenrivett and don’t think that they’re showing us EAS without telling us (like I was suggesting before) as it can be a 20-25 knot difference in cruise, and I think it’s a significant enough difference (hopefully, at least) they wouldn’t sneak it by us.

But I did look at what my manual says, and (not explicitly, but suggestive language) that what is displayed on the PFD is CAS.

[Vessbot]

A neat aside, on the SR-71…

[EXDAC]

Originally Posted by Vessbot

But I did look at what my manual says, and (not explicitly, but suggestive language) that what is displayed on the PFD is CAS.

I don't know that all aircraft use the same terminology but what Honeywell and Douglas displayed on the MD-11, MD-90, and 717 PFD was Computed airspeed. I could provide a bit more detail on the derivation if needed. I would argue that the indicated airspeed was CAS (Computed Air Speed not Calibrated Air Speed). TAS is a separate numeric readout that is clearly identified as TAS on the flight decks I worked on.

[Alex Whittingham]

It's a CS.25 requirement I think, CAS. I vaguely recall one type (Embraer?) showing EAS.