Boeing PFD IAS or CAS
 

Scratching my head here. Is Boeing PFDs using IAS or CAS? I always assumed it was IAS because thats what its experiencing when flying through the atmosphere and reading the air density. Or the air molecules going over the wing at higher altitude which is important not TAS. As we know higher up air density decreases so to maintain constant IAS TAS must increase

Does it matter? The two are not too far apart. But given that CAS is IAS corrected for instrument and position error what do you reckon? Now consider that modern aircraft have ADC's, the product of which is a input for a digital display and TAS (something that requires CAS), what do you reckon now?

I'm more interested in why this is a Boeing-specific question.

On Boeing aircraft it's CAS and I would imagine any other EFIS aircraft to have the same.

Thanks guys. My thoughts are why not TAS?

CAS is one of the primary reference points, as it describes the dynamic pressure acting on aircraft surfaces regardless of the existing conditions of temperature, pressure altitude or wind.

What benefit would a pilot gain by having TAS displayed instead of CAS or IAS?

A better question would be, why not display EAS?

Why does it matter (IAS vs CAS vs EAS)? Who is going to do continuous conversions in his head during the flight?

If the speeds in the manuals match the speeds displayed on the PFD, WHY DOES IT MATTER?!?

It doesn't really, except the type I fly there are a few ifs and buts regarding speed as you get higher. For instance drift down speed is 20 knots faster above FL270. Suspect if speeds were given in EAS then these adjustments wouldn't be necessary.

I suppose out of all this TAS is the important one as its in the Lift formula.

I was only curious as to what the PFD showed. My thoughts where the aircraft will stall at the IAS rather than TAS. As IAS is reading the oncoming air?

Aircraft will stall at a particular Angle of Attack.

I'm currently studying POF and thats where my original question came from.

Pilots don't go around constantly calculating lift using TAS and a formula though.

IAS is more relevant than TAS for stalling but is not the whole story. From a piloting point of view TAS is really only relevant to navigation, but IAS, though imperfect, is relevant to a whole raft of performance related issues. E.g, stalling, gliding, holding, engine out performance, best economy, best range, best endurance etc. As an approximate guide to how the wing is performing in normal ops, IAS or CAS are very useful.