EFIS and Speed reference

[GuinnessQueen]

Dear all,

A quick question, which I'm not finding particularly easy to answer.

With all the gadgets and gizmos available in glass cockpits, what is the speed that is presented on the speed tape.

Is it IAS, or does the ADC do a few tricks allowing the actual presented air speed not to be IAS, but rather TAS or EAS?

Thanks in advance

GQ

[Pattz]

In the Boeing 737-3/4/500 it is Calibrated Air Speed.
The ADC has therefore adjusted for position and instrument error.

[The Greaser]

Isn't it by definition indicated airspeed?

[seat 0A]

The Indicated Airspeed is calibrated, so it can be indicated as Calibrated Airspeed:confused:

[GuinnessQueen]

Thanks for the replies,

I also found another source (also for B737) that stated EFIS PFD presents CAS.

So while it might literally be IAS, it isn't really!!

Cheers

GQ

[Old Smokey]

Typically (I can't speak for all types) it is CAS that is Indicated on EFIS instruments, and typically, IAS indicated on Standby instruments.

Unfortunately, EAS is not (but could be) presented on EFIS aircraft. It's about time that it became standard, as they said of Steve Austin, "We have the technology"...........

[Mad (Flt) Scientist]

On our types it's the closest approximation we can give to CAS within the limitations of the air data system and knowledge of the static and/or total source errors. Therefore it is IAS on all instruments, but the IAS on the electronic gadgets is usually rather closer to CAS than the IAS on the standby. Although with some fancy standby systems, there's a slightly dumb ADC incorporated, so even the standby 'IAS' is a decent stab at CAS.

It's not really practical to design a system for perfect airspeed indication, nor is it necessary.