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onduty

Speed Confusion - IAS/KIAS/TIAS

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Hi Dear All,

It's been bothering me for long time now: in original, RW manuals and even in PMDG manuals there are two types of speeds given in the Limitations section. IAS and TIAS. Never encountered TIAS in my long-long RW aviation career. Anybody nows the meaning of TIAS? (Perhaps CAS by today's terminology?) What is indicated on the airspeed instrument, TIAS or IAS? In the "instrument markings" section PMDG refers to KIAS (!??). If we can read IAS on the ASI why do we need another value like TIAS? What is supposed to do with that?

I just hope that my questions are clear enough (its late night here after a long day in RW aviation).

Appreciate your educated help,

 

Tamas

 

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Posted (edited)

The terms, such as TIAS, are not encountered very often in modern text but the POH dates from the DC6 Introduction.  Common then and today is IAS - Indicated Airspeed.  I'll skip the variations on this such as calibrated airspeed and only add that in the POH charts you'll also see DIAS - Dial Indicated Airspeed, which is the same as IAS.  Simply, this is the value read directly from the airspeed instrument.  KIAS is same as specifying IAS in knots and is pretty common too.

TIAS is True Indicated Airspeed, which is the indicated airspeed corrected for the ambient atmosphere (pressure and temperature for the most part).  Today you will often see this expressed as KTAS or Knots True Airpeed.  TIAS is basically a military term and I believe is derived from the differences between a true airspeed based on calibrated speed vs based on indicated speed. For our purposes TIAS is equal to IAS at sea level standard atmosphere conditions.  The POH used TAS rather than TIAS in the performance tables because I suspect the values are based on calibrated speeds.

Edited by downscc

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On 5/10/2018 at 11:50 PM, downscc said:

The terms, such as TIAS, are not encountered very often in modern text but the POH dates from the DC6 Introduction.  Common then and today is IAS - Indicated Airspeed.  I'll skip the variations on this such as calibrated airspeed and only add that in the POH charts you'll also see DIAS - Dial Indicated Airspeed, which is the same as IAS.  Simply, this is the value read directly from the airspeed instrument.  KIAS is same as specifying IAS in knots and is pretty common too.

TIAS is True Indicated Airspeed, which is the indicated airspeed corrected for the ambient atmosphere (pressure and temperature for the most part).  Today you will often see this expressed as KTAS or Knots True Airpeed.  TIAS is basically a military term and I believe is derived from the differences between a true airspeed based on calibrated speed vs based on indicated speed. For our purposes TIAS is equal to IAS at sea level standard atmosphere conditions.  The POH used TAS rather than TIAS in the performance tables because I suspect the values are based on calibrated speeds.

My head is sore just reading this..... 🙂
 

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5 hours ago, Jude Bradley said:

My head is sore just reading this..... 🙂
 

Hi Jude, is that a comment on the clarity of my explanation or the complexity of the topic?  Perhaps I distilled the subject into the bare essence when an explanation of how the airspeed instrument works and how the information it provides is interpreted. A very good source for this and all the basics is the AIM: https://www.faa.gov/air_traffic/publications/

They call it the Aeronautical Information Manual now, I still call it the Airman's Information Manual because it is written for men and women learning basic airmanship.

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Hi,

After reading through Dan's explanation and the performance/limitations sections of RW manuals again, I rather think that TIAS = CAS (in today's term), rather than TAS, because:

1. In the performance section Douglas uses TAS as a separate term in  several places.

2. It would not be practical to use TAS for speed limits, because ther is no direct instrument indication for the pilot.

3. In many manuals of modern aircraft (rather light airplanes) the speed limits are given in IAS and CAS.

(Frankly, after many years of experience with GA aircraft I'm still not sure what is the benefit to have CAS  values infligth.)

 

Tamas

 

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19 hours ago, downscc said:

Hi Jude, is that a comment on the clarity of my explanation or the complexity of the topic?

 

 

Hi Dan,

No, your explanation very clear and educational. thank you. I just meant that 2nd reading was required. 🙂
 

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11 hours ago, onduty said:

Hi,

After reading through Dan's explanation and the performance/limitations sections of RW manuals again, I rather think that TIAS = CAS (in today's term), rather than TAS, because:

1. In the performance section Douglas uses TAS as a separate term in  several places.

2. It would not be practical to use TAS for speed limits, because ther is no direct instrument indication for the pilot.

3. In many manuals of modern aircraft (rather light airplanes) the speed limits are given in IAS and CAS.

(Frankly, after many years of experience with GA aircraft I'm still not sure what is the benefit to have CAS  values infligth.)

 

Tamas

 

This makes a lot of sense Tamas. So much that I dug deeper and found a copy of T.O. 1C-118A-1, the "dash-one" for this aircraft, and in the Airspeed Limitations section (pg 5-2) the following note is offered:

Quote: Limit markings on the airspeed indicator vary with different C-118A aircraft. On some the limits are for indicated airspeed (IAS), while on others the limits are for equivalent airspeed (EAS), which is also referred to sometimes as true indicated airspeed (TIAS). End Quote

Thus, TIAS = CAS at low speeds only, above 200 kts where compression comes into play there is a difference between CAS and EAS and here TIAS = EAS.  This is where may have to read the topic more than once:  I incorrectly presumed that TIAS was equivalent to TAS where in fact it is not but rather equal to EAS.  The difference between EAS and TAS is that EAS does not compensate for altitude (air density).

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Thank you Dan, that makes sense. I think that's the answer to my original question.

 

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