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HeliTorque :: View topic - IAS, CAS & TAS Relationship
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HeliTorque Forum Index » Ground School

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IAS, CAS & TAS Relationship
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haggishunter
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PostPosted: Tue Jul 14, 2009 6:42 pm    Post subject: IAS, CAS & TAS Relationship Reply with quote

After having an issue with an Air Data Computer this morning resulting in one ASI reading up to 50kts different to the other one, could someone open their Bristol notes (Flight Instruments) and remind of what the order is for these speed increasing with with altitude.

We knew which Computer had the error and flew off the other one, but it annoyed me that I couldn't remember... and my coffee got cold!

My Flight Instruments folder is in the UK, so if someone could be so kind to remind me I would apreciate it.

Cheers,

HH
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afterburner
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PostPosted: Tue Jul 14, 2009 10:41 pm    Post subject: GS Reply with quote

IAS is the primary.......what the instrument reads.

CAS is the IAS corrected for installation error. It is more or less the same as the IAS. Reference the calibration card.

Now TAS changes with altitude. As you climb, air thins, less ram air into the pitot, and the IAS is actually less, therefore, TAS is greater than IAS at altitude.

If you add 2% per thousand feet to the IAS, you should be pretty close.
ex: At an IAS of 100 at 5000 feet, TAS is expected to be approx 110.

Simple.

Laughing
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haggishunter
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PostPosted: Tue Jul 14, 2009 10:55 pm    Post subject: Reply with quote

AB many thanks sir, I thought that but wanted clarification!

Good man!

HH
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PostPosted: Wed Jul 15, 2009 12:54 am    Post subject: Ground Reply with quote

HH,

No charge. Anytime.

Fly safe,
Gotta go now.........
Helicopter
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