Calculating TAS

[uwochris]

Hey guys,

It's my understanding that TAS is the equivalent airspeed corrected for density changes.

Here is a question from a work book that I can't quite figure out (or understand):

Pressure Alt: 33,000 ft.
IAS= 267 kts.
Instrument and Position error: +8 kts.
Compressibility error: - 13 kts.
Indicated OAT: -28 C.
Temperature Rise: 20 C.

a) 456 knots.
b) 477 knts.
c) 427 knts.
d) 433 knts.

To calc TAS, I used IAS+ instrument error- comp error (267 + 8 - 13= 262 knots). Then I used my E6B to calculate TAS using 33000 as my pressure altitude and -28C as the OAT. I get something close to 470 knots.

The correct answer, however, is a) 456 knots. What I do not understand is the "temperature rise" statement. What is this? How do I factor it into the TAS calculation.

Can anyone explain how to calculate the right answer?

Thanks,

Chris.

 

[VNugget]

Just like the ASI, thetemperatureprobealso has its own errors athigh speed, which is what the temperature rise is. If we subract thetemperature rise from the indicated OAT, we get atrue OAT of -48. Now aEAS of 262 becomes a TASof 456.

If you read this before I edited it, I said something really stupid. Ignore it.

P.S. the problem should really use the word "correction," not "error." Those are opposites.

 

[mattpilot]

If you read this before I edited it, I said something really stupid. Ignore it.

HAHA.. no shat - i was about to correct you lucky bastard i didn't hit the quote button


ANyhow.... only problem i have is why do you Subtract the temp from -28 and not add it?

By subtraction you get -48, when you add you get -8.

If temperature increases (pitot tube heats up), why would you calculate for it getting colder?

I realize you got the correct answer, it just confuses me.

 

[VNugget]

HAHA.. no shat - i was about to correct you lucky bastard i didn't hit the quote button

Looks like I escaped certain humiliation by the breadth of a hair

ANyhow.... only problem i have is why do you Subtract the temp from -28 and not add it?

By subtraction you get -48, when you add you get -8.

If temperature increases (pitot tube heats up), why would you calculate for it getting colder?

I realize you got the correct answer, it just confuses me.

Well we're not calculating for it "getting colder" .. quite theopposite. The reading gets (erroneously) warmer, so we subtract theerror from the indication to correct for that. That same "correctionvs. error" issue again. Think about your starting point and your endingpoint; your gut instinct was right in this case.

 

[mattpilot]

Well we're not calculating for it "getting colder" .. quite theopposite. The reading gets (erroneously) warmer, so we subtract theerror from the indication to correct for that. That same "correctionvs. error" issue again. Think about your starting point and your endingpoint; your gut instinct was right in this case.

Ah i see.. thanks.. indicated OAT probe is already heated by 20 degrees thats why we subtract 20 to make it colder. I had that backwards... kinda like you with IAS and calibration error in your original post hehe

 

[Donsa320]

Or you could buy a Jeppesen CR computer and do it the way we all did it before the glass cockpits came along.