supermunk
10th Nov 2000, 16:18
Can anyone point me in the right direction to find the accurate formulae to cinvert IAS to TAS both direct and via the Mach route. I've got the formulae from Ed Williams' Aviation formulary but it seems to give consistent high figures compared with my trusty RAF MK4B computer. Alternatively, if anyone's cruising at Mach 0.82 at FL350 ISA, let me know what the FMS says for TAS.
Remember, it has to go up, but landings bite if you get them wrong.
There is a very simple, accurate mental formula you can use if you have the M and RAT or TAT.
M .80 and RAT of -20 yields a TAS of 468. (You have to memorize that.) This will be your standard from which to start your mental computation.
Now, for each .01 M, faster or slower, there is a 5 kt difference in TAS. For each 1 C degree of difference in RAT (TAT), there is a one kt difference in TAS. (Colder is slower, warmer is faster.)
A couple of examples: 1. M.84, RAT -15. M.84 is 20 kts faster the the .80 standard, so you add that to 468, so we have 488 so far. Now, -15 is 5 degrees warmer than your -20 standard, so we add an additional 5 kts. Your answer is 493 kts TAS.
Example No. 2. M.79, RAT -27. M.79 = 468-5, or 463. Now subtract 7 kts for the colder RAT and you come up with TAS of 456.
The nice thing about this simple mental calculation is that you don't care about ISA deviations or IAS or altitude! With a little practice, anyone can do it in their head in a few seconds.
If you want to do it from ISA (or SAT or OAT), then you have to know that the RAM rise at .80 is +30C. (At .82 it's +32) So, in your example, M.82 indicated M at FL 350 ISA, we'll calculate it this way:
M.82 is 2x5, or 10 kts faster than our standard of 468 for the computation, so we have = 478 (so far). ISA at FL 350 is -55C. Add 32 for the difference between SAT and RAT, now we have -23 which is -3 difference from OUR standard of -20 for our calculation. So, the result we come up with is 475 TAS.
I've found this to be very accurate (within 2 kts) for jet cruise conditions.
For intermediate altitudes or if you MUST use IAS, I'm sure someone else can give you a formula. I'd be surprised if it would be as accurate or as easy to use, though.
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[This message has been edited by quid (edited 12 November 2000).]
nose-cabin
12th Nov 2000, 05:05
a quid
example 1 is 493kts.
I think
Oops!
A guy should at least get his examples correct! http://www.pprune.org/ubb/NonCGI/eek.gif
Thanks for the catch, I've edited it.
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nose-cabin
12th Nov 2000, 19:34
QUID
Thanks that is a good guide.
I usually multiply altitude in thousands
by 3 when below 15,000ft and add to IAS.
Works fairly well.
ie at 6,000ft and if the IAS is 160
then TAS is pretty close to 178kts.
another quick x-wind check is
30 deg off heading mult wind by .5
45 deg = .7
60 deg. = .9
not too far off the mark.