ISA rule of thumb

I've seen Captains that can figure out ISA in their heads, I still have take a minute to and look at that graph chart......and follow those lines. What is the rule of thumb?
 
I've come up with this chart, but it's still a little clumsy.

hPa (mbar) ......... height (metres) ......... height (feet) ......... temperature (degC)
»...... 10 ................... 31 055 ................ 101 885 ................. -45.4
»...... 20 ................... 26 481 .................. 86 881 ................. -50.0
»...... 30 ................... 23 849 .................. 78 244 ................. -52.7
»...... 40 ................... 22 000 .................. 72 177 ................. -54.5
»...... 50 ................... 20 576 .................. 67 507 ................. -55.9
»...... 70 ................... 18 442 .................. 60 504 ................. -56.5
».... 100 ................... 16 180 .................. 53 083 ................. -56.5
».... 150 ................... 13 608 .................. 44 647 ................. -56.5
».... 200 ................... 11 784 .................. 38 662 ................. -56.5
»... (226/ISA TROP.. 11 000 .................. 36 091 ................. -56.5)
».... 250 ................... 10 363 .................. 33 999 ................. -52.3
».... 300 ..................... 9 164 .................. 30 065 ................. -44.5
».... 400 ..................... 7 185 .................. 23 574 ................. -31.7
».... 500 ..................... 5 574 .................. 18 289 ................. -21.2
».... 600 ..................... 4 206 .................. 13 801 ................. -12.3
».... 700 ..................... 3 012 .................... 9 882 ................... -4.6
».... 800 ..................... 1 949 .................... 6 394 ..................... 2.3
».... 850 ..................... 1 457 .................... 4 781 ..................... 5.5
».... 900 ........................ 988 .................... 3 243 ..................... 8.6
».... 950 ........................ 540 .................... 1 773 ................... 11.5
».. 1000 ........................ 111 ....................... 364 ................... 14.3
». (1013.25/ISA MSL ....... 0 ........................... 0 ................... 15.0)
».. 1050 ...................... - 302 .................... - 989 ................... 17.0
 
Let's use FL 310 as an example.
So we take 31 and multiply it by 2 and get 62. That 62 will be subtracted from 15 to get -47. My charts say -45, which is close enough for me for me to figure out cruise performance. That was easy!
 
But here is one more thing that's bugging me. Engine performance data is based on TAT, but some of our planes give SAT, my Check Engineer says just subtract between 22 and 25 degrees from SAT to get TAT. My CR-2 Manual dances all around the subject *WITHOUT* showing how to convert them.

Any help?
 

onthebeach

New Member
SAT / TAT; conversions

If you have access to SAT and TAS, then here is a formula to compute TAT: square TAS & divide result by 9500. add the number you get to SAT to obtain TAT.

example: SAT of -56 C & TAS of 450 knots.
450 sqrd = 202,500
202,500 / 9500 = 21.3
TAT = -35 C (approx.)

Note: this formula works only when dealing with "knots" and "degrees Celsius."
 
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