This is a splitting hairs kind of question - apologies - but I'm keen to know the answer.

Using ECHO, let's suppose we're working out the TAS given 2,500 kg gross weight at 10,000 feet and 55% power with OAT at cruise 5 deg. There are two approaches:

• Determine DH (given PH and ISA variation) and use the ISA section of the TAS table
• Use the PH, and interpolate b/w the ISA and the ISA+20 tables

Using method 1 (my preference as I like bringing everything back to DH where I can):
At 10,000 ft, ISA = 15 - 10x2 = -5
If OAT at cruise is 5 degrees, ISA deviation is +10
DH = PH + ISA variation x 120 = 10,000 + 10 x 120 = 11,200
Thus, we interpolate b/w 170TAS for 10,000 and 176TAS for 15,000 and we obtain a TAS of 171.44 (= 170 + 6*12/50)

Using method 2:
With ISA deviation of +10, we average 170 TAS (at 10,000 in ISA with 55% power) and 173 TAS (at 10,000 in ISA+20 with 55% power) and get 171.5

Now I know this difference is completely immaterial in any practical sense. However, it does give rise to differences in the preferred power setting to maximise GNMPG in a purely theoretical sense. For example, consider the section "Choosing the power setting for best range" in the BT book (page 104 in the May 2015 ed). Using method 1, the best power setting for GNMPG with 25 knot head wind is 45%. But using method 2, it's 55%.

Is it likely in an exam a question will arise that relies on this level of accuracy? Is there are recommended approach for using the tables?

Thank you.

It is probably simpler to convert to density height. However remember that in the CASA exam there will be no TAS tables or power tables given. The CASA exam simply gives you a TAS and a fuel flow as part of the question. I based the book on the tables because the syllabus says you should be able to extract figures from the performance tables.

In this regard, the exercises in the book go beyond the requirements of the actual CASA exam. Also, there are obvious limits to the use of tables for absolute accuracy since tables give only a selected sample of weight, height and power values. For greater accuracy you should be using graphs - but that is really going a bit too far for practical purposes.

Thank you!