Hi,

I have a question on exam accuracy, particuarly if the question is 'enter the value' as opposed to multi-choice.

Consider:

**Exercise 4.4 (Page 82 , answers page 125 on the Performance e-text)**What TAS could be expected in an Echo when cruising at 5000ft with an OAT of plus 15 degrees Celsius, if gross weight is 2500kg and 75% power is used

My approach was (different to the Answer approach, but hopefully valid)

Pressure Height = 5000 feet

ISA Temperature = 15 - 2(5000/1000) = 15 - 10 = +5 degrees Celsius

ISA Deviation = OAT - ISA Temperature = 15 - 5 = plus 10

Density Height = Pressure Height + 120(ISA Deviation) = 5000 + 120(10) = 6200 feet

Since we have Density Height use the ISA Chart on the supplement - using 6200 feet values

- TAS(ISA; 5,000 feet; 2,500kg, 75% Power) = 192 kts
- TAS(ISA; 10,000 feet; 2,500kg, 75% Power) = 200 kts

This is 8 kts increase over 5000 feet; or 1.6kts/1000 feet;

So for 6200 feet TAS = 192kts (at 5000 feet) + [1.6x1.2] (for the extra 1200 feet) = 192+(1.6*1.2) =

**193.9 kts**

The Answer on page 125 takes a different (and valid) approach by interpolating the TAS at Pressure Height 5000 at ISA, and Pressure Height 10,000 at ISA+20 to get the TAS at Pressure Height of 5000 at ISA +10. All of this is at 2,500kg, 75% Power. The answer here is

**193.5kts**
MY Query:

Which approach is correct in an

*exam situation?* The second approach worked well as ISA + 10 is in the middle between ISA and ISA+20.

If it was multi-choice I could select the right one, but I'm not sure how PEXO would mark me for 193.9 (my method) vs 193.5 (other method)... or is there an error in my calcs?

Cheers

SJM