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## ETAS and Final Reserve rate

• Rac747
• Topic Author

### Rac747 created the topic: ETAS and Final Reserve rate

Hey everyone, I'm sitting the performance exam and Friday and I just have some queries regarding use of ETAS and final reserve calculations.
When doing a CP or PNR question I use the rule of thumb explained in Bob's performance book which is use ETAS if the drift is greater than 10 degrees. In other exams I've done etas is used for drift angles as low as 7 degrees. I use an eb-6 calculator and there is no mark for 7 degrees. Based off a rough extrapolation this amounts to a couple knots of variation, but I know how important accuracy is in this exam and I don't want to be caught out.
When calculating final reserve will casa specify what rate to calculate it at? In Bob's online exams it specifies final reserve to be calculated at cruise rate. In other exams I have seen questions where they use the holding rate without being specific in the question about which rate to use. When I'm hour building, I always calculate my 30 minute final reserve at cruise rate and my instructor has never pulled me up on it.

Any help would be much appreciated. Thankyou

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 876

### John.Heddles replied the topic: ETAS and Final Reserve rate

use ETAS if the drift is greater than 10 degrees.

Using ETAS always gives the mathematically correct solution regardless of drift.

However, once you get down to smaller drift angles, the difference between using ETAS or not becomes less and less. CASA uses ETAS down to 5 degrees. Given that you are going to set up the computer to give you the ETAS answer anyway, why not just run it all the time ?

I'll leave Bob to answer the rate question so I don't confuse the issue at all.

Engineering specialist in aircraft performance and weight control.
The following user(s) said Thank You: Rac747

• Posts: 2468

### bobtait replied the topic: ETAS and Final Reserve rate

Keep in mind that there still is a real world out there where real pilots understand that there is a common sense limit to accuracy in such calculations. If you use an E6-B type computer you don't need to worry about effective TAS anyway.

You don't have to stipulate which computer you use in the exam, so CASA's marking scheme must allow for your ETAS being half a knot out.

What next? Do we allow for the fuel used during the 180 degree turn necessary to return to base in the case of a PNR. Maybe a half loop with a half roll would be a safer option to reverse direction!!

Regarding the calculation of final reserve fuel, as far as I am aware, the fuel flow to be used in the calculation would have to be given in the question since the regulations require only that you can remain airborne for specified time. There is no rule regarding the actual amount of fuel required. So there would be many 'correct' answers depending on the power setting you chose.
The following user(s) said Thank You: Rac747