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Establishing the take-off weight limit - Exercise 5.1 (p.142)

dontgiveup created the topic: Flying for Maximum Endurance - Questions in the book (p.109)

For questions on flying for maximum endurance, I realise that contingency fuel is not accounted for in the answer.
E.g. for No. 6 of p.109 (of the e-book, or see below), the answer says the margin is 11.4 gal, and the max time that the ECHO could hold is 39.8 min (11.4 gal/ 17.2 gph ).

E-book p.109 - No.6
The trip fuel for a flight from A to B is 66 gal. What is the maximum time that an ECHO could plan to hold over the destination aerodrome
at 35% MCP if start up was with 102 gal?

Shouldn't the 10% contingency be applied to the 11.4 gal as well?
i.e. 11.4 gal / 1.1 / 17.2 = 36.15 min?

(same question for #3 - or is it because the questions says "plan for", that's why the 10% contingency fuel does not need to be considered?)

bobtait replied the topic: Flying for Maximum Endurance - Questions in the book (p.109)

According to my copy of the book, the contingency fuel has been accounted for. eg
The flight fuel is given as 66 gallons.
So minimum fuel required for the planned flight is 66 x 1.1 + final reserve fuel + taxi.
(multiplying by 1.1 accounts for the contingency fuel).
So minimum fuel required is 66 x 1.1 + 15 + 3 = 90.6 gallons
You started up with 102 gallons, so you have 102 - 90.6 = 11.4 gallons of margin fuel available.
You can plan to burn all of that holding if you need to.
The lowest fuel flow available is 17.2 gallons at 35% power.
So maximum time you can plan to hold is 11.4 gallons at 17.2 gallons/hour = 39.8 minutes.

Note that once you are over the destination aerodrome, the contingency fuel does not apply. Contingency fuel accounts for unexpected headwinds, diversions en route etc. It applies only to the trip fuel - not the margin fuel.

dontgiveup replied the topic: Establishing the take-off weight limit - Exercise 5.1 (p.142)

Thanks for the quick reply, Bob!
I got another similar question.

For Exercise 5.1 (e-Book p.142),
10% contingency fuel does not apply to the trip fuel to calculate the Fuel at Take-off (for subsequently adding to MZFW of 2630 kg for comparison)? How come?

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Edit: Answering my own question
10% contingency fuel DOES apply to the trip fuel. See below forum question for details.

Speaking of Exercise 5.1, for Exercise 5.1 #8,
should the third figure in the worked answer be:
MZFW (2630 kg) + Min. Fuel (58 + 15 + 3 gal = 76 gal, OR 206.72 kg)
= 2836.72 kg? [Edit: this was wrong because 10% contingency should be added to the trip fuel of 58 gallons
AND taxi fuel should not be included in the calculation - reason see Bob Tait's reply below]

Why is the figure calculated in the worked answer 2845kg instead?
Was the taxi fuel 3 gal double counted somehow? Because I reckon the figure would be right if I add 3 more gallons to Min. Fuel.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Edit: Answering my own question
The correct calculation for Exercise 5.1, Question 8 should be:
MZFW (2630 kg) + Min. Fuel
= 2630 kg + Trip Fuel with contingency (58 x 1.1 = 63.8 gal, OR 173.536 kg) + Reserve (41 kg)
= 2630 kg + 214.536 kg
= 2844.536 kg
= 2845 kg (rounded off) = answer in the eBook (ref: textbook p.141, Fig 5.6)

Conclusion: I should RTFQ as well as RTFB - B for book.

bobtait replied the topic: Establishing the take-off weight limit - Exercise 5.1 (p.142)

When you do the ZFW check, you take the ZFW limit of 2630 and add the weight of all of the fuel on board at TAKE OFF. If you have calculated the minimum fuel required at START UP, you would have included the taxi fuel.

However, 2950 is a TAKE OFF weight limit, so when you are checking to find the maximum TAKE OFF weight, you are considering the weight of the aircraft at TAKE OFF, not at START UP. So the taxi fuel would have been burnt.

The question gave you the fuel on board at TAKE OFF, so the taxi fuel has been burnt.