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## Auxiliary fuel burn and CofG

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### heidif created the topic: Auxiliary fuel burn and CofG

Question:

An Echo is loaded at take-off so that the centre of gravity is exactly on the forward limit of the permitted range.

If fuel is now burnt from the auxiliary tanks, the centre of gravity will-

Select one:
a. move aft and out of the envelope
b. move forward and out of the envelope
c. move forward and remain in the envelope
d. move aft and remain in the envelope

The answer is c

Is this answer still correct if the weight of the plane is 2360kg or less?

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### jukzizy replied the topic: Auxiliary fuel burn and CofG

if it is less then you have to use 2400 Kg since it is a forward limit question.

Forward limit questions are best to be plotted on chart than using formula

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• John.Heddles
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### John.Heddles replied the topic: Auxiliary fuel burn and CofG

As is stands, the question doesn't have a correct answer unless the question adds a few caveats. I have no idea whence you came across the question but it is evident that the person who set the question up just wasn't thinking straight ....

Three cases to consider -

(a) in the high weight range so that all calculations remain in the high weight region where the limit varies with weight. In this case, the CG moves forward as fuel is burnt but the limit moves forward at a faster rate so the CG becomes more conservatively inside the envelope compared to the limit. The answer would be (c).

(b) in the mid-range weight so that the starting point is above 2360 kg and the end point below. In this case, the outcome depends on the quantity of fuel burnt and the starting point. Either (b) or (c) can be made to be the answer.

(c) in the low weight range so that all calculations are for the constant weight limit. In this case, the CG moves forward as fuel is burnt and outside the forward limit so (b) is the answer.

For your specific question relating to the weight below 2360, if option (b), above, applies, you need to run some sums. If option (c) applies, there is no need to run any sums and the answer is self evident.

Engineering specialist in aircraft performance and weight control.

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### heidif replied the topic: Auxiliary fuel burn and CofG

Thanks so much John.......your explanation is exactly what I was thinking. I ran my sums for an aircraft below 2360kg and chose option B. Didn't use the graph for over 2360kg, so I got the question wrong.
The question was in the Bob Tait Exam Prep for CFPA.
Thanks for responding.
Heidi

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• John.Heddles
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### John.Heddles replied the topic: Auxiliary fuel burn and CofG

Setting the question for the sloping section of the limit line is the logical thing. However, to exclude the other considerations, the question really needed a caveat to restrict the area of interest to the sloping section. I would imagine that was just an oversight, keeping in mind the level of workload Bob and Stuart run with day in day out ...

Engineering specialist in aircraft performance and weight control.

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### heidif replied the topic: Auxiliary fuel burn and CofG

Yes I totally understand, and appreciate all their hard work to help me pass my exams

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### Stuart Tait replied the topic: Auxiliary fuel burn and CofG

I'll chase that question down and make amendments to make it less ambiguous

Oh and congratulations Heidi on a solid pass

Cheers

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### bobtait replied the topic: Auxiliary fuel burn and CofG

For the Echo you wouldn't have any fuel in the auxiliaries unless the mains were full. It's very unlikely that the aircraft would weigh less than 2400kg with full mains and any normal payload. We'll change the wording to indicated that you have a pilot and five passengers on board with an empty weight of 1990kg.

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### heidif replied the topic: Auxiliary fuel burn and CofG

Thanks Stuart. I was pretty pleased with myself and with Bob for his fantastic videos and teachings to help me get there.

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