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Reviews Questions Set 2 - Q4

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Airspeed created the topic: Reviews Questions Set 2 - Q4

The answer for this question says that if your gonna have an aft limit balance problem, it will most likely occur at ZFW, but the questions says the plane is already loaded with full fuel, so why does the answer remove fuel, calc ZFW, then add the new passengers.

My initial thought was, ok, I have full fuel, bags in the back, an empty front cargo area and two new passengers, so I have to figure out what the balance is with the new PX and then figure out what I have to transfer into the front compartment. I came up with 9.5Kg to shift, but as there are only 25 kg packages, then I'd have to move 1x25kg package.
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Airspeed replied the topic: Reviews Questions Set 2 - Q4

I the ZFW used because fuel burn up to destination might put the plane out of balance?
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bobtait replied the topic: Reviews Questions Set 2 - Q4

The ECHO fuel data in the supplement says that the main tanks should be filled first and any additional fuel should be placed in the auxiliary tanks. The main tanks are well ahead of the aft c of g limit, so with full fuel, it's possible for the c of g to be well inside the envelope with the fuel on board, yet outside the aft limit with the fuel removed.

Also all of the compartment arms and seats (except for the nose locker) are behind the c of g aft limit. So any c of g problem is likely to be:

the forward limit with full fuel on board

the aft limit with cargo and passengers and no fuel on board.
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Airspeed replied the topic: Reviews Questions Set 2 - Q4

Thanks Bob.

So, first, always check aft limit with no fuel and required cargo or PX, then add fuel and check again?
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  • John.Heddles
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John.Heddles replied the topic: Reviews Questions Set 2 - Q4

First, the basic requirement is to ensure that the CG is within the envelope at all stages of flight (CAR(1988) 235(9)).

For a simple aircraft, this usually is addressed adequately by checking the CG intersection at takeoff (ramp weight should be close enough) and zero fuel weight as, for a prismatic tank, the fuel arm is constant and the burn on the wt x IU envelope will be a straight line.

There are a few caveats which need to be kept in mind, however.

If the small, simple aircraft has a (very) strange envelope with any vertices at the forward and aft limits which are re-entrant, you need to plot the fuel burn to check that you don't start and finish inside the envelope but venture outside during the progress of the flight. This would be very unusual but is noted for interest. Certainly, I can't ever recall having seen a light aircraft with such an envelope.

If the aircraft has multiple tanks, then there may be a potential for the fuel burn to take the CG outside during flight - particularly near the point where the aux fuel is expended and you are changing to mains. If you have more than two tank arms, then things can get more complex. In this case, it usually is necessary (for pilot calculations) to plot the fuel burn to check what's what with the CG movement during flight.

If the fuel tanks are not prismatic, then one often sees a fuel CG characteristic curve which is not a straight line and may be a strange shape depending on the fuel tank design. Depending on the circumstances, there is a potential for the fuel burn curve to migrate outside, and then back inside, the envelope during flight. If the pilot is doing his/her own sums, this can be checked, again, by plotting the fuel burn on the envelope.

If you have a WCO generated loading system, the WCO should have covered such potential problems in the loading system detail.

So, traps for young players, on occasion.

Second, Bob is offering comment specifically for the Echo (and that's reasonable, as you are interested in the exam scenario and the specific question relates to that aircraft). Be aware that the comments are not general and, generally, will not be transferable to an aircraft which you might have to load out there in the real world.

Although geared mainly for trimsheets, you may find a fairly lengthy thread of some use to review -
bobtait.com.au/forum/rpl-ppl/5236-aircra...loading?limitstart=0

Engineering specialist in aircraft performance and weight control.
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