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Loading System Limits

  • basketball
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basketball created the topic: Loading System Limits

Howdy Folks,

Was hoping someone could enlighten my little brain with relation to the loading systems and limits.

Firstly with Alpha, example 2 on page 5.23, we solve the max luggage that can be added when the plane is at t/o weight by drawing some lines on the graph and seeing where it intersects the envelope. In the books example, the max luggage that can be added is 20kg, and we say anything more will put the plane out of the CG envelope at t/o. However what about when fuel is burnt in flight? I know in this instance the fuel is right on the datum, so we basically draw a line straight down from the point we derived earlier, but in practice would you check fuel burn just to make sure you are in the CG through the flight?

In the case of Bravo, the example on page 5.24 and 5.25, has the t/o weight and moment in the envelope and from there we conclude the plane is safe to fly. How come we don't do another check to see that when fuel is burnt during flight it remains with in the envelope?

Also, with the second example on Bravo, page 5.26, we have ZFW, and then from there work out max luggage that can be added. We get 40 pounds, so if we add 40 pounds we will be right on the aft limit. This would obviously change if fuel is added, how come we don't check fuel aswell in this instance to see the effect when added, since we need fuel to fly right?

Am i missing something? Am i seeing the problems wrongly? I understand how to get the answers, just not sure why we don't do a cg for t/o weight, and weight after expected fuel burn i.e. landing condition.

Thanks,
Brendan
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Richard replied the topic: Re: Loading System Limits

Hi Brendan,

Yes, you do always need to check for the effect of fuel burn, in fact it is a legal requirement. You must ensure the aircraft is in balance "during all phases of the flight" or, in other words, at take off and after theoretically burning off every last drop of usable fuel. Of course you won't actually burn all the fuel but if the TOW and ZFW points and the line between them remains within the CofG envelope, you can be sure the aircraft can't "burn itself out of balance" during the flight.

Let's look at your examples one by one:

Alpha, Example 2, pg 5.23:
You have found the maximum amount of baggage you can add and that has given you a point right on the forward limit of the envelope. Let's call that point "A". So, A is the new TOW weight CofG position with the extra 20kg of baggage loaded. There will now be a new ZFW CofG position too and this will be directly below "A" at a weight of 1420kg (the old ZFW of 1400kg + the new 20kg of baggage in the nose). Let's call the new CofG position for ZFW point "B".

As the aircraft burns fuel during flight, the CofG position will slide down the line from A towards B and since this line remains within the envelope the aircraft will remain in balance. You can plot this to be thorough but in this particular example there really isn't any need since it was obvious the new ZFW position would also be within the envelope.

Bravo example p 5.24-5.25
This is just a demonstrations of how to use the Bravo chart and load sheet. In practice you would still have to check the CofG position for ZFW and at TOW to ensure the flight remained within balance at all times.

Bravo example p 5.26
In this example we're finding maximum baggage possible so the CofG position at ZFW is the relevant point to consider in the question. The example makes no mention of the intended fuel load so we can't do anything about that. We can only work with the information we have.

If you're interested, we can have a bit of fun and work out the aft limit and look at the effect of adding fuel on CofG position. The aft limit is a constant for any weight in Bravo so find some weight where the moment index can be easily read and work it out. Bear in mind we're having to interpret lines on a graph so it's not going to be too accurate.

e.g. at 2200lbs we have a max permissable moment index of 204. CofG position = (204 x 1000) / 2200 = 92.7
e.g. at 1900lbs we have a max permissable moment index of 176. CofG position = (176 x 1000) / 1900 = 92.63
e.g. at 1525lbs we have a max permissable moment index of 150. CofG position = (150 x 1000) / 1625 = 92.3

We can see the real aft limit lies somewhere between 92 and 93 inches aft of the datum in Bravo. The fuel tanks are on 91 inches so adding fuel will therefore tend to bring the CofG forward from the aft limit.

Cheers,

Rich
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  • basketball
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basketball replied the topic: Re: Loading System Limits

Thanks for the great explanation Richard. I have no excuses not to smash the performance exam now! (Well the loading bits anyway :D)
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Richard replied the topic: Re: Loading System Limits

Glad to help B)
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