737NG Dual center tank pump fail
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737NG Dual center tank pump fail
Hi Everybody,
My first question on this forum.
Does anybody here know where to find how much fuel one can burn out of the wing tanks after both Center Tank pumps fail before one would be so far out of CG and not be able to continue flight?
The scenario I m looking at the moment is about 5t stuck in the Center Tank and roughly an hour flight so 2000kg more or less burn, 1000 a side on the wings.
Your comments are highly appreciated. Been looking through the manuals and the old interweb but havent come up with anything definitive sofar. QRH just says He you probably wont have enough fuel to continue.
My first question on this forum.
Does anybody here know where to find how much fuel one can burn out of the wing tanks after both Center Tank pumps fail before one would be so far out of CG and not be able to continue flight?
The scenario I m looking at the moment is about 5t stuck in the Center Tank and roughly an hour flight so 2000kg more or less burn, 1000 a side on the wings.
Your comments are highly appreciated. Been looking through the manuals and the old interweb but havent come up with anything definitive sofar. QRH just says He you probably wont have enough fuel to continue.
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Aircraft still flies if you are slightly out of CG envelope and I'm almost sure it's easier to fly it 1% out of envelope than inside the envelope with dual engine flameout due to fuel starvation...
As you said it yourself, lack of fuel will be your first and most important problem...
As you said it yourself, lack of fuel will be your first and most important problem...
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"The scenario I m looking at the moment is about 5t stuck in the Center Tank and roughly an hour flight so 2000kg more or less burn, 1000 a side on the wings."
You should be able to work this out using a loading chart? It does depend somewhat on your ZF CofG!
You should be able to work this out using a loading chart? It does depend somewhat on your ZF CofG!
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CG vs Fuel (kg) 737NG
For those interested here is the effect on MAC% when fuel is being added/used by the aircraft.
As clearly can be seen, from empty (0%) the centre fuel addition moves MAC rearwards to -9% then the centre fuel moves the MAC forwards up to +11%.
On the BBJ where AUX tanks are fitted an additional parallelogram is added to the 21 tons above the centre fuel amount to take these variable configuration tanks into account.
The result of the AUX fuel can easily take the MAC out of limits for the aircraft thus care must be given to how much in which tank and where the hold capability is used as the aircraft operates in the envelope higher than where many airlines publish their maximum weight, in an area than is confined by approximately 4% MAC between forward and aft limit at MTOW.
For those interested here is the effect on MAC% when fuel is being added/used by the aircraft.
As clearly can be seen, from empty (0%) the centre fuel addition moves MAC rearwards to -9% then the centre fuel moves the MAC forwards up to +11%.
On the BBJ where AUX tanks are fitted an additional parallelogram is added to the 21 tons above the centre fuel amount to take these variable configuration tanks into account.
The result of the AUX fuel can easily take the MAC out of limits for the aircraft thus care must be given to how much in which tank and where the hold capability is used as the aircraft operates in the envelope higher than where many airlines publish their maximum weight, in an area than is confined by approximately 4% MAC between forward and aft limit at MTOW.
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"The scenario I m looking at the moment is about 5t stuck in the Center Tank and roughly an hour flight so 2000kg more or less burn, 1000 a side on the wings."
You should be able to work this out using a loading chart? It does depend somewhat on your ZF CofG!
You should be able to work this out using a loading chart? It does depend somewhat on your ZF CofG!
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Not all airlines carry those loading charts anymore in flight deck...
A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
Using 2t from wing tanks results in a forward movement of ~2% MAC.
A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
Using 2t from wing tanks results in a forward movement of ~2% MAC.
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Not all airlines carry those loading charts anymore in flight deck...
A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
Using 2t from wing tanks results in a forward movement of ~2% MAC.
EDIT: "~7% MAC." Are you confusing trim index units with %MAC?
Last edited by BOAC; 3rd Oct 2014 at 14:11.
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Not too much rocket science if recognising the fact that the Center Wing Tank is located nearly on the C of G.
A number of airlines routinely leave tankered fuel in the Center Tank for landing provided you have sufficient ZFW margin to do so. This limits the amount of frost on the upper wing and enables warm fuel to be placed in the main tanks.
A number of airlines routinely leave tankered fuel in the Center Tank for landing provided you have sufficient ZFW margin to do so. This limits the amount of frost on the upper wing and enables warm fuel to be placed in the main tanks.
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BOAC,
Loadsheets do not all have these charts. Operators can have adjusted load sheets giving them uncorrected trim, then correct for flap and thrust rating used for departure to achieve trim for departure. As for the graphs, they are subsequently not required as long as each weight category has band in which they must fall (FWD/AFT limits).
Forward MAC compared to normal (empty) when landing.
Correct, it is normal for this to happen. Nothing unto wards about it. Merely stated that this happens when using centre fuel. Simple.
I suggest you look at graph and read inputs: Weight vs MAC/CG movement
Certainly not BOAC, as a trim index unit of +7 would take an aircraft correctly trimmed outside the green band on 737. The trim index unit for 5 tons of centre fuel is actually about +0.5.
I would love to expand on this data and can show all the references in the according W&B manuals should this be required. Thus I am not sure where your response above comes from.
The reason the chart (see link above) is they way it is, is because the centre tank is NOT exactly a square box nor is it exactly located at balance point of aircraft, nor is wing fuel in mid CG of aircraft but due to the tank design has a forward/aft movement according to the amount of fuel carried.
Dogma, it is indeed not rocket science and indeed many airlines land with centre tank fuel, generally 737 aircraft are aft restricted, thus by leaving centre fuel it moves CG forward, away from the aft restriction.
Quote:
Not all airlines carry those loading charts anymore in flight deck...
- so what are they going to do if they divert to a non-company station - wait for mummy to come along and help?
Not all airlines carry those loading charts anymore in flight deck...
- so what are they going to do if they divert to a non-company station - wait for mummy to come along and help?
Quote:
A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
- no it does not. That weight is already there.
A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
- no it does not. That weight is already there.
Quote:
Using 2t from wing tanks results in a forward movement of ~2% MAC.
- thus we are talking -2% MAC? Hardly earth-shattering, I suggest.
Using 2t from wing tanks results in a forward movement of ~2% MAC.
- thus we are talking -2% MAC? Hardly earth-shattering, I suggest.
I suggest you look at graph and read inputs: Weight vs MAC/CG movement
EDIT: "~7% MAC." Are you confusing trim index units with %MAC?
I would love to expand on this data and can show all the references in the according W&B manuals should this be required. Thus I am not sure where your response above comes from.
The reason the chart (see link above) is they way it is, is because the centre tank is NOT exactly a square box nor is it exactly located at balance point of aircraft, nor is wing fuel in mid CG of aircraft but due to the tank design has a forward/aft movement according to the amount of fuel carried.
Dogma, it is indeed not rocket science and indeed many airlines land with centre tank fuel, generally 737 aircraft are aft restricted, thus by leaving centre fuel it moves CG forward, away from the aft restriction.
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A confident and dare I say welcome post by Skyjob, important stuff this mass and balance business.
Could you afford us any info on where you cut your teeth and experience on the 737NG?
Could you afford us any info on where you cut your teeth and experience on the 737NG?
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The chart in the link above is for the entire fuel load. The aircraft is supposed to be loaded with main tanks first, then the centre tank. And fuel is supposed to be used from the centre tank first & then from the mains.
So on this chart, the first 7.6 - 7.8 tonne is the main tank fuel, which moves the MAC forward as it is loaded & back as it is burnt. The rest is the centre tank fuel, which moves the MAC rearward as it is loaded & forward as it is burnt.
There is no chart that displays what happens when fuel remains in the centre tank while main tank fuel is burnt, as this is not normally what happens. However, from the chart you can deduce that MAC will move rearward as main tank fuel is used. Using the example in the initial question, with full mains & 5 tonne in the centre tank, the moment for fuel would be about +1.6 MAC. Then, if the centre tank pumps both fail at that point, MAC would be moving rearwards as fuel was burnt from the main tanks. A 2 tonne burn from full tanks would equate to about -5.3 movement in MAC. (from about +9 MAC back to +3.7 MAC). So +1.6 less 5.3 gives a -3.8 MAC movement for fuel burn.
As has been said, the effect of this movement would depend on how the aircraft was initially loaded. I don't believe that it is likely to take the aircraft outside C of G limits. Boeing obviously don't think so as well, as there is no mention of C of G problems in the QRH for both centre tank pumps failing. In fact, that checklist is very low key, except for a gentle reminder not to run out of fuel! I guess the only time you would have to be aware is when the centre tanks pumps fail shortly after departure & the aircraft was loaded close to the aft C of G. In that case, you would most likely be considering a return to the departure airport anyway. In that case, you would only have to be careful if you were planning to burn off fuel prior to the landing.
So on this chart, the first 7.6 - 7.8 tonne is the main tank fuel, which moves the MAC forward as it is loaded & back as it is burnt. The rest is the centre tank fuel, which moves the MAC rearward as it is loaded & forward as it is burnt.
There is no chart that displays what happens when fuel remains in the centre tank while main tank fuel is burnt, as this is not normally what happens. However, from the chart you can deduce that MAC will move rearward as main tank fuel is used. Using the example in the initial question, with full mains & 5 tonne in the centre tank, the moment for fuel would be about +1.6 MAC. Then, if the centre tank pumps both fail at that point, MAC would be moving rearwards as fuel was burnt from the main tanks. A 2 tonne burn from full tanks would equate to about -5.3 movement in MAC. (from about +9 MAC back to +3.7 MAC). So +1.6 less 5.3 gives a -3.8 MAC movement for fuel burn.
As has been said, the effect of this movement would depend on how the aircraft was initially loaded. I don't believe that it is likely to take the aircraft outside C of G limits. Boeing obviously don't think so as well, as there is no mention of C of G problems in the QRH for both centre tank pumps failing. In fact, that checklist is very low key, except for a gentle reminder not to run out of fuel! I guess the only time you would have to be aware is when the centre tanks pumps fail shortly after departure & the aircraft was loaded close to the aft C of G. In that case, you would most likely be considering a return to the departure airport anyway. In that case, you would only have to be careful if you were planning to burn off fuel prior to the landing.
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Oakape - think the C of G moves forward with the full remaining in the Center Wing Tank and burning from the Main Tanks. You are talking about changes in the % of the Mean Aerodynamic Cord : so the MAC% decreases as the Main Wing Tank Fuel is burnt
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Thus I am not sure where your response above comes from.
There is no chart that displays what happens when fuel remains in the centre tank while main tank fuel is burnt
I have never seen a chart like the one you posted, sj, and I would be VERY wary of using it since the MAC change from empty tanks to full tanks (19%MAC) is prettty well outside the scope of the 737NG envelope at some weights (700, anyway). That has an absolute max range of 20%MAC so not a lot of margin for error.
Last edited by BOAC; 4th Oct 2014 at 08:40.
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BOAC - Not sure the % MAC change is as great as you suggest. His graph just shows the effect of increasing fuel load filling the wing tanks first and then the centre wing tanks. Full Wings +9% and then moves (decreasing MAC) forwards with the addition of centre tank fuel.