View Full Version : 737NG Dual center tank pump fail
DutchExpat
2nd Oct 2014, 20:39
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.
FlyingStone
2nd Oct 2014, 21:15
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...
Skyjob
2nd Oct 2014, 21:36
Your Performance department has details of how CG is affected based on fuel in the tank. It travels forwards the fuller it is...
B737900er
3rd Oct 2014, 01:31
Boeing say fuel burn will only change the CG by 4+/- % so I guess it depends how you loaded the bird first.
"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!
Skyjob
3rd Oct 2014, 11:12
CG vs Fuel (kg) 737NG (https://drive.google.com/file/d/0B_b6TyjPLrfBNjQ1NDA0ZTktNDI0NS00NmI3LTg4OWMtZGU0ZDJhOTQ4NGN j/view)
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.
BUGS/BEARINGS/BOXES
3rd Oct 2014, 12:16
"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!
Bingo BOAC
Skyjob
3rd Oct 2014, 13:30
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.
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?
A 5t deadweight in centre tank results in a movement forward of ~7% MAC. - no it does not. That weight is already there.
Using 2t from wing tanks results in a forward movement of ~2% MAC. - thus we are talking -2% MAC? Hardly earth-shattering, I suggest.
EDIT: "~7% MAC." Are you confusing trim index units with %MAC?
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.
Skyjob
3rd Oct 2014, 18:02
BOAC,
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?
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).
Quote:
A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
- no it does not. That weight is already there.
Forward MAC compared to normal (empty) when landing.
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.
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
EDIT: "~7% MAC." Are you confusing trim index units with %MAC?
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.
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?
Oakape
3rd Oct 2014, 19:15
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.
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
Thus I am not sure where your response above comes from. - only a 'few' years using dropline trim sheets (D-LTS) and on those for 737NG, 5T in the centre tank represents about 2%MAC nose down (6 'trim units' on my last company charts)There is no chart that displays what happens when fuel remains in the centre tank while main tank fuel is burnt - most D-LTS will show that. Incidentally, CofG moves FORWARD as main tank fuel is burnt on the 737, or it did on the ones I flew!
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.
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.
Skyjob
3rd Oct 2014, 20:02
I think BOAC you are reversing MAC and trim.
6% MAC equating to 2 trim units, yes I would buy that...
2% MAC equating 6 trim units, no way.
and then moves (decreasing MAC) forwards with the addition of centre tank fuel. - how about -10%? [10+9=??]
trim units - have you ever used a D-LTS? 'Trim Units' on a D-LTS traditionally run from 0 to 100.
A and C
3rd Oct 2014, 20:50
As the centre tank on the -800 extends out as far as the engine pylons I'm not sure how sucsesfull it would be using it to keep fuel warm ?
It is primarily the (lack of) wing thickess that causes the fuel-frost icing, and the ctre tank is in a thicker part of the wing than the outer mains. It works!
Sorry thread creep my bad.
The Center tank on the NG doesn't extend that far and the issues helped by adding warm fuel to the Wing Fuel. Less ice outside fuel stick 4
DutchExpat
3rd Oct 2014, 21:09
Oakape thank you for your great answer The scenario the have for us is dispatching with one center tank inop low vis and 2 rwy out in Madrid BCN cavok.
Option do a low vis approach or divert. One will be risk of missed and making your cg problem worse The other your accepting to take a bigger cg change. No right or wrong both are acceptable as far as I know.
But I thought it an interesting scenario and greatly appreciate your answer!
Might want to read Oakapes post again, its the reverse of what actually happens. 100% of MAC is at the trailing edge.
Not that I think it matters on the 737NG but would be happy to be proved wrong
Option do a low vis approach or divert. One will be risk of missed and making your cg problem worse The other your accepting to take a bigger cg change. No right or wrong both are acceptable as far as I know. - can I clarify this scenario? Are you are expecting to land at destination with 5T in the ctre tank having had an early No 2 pump fail or what?
NB Oakape's C of G movements are wrong as has been pointed out.
Oakape
3rd Oct 2014, 22:20
Yep, had the movement back to front! Sorry about that. Trying to put it together in too big a rush, as was going out (no excuse! lol). Of course, increasing MAC is C of G moving towards the rear. So should read -
So on this chart, the first 7.6 - 7.8 tonne is the main tank fuel, which increases the MAC & moves the C of G rearward as it is loaded & forward as it is burnt. The rest is the centre tank fuel, which decreases the MAC & moves the C of G forward as it is loaded & rearward 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 forward 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 forwards 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 decrease in MAC & forward movement in C of G 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 forward C of G. In that case (both centre tanks failing shortly after departure), you would most likely be considering a return to the departure airport anyway, as you most likely wouldn't have enough fuel to complete the trip & then you would only have to be careful if you were planning to burn off fuel prior to the landing.
chksix
4th Oct 2014, 06:55
A question from a 737 enthusiast here if I may.
Isn't the green band mainly a take off limit? The true aft limit for flight would be 15-17 Units of trim?
I think some are more than a little confused here. 'Trim units' refers to the units used in calculating c of g on a load chart and are based on the moment arm for the particular load item, be it SLF/fuel/food etc. They have nothing directly to do with 'stab trim units' where your green band lies. These are derived from the resulting C of G from the loading chart or computerised loading programme.
chksix - to answer your question - on the -700, the absolute aft limit is 30% MAC but decreases rapidly as you move away from a weight of 64T. The absolute forward limit, for interest, is just over 10% and ditto.
As I posted, with a D-L TS it is very easy to see the effect of trapped fuel on C of G.
Aluminium shuffler
4th Oct 2014, 09:43
My view is that this is a fairly futile debate. In the event of losing both pumps in that tank, you have no choice but to use the wing tank fuel and accept whatever trim change occurs. The change in trim between the time of pump failure and landing isn't going to be any worse than that when you do a sector with wing tanks only, because that CoG change is all that is occurring.
SO, as stated before, range and endurance will be the relevant considerations, not that your useable fuel has been drastically cut. You'd want to land asap, even overweight, as the wing fuel has a bending moment that counteracts the mass in the fuselage, as you are now likely to be above the MZFW on landing, and of course that means choosing a field with the required performance attributes and landing as gently as possible.
As for the green band, I think that is indeed for take off only and in a non-normal, standard limitations no longer apply; what is you alternative to flying outside the standard trim limit in this event?
SO, as stated before, range and endurance will be the relevant considerations, - not quite as simple as that for the OP if you read post #23 - we still need to know a little more about this sim 'scenario' and we can then 'quarterback' the answers:). Regrading the rest of your post, it is not directly comparable to just "do a sector with wing tanks only, because that CoG change is all that is occurring." since there is now a fixed moment that was not there 'at planning' and now will be there at landing - whether that is significant is what this is all about, really.
The ZFW(/M) point is totally relevant and is, of course, part of the considerations in the method of trying to avoid fuel frost by retaining ctre tank fuel on landing on the NG.
you are now likely to be above the MZFW on landing - actually you could be 'above' it at the moment of second pump failure!
Aluminium shuffler
4th Oct 2014, 16:16
I really don't see the issue. The centre tank is close enough to the CoP that trim requirements will be well within the available range, after all you can get airborne with 7.8t and deal with the CoG change in flight, and the CoG change here will be comparable. The start point of the CoG may start and finish a little futher forward with this addition 5t in the centre, but it's highly unlikely to have any adverse effect once in flight. Ultimately, there is little you can do should it actually happen, so any calculated trim and MAC is pointless. Trim what you need. Stability will be greater with a fwd CoG, and handling may be affected by needing a bigger control input for the flare, so if the aircraft needs a lot of NU trim before descent, you could move some passengers aft to compensate, but unless you really want a pointlessly academic hypothetical debate, I would suggest it's enough to just say land asap at a field that will allow a gentle landing at high weight.
I really don't see the issue. - nor me, as I posted earlier. I would still like to hear from the OP about the 'scenario' though.