737NG Dual center tank pump fail
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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!
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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!
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!
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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
Not that I think it matters on the 737NG but would be happy to be proved wrong
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Originally Posted by Dutch
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.
NB Oakape's C of G movements are wrong as has been pointed out.
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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.
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.
Last edited by Oakape; 3rd Oct 2014 at 22:39.
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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.
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.
Last edited by BOAC; 4th Oct 2014 at 08:40.
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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 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?
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Originally Posted by AS
SO, as stated before, range and endurance will be the relevant considerations,
. 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
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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.
