737NG: ctr tank & density error?
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737NG: ctr tank & density error?
Fairly new to the 737NG and was wondering about the following:
Common technique in my company is to increase the desired uplift of fuel by 100-200kg due to the fact that ~200kgs will be "lost" as soon as the CTR tank is empty inflight. Fine to me, but nobody seems to have a reasonable explanation for this. I've heard several talking about an error in the density sensors of the tanks, others mention the fact that this technique only applies when the fuel at the point of departure is hot due to high OAT....
There's nothing in our books about it... could anyone shed some light on this for me? Why is there an error in the fuel qty indications and what exactly does it have to do with the ctr tank and/or the density sensors?
thx in advance
tadex
Common technique in my company is to increase the desired uplift of fuel by 100-200kg due to the fact that ~200kgs will be "lost" as soon as the CTR tank is empty inflight. Fine to me, but nobody seems to have a reasonable explanation for this. I've heard several talking about an error in the density sensors of the tanks, others mention the fact that this technique only applies when the fuel at the point of departure is hot due to high OAT....
There's nothing in our books about it... could anyone shed some light on this for me? Why is there an error in the fuel qty indications and what exactly does it have to do with the ctr tank and/or the density sensors?
thx in advance
tadex
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Not sure about the NG, but on the 'classic the gauge tolerance on all tanks is 2.5% of full-scale reading, i.e. 0.025 * 7000 = 175kg. The CTR tank is bigger than the wings tanks so the possible error is larger, thus potentially more significant.
When trying to measure small amounts in the CTR, the existing error can be at least as big as the amount you're tyring to read, so accurate readings are difficult. When we uplift anything into the centre tank, company SOPs are to uplift the reqd volume of fuel from the bowser and check the gauges are within limits, rather than using the gauges to uplift only to find when airborne that the CTR tank's error is in the 'wrong' direction; thus leaving you close to 200kg short.
Sorry if this is barking up a slightly different tree, and that it doesn't really answer why there is an error in the first place, except to say perhpas that nothing's perfect.
When trying to measure small amounts in the CTR, the existing error can be at least as big as the amount you're tyring to read, so accurate readings are difficult. When we uplift anything into the centre tank, company SOPs are to uplift the reqd volume of fuel from the bowser and check the gauges are within limits, rather than using the gauges to uplift only to find when airborne that the CTR tank's error is in the 'wrong' direction; thus leaving you close to 200kg short.
Sorry if this is barking up a slightly different tree, and that it doesn't really answer why there is an error in the first place, except to say perhpas that nothing's perfect.
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From what I understand the 737 tank sensors do not compensate for different S.G. of fuel being uploaded, therefore it is common to see the wings showing less than full when they are actually full (valves automatically shut whilst fuelling when the tank is full).
We calculate an uplift litreage using the S.G. and round it up to the next hundred. That becomes a minimum uplift from the bowser, no matter what the guages say in Kgs.
PP
We calculate an uplift litreage using the S.G. and round it up to the next hundred. That becomes a minimum uplift from the bowser, no matter what the guages say in Kgs.
PP
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Grazie amigos
Now for Greek and Turkish ...........................
Paolo - we all know what it NORMALLY is, it is knowing whether the fuel has been undergound at -4 as in Krakov a few years back or in a bowser in Egypt for a day or so at +38. I find 'a bit' over ' and 'rounded up' normally does ok + a bit of economic flying.
Now, did I mention Egyptian......................?
Now for Greek and Turkish ...........................Paolo - we all know what it NORMALLY is, it is knowing whether the fuel has been undergound at -4 as in Krakov a few years back or in a bowser in Egypt for a day or so at +38. I find 'a bit' over ' and 'rounded up' normally does ok + a bit of economic flying.
Now, did I mention Egyptian......................?
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I go back to my previous post and re-iterate, it is down to the sensors not compensating for whatever the density is, unlike on the 75/767 that I have also flown, which do. On them, when the tank says full (valve shuts whilst fuelling) the guage reads full quantity, not short or over, but full.
It matters not what the actual density is because 737 fuel guages will only read correctly at whatever the 'design' density was. Knowing the actual density of the fuel you are loading will still not get your guages to read correctly!
PP
It matters not what the actual density is because 737 fuel guages will only read correctly at whatever the 'design' density was. Knowing the actual density of the fuel you are loading will still not get your guages to read correctly!
PP
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the valves will auto-shut at a preset quantity,not necessary at maximum standard. On our 737 ,classic and NG,the tech's modified this quantity,in order to avoid overfilling the wing
,and now,for ex,the valves on the wing tank on a 300 will close at 4400kg instead of 4600 kg,which was set before.
,and now,for ex,the valves on the wing tank on a 300 will close at 4400kg instead of 4600 kg,which was set before.
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Pete,
Sorry if I was misleading. The procedures I mentioned aren't designed to get the gauges reading right, simply to ensure that you load the correct amount of fuel even though the gauges may not be accurate (usually within a tolerance of 100-200kg).
Don't know if I understood the original question right, though.
Sorry if I was misleading. The procedures I mentioned aren't designed to get the gauges reading right, simply to ensure that you load the correct amount of fuel even though the gauges may not be accurate (usually within a tolerance of 100-200kg).
Don't know if I understood the original question right, though.
Tadex,
It is my experience that fuel is not “lost” when the centre tank is empty in-flight but that the centre tank fuel will indicate up to 200kgs less in the climb due to the body angle. This returns in part when you accelerate above FL100 and all the way once you have established in the cruise. Conversely you may get some re-appearing in the descent if you did not drain the centre tank fully in the cruise. However this will disappear again at approach body angles. I have never got so low on fuel to find out if this +/-200kg is usable or not – and I hope I never do!
As regards fuel tank accuracy & compensation techniques, to quote from my own site: “The classic fuel gauges will compensate for changes in sg and have an accuracy of ±2½%. The NG fuel quantity indicating system uses a microprocessor to analyse a capacitance signal from units in each tank. The signal contains data on fuel quality and temperature, which is then used to calculate density. The processor then sends the fuel-weight signal to the flightdeck displays and FMC with an accuracy of 1%.”
S&L
It is my experience that fuel is not “lost” when the centre tank is empty in-flight but that the centre tank fuel will indicate up to 200kgs less in the climb due to the body angle. This returns in part when you accelerate above FL100 and all the way once you have established in the cruise. Conversely you may get some re-appearing in the descent if you did not drain the centre tank fully in the cruise. However this will disappear again at approach body angles. I have never got so low on fuel to find out if this +/-200kg is usable or not – and I hope I never do!
As regards fuel tank accuracy & compensation techniques, to quote from my own site: “The classic fuel gauges will compensate for changes in sg and have an accuracy of ±2½%. The NG fuel quantity indicating system uses a microprocessor to analyse a capacitance signal from units in each tank. The signal contains data on fuel quality and temperature, which is then used to calculate density. The processor then sends the fuel-weight signal to the flightdeck displays and FMC with an accuracy of 1%.”
S&L
When I first operated the B737-300 for Orion Airways many moons ago I also contemplated this anomoly. My research at the time was that the capacitance type gauges were accurate but I decided to drip the wing tanks having filled them to capacity. (I managed to borrow a step ladder from the friendly refueller at CFU!).
Althought the gauges showed 9,300 kgs the dripsticks showed 9,100 kgs! I managed to persuade the company to lower the full wings fuel figure to 9,100 kgs since the early B737s were very obstacle limited on take off and the 200 kg could be the difference between a tech stop or not!
I suppose the academic way to refuel is to drip the tanks prior to commencing the refuel, obtain the S.G. and calculate the fuel volume required (eg litres), pump the fuel in and then, to be belt and braces, drip the tanks afterwards! A gross error check would then be made by referring to the fuel gauges. In the "olden days" on the B707 we had a flight engineer who did precisely that!
Oh, by the way, never had a problem getting the SG from the refueller in circa 25 years of operating!
Althought the gauges showed 9,300 kgs the dripsticks showed 9,100 kgs! I managed to persuade the company to lower the full wings fuel figure to 9,100 kgs since the early B737s were very obstacle limited on take off and the 200 kg could be the difference between a tech stop or not!
I suppose the academic way to refuel is to drip the tanks prior to commencing the refuel, obtain the S.G. and calculate the fuel volume required (eg litres), pump the fuel in and then, to be belt and braces, drip the tanks afterwards! A gross error check would then be made by referring to the fuel gauges. In the "olden days" on the B707 we had a flight engineer who did precisely that!
Oh, by the way, never had a problem getting the SG from the refueller in circa 25 years of operating!
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Althought the gauges showed 9,300 kgs the dripsticks showed 9,100 kgs!
BTW All airlines I have flown with use a check on lires/kg based on 'standard' SG (0.8) and this must be within 300kg (737). Where it differs from 300kg the actual SG is then used and if still >300kg a drip is performed unless the discrepancy can be explained eg long-term APU usage. I have always found this adequate. I have achieved 9400kg in the wings with very cold fuel (KRK again).
BOAC - its a quite a few years ago now so the memory may be a bit hazy but we filled the wing tanks till the valves shut off - the gauges totalled 9,300 kg but when I dripped the tanks we had 9,100 kgs - now I cannot recall what the calibration is on the dripsticks, so maybe if its kgs that assumes a certain sg?
Agree the best way is calculate the required uplift based on actual sg and pump it in and then do a gross error check.
Agree the best way is calculate the required uplift based on actual sg and pump it in and then do a gross error check.
