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A320: Fuel gravity

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A320: Fuel gravity

Old 14th Jan 2020, 17:39
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Question A320: Fuel gravity

Hello community,

I have a question: Failure of both wing tank pumps on the left side in FL300. Do we always have to apply the fuel gravity procedure to get the fuel? Why cant we just open the crossfeed? and why is there such a big difference between FL300 and FL150? If i was 3 seconds in FL300 i can resume in FL300? I think i did not understand the topic sorry.

Thanks folks!!!!

SW
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Old 14th Jan 2020, 19:04
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Originally Posted by Speedwinner View Post
Hello community,

I have a question: Failure of both wing tank pumps on the left side in FL300. Do we always have to apply the fuel gravity procedure to get the fuel? Why cant we just open the crossfeed? and why is there such a big difference between FL300 and FL150? If i was 3 seconds in FL300 i can resume in FL300? I think i did not understand the topic sorry.

Thanks folks!!!!

SW
Assuming you are going to stay airborne for a while, opening the crossfeed would lead to imbalance, because you would be feeding both engines from the right side tank only.
The reason for selecting cross feed on is to provide positive pressure on the failed side until you get to your adjusted ceiling. Once there you gravity feed with the xfeed closed.
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Old 15th Jan 2020, 04:14
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Originally Posted by Speedwinner View Post
Hello community,

I have a question: Failure of both wing tank pumps on the left side in FL300. Do we always have to apply the fuel gravity procedure to get the fuel? Why cant we just open the crossfeed? and why is there such a big difference between FL300 and FL150? If i was 3 seconds in FL300 i can resume in FL300? I think i did not understand the topic sorry.
Thanks folks!!!!
SW
The refueled tanks on ground are like beer bottles with the cap on. There is lot of air absorbed in it. As the aircraft climbs the atmospheric pressure keeps dropping which is like partially opened cap. The air keeps bubbling out and can create a vapour lock but due to the pressure created by fuel pumps the supply to the engine is not disrupted. By the time the aircraft climbs to FL 300 and above and stays there for at least 30 mts. all dissolved air bubbles out. So if fuel pumps fail now, the FL if above 300 can be maintained. If time spent was less than 30mts then you put the cap back on the bottle by descending to 300(higher pressure), if not climbed to 300 at all then there is lot more air in the fuel and you got to descend to even higher atmospheric pressure of 15000. If only one side pumps fail then you have to see if you are in gravity feed case. If not then initially cross feed for short duration but it will create imbalance. So you will have to compare that with extra consumption at lower level and choose the appropriate course.

Last edited by vilas; 15th Jan 2020 at 06:40.
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Old 15th Jan 2020, 07:20
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Thanks!!!beer bottle is a very good example!
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Old 15th Jan 2020, 07:34
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Originally Posted by vilas View Post
The refueled tanks on ground are like beer bottles with the cap on. There is lot of air absorbed in it. As the aircraft climbs the atmospheric pressure keeps dropping which is like partially opened cap. The air keeps bubbling out and can create a vapour lock but due to the pressure created by fuel pumps the supply to the engine is not disrupted. By the time the aircraft climbs to FL 300 and above and stays there for at least 30 mts. all dissolved air bubbles out. So if fuel pumps fail now, the FL if above 300 can be maintained. If time spent was less than 30mts then you put the cap back on the bottle by descending to 300(higher pressure), if not climbed to 300 at all then there is lot more air in the fuel and you got to descend to even higher atmospheric pressure of 15000. If only one side pumps fail then you have to see if you are in gravity feed case. If not then initially cross feed for short duration but it will create imbalance. So you will have to compare that with extra consumption at lower level and choose the appropriate course.
so if I have a fuel leak and it is stopped by cutting the engine I can use the crossfeed right? How is the feeding then? Is it equal or which tank will empty faster?

and it is better to fly lower to use gravity fuel feeding instead of the crossfeed?
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Old 15th Jan 2020, 09:03
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Originally Posted by Speedwinner View Post
so if I have a fuel leak and it is stopped by cutting the engine I can use the crossfeed right? How is the feeding then? Is it equal or which tank will empty faster?

and it is better to fly lower to use gravity fuel feeding instead of the crossfeed?
I think you are mixing up different issues. In case of fuel leak from engine which stops after shutdown if you need the fuel of the failed side tank then you can cross feed to live side, sure! Follow the cross feed procedure. For this situation there are two procedures one with fuel pumps working and the other pumps inop.
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Old 15th Jan 2020, 12:54
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Originally Posted by vilas View Post
The refueled tanks on ground are like beer bottles with the cap on. There is lot of air absorbed in it. As the aircraft climbs the atmospheric pressure keeps dropping which is like partially opened cap. The air keeps bubbling out and can create a vapour lock but due to the pressure created by fuel pumps the supply to the engine is not disrupted. By the time the aircraft climbs to FL 300 and above and stays there for at least 30 mts. all dissolved air bubbles out. So if fuel pumps fail now, the FL if above 300 can be maintained. If time spent was less than 30mts then you put the cap back on the bottle by descending to 300(higher pressure), if not climbed to 300 at all then there is lot more air in the fuel and you got to descend to even higher atmospheric pressure of 15000. If only one side pumps fail then you have to see if you are in gravity feed case. If not then initially cross feed for short duration but it will create imbalance. So you will have to compare that with extra consumption at lower level and choose the appropriate course.
I must say I will steal Your beer bottles explanation for my sessions as it adds 2 of the 3 favorite things for pilots : fuel and beer. Iíll skip on the third one.
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Old 15th Jan 2020, 13:25
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Agree with Sonicbum. Best analogy I have read about fuel gravity. Thank you Vilas.
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Old 15th Jan 2020, 18:01
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Originally Posted by pineteam View Post
Agree with Sonicbum. Best analogy I have read about fuel gravity. Thank you Vilas.
You are always welcome!
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Old 16th Jan 2020, 21:17
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Note that the procedure L TNK 1+2 pump Lo press does not ask to apply the gravity fuel feed procedure immediately and you can stay with X feed ON until FL 390. Whether or not to go on gravity will depend on the fuel required to continue your flight.

You are feeding the failed side from the functioning side and this is at the price of double fuel consumption on the operating side . It's up to you to continue to FL 390 as long as you have X feed ON . You have to decide until which point you can do that and when you have to say STOP I need that fuel on the operating side and starts gravity fuel feeding procedure ! This is where you switch OFF the Xfeed and apply the gravity fuel feeding procedure because you need the fuel from the operating side. So crossfeed OFF is your decision !

To elaborate more on this subject let's take a practical example:

Flight Time 2:40 Block Fuel 10600 Kg ( 1000 Kg extra) : Outer/700 Left Tank 4600 Center : 0Kg \ Right Tank 4600 Kg\ Outer : 700 Kg

You get Left tank pump 1+2 low pressure as you pass through FL 200. The question is can you continue the flight?
The procedure does not state to apply the gravity fuel feed immediately but only when affected tank fuel is REQUIRED. But when is it required? and when will you decide to apply the gravity fuel feed procedure in order to consume on the failed side ?

Initially you apply the Left Tank PUMP 1+2 fault procedure and put the X feed to ON. Then you get to the point "When TNK Affected fuel required ... TNK affected feed gravity only". But when is it required ? the answer is :when you reach the minimum fuel on the operating side to continue your flight.

You will certainly need the affected tank to continue your flight as you are feeding 2 engines from the right tank. This is where it becomes interesting.

As you pass FL 200 you check the Fuel USED 1+2 and you realise have burnt 1000 Kg so far . You then have on your tanks : Left Inner Tank 4100 Right Inner Tank 4100.

Your planned FL is FL390 and you need to stay 30 minutes on Xfeed after you pass FL300 . In other words, without Xfeeding the left tank will not be used until 30 Minutes after you passed FL 300 . Are you able to do it ?

You check your OFP and it states that you will pass FL 300 after 14 minutes of flight time .

You add 30 minutes to those 14 minutes and you get a total flight time of 44 minutes

You check your OFP at 44 minutes EET and you see a planned fuel burn of 2500 Kg.

You deduct those 2500 Kg from the fuel used at the time of the failure . So 2500 Kg - 1000 Kg = 1500 Kg.

That means you will have to burn additionally 1500 Kg on the right tank from the moment of the failure until you pass FL300 + 30 minutes.

The question is , will you have sufficient fuel on the right tank to continue your flight ? and affter 30 Minutes above FL300 ( 44 minutes total time) what will be left on the right tank ?

4100 Kg ( remaining fuel at the time of the failure) - 1500 Kg = 2600 Kg

Right tank after 30 min above FL 300 will have 2600 Kg + 700 Kg ( Outer tank ) = 3300 Kg . is it enough ?

At FL 300 +30 Minutes ( which is 44 minutes of EET ) you will switch OFF the X Feed and apply the gravity fuel feeding procedure . You can stay at FL 390 and the left tank will be on gravity.

You check the OFP and realise that the minimum fuel requied on board at that time ( 30 Min after FL300 which is EET 44 minutes ) is 6500 Kg that means 3250 Kg on each side and you have 3300 on the right tank !

So you have enough fuel to continue the flight after Xfeeding the failed side for 30 minutes above FL300 .

Because you were crossfeeding distribution will look like this at EET 44 minutes ( 30 minutes after FL300 )
Outer /700 Kg / Inner :4100 Center : 0 Kg \Inner : 2600 Kg \ Outer : 700 Kg
FOB= 8 100 Kg

There you go , you have the fuel to continue your flight but you don't want to be on minimum fuel on the right tank and have the extra fuel on gravity! At least you are able to demonstrate it .

You do not want to continue your flight on gravity only as you might encounter turbulence and experience negative g factor. Gravity procedure states to avoid negative G factor , but how can you avoid it when you encounter turbulence ?

By the way the max imbalance for landing is 1500 Kg and you were within limits but now the decision is yours !

Last edited by Citation2; 16th Jan 2020 at 21:35.
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Old 17th Jan 2020, 04:17
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By the way the max imbalance for landing is 1500 Kg and you were within limits but now the decision is yours !
According to airbus aircraft can be landed with one tank full and other tank empty.

Last edited by vilas; 17th Jan 2020 at 07:46.
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Old 17th Jan 2020, 06:23
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Great post Citation2. It is a comprehensive explanation with regards to fuel considerations for gravity fuel feed.
For the case of center tank is not empty, we can use fuel from it initially, until the inoperative side wing tanks are filled up to 5500kg due to the fuel recirculation. Are there any adverse effects if we continue to use fuel from the center tanks? Would there be a spill resulting from overflow?
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Old 17th Jan 2020, 07:31
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Originally Posted by dream747 View Post
For the case of center tank is not empty, we can use fuel from it initially, until the inoperative side wing tanks are filled up to 5500kg due to the fuel recirculation. Are there any adverse effects if we continue to use fuel from the center tanks? Would there be a spill resulting from overflow?
That will depend on whether centre tanks are new type or old. In new design centre tank cannot feed engines directly. It will have to be through inner tanks only. Center tank fuel keeps emptying into inner tanks automatically when the inner tank fuel reaches the under full sensors. It can also be controlled manually with mode selector in MAN. In the old design the centre tank will not drain into inner tank but feed the engine directly itself first anyway. There is no question of overflow.

Last edited by vilas; 17th Jan 2020 at 08:14.
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Old 17th Jan 2020, 13:43
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Originally Posted by dream747 View Post
For the case of center tank is not empty, we can use fuel from it initially, until the inoperative side wing tanks are filled up to 5500kg due to the fuel recirculation. Are there any adverse effects if we continue to use fuel from the center tanks? Would there be a spill resulting from overflow?
No issue with overflow or spill. During normal ops, the CTR TK fuel is used first as well. The fuel recirculates to the outer tank and overflows to the inner tank. The inner tank should not be completely full, because of the TO being done with fuel from the inner tanks. I assume that the inner tank will not be completely refilled before the CTR TK is empty.
There is also a vent surge tank in the wings that allows for a 2% fuel expansion.

The likelihood of reaching your destination increases if you have fuel in the CTR TK because the fuel in the INR TK is not needed initially. Gravity feeding can be postponed until the CTR TK is empty and the 30 minutes above FL300 are exceeded.
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