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 20:35.