Fuel Imbalance and Transfer Logic in a Multi-Tank Transport
A four-engine transport is in cruise at FL350 with a total fuel of 18 000 kg. The left inboard tank indicates 4200 kg, the left outboard 1800 kg, the right inboard 3900 kg, and the right outboard 2100 kg. The fuel management system is in automatic mode, transferring from outboard to inboard tanks to maintain balance. The left inboard quantity is decreasing faster than the right inboard, and the left outboard is not transferring. The crew notes a left inboard low-quantity caution at 4000 kg. The aircraft has a crossfeed capability and a manual transfer mode. The imbalance limit is 1000 kg between left and right total. The crew must decide whether to crossfeed, manually transfer, or continue.
1. Calculate the left-right imbalance and determine whether it exceeds the limit, and explain the significance of the outboard tank not transferring.
Read the worked answer
The left total is 4200 plus 1800 equals 6000 kg. The right total is 3900 plus 2100 equals 6000 kg. The total imbalance is zero, so the aircraft is laterally balanced despite the individual tank differences. However, the left inboard is decreasing faster than the right inboard, which means the imbalance will develop over time. The left outboard not transferring is the root cause: the automatic transfer logic is not moving fuel from the outboard to the inboard, so the left inboard will continue to fall. The low-quantity caution at 4000 kg is a warning that the left inboard is approaching a level at which the engine feed may be compromised. The significance is that the automatic system has failed in one branch, and the crew must intervene before the imbalance develops. The judgement is to investigate the transfer fault and use manual transfer or crossfeed to correct the trend.
2. Explain the options of crossfeed and manual transfer, and state which is appropriate given the fault.
Read the worked answer
Crossfeed connects the left and right fuel systems so that engines can feed from either side. It is used to correct an imbalance by feeding both sides from the heavier tank. Manual transfer moves fuel from the outboard tank to the inboard tank within the same side. In this case, the problem is not an imbalance between left and right but a failure to transfer from the left outboard to the left inboard. Crossfeed would not fix the transfer fault and could complicate the situation by moving fuel across the aircraft. Manual transfer is the appropriate action because it directly addresses the failed transfer. The crew should select manual transfer for the left outboard to left inboard and monitor the quantities. If manual transfer also fails, the crew should consider crossfeed to keep the left engines fed from the right side, but that would create an imbalance. The judgement is to use manual transfer first, then crossfeed only if the left inboard continues to fall.
3. State the decision logic for continuing versus diverting if the manual transfer does not restore the left inboard quantity.
Read the worked answer
If manual transfer does not restore the left inboard quantity, the left inboard will continue to fall toward the low-quantity caution and eventually the engine feed limit. The crew should then use crossfeed to feed the left engines from the right side, which will create a left-right imbalance. The imbalance limit of 1000 kg will be reached as fuel is consumed asymmetrically. The crew should monitor the imbalance and plan a diversion before the limit is exceeded. The decision logic is: if manual transfer works, continue; if it does not, crossfeed and divert to the nearest suitable airport. The diversion should be planned while there is still enough fuel to reach it with reserves. The crew should not wait for the low-quantity caution to become a low-pressure warning. The judgement is to divert if the transfer fault cannot be corrected, because the fuel management system is degraded and the aircraft is losing lateral balance.