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One for the fuel systems specialist.

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One for the fuel systems specialist.

Old 24th Nov 2017, 21:36
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RMC
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One for the fuel systems specialist.

As an ex Test Engineer On Airbus and British Aerospace 125 I post this here because I suspect it is the only place I might gain an answer to this question. I currently fly the 787 and there is a debate in the instructor community as to whether we should have the fuel system schematic up as centre tank contents approach low and the centre tank pumps are selected off.
On the leave the schematic hidden side they say the aircraft will tell you if there is something wrong (fuel low pressure waning) they believe that because the main wing tank pumps are permanently selected on they are running throughout the flight. That the main tank fuel lines are permanently pressurised by the wing tank pumps (but fuel only comes out of the centre tank first because centre tank pump pressure is higher).

Whilst it is clear the centre tank pumps will put out a higher pressure than the wing tank pumps when all are operating the 787 Flight Crew Operations​ Manual​ schematic description says
“Fuel Pump​(green rectangle)​ - Fuel​ pump switch is ON.​
Fuel Pump​(green rectangle ​with​ line)​ - Fuel​ pump switch is ON and​ pressure ​sensor detects​ pressure greater than 7 to 8 ​ psig” this implies that when the centre tanks are running and there is no green line coming from the wing tank pumps those lines are unpressurised.

Interested in both general and type specific thoughts.
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Old 26th Nov 2017, 09:54
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RMC, a generic view would consider that just because something can be displayed, should it be; particularly if there other means to alert a condition which requires action.
Guidance for designing display formats includes the importance of identifying those items which declutter the picture, both physically and mentally. The human has a great capacity to ‘see’ many things at the same time, but due to the process of perception, not to understand everything or consider the importance of each, thus the risk of subconscious mental clutter or distraction.
For a fuel tank display, a brief flicker in change of fuel quantity can distract the mind from what may be a much more important task.

Too often operators generate SOPs because in isolation they appear to be a good idea. Unfortunately in many modern aircraft with highly integrated systems and displays, situations can be encountered which have not been or cannot be thought through beforehand.
The general recommendation is to follow the manufacturers guidelines or ask if none are apparent.
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Old 26th Nov 2017, 17:03
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Too often operators generate SOPs because in isolation they appear to be a good idea. Unfortunately in many modern aircraft with highly integrated systems and displays, situations can be encountered which have not been or cannot be thought through beforehand.
The general recommendation is to follow the manufacturers guidelines or ask if none are apparent.
Very good advice.

However, I think that there must be a degree of latitude allowed, precisely because ‘situations can be encountered which have not been or cannot be thought through beforehand’.

Most guys and gals in my outfit display the Fuel Synoptic when switching off the CTR pumps in our 787s, even though there is no requirement so to do.

There is always a slight ‘frisson’ during the brief period after they have been switched off and before the main pumps have taken over, when there are no ‘green lines’ going to the engines. I guess that illustrates the time delay between sensing and displaying.

Another limitation inherent in such displays is that the ‘flow lines’ in electric, hydraulic, pneumatic and fuel systems generally reflect the position of switches and valves and do not necessarily indicate ‘flow’ within the system. So for the 787 fuel system, the presence of a green flow line indicates that the pump is on and that pressure of 7 to 8 psig is sensed. It does not, in itself, confirm that fuel is actually flowing in the line.

An example where non-display of the synoptic is standard is the Flight Controls display. After engine start, the flight controls are exercised but the synoptic is not displayed, the theory being that any malfunction will be annunciation by EICAS. It could also be that the display may not reflect totally the full movement of all the control surfaces. I remember a similar issue on the 747-400, where the spoiler display only reflected the position of two (non-symmetrical) spoilers in flight. The resultant effect indicated that only one spoiler was actually moving, which was incorrect.

To sum up; I would encourage crews to display and take notice of any information that enhances SA, provided that they are aware of any limitations inherent in that display.
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