OVRD/JETT Main 2&3 Pump indication

[ASCTU744]

I started the "cleared for takeoff" situ and removed AC power, when I turned AC power back on I noticed that the fuel synoptic indicated that the OVRD/JETT pumps were on (see images). After engine start the same indication was displayed until 5-6 minutes after AC power was established (blue armed pump symbols on the synoptic).


Aren't the ovrd/jett pump symbols supposed to be armed (blue)? Or is this an airline specifc config?

[Hardy Heinlin]

You need to refer to a known situ file for a common start of the discussion and then describe your further scenario completely and exactly step by step.

[ASCTU744]

Edited, sorry for the unclear post.

[Hardy Heinlin]

removed AC power, when I turned AC power back on

What exactly did you do?

You need to talk in "checklist language".

"Remove AC power" may mean anything.

[ASCTU744]

https://www.youtube.com/watch?v=FVB6txhQ4Sw

When the aircraft is disconnected from all AC sources (APU, EXT, Engines) and then reconnected to any AC source, the OVRD/JETT pump symbols on the synoptic seem to indicate a (for me) weird indication. After 5 minutes the symbology of the OVRD/JETT pumps recovers to normal (blue color; armed, because the CWT pumps are running). When a new situ is loaded (Let's say Sydney-KLAX) where AC power is established the "bug" reappears.

I hope that my video makes the situation clear.

So in summary:

-Load 'Clear for T/O' situ
-Observe FUEL synoptic
-Disconnect all AC power sources (EXT, APU, ENG)
-The FUEL synoptic lacks info due to loss of sensor data
-Re-apply any AC power source
-Observe green OVRD/JETT pumps on the synoptic, while CWT pumps supply the engines
-Wait 5 minutes
-OVRD/JETT pump symbols become amber (pumps shutting down) and then armed (blue).

Is this a feature of the 747 or an airlines specific option?

Thx in advance,

-Nadir

[Hardy Heinlin]

As far as I recall, this is a Boeing feature (not a bug) and related to certain 5 minute time delays.

You may monitor the timers on: Instructor > Analysis > Miscellaneous at item "CWT delays L;R;Final:"

CWT = Center Wing Tank
L = Timer for left CWT
R = Timer for right CWT
Final = A specifc timer at the end of the entire delay logic

E.g.
"5;5;5" means that all timers have run for 5 minutes (which causes further logic states).
"5;5;0" means that the final timer is reset to zero
"5;5;*" means that the final timer is running
The display just indicates 0, *, and 5. The minutes in between are not displayed.

[ASCTU744]

Thank you,

I usually use PSX for system review after studying 744 systems, but end up learning new things from PSX XD.

This was the case with the IRS 5 minute timer, EEC ALTN mode, this system-logic and much more.

All the best,

Nadir

[John H Watson]

When I load up the "cleared for takeoff" situ, I get green O/J pumps prior to power removal. And the opposite occurs when I cycle power. They turn to blue after approx. 5 minutes.

I have version 10.172.

[ASCTU744]

When I load up the "cleared for takeoff" situ, I get green O/J pumps prior to power removal. And the opposite occurs when I cycle power. They turn to blue after approx. 5 minutes.

I have version 10.172.

Before you opened "cleared for takeoff" did you have a situ loaded where the aircraft isn't on AC power? (I.e. cold and dark). If you did, reloading the situ file should give you blue O/J pumps before removing AC power.

[Hardy Heinlin]

When I load my Basic 004 - Cleared for takeoff.situ, I always get the same correct setting, no matter what situ was loaded before.

The time delay values at situ start are stored in situ files under the variable name "FuelCwtTd". 1200 corresponds to 5 minutes.

The pump status also depends on the flap selection.

[ASCTU744]

When I load my Basic 004 - Cleared for takeoff.situ, I always get the same correct setting, no matter what situ was loaded before.

The time delay values at situ start are stored in situ files under the variable name "FuelCwtTd". 1200 corresponds to 5 minutes.

The pump status also depends on the flap selection.

I tried it again and the previous situ indeed doesn't have an effect on the O/J pumps. Does this delay feature have a technical name I can lookup in the manuals?

[Hardy Heinlin]

I don't have a name for it, but if you have schematics on the fuel system you may find it in the context of the CWT pump control logic.

Timer L is reset to 0 if CWT pump L switch is off, else timer L runs to 5 min.
Timer R is reset to 0 if CWT pump R switch is off, else timer R runs to 5 min.
Final timer is reset if (timer L = 5 min AND pump L low pressure) OR (timer R = 5 min AND pump R low pressure),
else final timer runs to 5 min.

I guess this is to avoid any short term destabilization in the pump system. It also affects the scavenge pump system.
I.e. in case of a short term low pressure it will not immediately reset the system.

[John H Watson]

Before you opened "cleared for takeoff" did you have a situ loaded where the aircraft isn't on AC power? (I.e. cold and dark). If you did, reloading the situ file should give you blue O/J pumps before removing AC power.

I loaded cold and dark, then loaded the takeoff situ. I still get green 2/3 tank O/J pumps on loading the situ file. Without any input, they eventually turned to blue (after a few minutes, perhaps).

Here's the logic from the Schematics...



I'm not sure how to read that logic. Do we assume a 1 output from the OR gate if zero has a 5 minute delay? Or is the output zero anyway?

I know an O/J pump immediately goes to cyan if one CWT is switched on with low CWT fuel.

[Hardy Heinlin]

Just to be sure: What do the outputs 8, 5, 11, 12 do when the related contact to the ground is closed? Do they make the symbol cyan or green?

[John H Watson]

Basically all the outputs do is energise the pump power relays.

The relays do, however, have auxiliary contacts which seem to tell the EIUs when the relays are energised or de-energised. I guess the synoptic colours come from data from various sources (fuel system control cards and relay position sensors)

Unfortunately, the pump circuits are spread over two pages. There are lots of things between 8, 5, 11 & 12 and the main pump power relays, including jettison control relays and CWT pump command relays. Also all aircraft should have the new safety circuits to stop the pumps from running with the fuel level too low or pump pressure low. 

For techies... SSM 28-31-04 page 102.1 Sheet 1 & 2.

[ASCTU744]

I'm not sure how to read that logic. Do we assume a 1 output from the OR gate if zero has a 5 minute delay? Or is the output zero anyway?

The 0/1 logic is explained in a scavenge pump schematic:

Just to be sure: What do the outputs 8, 5, 11, 12 do when the related contact to the ground is closed? Do they make the symbol cyan or green?

Even if those outputs command the pumps to run it still doesn't really tell anything about synoptic status, when the APU runs the synoptic symbols for boost 2 aft (or 2&3 aft) also remain(s) blank when the respective switches are off.

[ASCTU744]

The FIM mentions this feature.

[Hardy Heinlin]

The transition to "0" after 5 minutes is also modelled in PSX. I think PSX agrees with all these descriptions and diagrams. You need to wait 5 minutes for the change from green to cyan.

[ASCTU744]

The transition to "0" after 5 minutes is also modelled in PSX. I think PSX agrees with all these descriptions and diagrams. You need to wait 5 minutes for the change from green to cyan.

I suppose the timer resets after:
-Power loss to the FSMC
-CWT fuel increase (when moving the slider Instructor>Aerodynamics>Fuel Qty up)

I just found out about the second one.
Is this a correct conclusion?

[Hardy Heinlin]

The timers have no effect when the control systems are unpowered.

The CWT fuel quantity may affect the low-pressure status of the pumps which affects the timer logic.