EXDAC
July 15, 2025, 19:51:00 GMT
permalink Post: 11923215

Quote:

Originally Posted by Blacksheep

Switches, schnitzels. There\x92s much talk about the switches being physically moved, but there\x92s a lot of digital jiggery-pokery between those switches and the EECs

Please illustrate your point with a schematic or block diagram from Boeing or some other other credible source. Perhaps first showing that the EEC is in the path between the fuel switch and the HPSOV.

It's never too late for any of us to learn something new about the fuel valve control architecture and sometimes facts may be hard to separate from the hamster droppings.

Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Switches High Pressure Shutoff Valve

EXDAC
July 16, 2025, 00:02:00 GMT
permalink Post: 11923356

Quote:

Originally Posted by T28B

From further back in this thread, and as addressed in previous threads, as to what the SAIB was addressing.

Can we please put this to rest?

References to the SAIB and to inadvertent switch activation should perhaps be put to rest but the quote you use is not a good basis for doing so.

The SAIB describes a defect in the installation of the locking mechanism that, if present, could allow inadvertent activation.

The SAIB does not state that the switch will snap back into cutoff.

The SAIB inspection procedure would not be possible if the switch snapped back to cutoff -

"1) Inspect the locking feature of the fuel control switch to ensure its engagement. While the airplane is on the ground, check whether the fuel control switch can be moved between the two positions without lifting up the switch. If the switch can be moved without lifting it up, the locking feature has been disengaged and the switch should be replaced at the earliest opportunity."

The person you quoted has either seen a different switch defect or the authors of the SAIB didn't know what they were talking about.

Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Switches Special Airworthiness Information Bulletin

EXDAC
July 16, 2025, 15:04:00 GMT
permalink Post: 11923790

Quote:

Originally Posted by Dimitris

Regardless of what happened and why regarding the switches going to off, there is now a documented failure mode of the system that needs mitigation (?):
Fuel switches off at less than XXX ft lead to unrecoverable AC.

I have seen no documentation of such a failure mode. There are many flight crew actions that can cause loss of the aircraft but these are not failure modes. In any event the need for mitigation of a catastrophic failure mode depends on the probability of its occurrence.

Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Switches

EXDAC
July 16, 2025, 15:09:00 GMT
permalink Post: 11923795

Quote:

Originally Posted by GarageYears

The problem here with inhibiting the fuel cutoff is that what happens if you have an engine fire less than your XXX ft? You still need to turn off that engine, right? Now you could say turning off BOTH should be inhibited... what if they are both on fire and there's a nice flat space in front of you?

Pull the fire handle(s)? Or do those that want the fuel switches to be inhibited also want the fire handles to be inhibited?

Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Fuel Cutoff Switches

EXDAC
July 16, 2025, 19:12:00 GMT
permalink Post: 11923945

Quote:

Originally Posted by GarageYears

The original SAIB came about because some switches on 737s were found to be installed incorrectly. When the switches were installed incorrectly they would not hold in the RUN position and would immediately snap back into CUTOFF.

Did you observe this "snap back" yourself?

Nothing in the SAIB suggests that this would happen with an improperly installed switch. In fact the SAIB inspection calls for the switch to be placed in both positions to test the locking mechanism. How would that be possible if the switch "snapped back"?

"1) Inspect the locking feature of the fuel control switch to ensure its engagement. While the airplane is on the ground, check whether the fuel control switch can be moved between the two positions without lifting up the switch. If the switch can be moved without lifting it up, the locking feature has been disengaged and the switch should be replaced at the earliest opportunity."

Subjects (links are to this post in the relevant subject page so that this post can be seen in context): RUN/CUTOFF Special Airworthiness Information Bulletin

EXDAC
July 17, 2025, 00:16:00 GMT
permalink Post: 11924069

Quote:

Originally Posted by GroundedSpanner

tdracer excellently summarised. But I'll confirm.
The switch is directly connected to the coils of a latching relay. That latching relay is directly connected to the coils of the spar valve. There is no digital logic device in the way,
The position of the switch is monitored (through a different set of contacts) by the EAFR (twice). The position of the spar valve is monitored by the EAFR.
Thus the recorder sees (twice) that the switch is moved, and that the valve moved in response.

The control path the engine HPSOV is likely far more important in his case as that valve is much faster acting the slow motor driven spar valve.

Subjects (links are to this post in the relevant subject page so that this post can be seen in context): EAFR High Pressure Shutoff Valve Spar Valves

EXDAC July 15, 2025, 19:51:00 GMT permalink Post: 11923215	Quote: Originally Posted by Blacksheep Switches, schnitzels. There\x92s much talk about the switches being physically moved, but there\x92s a lot of digital jiggery-pokery between those switches and the EECs Please illustrate your point with a schematic or block diagram from Boeing or some other other credible source. Perhaps first showing that the EEC is in the path between the fuel switch and the HPSOV. It's never too late for any of us to learn something new about the fuel valve control architecture and sometimes facts may be hard to separate from the hamster droppings. Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Switches High Pressure Shutoff Valve
EXDAC July 16, 2025, 00:02:00 GMT permalink Post: 11923356	Quote: Originally Posted by T28B From further back in this thread, and as addressed in previous threads, as to what the SAIB was addressing. Can we please put this to rest? References to the SAIB and to inadvertent switch activation should perhaps be put to rest but the quote you use is not a good basis for doing so. The SAIB describes a defect in the installation of the locking mechanism that, if present, could allow inadvertent activation. The SAIB does not state that the switch will snap back into cutoff. The SAIB inspection procedure would not be possible if the switch snapped back to cutoff - "1) Inspect the locking feature of the fuel control switch to ensure its engagement. While the airplane is on the ground, check whether the fuel control switch can be moved between the two positions without lifting up the switch. If the switch can be moved without lifting it up, the locking feature has been disengaged and the switch should be replaced at the earliest opportunity." The person you quoted has either seen a different switch defect or the authors of the SAIB didn't know what they were talking about. Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Switches Special Airworthiness Information Bulletin
EXDAC July 16, 2025, 15:04:00 GMT permalink Post: 11923790	Quote: Originally Posted by Dimitris Regardless of what happened and why regarding the switches going to off, there is now a documented failure mode of the system that needs mitigation (?): Fuel switches off at less than XXX ft lead to unrecoverable AC. I have seen no documentation of such a failure mode. There are many flight crew actions that can cause loss of the aircraft but these are not failure modes. In any event the need for mitigation of a catastrophic failure mode depends on the probability of its occurrence. Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Switches
EXDAC July 16, 2025, 15:09:00 GMT permalink Post: 11923795	Quote: Originally Posted by GarageYears The problem here with inhibiting the fuel cutoff is that what happens if you have an engine fire less than your XXX ft? You still need to turn off that engine, right? Now you could say turning off BOTH should be inhibited... what if they are both on fire and there's a nice flat space in front of you? Pull the fire handle(s)? Or do those that want the fuel switches to be inhibited also want the fire handles to be inhibited? Subjects (links are to this post in the relevant subject page so that this post can be seen in context): Fuel (All) Fuel Cutoff Fuel Cutoff Switches
EXDAC July 16, 2025, 19:12:00 GMT permalink Post: 11923945	Quote: Originally Posted by GarageYears The original SAIB came about because some switches on 737s were found to be installed incorrectly. When the switches were installed incorrectly they would not hold in the RUN position and would immediately snap back into CUTOFF. Did you observe this "snap back" yourself? Nothing in the SAIB suggests that this would happen with an improperly installed switch. In fact the SAIB inspection calls for the switch to be placed in both positions to test the locking mechanism. How would that be possible if the switch "snapped back"? "1) Inspect the locking feature of the fuel control switch to ensure its engagement. While the airplane is on the ground, check whether the fuel control switch can be moved between the two positions without lifting up the switch. If the switch can be moved without lifting it up, the locking feature has been disengaged and the switch should be replaced at the earliest opportunity." Subjects (links are to this post in the relevant subject page so that this post can be seen in context): RUN/CUTOFF Special Airworthiness Information Bulletin
EXDAC July 17, 2025, 00:16:00 GMT permalink Post: 11924069	Quote: Originally Posted by GroundedSpanner tdracer excellently summarised. But I'll confirm. The switch is directly connected to the coils of a latching relay. That latching relay is directly connected to the coils of the spar valve. There is no digital logic device in the way, The position of the switch is monitored (through a different set of contacts) by the EAFR (twice). The position of the spar valve is monitored by the EAFR. Thus the recorder sees (twice) that the switch is moved, and that the valve moved in response. The control path the engine HPSOV is likely far more important in his case as that valve is much faster acting the slow motor driven spar valve. Subjects (links are to this post in the relevant subject page so that this post can be seen in context): EAFR High Pressure Shutoff Valve Spar Valves

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