Boeing 737 - Trailing Edge Flaps & TE Flaps Bypass valve
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Boeing 737 - Trailing Edge Flaps & TE Flaps Bypass valve
Hello everybody ! 
I am making a 737 type rating and I realized that the way the FCOM describes the Trailing Edge flaps may be "oversimplified". It is probably not a big matter in terms of operational consequences, but I would like to share what I've found to see what you think about it :
As a quick reminder, there are two important parts to control the trailing edge flaps:
- The Trailing Edge Flap Control which, following a flap selection, directs the hydraulic pressure from system B to a
- Trailing Edge Drive Unit which rotates some torsion links to extend or retract the flaps.
In between the two (the TE Flap Control and the TE Drive Unit) there is a so called "TE Flap Bypass valve", which is used when an asymmetry / a skew condition is detected to remove hydraulic power to the TE Drive Unit to prevent the flaps from operating.
The issue I found is that in the FCOM, Boeing presents the Bypass valve as being in line between the TE Flap Control and the TE Drive Unit, so to remove the hydraulic power to the TE Drive Unit you need to CLOSE the bypass vale. And that's exactly what the Boeing FCOM says.
But if it is called a BYPASS valve, it means it has to BYPASS something right ?
In another (unofficial) publication, The 737 Handbook, the system is however presented with a bypass valve "in parallel" with the TE Drive Unit. To remove the power, the bypass valve therefore OPENS to allow hydraulic fluid return without actuating the TE Drive unit.
I suspect The 737 Handbook is right, as the valve is called a BYPASS valve.
I think Boeing simplified its system description in the FCOM (no return lines on schematics, in descriptions...), so the "parallel" bypass valve became a simple "in line" valve, and it operates to the opposite of what it actually does in the actual aircraft.
Operationally speaking it doesn't make a big difference, the idea being that hydraulic power is removed "somehow". But talking about this system now appears trickier than I initially thought, as it can lead to some confusion concerning the valve position.
I wished I had a 737 AMM !
What are your thoughts ? Am I right to think that in the actual aircraft, the bypass valve is mounted "in parallel" with the TE Drive Unit ?
ps: I had a small sketch ready to illustrate what I'm saying but unfortunately I don't have enough "seniority" on PPRuNe to post it, so I hope you understood what I meant !
I am making a 737 type rating and I realized that the way the FCOM describes the Trailing Edge flaps may be "oversimplified". It is probably not a big matter in terms of operational consequences, but I would like to share what I've found to see what you think about it :
As a quick reminder, there are two important parts to control the trailing edge flaps:
- The Trailing Edge Flap Control which, following a flap selection, directs the hydraulic pressure from system B to a
- Trailing Edge Drive Unit which rotates some torsion links to extend or retract the flaps.
In between the two (the TE Flap Control and the TE Drive Unit) there is a so called "TE Flap Bypass valve", which is used when an asymmetry / a skew condition is detected to remove hydraulic power to the TE Drive Unit to prevent the flaps from operating.
The issue I found is that in the FCOM, Boeing presents the Bypass valve as being in line between the TE Flap Control and the TE Drive Unit, so to remove the hydraulic power to the TE Drive Unit you need to CLOSE the bypass vale. And that's exactly what the Boeing FCOM says.
But if it is called a BYPASS valve, it means it has to BYPASS something right ?
In another (unofficial) publication, The 737 Handbook, the system is however presented with a bypass valve "in parallel" with the TE Drive Unit. To remove the power, the bypass valve therefore OPENS to allow hydraulic fluid return without actuating the TE Drive unit.
I suspect The 737 Handbook is right, as the valve is called a BYPASS valve.
I think Boeing simplified its system description in the FCOM (no return lines on schematics, in descriptions...), so the "parallel" bypass valve became a simple "in line" valve, and it operates to the opposite of what it actually does in the actual aircraft.
Operationally speaking it doesn't make a big difference, the idea being that hydraulic power is removed "somehow". But talking about this system now appears trickier than I initially thought, as it can lead to some confusion concerning the valve position.
I wished I had a 737 AMM !
What are your thoughts ? Am I right to think that in the actual aircraft, the bypass valve is mounted "in parallel" with the TE Drive Unit ?
ps: I had a small sketch ready to illustrate what I'm saying but unfortunately I don't have enough "seniority" on PPRuNe to post it, so I hope you understood what I meant !
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When the bypass valve is in the normal position, it lets hydraulic power from the flap control valve go to the TE flap hydraulic motor. When the bypass valve is in the bypass position, it connects the hydraulic lines on both sides of the hydraulic motor to each other.
This stops operation of the hydraulic motor, prevents a hydraulic lock, and lets alternate flaps operation backdrive the hydraulic motor.
This stops operation of the hydraulic motor, prevents a hydraulic lock, and lets alternate flaps operation backdrive the hydraulic motor.
Just a general point: The system schematics and diagrams made available to pilots are simplified.
Pilots don’t need to see or know about every hydraulic joint, every sub-bus bar, or every bracket and pulley. We need to have an overall understanding of what the system does, how it works, and what the consequences are if a part fails.
So a hydraulic “bypass valve” might be shown as an in-line valve, or have no alternate fluid return, but its function should be clear enough for our purpose of flying the plane
A case in point is the two electrical diagrams of the 737 Stab trim circuits posted on one of the MCAS threads.
Pilots don’t need to see or know about every hydraulic joint, every sub-bus bar, or every bracket and pulley. We need to have an overall understanding of what the system does, how it works, and what the consequences are if a part fails.
So a hydraulic “bypass valve” might be shown as an in-line valve, or have no alternate fluid return, but its function should be clear enough for our purpose of flying the plane
A case in point is the two electrical diagrams of the 737 Stab trim circuits posted on one of the MCAS threads.
