A. 加油总管由一个单一的总管组成，其上安装有进气口适配器以及飞机每个油箱的单独控制阀和单向阀。在加油操作期间，加油总管控制并分配从单一进气口流向飞机所有油箱的燃油流动。
B. 加油总管安装在右翼主梁的前侧。它通过插入梁孔并由螺母固定的燃油管路接头端部固定在机翼梁上。加油和服务的通道通过机翼固定前缘上的一个铰链门提供。
C. 总管的外部连接通过进气口适配器完成，这些适配器可以接受标准的MS29520-1翼下加油喷嘴以及所有型号的Parker Commercial Nozzle F100。适配器端口由一个内部阀座组件密封，该组件通过配合喷嘴的操作打开以实现流动。除了进气口（适配器）之外，总管还包含四个燃油管路接头端部。其中三个接头端部连接到飞机的各个油箱。第四个接头端部通过一个截止阀连接到一个外部排放管路，以便在加油操作期间将溢出的燃油排出飞机。
A. 从总管到各个燃油箱的燃油流动由每个油箱的单独控制阀控制。这些控制阀通常处于关闭状态，并通过驾驶舱燃油指示器操作的相同传感器发出的电信号打开。当每个油箱被加满时，传感器会中断电信号，相应的控制阀会关闭流向其油箱的燃油。每个阀门上的手动超控按钮可用于加满油箱，或在电气故障时操作阀门。
B. 每个燃油箱管路中还包含一个单向阀，可防止燃油从油箱回流。这一特性使得在无需排空系统燃油的情况下，可以拆卸和更换控制阀、适配器或整个总管。
1) Boeing has no objection to XXX waiting until the next maintenance opportunity to inspect the fuel tank for the missing flappers.
2) Boeing has now received several new reports regarding the fueling manifold check valves. Boeing analysis of two broken check valves showed that the parts failed due to fatigue. Two of these valves were from airplanes delivered after 2003. We are considering revising the Ref /A/ FTD, but we do not have enough information to provide any fleet wide recommendations. Boeing is working with the manufacture to understand these recent failures and will release new service information as necessary.
3) Note that the Ref /A/ FTD recommends that operators look for separated flappers and report any finding when they do a fuel tank entry. Ref /A/ does not require airlines to inspect the fueling manifolds unless a flapper is found in the fuel tank.
Unquote
The following was also sent to another airline with a similar condition
Quote 737NG-FTD-28-03004, dated 19 Sep 2014 contains the latest information available for locating missing check valve flappers. However, we recommend the inspections also be extended to the areas near the Fuel Quantity Indication System (FQIS) tank units to make sure the flapper parts will not come in contact with the tank units as contact with a tank unit could result in erratic fuel quantity indication. XXX Airlines should also be aware that loose parts in the fuel tank may chafe against the tank structure over time. This chafe location could be more prone to corrosion if the primer is removed from the structure.
Additionally, Boeing would like to note that the fuel boost pump inlet screen will protect the boost pump and the engine fuel system from FOD larger than a 1/4 inch. If after a thorough tank inspection for the loose / missing parts is completed and no loose parts are found Boeing would have no objection to returning the datum aircraft to service.
An operator reported finding two flappers lying inside the center tank. See attached photo “Broken Flapper” and item number 120 IPL in reference CMM. These flappers had broken off of the Center Tank refueling check valve and were found inside the center tank. The flappers were discovered while performing unrelated service bulletin work in the center tank. Multiple valves that showed signs of wear have been analyzed at Parker.
Parker changed the manufacturing process of the flapper from cast to machined. This change was delivered on new 737NG airplanes sometime in 2000 or 2001. Boeing cannot determine the exact incorporation point of the machined check valve assembly because the part serial number is not available for airplanes delivered before Jan 2003. Refuel manifold P/N 2670150-7 (Boeing P/N 10-61393-7) serial number 4034 through 4681 were delivered with the machined flat check valve assemblies. See the reference Parker Service Information Letter for more information regarding this change. The part number of the check valve remained the same when inlet was changed from cast to machined flat and to “V” shaped machined inlet.
It is suspected that the flat machined configuration has sharper (less rounded) edges which may cause the fuel flow around the flappers to become more turbulent. During high flow rates the flappers may bang against each other more severely thereby causing damage to the PTFE washers and the seat, and after extended exposure to this condition, the flappers may separate from the check valve assembly. Several years after the change from cast to machined flat flapper assembly, the check valve inlet was changed by adding a “V” shaped inlet to minimize turbulence at the valve. See attachments for the previous and the current valve inlet type. Manifolds with the “V” shaped check valve inlet was delivered on 737NG airplanes starting in mid 2003.
Status Description
The machined check valve assembly with the “V” shaped inlet has been delivered on approximately 4000 manifolds. Some of these manifolds have been in-service for more than 10 years. Boeing received several reports of failed check valve flappers with the “V” shaped inlet. Parker tested different flapper materials to reduce the wear of the flappers. Aluminum 7075 was selected as the new material for the flappers. This material change was made starting with production refuel manifold P/N 2670150-7 (Boeing P/N 10-61393-7) serial number 9362 and above. Since the part number of the refuel manifold was not changed, there is no “and on” airplane line number for this change. Refuel manifolds with the improved flappers were delivered in Boeing production starting in September 2014.
Interim Action
Parker completed flapper material evaluation.
Final Action
The check valve flapper material was changed to aluminum 7075. This change is included in production refuel manifolds P/N 2670150-7 serial number 9362 and on. Airplane delivery with the improved fuel check valve flapper began in September 2014.
Operator Action
While performing other work in the center tank, operators are encouraged to look for separated flappers and report any findings. Also, operators are encouraged to inspect the flapper and seat condition when the valves are removed for maintenance.
Boeing received reports of difficulty finding the missing flappers. Below is a description of where in the fuel tank the flappers are typically found:
Left Main Tank
Flappers are found just outboard of left wing rib 5 near the refuel discharge line just aft of the front spar or in the refuel line near the check valve. See IPC 28-20-51-01 items 130 and 105 for the refuel tube and check valve.
Center Tank
There are three locations where fuel is discharged when refueling the center tank. Two locations are close together just inboard of the side of body rib for the right wing in the main part of the center tank. This tube shows in IPC 28-21-52-06, item 70. One discharge port is located aft of the aft span wise beam and the other is in the same tube just forward of the aft span wise beam in the body part of the center tank. Boeing also notes that the center tank includes the inboard part of the wing out to rib 5. This is referred to as the cheek tank. There is a third refuel discharge port just out board of rib 4 near the rear spar of the right wing just outboard of wing station 180. The third discharge port shows in IPC 28-20-51-03, Item 230. This discharge port can be accessed through wing access panel 631BB.
Right Main Tank
Flappers are found at or near the refuel discharge tube or in the tube. This tank has a very short refuel tube, about 3 inches, and a diffuser. The fuel discharges in the same bay as the refuel manifold near wing station 353. See IPC 28-20-51-04 for an illustration of the right main tank refuel discharge tube.
