自有案例+网络公众号+FTD 737NG-FTD-28-03004
2025年4月，有飞机（机龄23年）加油站油箱单向活门碟片丢失。特展开说明。
一、基本原理
单向活门位于加油站后部
对应的加油关断活门和单向活门（瓣分离）。
单向活门正反面。
总管活门工作原理：
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。
二、单向活门失效
执行燃油箱相关工作，拆除油箱盖板后，执行油箱内部检查或恢复盖板前，需检查外来物，预防FOD，如3C检“57-160-02-01右外机翼接近口”工卡等，多次发现FOD：银色半圆形塑料或铝制蝶阀片，背面为橡胶材质的黑色，有两个接耳，检查发现为中央油箱加油出口单向活门：件号：2670137或2670137-101。
该问题波音机队历史上多次出现，为产品设计缺陷，也进行过改进，但效果不明显。参见FTD 737NG-FTD-28-03004。
1、从厂家的表述看，首先将制造工艺从铸造修改为机加工（2001年）。
2、后来又怀疑平板机加工配置具有更锋利（不那么圆润）的边缘，这可能导致燃油在阀瓣周围流动时更加湍流。在高流量时，阀瓣可能会更严重地相互碰撞，从而导致PTFE垫圈和阀座损坏，经过长时间暴露在这种情况下，阀瓣可能会从检查阀组件上脱落。在从铸造改为机加工平板阀瓣组件的几年后，通过增加一个“V”形进气口来改变检查阀的进气口，以最小化阀门处的湍流。从2003年中期开始，737NG飞机开始交付带有“V”形检查阀进气口的总管。
3、Parker公司测试了不同的阀瓣材料，以减少阀瓣的磨损。选择了7075铝合金作为阀瓣的新材料。这一材料变更从生产加油总管件号2670150-7（波音件号10-61393-7）序列号9362及更高版本开始实施。带有改进阀瓣的加油总管从2014年9月开始在波音生产中交付。
三、根本原因分析
三个油箱的加油单向活门件号一样，都是2670137，或2670137-101，因此，打开压力相同。三个油箱的加油关断活门件号一样，即控制活门，都是2670136，因此，打开压力相同。参考CMM，总管的压力功率和中央油箱以及主油箱的流量差异如下所示。
那么是怎么来实现流量的差异，主要是通过下游管路。
1、1号油箱下游为1.5英寸的直管，通过2号油箱、中央油箱，直达1号油箱，出口有一个沿管道方向的活门32C194-3（弹簧加载关位的单向活门），和垂直管道方向的活门60B92604-6（排出集水和管路余油）。
2、2号油箱单向活门下游是一节外径1.5英寸的直管（344A2401-1），带一个扩散孔DIFFUSER – ORIFICE（344A2001-3，1.050 TO 1.060）直接伸在2号油箱里。
3、中央油箱先是一段1.5英寸的直管，而后焊接一段喇叭管，之后变为2.5英寸的直管，直到中央油箱的右侧出口（右翼4号肋板外侧，图中C处275，其中出口为2.0英寸的通管），由于一部分燃油已从此口流入中央油箱，流量减小，所以此后275经过一节喇叭管，之后变为2.0英寸弯管，直至中央油箱加油管路的中央出口，出口有两个，都是管径为1.5inch的通管（无活门或滤网等部件），在右侧翼身肋板内侧，前后隔板各一个。
可以看到中央油箱加油管路更粗，出口更多，且均为通管（出口没有单向活门或DIFFUSER ORIFICE），以此保证了控制活门打开后更大的流量，但三个油箱出口单向活门处管径相同，所以中央油箱处的流速更快，大流速意味着对单向活门的冲击更大，开度更大，也更容易来回摆动磨损，所以，中央油箱出口单向活门更容易损坏。
四、碟片常见丢失位置
1、左右大翼2号肋板1号油尺附近
此处是中央油箱翼身段的最低点，左翼身段虽无出油口，但蝶阀较薄，加油时会随油液流动到此。从大翼最内侧的油箱盖板531AB、631AB接近，向内看检查，可能在加强筋stringer接头下。
2、中央油箱机腹舱内
机腹段前后分三个隔断，空间较大，但比较空旷，注意检查肋板、纵梁、燃油泵抽油口等地方。从空调舱内中央油箱盖板131AB接近，需拆下部分管道或部件。
3、其他翼肋附近
常见翼身结合肋板（SOB Rib，side of body rib，即1号肋板）外侧附近，需从531AB、631AB进入油箱内部，穿过2、3号肋板的开口处向内检查。
五、案例说明
有飞机反映2号油箱溢油后重新加油完成后，发现在所有活门和泵关闭的情况下，2号油箱向中央油箱自动倒油。从8600LB降至7500LB后停止。
一）、验证过程：
1、 将中央油箱人工倒油至2号油箱（打开抽油活门，2号加油活门，中央R泵），结果为2号油箱油向中央油箱流入。
2、 打开2号油箱两个泵，中央R油箱泵，无油量传输。
3、 将1号油箱人工倒油至中央油箱，可能正常倒油。但完成倒油关闭所有泵和活门后，2号油箱又开始自动向中央油箱倒油，倒油至5700左右自动停止。
4、将1号油箱人工倒油至2号油箱，发现打开泵和活门后，油量不传输。
5、 （1）打开1号油箱泵（未开交输活门），中央加油活门，抽油活门，结果为2号油箱向中央油箱倒油
（2）只打开中央加油活门、抽油活门，结果为2号油箱向中央油箱倒油
（3）只打开中央加油活门，结果为油量不传输
（4）只打开抽油活门，结果为2号油箱向中央油箱倒油
6、 拉出左右大翼双侧通气油箱旁通活门，正常，无大量油液渗漏。
7、单开抽油活门，加油活门全都在关位，泵也都关了，确认30分钟串油中央油箱810磅
8、交输活门打开，1号和2号油箱的4个泵都打开，抽油活门打开,加油活门全部关闭。3分钟，330磅串至中央油箱
9、 中央油箱倒油至左油箱，正常，倒油左油箱至7710磅。2号油箱油量不变。
10、加油车加油2号油箱，只开2号油箱加油活门，2号油箱加了230磅，中央油箱加了4080磅
11、循环往复作动中央油箱加油活门及超控手柄，关闭所有活门，关闭所有加油泵，单独打开抽油活门，观察10分钟，无串油。 13 打开抽油活门，打开1号油箱双泵，打开交输活门，所有加油活门关闭。10S后1号油箱油开始往中央油箱串，确认中央油箱加油活门失效。
二）、检查结果
中央油箱和右主油箱的单向活门和加油活门均失效。中央油箱单向活门双碟均丢失不见（见文首图片）、其中右主油箱的单向活门单碟失效在开位，单碟失效在关位，见下图所示。
三）、故障说明
从原理看，这是极度罕见的一种失效模式。任一油箱加油都有两道防线，加油活门和单向活门。在加油活门的上游，加油站组件的腔体相当于形成一个共用的油池。无论是单向活门还是加油活门只要好的，都能够防止油液的串油。中央油箱和右主油箱加油活门和单向活门均失效，相当于右主油箱和中央油箱直接连通了。那么不打泵情况下，就看液面高低情况。以及打泵给管路增压的情况。
四）、在针对丢失后能否放行的问题上
历史波音给出过合适时机检查的NTO
Ref /A/ 737NG-FTD-28-03004
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.
五）、事前能否识别的问题
从系统原理角度逆推，我们认为有以下特征
1、任意单向活门的丢失都不会导致事前出现任何的故障表象。只会在油箱检查的时候发现有碟片丢失的问题。
2、任何的加油活门失效，应该是能有所表现的。因为意味着在加油的过程中，失效的加油活门无法关闭，那么该油箱的油量会持续的增加，主油箱就会发生溢油的事件，而中央油箱会发生比预设加油量多的情况。
3、单一油箱加油活门和单向活门均失效，会发生第2条的现象。
4、中央油箱和任一主油箱加油活门和单向活门均失效，当主油箱油面高的时候，会发生油箱间的流动，直到油面平衡。
附：FTD 737NG-FTD-28-03004原文
Document Description
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.
