最近某737NG飞机，石家庄短停反映左后厕所灯不亮，办理故障保留。后判断故障为S22故障，转无航材保留……新构型天空内饰前卫生间，镜灯测试位时开关正常，灯是好的，关门锁定后就是不亮，门锁也拆开看了里面好了，电路也是通着的，这个怎么解，手册没查到？
所以抽空分析一下安装了B/E公司厕所的天空内饰737NG飞机，厕所灯光不亮的问题。
事件背景2020年12月15日，天空内饰的737-800飞机，石家庄短停左后厕所灯不亮，依据MEL38-02B办理时间不足故障保留。
18日值班看这个保留转为了无航材，故障件给的是S22开关：其实如果稍微看一下这个电路图，就知道不可能是S22开关的问题，最可能的是R24继电器的问题。
今天再看了一下这个保留，发现果然确认为R24继电器故障，又继续转为了无航材保留。
询问了下石家庄当天办保留的人员，当时的故障现象是只有镜灯上部亮一小截，其他灯都不亮，锁闭厕所门后也不亮：
天空内饰737NG飞机厕所灯为LED的，分为BLUE WASH LIGHT、OVERHEAD LIGHT、MIRROR LIGHT ASSEMBLY，具体如下：L22 MIRROR LIGHT
L25 AUXLIARY LIGHT
如图所示，红色是电源线，绿色是地线，可以看出蓝色染色灯、顶灯、镜灯的电源是插钉是共用的，来自于115V AC GND SVC BUS 1，跳开关P18-3 E14。在厕所内部受R24 UTILITY SHUT OFF RELAY 继电器控制，R24吸合，灯就得电。
而R24继电器吸合受115 VAC WATER HEATER PWR控制。如果航后放完水后，严格按AMM操作，拨出厕所水加热跳开关，R24继电器失去吸合，这几个厕所灯就会熄灭，就是这个原因。
▲ D厕所水加热器跳开关是我测试拨出的
另外还有R22 SERVICE RELAY，这个继电器吸合后会旁通掉R24继电器，使电源通过其吸合触点A2-A1供到厕所灯上。
R22继电器受厕所门开关S21和SERVICE LIGHTS SWITCH S22控制。S22是一个三位开关，ON/AUTO/OFF，正常在中间AUTO位置。
当厕所门未锁闭时，此时，如果我们把S22放到ON位：
R22继电器会吸合，R22吸合后，其A2-A1导通，可以旁通R24继电器，此时如果拨出厕所水加热跳开关，厕所灯还能亮。另外，由于C2-C1导通，此时，将S22开关放回AUTO，R22继电器还是会保持吸合。将S22放到OFF位可使R22断电断开。
R22吸合后，将厕所门锁闭，S21接通后，由于R22的B1-B2接通，也能使R22保持吸合状态。并且此时R22不受S22开关控制。
R22吸合后，也会使R23通电吸合。
LAV OCCUPIED RELAY R23，从名字也能R23 继电器控制的厕所占用信号。
除R22继电器吸合后，会使R23通电吸合外，厕所门开关S21闭合后，也会使R23吸合。
R23吸合后，提供接地TOKEN信号（SIGNAL GROUND TOKEN SINGAL IN）给顶灯和镜灯，从而使其点亮。
另外，R23吸合后，其A1-A2触点导通，提供接地，使厕所外部的占用灯点亮。
顺便也看看L25 AUXLIARY LIGHT吧，这个灯容易被忽略，因为其位于镜灯上部特别小的一截：
其电源来源参考SSM 33-27-11：P18-3 A8跳开关，正常就是亮的，没有开关控制。
维护提示当遇到天空内饰厕所灯不亮时，利用SERVICE LIGHTS SWITCH S22可以很容易判断故障。S22开关位于镜子后面。比如此案例中，将S22放到ON位，如果厕所灯能亮，就说明灯的上游电源和下游线路部分都没有问题，故障原因是R24继电器没有吸合。最可能原因是：另外个人建议：碰到天空内饰厕所灯不亮时，如果将S22开关放到ON位后，厕所灯能亮，可以不用按MEL办理保留锁闭厕所，开缺陷卡控制就行，唯一影响就是厕所外面的占用灯会一直亮。
先在SSM/WDM 33-26-31上找到厕所的设备号，比如左后厕所LAV D是Y7，设备号就是Y007：
R24继电器就是Y007R24，可以得到件号是M83536-11-002L：
A B737-700 captain reports the MEL does not address lavatory mirror lights.
Inbound flight crew requested maintenance on arrival; but failed to write up the item. They informed me as they were leaving; that maintenance was coming to the aircraft to repair a light. When maintenance arrived they informed me that both the aft lavatory occupied sign and aft lavatory mirror light were inoperative. They placed the occupied light on a maintenance note. They MEL'ed the mirror light in the aft lavatory. They informed me of their action just prior to push back. After reviewing MEL 33-2 cabin lighting; I advised ZZZ1 maintenance along with ZZZ2 maintenance that MEL 33-2 makes no reference to lavatory lighting at all. ZZZ1 maintenance told me that the lavatory lighting issues fall under MEL 33-2 (cabin lighting). I again informed him as to my disagreement with lavatory lighting being released under MEL 33-2 due to the fact the MEL 33-2 makes no reference to lavatory lighting. ZZZ1 maintenance told me that is the way we have been doing it over 20 yrs captain. Now; do you want to cancel a flight just for a lavatory light? That is the only relief we have under the MEL. I told him that I would research this issue with others in the company and call him back. He wanted an answer there and now. I called dispatch; after consulting with dispatch and reviewing the MEL. The dispatcher wanted to release the aircraft with the aft lavatory locked out (inoperative). But ZZZ1 maintenance would not hear of it. They told him to take the aircraft as is under MEL 33-2. Unable to contact my chief pilot; the dispatch supervisor called me to inform me that he saw no problem with releasing the aircraft under MEL 33-2; and that lavatory lights did not represent a safety issue. At this point; I was getting nowhere with either ZZZ1 maintenance or dispatch. Lavatory lighting did not represent a safety issue; and further discussions with either maintenance or the dispatch supervisor would not have accomplished any further reasonable or useful information. I informed the first flight attendant as to the status of the aft lavatory; and to try to minimize its use. Having exhausted all possible avenues at this point in time; a decision was made between the dispatcher and I to continue the flight as safety was not a concern; and to take this matter up with ZZZ1 maintenance at a later time. Later I called my chief pilot and informed him as to what had taken place. To correct this disagreement in the future; a change to the master MEL making clear reference to lavatory lighting either under MEL 33-2 or any other under an additional MEL would more than correct any disagreement between maintenance and flight crews. Inbound flight crew to complete log entries maintenance to stop using the term 'we have been doing it for ___ yrs that way.' this type comment does not make it correct nor useful! Correct the MEL to reflect lavatory lighting issues either under MEL 33-2 or under another MEL that directly concerns lavatory items.这是一个机组的报告信息（NASA ASRS），当时是一架737-700厕所，后厕所占用信号和镜灯不工作，机务MEL 33-2，机组觉得MEL 33-2不适用这种情况。另外参考737NG-FTD-25-17001，厕所厂家B/E也发布了相应修改线路的SB和新构型水加热器，以避免出现此种情况。但是我看了下，飞机的SSM里并没有区别，还是会R24继电器还是会受到厕所水加热器电源的影响。 
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