SB 73-0241
2025年2月，近期机队反映多架飞机伺服燃油加热器端盖漏油的问题。整体均发生在装机时间较长的部件上，超过25000FH。
针对该问题，厂家在WTT会议材料中表示，调查分析表明端盖的两个封圈 (P/N: M25988/4 and P/N: AS43013235）有以下特征：
1、O型圈变硬且被挤压，无法确保表面密封。
2、变色的O型圈（灰色而非蓝色）呈现出表面粗糙。
3、没有过热或化学降解的迹象。
4、永久变形，无法恢复到原来的形状。
因此判断是由于热循环导致的O型圈失效。
实物分解如下图所示：
厂家建议按15000FH为软时限，执行定期的封圈更换。以减少此类事件的发生。
当前机队SX-73-2023-06-001采用被动更换的方式执行。通过可靠性决议按25000执行主动措施。
临时措施可以参考CMM的力矩要求对螺栓进行重新磅紧的方式。有案例表明50螺栓有松动的情况。但从目前的实际案例看，拧紧加温后能解决呲油的问题，还是会存在渗漏。
针对有多客户询问是否可以在翼更换封圈的问题。从CMM来说，执行封圈更换后，需执行5个测试，涉及单通道的供压和渗漏检测等，航线是无法完成的。从风险来说，国内航司发生过由于钢丝螺套脱出导致的燃油渗漏，从而触发空中关车的事件。因此不建议实施。在与CFM沟通后的回复也支持这一分析。
With regards to your request, please see the following final answer:
With reference to your request below, please find hereafter our Customer Support specialist’s final answer:
CFM has already worked on recommendations to prevent Servo Fuel Heater (SFH) leakages. The internal O-rings, P/N M25988/4235 (item 45 of CMM 73-71), were identified as the root cause of the leakages due to ageing and thermal compression. Investigations and reporting have shown that fuel leakages typically occur when the Servo Fuel Heater (SFH) reaches 18,000-20,000 flight hours TSN.
To address this, we released SB 73-0041 (Category 7) recommending removal and overhaul of the Servo Fuel Heater at 15,000 flight hours, in accordance with the Airworthiness Plan guidelines and recommendations.
More details are available in the attached latest symposium presentation.
It is not recommended to replace the O-rings on-wing as this hardware is located downstream of the fuel filter and contamination may occure during on-wing maintenace. Additionally, performing the replacement on-wing increases the risk of maintenance errors (e.g., incorrect torque). Furthermore, once the unit has been opened, all leakage tests required by the CMM must be performed, and these tests can only be carried out on a test bench.