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前文再续,书接上回 ▼
之前一篇推文 提到APU前次关车不当 导致运行中MAINT灯亮的问题
那么问题来了: 什么是正确的APU关车呢?
关断APU的程序 AMM PP写的太过简单
按手册来分别是:
❶断掉APU的引气和电负载
❷确认汇流条转换电门在AUTO位
❸APU主电门放OFF位。APU将进入60秒的冷却循环
❹将左前燃油泵电门放OFF位。 (如果是航后封机还需要确认其他一些电门位置)
❺按需将电瓶电门放OFF位。 注意必须等APU完成冷却循环并 且发动机停止工作后才能将电瓶电门放OFF位
这部分AMM手册,很明显 没有讲清楚关APU的关键
其实,这里面关键 阮工在上一次已经露了两点
好害羞的样子
(哦不,是已经说了两点) ▼
分别是冷却循环(cool down)和
电瓶电门关闭时间过早
没看过的小伙伴可以先回顾一下
接下来,阮老司机将为大家详细讲讲这两点
----------------------华丽的分割线---------------------
开讲之前,先和大家分享一个 我最近才琢磨出的道理
怪不得最近我媳妇的心思我一猜就中 关键是要看透这两点 ----------------------------------------
好了,接下来 阮工将教你看透这两点
◉ 冷却循环
1. 冷却循环是什么?
关过APU的小伙伴会发现,当我们把APU主电门放OFF位时,APU不会立即减速关车,而是会等一段时间才开始有明显减速关车的动静,这一段时间就是冷却循环。
An APU shutdown causes a cool down cycle. The cool down cycle time is 60 seconds. The time starts when the APU switch is put to the OFF position.The ECU does these steps for a cool down:
• Removes the ready-to-load signal移除准备接负载的信号 • Closes the bleed air valve关闭引起活门 • Closes the inlet guide vanes (15 dedegrees)关闭IGV至15° • Opens the surge control valve打开防喘活门 • De-energizes the starter-generator关断启动发电机励磁 • Starts the 60 second timer.启动60秒计时
◎内容来源:SDS 49-00-00
▲冷却循环完全由ECU控制
2. 冷却循环的作用
The 60 second cool down is required to prevent coke in the turbine bearing and fuel nozzles.The reason for the cool-down is to lower and stabilize the internal core temperature to help prevent coking of the fuel nozzles, turbine bearing seal and the cool down timer is there to achieve this.
冷却循环的作用是让APU在无气和电负载下运转一段时间,使核心机的温度降低并稳定,避免温度突然降低造成潜在的热冲击和内部旋转组件摩擦,从而防止APU涡轮轴承和燃油喷嘴结焦,提高APU的使用可靠性和寿命。
在APU运转过程中,断了直流电,比如关闭电瓶、拉APU灭火手柄,APU会立即关车,不会有冷却循环。如果APU没有完成冷却循环进入了关断模式此时会产生一个失去直流电的故障代码49-11019:
电瓶电门关闭时间 ◉ 冷却循环过后,APU才开始减速关断: During shutdown of the APU, these steps occur: At 30 percent speed, the APU air inlet door starts to close (closes immediately for APU fire). At less than 7 percent speed, an APU restart can be initiated.
那么到底我们该在什么时候电瓶电门呢?
其实正确的方法也在SDS里: The APU fuel shutoff valve and air inlet door will close for a normal or protective shutdown.Wait 40 seconds after the EGT goes below 300 C for the inlet door and fuel shutoff valve to close before you move the battery switch to the OFF position.
EGT降低到300°以后,再等至少40秒才能关电瓶。
Q 我都是在CDU上看APU的转速(INPUT MONITORING页面),降到7%以后开始关电瓶,那这么做对吗?
阮工 关于这个问题,早之前我也问过波音。转速降到7%时,APU从运转模式进入到关断模式,但进气门有可能还是没有完全关闭的。波音建议让进气门完全关闭,避免FOD(鸟,冰雪等从APU进气口进入造成损伤)。
所以建议大家还是按手册标准来:待APU EGT温度将低到300℃后,再等40秒关电瓶。
SR 4-3134108673,感兴趣可看看
Boeing recommends waiting at least 40 seconds after EGT goes below 300C during a normal shutdown before moving the battery switch to OFF position. The APU inlet door will not fully close until all of this 40 second interval has elapsed.
The APU will generate a DC Power Loss fault code, 49-11019, if it is still in Run mode when the battery switch is moved to OFF. The APU exits Run mode and goes to Shutdown mode shortly after rolling down below 7% speed. If the battery switch is moved to OFF during shutdown mode, no fault code should be generated even if the full 40 second shutdown cycle has not finished and the APU inlet door has not fully closed. Boeing has no technical objection to selecting the battery switch to OFF before the APU inlet door has fully closed.
However for best practice, Boeing recommends allowing the APU inlet door to fully cycle closed in order to help minimize the possibility of foreign objects, birds, snow etc. getting inside the APU inlet duct while the airplane is parked.
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你平时关APU的姿势对吗?
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