SR 3-5400024037
2022年8月，55*8飞机机组反映再现时马赫配平失效灯亮。DFCS核实为B通道有22-11212 故障信息，判断为B通道马赫配平系统故障，因停场时间不足且无运力调整，依据MEL保留B通道马赫配平系统和自动着陆系统，完成M项，办理SG DD保留。后此次起飞时收襟翼加速过程中 （大约是5度往0度收），VNAV方式下，目标空速应该为230，但是在收的过程中就直接到了250。CDU便签栏显示BUFFET ALERT信息。后DFCS测试，A通道自检通过，B通道有22-11212 FLAP POS INV信息，可疑部件为FLAP POSN-2。执行库测试60.33,A,B通道都通过。执行60.34库测试，60.34页面显示有22-11212 FLAP POS INV信息，可疑部件为FLAP POSN-2。执行襟翼校装测试，A通道在范围中，B通道均不在范围内。量线D229 PIN9和PIN10为断路状态，判断T428襟翼传感器故障。北京航后量线检查T428襟翼传感器故障，更换T428襟翼传感器。完成 BITE Library Test库测试60.33和60.34，测试正常。完成襟翼校准装测试，结果符合要求。依据FIM量线，检查正常。检查相关插头无腐蚀、脏污、损坏及内部销钉损伤等不正常现象。马赫配平系统B通道不工作故障也一并消失。就此与波音做了沟通。
Q1: Which computer provides the function to produce BUFFET ALERT” message. DEU or FMC ? Whether the flap system will affect its function. How does it affect ?
R1: The BUFFET ALERT message is displayed by the FMC when the current aircraft condition has a buffet margin less than that selected on PERF FACTORS page.
The following provides the logic for FMC display of the Buffet Alert message:
(Current speed is greater than high speed buffet with a pad or current speed is lower than low speed buffet with a pad or the current altitude is greater than the buffet limited max altitude with a pad) and (Airplane is in Air and G/S is greater the 60 knots and Flaps are up).
Therefore the message is inhibited when flaps are down and the message does annunciate both high and low speed buffet conditions when flaps are up.
The FMC Buffet limit is only for Flaps up. Below approximately 20,000, feet FMC flaps up buffet limits begin to be washed out so as not to conflict with the SMYD provided limits at lower altitudes.
High speed buffet is displayed as noted in the logic and is based upon the current speed approaching the high speed buffet limit.
Q2: Which system is the target airspeed derived from? FCC or FMC?
R2: With VNAV mode engaged the target airspeed is calculated by the FMC. The FMC performance management function uses airplane and engine models and optimization algorithms to provide flight regime control of the airplane through speed and thrust consistent with the performance mode selected or with constraints imposed by Air Traffic Control. Inputs to the performance management function include total fuel, mode selections and engine bleed information, altitude, airspeed, Mach and temperature, progress along the flight plan and pilot inputs from the CDU. The outputs are advisory data to the crew and target values of Mach, CAS and target thrust settings for optimum control of the airplane.
Q3: Is the fault Flaps are up signal from which component?
R3: Flap position signals are provided to the FMC from the DFCS.
