737NG液压油量指示在飞行中的正常变化
某737-700飞机，起飞至巡航后，机组观察到A系统液压油量减少至79并稳定，压力正常，以为液压系统A有渗漏，选择返航。
落地后机务打压检查A/B系统液压油量100/104，依据手册执行液压系统外部渗漏检查，未发现渗漏，译码显示正常，飞机正常放行。
AHM上ACMS油量报文
关于737NG液压油量在飞行中的变化，之前我发过一次：
也可参考波音737NG-FTD-29-01003
Reservoir quantity changes during flight
为了避免类似事件，这里再发一次这篇文章▼
有时候会碰到飞行员反映空中收起落架后，A系统油量低，后面又恢复正常。
嗯，只要系统油量没有持续减少的话，这是一个正常现象。
1. Before takeoff extending the Leading Edge Flaps to extend and the Leading Edge Slats to Intermediate position causes 10 % drop in System B hydraulic reservoir level.
起飞前，前缘半放出，B系统减少约10%。
2. Retraction of the landing gear causes a noticeable drop in system A hydraulic reservoir level. Retraction of the landing gear causes roughly 20% drop in indicated system A reservoir quantity.
收起落架后，A系统减少约20%。
3. Thermal contraction during cruise causes an additional drop in reservoir level. Operators have reported 5 to 10% further drop in system A reservoir level after some time spent in cruise, and calculations indicate that thermal contraction after prolonged cruise at low temperatures could cause as much as 20% drop.
巡航低温，热胀冷缩，油量指示减少5-10%，时间长甚至会减少20%。
4. Extending the Flight Spoilers causes 6 % drop in System A and System B reservoir quantity.
飞行中扰流板伸出，A、B系统油量减少6%
5. Leading edge flap extension to extend and Leading Edge Slats extension to Full Extend position just prior to landing removes an additional 20% from the reservoir.
准备着陆前，前缘全伸出油量又会减少20%。
6. Extending the Landing Gear causes roughly 20 % rise in system A reservoir level.
起落架放下后，A系统油量增加20%。
7. During Landing applying Speedbrakes (Flight + Ground Spoilers) causes 10 % drop in System A and 6% drop in System B reservoir level.
着陆期间使用飞行和地面扰流板，A系统油量减少10%，B系统减少6%
8. Retracting the Leading Edge devices during taxiing causes 20 % rise in system B reservoir level.
滑行时收回前缘，B系统油量增加20%。
9. Operating the Ailerons, Elevators, Rudder does not cause observable change in system A and B reservoir level.
操纵副翼，升降舵和方向舵不会使A、B系统油量有明显变化。
10. Operating the Trailing Edge Flaps does not cause change in the reservoir level because it is driven by a hydraulic motor, not an actuator.
操纵后缘襟翼不会引起液压油量变化
Experiencing in-flight reservoir level changes as described above should not be concerned, as long as the reservoir levels return to normal after the airplane has been on the ground for long enough for the hydraulic fluid to warm to ambient temperature. When comparing reservoir levels after flight to levels before flight, the airplane should be in the same state as it was before flight. The leading edge, landing gear, spoilers, and thrust reversers should all be in the same position, the pumps should be on/off as they were before, and the brake accumulator should be properly precharged.
飞行中遇到以上这些液压油量的变化不用担心，飞机回到地面后油量又可以恢复正常。我们比较飞行前后液压油量要考虑温度、舵面位置、泵是否工作的影响。
Fluid consumption of different Hydraulic components: The EMDP provides a hydraulic flow rate of 6 gpm. The EDP provides a hydraulic flow rate of 37 gpm. If you use only EMDP the EMDP Low Pressure Light will illuminate if you apply too much load to the system. If you excersize with the aileron, rudder, elevator (applying quick movement, great deflection of these flight controls) it can be enough to illuminate of the EMDP Low Pressure Light.
EMDP的额定流量是6 gpm (5.7 gpm at 2700 psi)，EDP的额定流量是37 gpm (36 gpm at 3000 psi )。如果仅用EMDP同时作动太多舵面的话，EMDP低压灯可能会亮。
In order to retract the MLG's within the time limits specified in the Airplane Maintenance Manual (AMM), the hydraulic system has to be capable of delivering 20 gallons per minute (GPM) of fluid flow. In a normal airplane operation, this flow is delivered by the engine driven pump (EDP), which has a maximum flow capacity of 36 GPM. This is supplemented by the System-A EMDP, which provides 6 GPM of flow capacity.
If only the EMDP is used to provide hydraulic power to retract the landing gear, it is unlikely that the MLG's will retract within the specified time due to the limited flow capacity of the EMDP. This is especially true if both gears are being retracted simultaneously.
Thus, a hydraulic power cart capable of delivering 20 GPM of flow must be used to test the MLG retraction times.
起落架收放测试时，如果仅用EMDP，收上时间会比手册要求的时间长。
类似的说明在FCOM里其实也有
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▐ 另外再补充两点▐
01 低油量仅在地面指示
4 白色的加油指示（RF：refill）
液压油量少于76%。只有当飞机在地面，并且两台发动机都关车或着后缘襟翼收上时才指示。
可以看出来，厂家为了避免正常运行中的液压油量低对机组造成干扰，将低油量信息进行了屏蔽。只在落地后双发关车和着陆后襟翼收上滑行时，才显示。
02 添加液压油注意放气
AMM TASK 12-12-00-610-801
Hydraulic Reservoir Servicing
You should bleed the hydraulic system if maintenance activity allows air to enter the system, or, if you add a large amount of fluid and believe that air has entered the system from the reservoir.
如果维修时使空气进入了系统，或者添加大量液压油时，注意给系统放气，避免后续运行中油量指示不准或系统低压。
TASK 29-00-00-870-801
Bleed the Hydraulic Systems
操作飞控舵面、反推、起落架手柄，对系统放气。
典型案例：
某737-800飞机，航后EDP漏油更换时，有大量液压油损失，更换后补加了3.5加仑，试车后发现A系统油量有减少，继续补加1.5加仑。
次日执行航班后，A系统液压油量为74，而航前记录值为106，检查无渗漏后补加至标准值，后续航班监控正常。