Boeing 737-800 – Loss of Hydraulic system B

Loss of hydraulic system B on the Boeing 737-800 is not a major failure. We still have the system A and the standby which are enough to fly the aircraft normally.

On the previous article, we have seen that loss of system A is mainly related to the manual gear extension. For the system B, it is mainly related to the flaps extension.

In my opinion, a loss of system B is a little more tricky than system A because there are more things to think about and also we are limited to flaps 15 landing.

Here, I would like to discuss:

  • What items are affected by a loss of hydraulic system B?
  • What are the consequences?
  • What are the main points to remember?

What items are affected by a loss of hydraulic system B?

Inoperative items Available or consequence
Autopilot B Autopilot A
Flight spoilers ( 2 on each wing) Roll rate and speed brake effectiveness may be reduced in flight
Yaw damper
Trailing edge flaps normal hydraulic system The trailing edge flaps can be operated with the alternate electrical system. Alternate flap extension time to flaps 15 is approximately 2 minutes.
Leading edge flaps and slats normal hydraulic The leading edge flaps can be extended with standby pressure. Once extended, they can not be retracted.
Autobrake Use manual braking
Normal brakes Alternate brakes
Engine 2 thrust reverser normal hydraulic pressure Thrust reverser will deploy and retract at a slower rate and some thrust asymmetry can be anticipated during thrust reverser deployment.
Alternate nose wheel steering Normal nose wheel steering

What are the consequences?

With a total loss of hydraulic system B, autopilot B is not available anymore. If it was engaged, it will disconnect. Autopilot A is available. The main consequence here is that we have only one autopilot left so we are restricted to CAT I landing as we need both autopilots for a CAT III approach (autoland). So, if the weather at our destination is below CAT I, this can lead to a diversion.

We have 4 flight spoilers inoperative (2 on each wing). Flight spoilers from system A are still operative. The consequence is a reduced roll rate and speedbrake effectiveness (rate of descent reduced).

Yaw damper is inop (yaw damper light illuminated) and the switch can not be reset to ON. Yaw damper prevents from dutch roll but if it doesn’t work, we can fly normally. It is recommended to avoid areas of predicted moderate or severe turbulence. If turbulence is encountered and passenger comfort becomes affected, reduce airspeed and/or descend to a lower altitude.

Note: In case of manual reversion only (loss of both system A & B), the yaw damper switch can be reset to ON and the standby hydraulic system powers the standby yaw damper.

The trailing edge flaps normal hydraulic system is inoperative. Instead, the trailing edge flaps can be operated by an alternate electric motor. Is is limited to flaps 15 and takes approximately 2 minutes to extend. It is not protected against asymmetry, so extension needs to be stoped if asymmetry is detected. The electric motor gets hot quickly if used continuously. After a complete extend/retract cycle, i.e., 0 to 15 and back to 0, allow 5 minutes cooling before attempting another extension.
The main consequence of a flaps 15 landing is a greater approach speed and a longer landing distance of course. This needs to be carefully studied with the performance QRH section or the Boeing OPT on the iPad.

For the leading edge flaps and slats, it can be extended with standby pressure. Really important, once extended, they can not be retracted. In this configuration, we might not be able to reach an alternate field if needed because of the increased drag and fuel consumption.
In the hydraulic reservoir, there is a little standpipe. If we lose the hydraulic fluid because of a leak, we still have sufficient fluid for operation of the PTU (Power Transfer Unit).
The purpose of the PTU is to supply the additional volume of hydraulic fluid needed to operate the auto slats and leading edge flaps and slats at the normal rate.

Autobrake is inoperative. We leave it on the OFF position and we use manual braking with the alternate system. If brake pressure drops below 1500psi, hydraulic system A automatically provides alternate brakes. So there is nothing to do.

Engine 2 thrust reverser inoperative. Engine 1 thrust reverser still works normally. But now for reverser 2, we are using the standby system. This one works with an electric motor driven pump which deliver 6 times less power than an engine driven pump. The consequence is that the thrust reverser 2 will deploy and retract at a slower rate and some asymmetry can be anticipated.

Finally, alternate nose wheel steering inoperative but we use normal nose wheel steering and nothing to do here.

What are the main points to remember?

To me, the main points to remember in case of loss of hydraulic system B:

  • CAT I restricted
  • No possible retraction of leading edge flaps and slats when extended
  • Trailing edge flaps electrically extended and limited to flaps 15

It is important to review performance and landing distance with the QRH or OPT.

2 thoughts on “Boeing 737-800 – Loss of Hydraulic system B

  1. Hi Johan

    When B system inop, when you extend flaps by using alternate switch. Do you extend continuously from up to 15 or step-by-step 1-5-15 as normal operation?

  2. Hi, thanks for your comment and good question!

    First, the switch must be held in the down position as it is spring-loaded, otherwise it comes back automatically to the neutral position. You would normally set the flap lever to 15 (so no need to do step by step) and then hold down the switch until it reaches the 15 position. Bear in mind that it will take approximately 2 minutes to extend, so quite long.

    Also, there is no asymmetry or skew protection through the alternate system. This means that you need to monitor the needles on the indicator of the flaps position. If flaps asymmetry occurs, release the switch immediately, that’s what the QRH says. The leading edge flaps and slats will extend through standby pressure. Once extended, they can not be retracted.

    You could retract the trailing edge flaps if needed. After a full cycle (0 –> 15 then 15 –> 0), you need to wait at least 5 minutes before extending again for cooling purposes.

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