Loss of hydraulic system A on the Boeing 737-800 is not a major failure. We still have the system B as well as the standby which are enough to fly the aircraft normally.
Here, I would like to discuss:
- What items are affected by a loss of hydraulic system A?
- What are the consequences?
- What are the main points to remember?
What items are affected by a loss of hydraulic system A?
| Inoperative items | Available or consequence |
| Autopilot A | Autopilot B |
| Flight spoilers ( 2 on each wing) | Roll rate and speed brake effectiveness may be reduced in flight |
| Normal landing gear extension and retraction | Manual gear extension is needed |
| Ground spoilers | Landing distance will be increased |
| Alternate brakes | Normal brakes |
| Engine 1 thrust reverser normal hydraulic pressure | Thrust reverse will deploy and retract at a slower rate and some thrust asymmetry can be anticipated during thrust reverser deployment. |
| Normal nose wheel steering | Alternate nose wheel steering |
What are the consequences?
First, if a total loss of hydraulic system A happens and we are flying with autopilot A, it will disconnect. We still can use system B and engage autopilot B. 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 B are still operative. The consequence is a reduced roll rate and speedbrake effectiveness (rate of descent reduced).
An important one is that we will need to extend the gear manually. This will take some time so we need to anticipate for that. There is a manual extension access door. When open, there are 3 handles which have to be pulled and the corresponding gear gets down by gravity and air loads. Interesting to know that during normal operations, if the manual extension access door is open, the landing gear cannot be retracted. In all cases here, with loss of hydraulic system A, when the gear is down, it can not be retracted.
Ground spoilers will be completely inoperative so landing distance will be increased. Remember flight spoilers from system A are inoperative. During normal operations, flight + ground spoilers are 12. With failure of system A, we only have 4 working (1/3). This is to consider seriously depending where we are landing.
Alternate brakes inoperative but it is not affecting at all cause normal brakes are available.
Engine 1 thrust reverser inoperative. Engine 2 thrust reverser still works normally. But now for reverser 1, 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 1 will deploy and retract at a slower rate and some asymmetry can be anticipated.
Normal nose wheel steering inoperative but alternate nose wheel steering is available. It is part of the NNC to activate it so we can use the tiller normally to control the aircraft on the ground.
What are the main points to remember?
To me, the main points to remember in case of loss of hydraulic system A:
- CAT I restricted
- Manual gear extension
Then we need to consider the landing performances depending the destination.
It is possible to get partial loss of hydraulic system A if there is a leak from the engine driven pump. Hydraulic fluid will leak until it reaches the standpipe. At the top of the standpipe, the reservoir quantity of system A will be approximately 20%. We will be able to use hydraulic system A normally in that condition.
