Hydraulic failure question

[killairbus]

So we need to presurize the 4th system first and depressurize it last to prevent fluid transfer. My question is, what happens when the 4th system's engine and demand pumps fail? Will the fluids from 1,2, and 3 spill over? How would a fluid transfer affect the flight controls and what could I expect to see on the pressure readings?

-Angelo Busato

[downscc]

13 hours ago, killairbus said:

So we need to presurize the 4th system first and depressurize it last to prevent fluid transfer. My question is, what happens when the 4th system's engine and demand pumps fail? Will the fluids from 1,2, and 3 spill over? How would a fluid transfer affect the flight controls and what could I expect to see on the pressure readings?

-Angelo Busato

My first impulse is to look at the QRH.., but then I thought why not switch to AUX 4? Check the QRH.

[skelsey]

The systems are not directly linked but IIRC it is possible for a small amount of fluid to transfer through the brake system. If thin happens a lot then over a long period of time and nobody tops up the systems you could end up with a low fluid level in one of the other systems (and obviously this would be compounded if there was a small leak in Sys 4).

If the Eng 4 pumps fail in flight there will be no appreciable transfer of fluid or hydraulic power from the other systems - as I say it is IIRC to do with the brake accumulators. I am however doing this from memory so excuse any errors! 

As Dan says, in any event your first port of call will be the QRH which will basically get you to try and restore pressure through an alternative means (ie the demand pump if the EDP has failed) or make the system safe if this is not possible (ie turning all the pumps off). Unless you had an EDP or ADP MEL'd at the start of the flight it would be very unusual to have both fail simultaneously - the more likely scenario would be a loss of fluid from a leak.

Hyd Sys 4 failure is a fun one as you will be dealing with secondary trailing edge flap extension, an alternate gear extension, reduced elevator and roll spoiler authority and no autobraking, amongst other things! Again, the QRH will list the inoperative systems.

[skelsey]

Just to clarify having looked up the details - as I recalled, the fluid transfers through the brake lines and specifically the anti-skid shuttle valve which separates systems 1 and 4. Pressurising system 4 first keeps the valve in place. Otherwise a very small amount of fluid (i.e. that within the tubing ahead of the shuttle valve) can transfer across - it's not a continuous flow or anything.

I was reminded, however, that there is a small amount of shared pipework downstream of the anti-skid shuttle valve and a leak in this area could cause the loss of systems 4, 1 and 2 sequentially - ouch! 

[killairbus]

Thanks!

[berts]

On 2/20/2019 at 5:57 PM, skelsey said:

Hyd Sys 4 failure is a fun one as you will be dealing with secondary trailing edge flap extension, an alternate gear extension, reduced elevator and roll spoiler authority and no autobraking, amongst other things! Again, the QRH will list the inoperative systems.

Although not relevant to a HYD Sys 4 failure on the 744, one of the many important considerations to think about with any loss of a hydraulic system is how it might affect the autopilot(s) and also your autoland capability in limiting conditions. 

There are also other things to think about with any system failure in flight that might not be covered in any great detail in a non-normal QRH procedure, but certainly will be during a pilot's conversion course and subsequent refresher training.  For example, taking Simon's explanation of an Hyd Sys 4 failure, how will this hydraulic loss affect the aircraft performance in the event of a go-around or diversion?   After the Wing Gear is extended using the Alternate system it cannot be retracted and this will result in extra drag affecting not only the aircraft's limiting speed and climb performance during a go-around, but also it's diversion range due to the increased fuel consumption.  Fortunately, the 744 hydraulic systems and system redundancy are an excellent Boeing design and the pumps are also very reliable, so hydraulic failures of any sort are relatively rare.  Fun, Simon? - well, yes - but maybe only in the PMDG 744?!! 😬     

[skelsey]

39 minutes ago, berts said:

Fun, Simon? - well, yes - but maybe only in the PMDG 744?!!

😄 Yes, indeed -- I should really have qualified 'fun' with some quote marks!!