I was thinking about the current workings of fly by wire, and it's my understanding that they still rely on hydraulic actuation to move the control surfaces, but simply replace the metal wire cable linked with an electronic cable that commands the hydraulic actuators instead.

 

With this system i assume a loss of hydraulics in one hydraulic system would mean the backup hydraulics would have to be used, reducing the amount of equipment that can be used? A wet hydraulic system is also heavy.

 

Now this is a total shot in the dark, but it seems to me a system based on electro mechanical actuators would be simpler, more reliable and easier to add redundancy into, and lighter. 

 

The system could use the electric wires to transmit inputs to a mechanical motor which then moves the control suface in question (lets say the rudder and rudder pedals). This system could have several redundant motors for the control surface (eg the rudder) that are independantly linked to a seperate electrical wire linked to the same control input (such as rudder pedals). This would mean that if one electrical wire was damaged or unable to send the input signal to the rudder then the second electrical wire would switch on and transmit that signal to the backup mechanical motors which actuate the rudder.

 

If a total loss of generator power occured the system would automatically switch to backup power from the batteries and could operate as normal until generator power from the APU could be brought online. 

 

My apologies if this is already a system in use but it seems at least on commercial airliners, hydraulic systems are still used in Fly By Wire to actuate control surfaces and these appear harder to build in redundancy than simple mechanical actuators.

 

 

The system would be, as usual, backed up once again by normal steel cables if all electrics failed.

 

My thoughts on this originated from reading about an MD-11 which had all three hydraulic systems lose fluid and they had to land the aircraft using engine power only. It then led me to think of the reductions in weight from a totally dry system.

 

Apologies if this is already in use, but i could not find anything on the web to suggest it was and a quick check on Airliners.net told me that all current airliners still use hydraulic actuation.

Electronics aren't powerful enough, and I don't think there's ever been a triple hydraulic failure yet on a FBW Airbus or Boeing 777/787.

 

On some aircraft they have electric motors as backups for the flap's hydraulic actuators, but they take ages as they just lack the power you get with hydraulics.

 

BTW, the Souix City accident you're thinking of involved a DC10 not an MD11...  :wink:

 

Regards,

Ró.

Electronics aren't powerful enough, and I don't think there's ever been a triple hydraulic failure yet on a FBW Airbus or Boeing 777/787.

 

On some aircraft they have electric motors as backups for the flap's hydraulic actuators, but they take ages as they just lack the power you get with hydraulics.

 

BTW, the Souix City accident you're thinking of involved a DC10 not an MD11...  :wink:

 

Regards,

Ró.

I see, so the current availability of electric motors just isn't strong enough for this job? Maybe the wear and tear would be high too.

 

And yes, of course it was a DC10. It was a few months ago i read about it now  :blush:

I see, so the current availability of electric motors just isn't strong enough for this job? Maybe the wear and tear would be high too.

 

And yes, of course it was a DC10. It was a few months ago i read about it now  :blush:

They'd need too big an electrical load to produce the force that hydraulics are great at producing with very little effort. That's the beauty of hydraulics. Also, certainly if your tanks went dry and you had to rely on the RAT, you'd be much better off with the limited flight controls that we have available to us with the current set up verses the control we'd have left if we tried to move the control surfaces only using motors running on electricity generated by the RAT.

 

Regards,

Ró.

Hi James,

 

Interesting post, the reason it has not been done is simply due to huge weight penalty. You could also argue reliability however weight is the main issue.

 

I am sure you can imagine what type of electrical motor would be required to raise the landing gear or move the rudder on a 747. Now on top of that add triple redundancy and you can already see the whole idea being thrown out of the window.

 

The triple hydraulic system loss you mention was on a DC10 not a MD11, NASA did land a MD11 using thrust only but this was purely a demonstration.

 

If you take an aircraft like the A320 for example, you have three independent hydraulic systems, on top of that you have backup electric pump on the yellow system, the blue system is also powered by a electric pump and ram air turbine.

 

So lots of redundancy on modern jets, and so far a dual system loss has been the worse case reported. Also if you look at those dual faults it has been down to incorrect handling from the flight crew, for example leaving the PTU running causing an overheat of the good system.

 

I have never heard of dual loss from actual hydraulic fluid leaking from both reservoirs.

 

Oops, Ro beat me to it

Thanks for the insight guys. Just had it on my mind and wanted to see what someone more knowledgeable than myself had to say about it.

I suspect the advantage of hydraulic is that once the system is pressurized, there is a good deal of energy available for immediate use as power.  Even with a storage system for electrics, I'm not sure you can extract the energy as efficiently.

 

scott s.

.

Also, hydraulics have (almost) no loss by distance, if you move alot of electricity through a long wire you loose energy. in addition, high power electricity is potensially very dangerous if you get a "leak", hydralic leaks are not that dramatic.

I suspect the advantage of hydraulic is that once the system is pressurized, there is a good deal of energy available for immediate use as power.  Even with a storage system for electrics, I'm not sure you can extract the energy as efficiently.

 

scott s.

.

Indeed, I'd forgotten that, hydraulics have accumulator tanks that can kick in at a seconds notice with no extra power required... Same can't be said for batteries... 

 

Regards,

Ró.

Thanks for your replies guys. Some interesting discussion there.

Be a man and fly an airplane with cables!

Be a man and fly an airplane with cables!

We have cables, manual reversion to the rudder uses cables, alternate gear extension too, so we've got cables...  ^_^ 

 

*If it's not Airbus, I'll kick up a fuss* - Still trying to get it to catch on... =/

 

Regards,

Ró.

We have cables, manual reversion to the rudder uses cables, alternate gear extension too, so we've got cables... ^_^

 

*If it's not Airbus, I'll kick up a fuss* - Still trying to get it to catch on... =/

 

Regards,

Ró.

Although I have to be affiliated to Boeing, I do love the flight on newer Airbii.

Although I have to be affiliated to Boeing, I do love the flight on newer Airbii.

LOL, welcome to the dark side...  ^_^    B)

Be a man and fly an airplane with cables!

Real men control roll by using a hip cradle to activate the wing warp system!

 

[grumpy old man mode (and I'm only 26 )]

Gammar peeve time: Airbus is a modern made up word, so unless your city's public transport system runs bii rather than busses, the correct plural is just Airbusses. Incidentally if it were a Latin word the correct plural would be Airbi, with only 1 i.

[/grumpy old man mode]