What is "fly by wire"???

[UA-RESURRECTED]

Most of your small GA piston airplanes probably use a mechanical linkage to connect the yoke to the control surfaces. I'm thinking something like cables and/or rods.

But what about the big and not so big jets and such? First, I always thought "fly by wire" meant there was no mechanical linkage between the yoke and the control surfaces. I take it I am wrong??? Also, what about hydraulics??? I'm thinking the yoke is connected to a hydro-unit of some sort, and hoses run from this unit to actuating devices on the surfaces??? What if there is a leak??? That could be deadly. Do they incorporate any type of failsafe???

Also, why hrdaulics in the first place??? I'm guessing you would have to REALLY manhandle that yoke to get the control surfaces to move in an airplane going +300kts???

 

[2000flyer]

You are correct. Primary flight controls are electrical signals via automatic flight control systems. Some aircraft have mechanical backup, most just have multiple redundant AFCS. If there is a leak, you have trim and very VERY heavy control forces.

2000Flyer

 

[profile]

Actually, there is NO backup to hydraulics in any of the bigger jets. There are multiple hydraulic systems, but if you lose all of them, like UAL did with a DC10 or JAL with a 747, all you have is engine thrust to control it.

FBW sends signals to the hydraulic actuators at the control surface, rather than have them controlled via cables from the control column.

Smaller jets usually have some sort of manual reversion where you can directly move the flight controls, but there is just no way to do that with the forces required for big jets.

 

[SkyWestCRJPilot]

On the CRJ the yoke and rudder pedals are linked to cables that extend to the control surfaces which then actuate hydraulic actuators that move the control surfaces. There are 3 hydraulic systems for redundancy but if you lose all three there is no manual reversion and with centerline thrust there is no real way to manuever by differential thrust like in the Sioux City crash. With the 3000 psi in the hyrdraulics there is no manhandling, rather it's quite responsive and light on the controls. The nose wheel steering is "fly by wire" with an electrical signal sent to the small hydraulic actuators to move the nosewheel.