Control Circuit stretch

[EngineOut]

Reading HTBJ, and there is reference to control circuit stretch on hydraulic powered controls. I was wondering if anyone could actually explain what that was.

Cheers.

[Kit d'Rection KG]

Errr,
That's when the control circuits (the cables connecting the flying controls together), errr... stretch... You know, elongate, er, get longer, errrm. Quite straightforward and not a problem for the line pilot, though worth knowing about for flight test crews.

[EngineOut]

this was in reference to hydraulic powered contols

[idg]

EO,
KG is spot on. All flying controls are hydraulically powered even in FBW a/c.
In conventional controlled a/c, cables run from the control column and rudder etc, to the servo valves at the controls which direct the pressure to the jacks to move the surfaces.
However these cable runs can obviously be very long and therefore there is the possibility of slack and stretch in the cables, not to mention the changing length of the airframe as the pressurisation 'blows up' the fuselage.
To combat these effects, designers need to keep the cables taut so that the smallest inputs on the controls create the correct movements in the servo valves. To this end they use devices called cable tension regulators which ensure that the cables are maintained at the correct tension. In older a/c this was just a big spring in the circuit but that has the dissadvantage of 'offsetting' the control as the spring flexed. Later designs are more complex and are usually duplicated to ensure correct 'centering' of the control.
I believe that the Jumbo has hydraulic assistance just for the cable runs cos they're so long!:ok:

[ICT_SLB]

Cable tensions are also important to proper Autopilot operation. If the cable tensions are too low, the AP Servo has to take up too much slack and so will tend to overcontrol - if they're too tight the servo may have difficulty overcoming friction in the control run & thus providing enough torque to control the hydraulic power drive units effectively. The correct cable tensions are normally given in the Maintenance Manual & checked on the production line prior to first flight & in service at major checks.