Are there any still commercial airplanes in use that are still steerable with complete loss of all electric systems? Do some commercial airplanes still have mechanical backup steering mechanisms?
Yes and Yes.
On most(*) Airbus aircrafts,
In case of a complete loss of electrical flight control signals, the aircraft can be temporarily controlled by mechanical mode.
This is called
MECHANICAL BACKUP and is the lowest level of control law (after Normal, Alternate and Direct Laws)
The aircraft can be steered:
- Pitch control is achieved through the horizontal stabilizer by using the manual trim wheel.
- Lateral control is accomplished using the rudder pedals.
The pilots are notified with:
A red MAN PITCH TRIM ONLY warning appears on the PFD.
Both controls require hydraulic power.
This means that if the aircraft looses BOTH electrical and hydraulic power, is no more controllable.
Usually there are pressurised hydraulic reservoirs that allow to have a minimum of hydraulic power even if there is total loss of electrical power, but if there is substantial damage to the aircraft that has severed the hydraulic lines, this is no more applicable.
(*): beyond the A320. Functionalities vary in the following manner: (emphasys mine)
Primary Flight controls:
Back-up Control Logical evolution of A320 / A340 / A340-600 family:
Full Fly-By-Wire, with a “Back-up” as an additional precaution to keep control of the aircraft during temporary loss of:
- all Primary Flight Control computers
- all Electrical power supply
A320 : full FBW controls, mechanical Back-up (Pitch Trim & Rudder)
A340/A330 : like A320, additional Yaw Damper to improve Dutch Roll
damping even in Back-up mode (BYDU with hydraulic micro generator)
A340-600 : like A340 for pitch, Rudder becomes fully Electrical (BPS + BCM : Back-up Power Supply + Control Module)
A380 : like A340-600 for Yaw control + BPS+BCM also power:
- Electrical Pitch Back-Up (elevators) linked to side-stick
- Electrical Roll Back-Up (ailerons) linked to side-stick
- Pitch Trim (Wheel is replaced by Switches)
For the A380, in particular:
Active Stability Control functions:
All levels of control laws (Normal, Alternate, “Direct” and even Back-up) include a Yaw and Pitch damping function