# How are fly-by-wire airliners controlled in case of complete electrical failure?

In this comment, it is postulated that even fly-by-wire aircraft (Airbus) are demonstrated to fly without any electrical systems.

All Airbus aircraft are demonstrated controllable with complete electrical failure. They would be pretty difficult to land if the electronics didn't at least partially recover (because the backup only allows controlling pitch and yaw and roll only via yaw-roll stability), but it would still be possible. [...] Jan Hudec Dec 21 '14 at 21:38

My question, assuming complete electrical failure, how exactly is this done? What controls have mechanical linkage (pitch trim? engines? flaps/slats?) Would it be possible, in theory, to land the plane in such a way that some passengers could walk away? (Perfect weather, miles of flat tarmac runways everywhere, cinematic pilot capabilities)

For the complete electrical failure in the aircraft, the following systems have to fail:

• The onboard power generation system, usually a synchronous generator has to fail in all the engines.
• The APU (Auxiliary Power Unit) has to fail.
• The RAT (Ram Air Turbine) should fail to deploy.
• The batteries and static inverters should fail (this won't help in controlling the aircraft, just added for sake of completeness).

The probability of this happening is very very low, and can be discounted. It is more probable that some other system (for e.g, hydraulics) fail before this happening. So far, to the best of my knowledge, no complete power loss has been reported in any fly by wire aircraft.

The reason the Airbus aircrafts are controllable in case of electrical failure is due to the fact that the fly-by-wire system has multiple redundancies that offer layered protection:

• Normal Law offers all the protections (like stall, load factor etc). This is the system in operation during normal flights.
• Alternate Law offers limited protection including stability, load factor and stall protections
• Abnormal Alternate Law limits the load factors and offers yaw damping only
• Direct Law is the lowest level of fly-by-wire system and directly transmits the pilot inputs to control surfaces, while still giving stall and load factor warnings
• In case all these fail, the system reverts to mechanical backup, where pitch control is achieved through the horizontal stabilizer and lateral control is accomplished using the rudder pedals.

In case of mechanical backup, the system will operate as long as hydraulic power is available. In case of electrical failure, as long as hydraulics are OK, the pilot should be able to control the aircraft.

However, usually, if there is no power, there is no hydraulic pressure (except reservoir, maybe) and the control is lost. In case of the Baghdad DHL aircraft, only hydraulics was lost and power was available.

There are some more points to consider in case of total electrical failure.

• In case the control surfaces are moved by electric power (like 787), power loss means loss of control.
• Power loss implies that the engine thrust cannot be controlled (of course, the engines won't be operational in that case anyway).
• Some fly-by-wire aircrafts are unstable by design. In these cases, any power loss will lead to loss of control and crash.
• I'm not sure there is any direct mechanical linkage in civil airliners. The amount of force required to operate such a system would be enormous.

Bottom-line is, in case of complete power failure, the aircraft is as good as dead. One example is a Fairchild Swearingen Metroliner (which was not fly-by-wire), that lost complete electrical power due to a lightning strike. The aircraft entered into a uncontrolled flight, which loaded the aircraft beyond limit, resulting in its disintegration.

• The uncontrolled flight was likely a spiral dive due to loss of instrument reference in IMC -- that's the main danger with a total power loss. Also, the DC-9 family uses servo tab primary controls (the rudder and elevator have hydraulic boost, but it isn't needed all that much), and Boeings from the 707 through the 737 have manual reversion in at least pitch and roll. – UnrecognizedFallingObject Sep 16 '15 at 4:54
• Does mechanical backup really require hydraulics? That doesn't sound very mechanical to me, and in fact I have heard in the past that there is an actual mechanical connection for just this purpose... – Lnafziger Sep 17 '15 at 17:03
• I believe the Boeing 737 is the largest airliner to have direct mechanical (ie. cable) linkage for its controls, and even that's indirect: the linkage controls servo tabs which in turn use aerodynamic forces to move the control surfaces. Still, you can (attempt) to fly a 737 even with complete failure of all power systems. – Mark Sep 18 '15 at 1:03
• @Mark -- 737 and DC-9 families, yes. (many RJs are actually non-reversion, interestingly enough, AFAIK, the ERJ-135/145 are the only reversion capable RJs in common use) – UnrecognizedFallingObject Sep 4 '16 at 16:49
• How do these statements line up? In case all these fail, the system reverts to mechanical backup and I'm not sure there is any direct mechanical linkage in civil airliners. The amount of force required to operate such a system would be enormous.? – zymhan Aug 11 at 4:17

Which controls have mechanical linkage differs between models.

In A320 the pitch trim and rudder have mechanical linkage. There is no mechanical backup for roll control; roll control is only possible via yaw-roll coupling.

Remember, that mechanical link really means hydraulic. Without hydraulic pressure the aircraft is not controllable. However at least in A320 the RAT drives a hydraulic pump for the blue system directly.

The mechanical backup is meant for the case of total failure of the flight computers rather than complete failure of electrical power, but that is well in line with what the question on which the commend was made asked.

As aerolinas already mentioned, complete electric failure is extremely unlikely with all the backups there are. But then, complete failure of all the flight computers is not much more likely and as far as I can tell never happened in practice.