In the case of a flame-out in both engines on the two-engine A320, does the aircraft revert to Direct law or does it maintain Normal Law?
An example is US Airways 1549 (the landing on the Hudson).
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3 Answers
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The airplane will no longer be in normal law mode if both engines fail and no other electrical generator is online.
The airplane needs a primary source of electrical power in order to remain in normal law (among other things of course). If both engines lose power, then the generators on those engines also lose power. The only other primary electrical source is the APU generator. In addition to the batteries, the RAT (ram air turbine) is also available to supply electric power (along with hydraulic power) in an emergency situation. In this mode, the flight controls reconfigure to alternate law until a generator is brought back online.
From the US Airways 1549 report, page 88:
...the captain started the APU, which improved the outcome of the ditching by ensuring that a primary source of electrical power was available to the airplane and that the airplane remained in normal law and maintained the flight envelope protections
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$\begingroup$ You say it changes to alternate law, but your quote says normal law $\endgroup$ Apr 7, 2015 at 23:03
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5$\begingroup$ @raptortech97 Starting the APU prevented it switching to alternate law. Because the APU was running, the plane stayed in normal law $\endgroup$– mjsApr 7, 2015 at 23:14
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1$\begingroup$ @mjs Ohhhh! That makes way more sense! $\endgroup$ Apr 7, 2015 at 23:15
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When both engines have failed, the A320 reverts to alternate law. However the pitch law is replaced by the direct law upon landing gear extension. The side stick movement directly controls the elevator position. It means that side stick input is proportional to flight control output. The THS is mechanically controlled from the trim control wheel. No protection is included.
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1$\begingroup$ do you have any source? (pilot handbook or similar?) $\endgroup$– FedericoMay 24, 2016 at 6:32
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According to https://www.skybrary.aero/index.php/Flight_Control_Laws
As is the case with ALT1, some failure cases that result in ALT2 will also cause the autopilot to disconnect. ALT2 is entered when both engines flame out, with faults in two inertial or two air-data reference units, with faults to all spoilers, certain aileron faults or with a pedal transducers fault.
In Alternate Law 2 (ALT2), Normal Law lateral mode is lost and is replaced by roll Direct Law and yaw Alternate Law. Pitch mode is in Alternate Law. Load factor protection is retained. In addition to those protections lost in ALT1 (Pitch Attitude and Low Energy Protection), Bank Angle Protection is also lost. In some failure cases, High Angle of Attack and High Speed Protections will also be lost.