Role of FBW Augmentation
While fly-by-wire augmentation provides the benefits of greatly reducing pilot workload and providing flight envelope protection, those are not the only reasons this technology has been developed for commercial transport aircraft. An additional driving factor (and in some ways the key motivation for FBW) is the opportunity to increase airplane performance. FBW provides handling qualities enhancement through augmentation thus enabling airplane configurations to be optimized for performance rather than handling.
Prior to FBW, commercial airplanes had to be configured to provide acceptable handling qualities without control system augmentation. Metrics such as stick force per g, stick force per knot, and maneuver response damping posed design constraints on cg range. In addition, wing design had to account for stall characteristics such as positive Stall ID and pitch stability. With FBW the control laws can be designed to augment the open-loop airplane characteristics such that the closed-loop response is acceptable. This allows pushing the cg further aft and designing wings for higher L/D (with less concern about stall characteristics) thus improving airplane fuel economy.
As a result, the truely open-loop (i.e., no computers involved) handling qualities of today's FBW airplanes are not sufficient to support certification. Reversionary control law modes are provided where the full-up normal mode system does not have sufficient availibility, but most often these include some level of augmentation to help improvide the handling qualities above what would be found with no augmentation at all. Because the probability of being in a reversionary control law mode is quite low, the handling qualities requirements that apply are not as stringent as for the full-up, every day normal system. Calls for the flight crew to have the ability to "turn all of the computers off" must be considered carefully as the handling characteristics they would encounter could be more than a handful.
Modern FBW commercial airplane control systems are designed to provide graceful degradation of levels of augmentation in response to detected failures. Of particular interest is loss of air speed and/or angle-of-attack. Multiple sensors and monitoring logic that compares signals from indepenent sources makes these systems robust to equipment failures through signal selection, fault detection logic. This leaves common mode failures (ones that corrupt equally all sources of a particular type of data) as the most serious and potentially dangerous. Severe icing or an encounter with a volcanic ash cloud are two scenarios that can cause bocked pitot probes leading to undetected erroneous air data and thus must be considered.
Pilots have long been taught to consider all of the sources of data that they have available to them and to be on the lookout for inconsistent data that may point to a sensor failure. Climbing with idle thrust while indicated airspeed is increasing is an example of a clear inconsistency that should cause the crew to question the airspeed indication. While some of the latest control systems include logic to detect such inconsistencies and select lower levels of augmentation that do not rely on the suspect data, pilots need to be able to make such determinations themselves.
Whether or not commercial transport control systems include provisions for pilots to select lower levels of augmentation is a design philosophy issue - one where A and B have taken different paths. Any procedures for crews to selectively down-mode to less augmentation must consider the handling qualities consequences. Going all the way to open loop is most likely not the best choice.