Old 8th Jan 2015, 19:09
  #19 (permalink)  
Join Date: Feb 2009
Location: Seattle
Posts: 379
Levels of Augmentation

Island Airphoto

I fully appreciate your concern that there have been far too many instances of crews pressing on with a full up control system when there appear to have been sensor data failures that went undetected and blindly followed by the crew to disasterous consequences. I think there are a number of improvements that are needed.
1. Better training of crews to recognize inconsistencies in airplane response data so that they are able to recognize potential sensor failures and adjust their control actions accordingly.
2. Refinement of control system signal selection, fault detection logic to perform signal consistency checks automatically to identify errors such as blocked pitot tubes that may equally corrupt all air data sources. Crews should then be alerted as to which data is suspect and control augmentation mode should revert to a configuration that does not rely on the data deemed to be in question.
3. Clear guidance to flight crews as to how the control system should be reconfigured if they suspect errant data.
4. Simulator experience flying the airplane manually throughout the flight envelope including exposure to any degraded levels of augmentation that involve significant handling qualities changes.

As to the point about open loop airplane stability, it is important to note that even the lowest level of control system configuration may require some level of augmentation. For instance, FBW augmentation has allowed airplane configurations with the certified cg range such that cg at its aft limit results in zero steady state elevator for a maneuver. This does not present a system that is wildly unstable, but does yield neutral pitch stabilty. In order to assure that the handling qualities experienced by the flight deck crew are at least acceptable, augmentation of some sort has been added to all modes. The B777 is a good example where the lowest level of augmentation (Direct Mode) includes inertial pitch rate feedback to the elevator.

I think it is more appropriate to speak of pilot selection of the lowest certified level of augmentation rather than pilot ability to turn off all of the computers. A subtle difference, but one that I feel we need to be clear about. We should not advocate having the crew manipulate the control system such that it is in a configuration that has not been fully tested. In the case of the B777, the handling qualiltes have been carefully and completely evaluated in Direct Mode (the lowest augmentation level that is both pilot selectable and can be automatically engaged in the event of detected failures) and found to be adequate for a low probability backup system. The B777 has not, however, been tested in a configuration where the elevators are commanded by pilot control column position alone.
FCeng84 is offline