Bluesideup
Just as a caveat, I have read most but not all the approximately 1300 posts on this thread. I have yet to read anything concerning the “Thrust Lock Mode” of the autothrust system. If it’s been previously addressed bear with me. For continuities sake, I’m starting at square one of how the system is supposed to work. As for my background, I flew the A-330 for a year and a half as a Captain prior to my retirement (60th birthday) about 1 year ago.
The problem with the autothrust system being inop is the way they interface with the thrust levers. When autothrust is in use the thrust levers are selected to one of 4 detent positions (TOGA, FLX/MCT, CL and IDLE) and don't move even though the engines may be delivering different amounts of thrust to maintain the selected mach/airspeed number. During cruise, they are in the CL or climb detent and have a power range from idle to max climb thrust. CL is the detent the thrust levers stay in 99% of the time. It is used for climb, cruise, descent and approach. To manually control the thrust levers you need to take them out of the CL detent, match the EPR setting with the thrust levers (there is symbology on the engine instruments which show you the position of the thrust levers and the actual EPR setting so that you can match them up before you disconnect the autothrust system). This provides a smooth transition between manual and autothrust use (no power surge). When the autothrust system failed on AF447, the thrust levers were in the CL detent. There is a difference if the system is turned off manually or due to a system failure. If the autothrust system is turned off manually using the pushbutton's on the thrust levers, the system will go to the max power setting of the CL detent unless the thrust levers are manually set as previously described. If there is a system failure the autothrust system goes into the "Thrust Lock Mode" and freezes the engine power at whatever thrust was being developed at the time of the failure. Thrust lock indications are a MASTER CAUT light, an amber flashing ENG THRUST LOCKED and AUTOFLT A/THR OFF,THR LEVERS.......MOVE ECAM messages, a single chime, and a STATUS page message. The chime is repeated every 5 seconds until pilot corrective action is taken. As you can imagine, when you have multiple failures, there's a lot of blinking lights, chimes, bells, whistles and other distractions not to mention a rather long list of ECAM and STATUS messages on the screens. The important thing to remember here is unless you reference the engine instruments, you don't know at what power setting the engines were actually producing at the time of the failure. The autothrust system on the Bus is OK, but remember the thrust levers never move so you lose one of the subtle cues normally found in the cockpit of most other aircraft. The autothrust system is normally very aggressive in trying to meet the speed demands placed on it. By this I mean you can get very large reductions or additions to thrust in order for the system to meet target speeds/mach settings. They are even more sensitive when turbulence is encountered and I've seen large variations in thrust setting in an attempt by the system to maintain target speeds. If you’ve ever ridden in the back of the A-330 in turbulence you will definitely notice the power changes. You also have to remember that power changes at max operating altitudes are very sluggish and it takes time to regain the target speed especially if the speed is bouncing around rapidly. In the AF447 situation, the PF (pilot Flying) slowed the aircraft from .82 to .80 mach. During this time he also encountered increasing turbulence and within a minute or so lost his instrumentation, autopilot, and autothrust. Until BEA releases the Flight Data Recorder readings and we know at what thrust setting the thrust lock mode froze the power, we won't know to what extent the autothrust system was responsible for possibly contributing to this accident. I hope this gives you a better understanding of how the system is supposed to work.