AF 447 Thread No. 5

[RetiredF4]

Trim

@takata and @ CONfiture

Please note again, that i didnīt comment at all on the subject, that the trimming feels like trimming an conventional aircraft or that it is worse or better or not possible.

I said (at least that was and is my intention), that it is not like concerning the process of doing it.

The SS has an artificial feel, i did not deny that. It tells you stick deflected NU, than trim NU to equalize, or vice versa. Itīs not comparable to a tactile feedback (be it natural or artificial), where the amount of mistrim and therefore the amount of necessary correction can be felt in the hand on the stick due to stick deflection from neutral and force required to keep it deflected. It is therefore airload related feedback.

That is IMHO one of the main reasons, why there is autotrim present and necessary in normal and alternate law, as in those laws SS deflection in pitch results in a change of loadfactor, which is kept stable when SS is back to neutral (as long as protections do not intervene). If i understand the system, the elevator is positioned by the flight control computer upon ss input, feedback from the response of the aircraft is fed back to the flightcontrol computer (not the SS) and elevator position is again adjusted by the flight control computer. With some delay the THS trim will zero out the elevator deflection. During this process (except the initial SS input) the SS is in neutral position. How should a pilot zero the elevator demand with manual trim without tactile feedback? Look on a gauge with elevator position? On behalf of the aircraft reaction? Therefore the need of autotrim.

That also poses problems to disable the trim in special flight conditions in normal or alternate law, where to start and where to end the input of autotrim? It would solve one problem and cause some others.

In direct Law there is a direct relationship between SS deflection and elevator deflection (some other factors like CG and configuration and ??? play a roll though), but neither tactile feedback to SS nor to the flightcomputers. Only the feedback from the behaviour of the aircraft is present. But this feedback is dependent on how fast the aircraft reacts to the inputs (as in this stall very slow to react to ND inputs), therefore trimming process (trim, check, trim again, check, trim some more, uups that was too much..) would be slower and a process needing more attention.

Unfortunately (in my oppinion) AF447 didnīt go into direct law, otherwise the aircraft behavior and the trim behavior could have changed the outcome of this accident.

Hope that clarifies my statement somewhat, no intention to post urban legends or to dsicredit airbus or to join any anti airbus comunities. Iīm a german tax payer and lot of german tax money is working with EADS.

Iīm just trying to understand the system in its detail like i was used in the aircraft iīve flown, but in looking on many posts with different understandings on two threads and reading nearly every available documentation from AB over a period of 2 years still keeps me mainly in the dark concerning lots of subjects.

And iīve the impression that the crew of AF447 found themselves in a worse situation, because all the time theyīve got where a few minutes.