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Back to AF447. Semantics can always be an issue. Neutralizing the elevator demand means just that, i.e. if continuous demand is made either NU/ND the THS will move in that direction until such time as the SS is placed in the neutral position. In Alt Law the Alpha protections are not available, and for this reason the elevator demand becomes a THS command as explained. At no time was the SS placed in the neutral position which would have enabled the autotrim function to maintain 1g, so effectively that function was over-ridden by the PF. Hence my reason for saying that 'autotrim' had nothing to do with it.
It has nothing to do with semantics, some of your statements are wrong. Airtren explained it already, let me try it again. Others feel free to correct me.
First some references out of LTTM (Technical training manual)
GENERAL
The pitch control is achieved by the two elevators and the THS via the computers, and controlled by the pitch side sticks orders or autopilot commands.
Max elevator deflection :
30° Nose up
15° Nose down.
Max THS deflection:
14° nose up (THS)
2° nose down.
In AP mode, the Flight Management, Guidance and Envelope Computers
(FMGEC) send the command orders to the FCPCs ; the FCPCs transmit them to the FCSCs
THS General
The aircraft has a Trimmable Horizontal Stabilizer (THS) , which has two elevators, for pitch trim control. The two elevators are attached to the trailing edge of the THS. The THS is attached to the rear fuselage and moves about an axis to permit pitch trim. The hydromechanical operation system of the THS (referredto as THS actuator) is controlled electrically by the Flight Control Primary Computers (FCPC) and mechanically.
Operation/Control and Indicating
There are three control modes for the THS:
autoflight (electrical control)
manual (electrical control)
standby (mechanical control)
In the autoflight mode the command signals fromthe autopilot are sent to the
FCPCs. The FCPCs transmit autotrim signals tothe electric motors which control the THS actuator.
In the manual mode the command signals from the side sticks are sent to the
FCPCs. The FCPCs transmit autotrim signals to the electric motors which control the THS actuator. The computers elaborate command orders to the servocontrols, depending on the different control laws.
In the standby mode the command signals are transmitted mechanically from
the control wheels to the override mechanism. The override mechanism cancels the autotrim signals from the FCPCs. It transmits the mechanical command signals directly to the hydraulic motors of the THS actuator.
ALTERNATE LAW WITHOUT PROTECTION
In this case, the pitch protections are lost except the load factor protection.
This alternate law without protection is activated in the FCPCs after a triple
ADR failure.
DIRECT LAW
In pitch Direct Law, all the pitch protections are lost.
The elevator deflection is proportional to stick deflection.
The autotrim function is lost and only the manual control of the THS is available.
Laws Reconfiguration - General
The reconfiguration of control laws is different in pitch axis and in lateral axis.
Control law reconfigurations are divided into two families :
- ALTERNATE
- DIRECT
In the event of loss of the normal control laws:
When the conditions required for keeping the normal control laws are no longer fulfilled, the control laws are reconfigured. The various degraded law states possible are (in flight or upon flare):
Roll and yaw:
- Yaw alternate law
Pitch:
- Nz law (with limited pitch rate and gains)
- Vc PROT law
- VMO2 law
- Pitch direct law
The laws called ”Alternate” are engaged when the protections related to the
normal laws (ALPHA 1, VM01) are lost.
The laws called ”Direct” are engaged when the Nz law is lost.
Pitch
The aircraft pitch control is achieved from the side sticks and in certain cases, from the pitch trim control wheels, which act on the elevators and on the THS, depending on the different laws.
Nz law
This law, elaborated in the FCPCs, is the normal pitch law engaged in the
flight phase.
Through a pitch action on the side stick, the pilot commands a load factor ;
the Nz law achieves this command, depending on the aircraft feedbacks, so
that:
- The short-term orders are achieved by the elevator servo controls.
- The long-term orders are achieved by the THS actuator (autotrim function).
The gains depend on the Vc, on the flap and slat position and on the CG
location.
In addition, the Nz law permits to achieve:
- A load factor limitation, depending on the flap and slat position.
- A bank angle compensation, for bank angles lower than 33°.
- A deflection limitation of the THS in the nose-up direction in the event of
the activation of the high angle-of-attackprotection, the excessive load
factor and the excessive bank angle exceeding.
The Nz law is such that the aircraft response is quasi independent of the
aircraft speed, weight, and CG location. If both ADIRUs are failed, the Nz
law is kept, but with limited pitch rate and gains. A consolidation of the vertical acceleration and pitch attitude rate is then performed via the two accelerometer units.
TURBULENCE DAMPING FUNCTION
General
The purpose of the Turbulence Damping Function implemented in the Electrical
Flight Control System is to damp the structural modes induced by atmospheric
turbulence.
Architecture
The Turbulence Damping Function consists of two lanes:
Longitudinal lane
The longitudinal Turbulence Damping command is computed by the FCPC1
(FCPC2 as a redundancy) as a function of the Nz accelerometer information.
It is added to the normal law command and transmitted to the associated elevator servo-controls.
Rear lateral lane
The rear lateral Turbulence Damping command is computed by the FCPC1
(FCPC3 as a redundancy) as a function of the informationof a specific Ny
accelerometer located at the rear bulkhead level.
It is added to the normal law command and transmitted to the associated yaw damper.
Specific equipment
The equipment specific to the Turbulence Damping Function are:
- the TURB. DAMP pushbutton switch
- the Ny front accelerometer
- the Ny rear accelerometer.
In the standby mode the command signals are transmitted mechanically from
the control wheels to the override mechanism. The override mechanism cancels the autotrim signals from the FCPCs. It transmits the mechanical command signals directly to the hydraulic motors of the THS actuator.
As we know from BEA, Alt2b was active, which means concerning the pitch that we lost some protections (Alpha1, VCprot..), the rest stays basically the same.
In layman term the SS commands loadfactor to the FCPC, where this demand is transfered to a n elevator deflection to achieve this demand. SS position does not represent elevator position. As can be seen in the FDR traces the elevators follow the computer demand, in our case the elevators stayed in the full NU position also when SS was not full up That´s because the computers tried to achieve the loadfactor demand. The THS was already full up and the aircraft could not achieve the desired loadfactor with full elevator NU. Later with relaxing of NU command and even with some ND command the reduction of the elevator from full NU was only by 15° to 15° NU, because that was enough to achieve the new corrected loadfactor demand. As the elevators where deflected NU, the trim command to the THS at that time would still have been NU instead of the necessary ND.
Only when the SS input would have been held ND long enough to change the loadfactor demand significantly (dont know if loadfactor protection would come into play) and the elevators would need a ND deflection to achieve this new load factor demand, then the THS trim would start to wind the THS down to neutralize the elevator position.
Why did the THS first not move, and later on continously full up? First the elevator authority was enough to execute the loadfactor demand with little deflection only, with decreasing speed the Computers ordered the elevators to further NU to follow the demand, and then the THS started to compensate for the elevator deflection. But the still decreasing speed and still present loadfactor demand led to a continuos NU command of the elevators and the THS in the following time frame.
To sum it up in my short words, pitch control in Alt2 is the same as in normal law with AP off, except vital protections lost. No change of Autotrim. The mainly NU input of PF ordered a loadfactor, which the computers could not achieve with elevators and autotrimming the THS in the decreasing speed environment. The few moments of reducing NU order or even giving ND order did not bring the elevator into the ND command region, therefore THS stayed all the way NU.
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As an example, go back and have a look at the the initial zoom climb. The initial elevator NU commands were not aided by the THS moving because the allowable 'g' in Alt Law was exceeded ([+1.25/-0.75] where did I get that? Don't know - must have read it somewhere). During the climb the THS moved to maintain the pitch attitude when the SS movements were nominally around the neutral position, but when the 'g' went negative it moved back to 3°NU and only started tracking toward maximum with continued SS NU as the aircraft proceeded to leave the flight envelope.
I couldn´t find the reference to this loadfactor protection either, although i remember some similar postings here that it was not active due to the nature of the speed failure? A
BEA in its report mentiones the ALT2B law as present, but does not describe in detail, what kind of protections had been lost and which ones still had been active. In my references i couldn´t find what the letter "B" in Alt2B stands for.
Would be interesting to know, how much unloading in term of g would have been accepted by those remaining protections. Because that again would influence elevator position and therefore autotrimming. Also if the turbulance dampening mode was activated and if that one would influence the response to inputs.
IMHO direct law from beginning might have led to an different outcome.
Sorry for the long post.