Lyman

From Owain,

2. At that sort of AoA the rudder power would be severely limited because (a) it was working at something like 70 deg effective sweep and (b) it was probably sat in the low energy wake of the stalled wing.

Pursuant to your observation, I would like to add that the Rudder/Vstab may have had little if any directional control as a system, and that the Rudder may have had an opposite effect to control deflection.

Would you rule out that Rudder may have reversed its effect, if not wholly, then intermittently?

The Rudder, at 17 degrees sweep from vertical, plus AoA, supplies just the sort of tangential bias from airflow from under the HS and even behind it, at this point? That the airflow past Rudder in this direction works opposite its intended force? Keeping in mind that the elevators, fully UP, do not shield this surface from under tailplane flow?

The Directional system is also in a transiently Stalled wake, that of the HS/elevators?

Perhaps off the wall, but the corner of the Rudder that exhibited damage was in this very unusual flow, and in effect, played leading edge to V/S system, collecting its drag from beneath and behind the HS/elevators? As such, it would be in vulnerable and undesigned for airflow, perhaps in some sort of leading edge flutter that corrupted the structure as seen in early photos?

roulishollandais

structural damage are calculation marge , then first permanent deformation, then rupture. Why to override ? to avoid the ground for instance...

Lyman

I think the limit load is set for the computers at a value that merely meets certifications for aircraft in general.

If I'm definitely nose down for the ground, I'll risk 2.75, or 3.0

roulishollandais

I still remember reading Pierre Baud beginning 1988 explaining that the system accepted both SS inputs to be added...

Dozy, you open the difficult problem of the specifications. Has the flight engineer to discuss them ? It is from his responsability to ask and ask and ask : "Do you really want that ? Do you realise that means also that ? aso".

An example was Ariane5 V501 crash : not only the rocket crashed due to an single wrong carry, but the engineer said later that after the both (wrong) failures of the two inertial systems the computation of the path had to be stopped ! It was the spec ! He obeid without discussion... (8 billions FF 1996).

DozyWannabe

I don't understand how exceeding 67 degrees of bank or pulling into a stall will help you avoid the ground.

That is supposed to be an abnormal operation though. Normal procedure only has one pilot in active control at any one time, and if necessary this can be enforced through the override switch.

I have it on good authority that the test pilots and engineers (aeronautical, electrical and software) worked together to an almost unprecedented level, and nothing made it into the spec unless all groups had reviewed and signed off on it.

Now that *was* rocket science. Very different disciplines involved with a very different design brief.

CONF iture

So simple that you get it wrong : Beside the load factor protection you might have other protections as well depending of the type of malfunction, and 2 levels of alternate law too ...
Also between ALT1 and 2 a confusing mixture of pitch anf roll control.

What is needed is to keep things simple :
Everything works fine - go for the normal law with all the grigri.
Something is in doubt - go straight to direct law to get rid of all those grigri

Your graceful degraduation is not wanted - keep things simple.

Machinbird

Not my airplane, but the following represents my thoughts on how the AF447 crew got so far out of sync with the aircraft.

Normal Law-makes sense
Alt 1 Law-graceful degradation, but the risk is that a heavily stressed pilot may not recognize his reduced protections. (NA to AF447.)
Alt 2 Law-graceful degradation-NOT.
It is an awkward stepchild. It is fairly clear from the narrative that Bonin never integrated the loss of protections into his flying. It would have been better if the aircraft had dropped into full Direct law. It is likely then that the stick response would have been sufficiently different that failure to recognize Direct law would be impossible. Direct law has the added benefit of the HS trim staying put at last setting unless changed by the crew.

It is fairly clear to me that the Crew of AF447 lost confidence in the flight control system's proper operation, and that doubt kept them from recognizing that they were stalled. The doubt began with the initial roll oscillation and escalated as the aircraft stalled. Flying in a unfamiliar part of the envelope was a contributing factor. (ALT2 at altitude)

When you put a guy in an unfamiliar situation, you are flipping coins as to how he will respond. Every so often you will come up tails. (i.e. the wrong answer).

Owain Glyndwr

Lyman

Well they were at least 250 kts too slow to get any sort of aeroelastic reversal and the sidelip/control deflections never got into the non-linear range let alone stall, and even stall is not the same as reversal, so I guess I would rule out rudder reversal.

Frankly, I haven't a clue what you mean by these remarks!

Don't be daft - take a look at a side elevation of an A330 at say 35 deg AoA. The HS sits well behind the fin LE and even if the HS were stalled there is no way that its wake would impinge on more than the very bottom of the rudder. And we know that the HS was operating with no more than about 10 deg AoA so it wasn't stalled and the elevators remained effective, so there wasn't any HS wake anyway.

As for the directional system being in a transiently stalled wake of the elevators, that is even more ludicrous since the elevator hinge lies at the same longitudinal station as the rudder TE at the fin root. No way that any elevator wake (even if it existed) could wash over the rudder.

You don't give up do you? That rudder damage was collected when the aircraft struck the water tail first and the APU was thrown through the top of the fuselage striking the rudder root TE. There is, and never has been, any evidence to support your wild theory of rudder failure in flight

A33Zab

Then keep it simple:

Normal, Alternate(PROT LOST) & Direct(PROT LOST)

Forget about the 'grigri' you still have in whatever Alternate....it states PROT LOST....that is what is the key in the ALt/Direct messages.

The 2 LAW/MODE you suggest is that referred to B. FBW? then you forgot to mention SECONDARY mode as degradation (i.a. no envelope protection) of the Normal Mode.
---

There is an simple and effective autotrim cancel 'feature'.... just hold the manual wheel.

HazelNuts39

I've been wondering what the Flight Director would command in pitch if (for whatever reason) it ignores the fact that airspeed is below VLS. I would think the FD would then command the change in pitch attitude that changes the current vertical speed to the selected value, i.e.:

ΔPitch=ΔFPA=ΔVS/TAS (1° =~ 700 fpm @ 400kTAS)

That ΔPitch, i.e. the angle between the aircraft symbol at the center of the attitude display and the FD pitch bar, was within ±1.5° in the 23 seconds after the FD became available. Would that explain the PF's sidestick inputs?

OK465

Exactly.

Assuming VLS not a factor, the FD commands are a function of the VS, not pitch attitude, (or FPA if that was the selection) in existence (which becomes the 'selected' and FMA displayed VS) when the bars returned.

This will remain so as long as there is no FCU selected change of the VS value, or vertical mode change thru pilot action, or the computations for the FD's again become unreliable.

As the airspeed decays at a given rate, the FD pitch bar will command a pitch rate commensurate with the pitch attitude required to maintain that VS (FPA) at successively lower airspeeds.

Eventually this becomes a losing proposition...

A33Zab

It remains Nz law, so the delta between NzCmd and NzAct = FD bar position. e.g. if you are on commanded trajectory the FD pitch bar will be neutral. (NZCmd = NzAct)

AMM:

Quote:

The pitch FD bar command is computed by using the measured vertical acceleration (NZ) and the NZFD command (pitch outer loop).
The gain and the limitation of the bar command are different according to whether HDG-V/S or TRK-FPA has been selected on the FCU.
With HDG-V/S selected, FD pitch control command is limited to plus or minus 22°5, and with TRKFPA selected,
FD pitch control command is limited to plus or minus 50°.

You need to calculate NzCmd from 'smoothened' SS input...

Max SS = +/- 16 degrees.

RetiredF4

Am i correct, that there is no such procedure, no such advice, no such recomendation and no such training for it?

Only in direct law it says "use manual trim only". But there, because the trim does the only correct thing, it stops working in automode.

HazelNuts39

OK465 and A33Zab,

Thanks for your replies. The question I have: In the time fragment shown, is the PF following the FD commands or is he chasing a pitch attitude?

A33Zab

I know, the only refrence made is in

FCOM 1.27.00 Description:

"Mechanical control from the pitch trim wheel has priority over electrical control."

and

Flight controls normal law.

Automatic pitch trim is frozen in the following cases:

– Manual trim order
- ......

I've read that during training crew is prohibited to touch the wheel,
IMO that's a shame, they should know the dangers and benefits of such action.

To be exact:

In Direct LAW it is PFD amber message "USE MAN PITCH TRIM"
in backup (Elevator not available) the message is red "MAN PITCH TRIM ONLY"

Lyman

A33Zab

in backup (Elevator not available) the message is red "MAN PITCH TRIM ONLY"

Where is the colon?

MAN PITCH : TRIM WHEEL ONLY

Nit pick?

OK465

HN 39:

If I interpret your graph correctly, your FD pitch bar plot represents the angular difference between the pitch attitude symbol (theta) and the FD bar, i.e. mostly within + or - a degree and one-half until around 72s.

If this is all referenced around your upper plot of a possibly active VS of +1500 displayed on the FMA, then you graph clearly shows that when aircraft vertical speed was below +1500 (48s-55s), the FD bar would be above the aircraft pitch symbol (theta), and vice versa when the aircraft VS was above +1500 (56s-67s), the pitch bar would be below the aircraft pitch symbol.

At 67s-72s the bottom began to fall out.

As to your actual question, since the trend for pitch is a generally consistent increase (not a specific attitude) with some minor variation (as would be expected control-wise with the speed decay) until the aircraft VS starts to go deeply negative, it sure looks like an attempt is being made to follow the FD until it can't be. I think as a practical matter the variations in SS input could be a result of the 'mushy' pitch response to the inputs as the stall is entered, as they are generally back and forth around neutral until the FD's are once again removed later. At which time the input is definitely only NU indicating lack of cues requiring pitch 'fine tuning'.

I hope I stated this clearly, and it is of course just an opinion...there may be other explanations which fit.

However, I've observed a number of pilots making their first attempts at PRM (precision radar monitor) breakouts where the FD's are initially turned off, but can as a matter of course inadvertently return or be reselected in other than the desired mode, and the results can be spectacular both vertically and horizontally, and they're not even stalled.

(Question: Where did your FD pitch bar info come from? Surmised?)

HazelNuts39

Calculated as (VSsel-VS)/TAS in consistent units, radians converted to degrees. Maybe that is too simplistic?

Lyman

OK465

IYO, is this the first time in the last two minutes PF has chased his tail? Or is it possible it is a close reprise of what got him to 38k in the first place?

lyman

OK465

HN 39:

Other than the possible oscillatory FD effects of a variable rate of decay of TAS due to pitch inputs, it looks to me very representative of the precision associated with someone attempting to fly a flight director, to the extent of making the hairs stand up on the back of my neck.

You're the physics dude, I defer to you on the details.

(Lyman: You need to check IR #3 for when the FD's were actually available. My opinions only refer to the insightful graph. )