Machinbird

Does the Ny rear accelerometer have a fixed mounting position or is there a range of adjustment to zero out lateral g sensing? If adjustable, are there any procedures in setting up this accelerometer that could be non-responsive if the aircraft was in Normal law instead of Alternate law? Is the Ny rear accelerometer used in Normal law?

I realize that this can be potentially be a time consuming area to research, but it might be useful to "sweep out" this corner of the Yaw control system for things that could induce a yaw input in Alternate2 law. FCOM search has been non-productive. TIA

bubbers44

Air France Flight 447: 'Damn it, we’re going to crash’ - Telegraph

bubbers44

This crash says the pilot not flying needs to be more aggresive in taking over when the other is AFU. A little forward pressure on the side stick would have solved the situation. Most Boeing pilots seeing a control wheel in their gut would do so instantly. Just say I have the airplane and problem is solved. Side sticks make that harder to do. You can not tell what the other pilot is doing from your seat. Airbus isn't going to change anything so I guess noticing the little annunciator is their only clue what the other pilot is doing. Guess this will be happening again.

Owain Glyndwr

I really don't see the problem here.

Ny is computed from an accelerometer mounted near the top of the rear bulkhead.
If it is to be used to estimate sideslip then the reading needs to be corrected for rudder sideforce (deflection) roll and yaw rates. If airspeed information goes south then so does the estimated sideslip.
So far as I can see, the only use of Ny and/or sideslip in the EFCS is a part of the lateral turbulence alleviation function.

In Alt2 there is no bank protection (and therefore no spiral stability term). Consequently the EFCS neither needs nor uses either Ny or estimated sideslip for roll control in this mode.
In Alt2 the rudder deflection is controlled by pedal angle and yaw damper commands. The yaw damper uses yaw rate; there is no Ny or sideslip term.

Consequently the EFCS doesn't use Ny at all when Alt2 is engaged.

The estimated sideslip is sent to the PFD.

That's it. Full stop.

mm43

Agreed, and thanks for your generous reply. However, it is somewhat difficult to follow what inputs the YD is getting, and what effect NCD data may be having on the Ny Accelerometer inputs to the YD calculation.

The DFDR Lateral Acceleration (sideslip) trace is reflected in the YD output and its amplitude appears to have increased marginally during the descent and has a natural period of around 7 - 8 seconds which only gets interrupted by prolonged left bank commands. As pointed out earlier, the clockwise heading change is greatest when the Ny driven oscillation is dampened by hard over and held left SS inputs. So my feeling is that the PF managed to dampen the Ny oscillations that could have easily resulted in a total LOC.

Having written the above, then noted your latest comments, I am no further ahead in resolving what I still consider is the YD driving the Rudder and "leading" the Roll/Bank. I've had a look at the Pitch Attitude trace and find that maximum yaw coupled with side-slip tended to occur when pitched up with a RH roll bias.

You say that, "In Alt2 the rudder deflection is controlled by pedal angle and yaw damper commands", and yet in the period we have been looking at, only YD commands have been presented to the rudder. How or why does the Lateral Acceleration trace then synchronize with the YD / Rudder traces? To answer my own question, I have previously posited that the aircraft was effectively 'fish tailing'.

Its not important to the outcome, though helpful in understanding what is actually happening in what appears on the surface to be an aircraft in a benign stall and retaining some lateral stability, i.e. avoiding the 'spiral dive'.

rudderrudderrat

Hi mm43,
I think the clockwise heading change was due to adverse aileron drag when the wing was stalled. The differential spoilers would have been ineffective in the turbulent flow above the wing. But the RH down going aileron would present more drag to the forward aircraft direction and induce a turn to the right, effectively "the wrong way".

I think that was first explained by Owain.

edit: I don't think PF managed to do anything other than mostly pull back during the stall. His roll commands were ineffective once the wing was stalled.

Owain Glyndwr

The answer to that I think is that there isn't any Ny accelerometer input to the YD calculation when Alt2 is operative. So as as I know, the sole short term input to the YD in this state is rate of yaw. Ny does affect the YD operation when in normal law in that the lateral turbulence damping commands are fed through the YD, and if you want to reduce motion at the back end of the cabin then an accelerometer on the rear bulkhead would be a good starting point but as I understand it this is a pilot selectable option

The DFDR trace is Ny, which is not simply the lateral acceleration due to sideslip - there is a substantial contribution from the gravity component when banked. And as I said, at high AoA bank generates sideslip directly although there will be additional slip associated with accelerations along the y axis coming from sideforce. Up to 02:13:00 there is very little rudder application, then it is active for about 45 secs after which it goes quiescent again, and while the rudder application was active there is little sense of any reduction in the rolling motion, so I'm not at all sure that your feeling is justified.

I'm sticking with my feeling that it is lagging.

That is simply because the PF made no rudder pedal movement surely?

They both synchronize with the aircraft dutch roll/roll oscillation motion - the measured lateral acceleration with bank angle plus some additional 'g' from yaw movements (assuming that Ny comes from that rear bulkhead accelerometer); the YD traces synchronize with yaw rate.

Yes I agree it is not a be all/end all point. One additional remark may be relevant though - when you get into or towards that high AoA/roll oscillation condition it tends to be a situation where roll due to sideslip is high and weathercock stability low, which is the condition for more positive spiral stability, which makes the reason for the continued roll right even more mysterious! [I've seen this before - in my youth we tested a free flight model which had this roll oscillation and even when we cut the complete fin off it simply wallowed in a straight line with no spiral divergence]

CONF iture

There is no such thing as 'the little annunciator' in flight.

The article has the merit to state the commun sense analysis regarding the sidestick concept by Airbus.

Never, the BEA would mention anything in that direction in the contributory factors chapter.

OK465

FWIW, with a reliable air data source, any sideslip will also cause the computed displayed winds on the ND to be in error. As a result, the GS displayed on the ND may also be erroneous during the uncoordinated condition.

These are normally such small variances, they are probably rarely noticed.

ironbutt57

Thought, neither of the two 757's lost due to inaccurate ADR..granted for different reasons had side sticks, and yet nether one was control regained...also a "little sidestick forward" might well not been enough to counter the THS position....might have indeed required full stick forward, AND manual dose down trim...

Machinbird

In the old days before automation, we pilots were confronted with a bunch of steam gages, one of which was the old needle-ball indicator. Ny is the analog of the old ball indicator. The rule was "step on the ball" to center it. All the jets I flew including a straight wing one had a yaw damper controlled by a yaw rate gyro. If Ny is used in the EFCS, it is not for yaw damping, but for biasing out lateral g through rudder deflection.

You seem to be in the possession of inside information regarding the arrangement of the 'bus control system, however how certain are we that the bus EFCS in Alt2 does not have authority to "center the ball" in Alt2, and only sends a signal to the PFD equivalent of the ball telling the crew to center the ball.
This seems to conflict with:

mm43

Hi rudderrudderrat,Yes, and that is exactly what I had concluded ...Owain Glyndwr;

Many thanks for sharing your experience in this debate.Maybe just a case where the overall drag and the wake vortex conspire to provide a stable outcome.

jcjeant

Hi,

A nice graphic representation



The BEA cartoon ....

bubbers44

CONF, so the Airbus people that say there is an annunciator showing what the other pilot is doing is not true? I have never flown an Airbus by choice so assumed the annunciation from Airbus pilots was a true statement. Maybe there is no annunciation of what the other pilot is doing. That is even worse than what I thought about one pilot knowing what the other is doing In a dark cockpit. Please tell me this is not true.

Owain Glyndwr

Machinbird

Sorry, I don't have any inside information, but I do have a downloaded copy of an A330 FCOM which states that for Yaw control in Alternate 2:

Dutch roll damping is provided with authority limited to 4 deg in CONF 0, 15 deg in other configurations.
Turn coordination is provided except in CONF 0

So for the AF447 case the EFCS does not centre the ball, although in most other cases if does.

mm43

Adding to the above;

From Airbus A330 Instructors Support Manual -It appears the comment on yaw applies to any of those conditions mentioned for ALTN 2.

CONF iture

Nothing but the truth. For the 30 sec period the PF was full back stick, both captain and PNF were unaware of such vital piece of information.

Correct but ...

• In the Birgenair case, the PNF had that priviledge to witness how the displacement of the yoke by the AP was consistant with its own side instrumentation. He tried to tell the captain but maybe was not confident enough to intervene ...
• The Aeroperu was a nightmare scenario where airspeed altitude V/S were all wrong. The end result was a CFIT where flight control displacements were minimal. In that case I agree, to have coupled yokes was not an advantage to the independant sidesticks (but neither a disadvantage).

For the THS, don't get me started again please. I am still amazed how the THS got almost full up :

1. automatically
2. under stall warning
3. the command by the sidestick for the period was more up than down but nothing that drastic

bubbers44

I have never flown an aircraft that one pilot did not know what the other was doing. What a strange way to design an airliner.

gums

Salute!

c'mon Bubbers.....

Airmanship!

Flew one jet with no second seat, so we flew "chase" and could only see/feel what the nugget was doing. No big deal, as just ask if he was putting in "x" stick.

In the Viper we flew chase for the guy's "solo" mission just to keep USAF happy. We had already flown a few hours in the family model. And like the 'bus, no visual or feeling what the nugget was doing with his sidestick.

I like the tacit feel and look of the yoke for a "crew" plane. but in this case the clue is to look at what the indicators were showing. 'course, seems the indicators were FUBAR and the crew had to deal with all the nuances of the reversion modes and.......

Gonna be a landmark finding report, as others have suggested.

ExSp33db1rd

Boeing even let you see what the Auto-pilot is doing, it was vaguely comforting on a dark night to see the controls columns and throttles gently rocking or rolling back and forth, just as they would do if the other pilot was hand-flying. Kept one in the loop, so to speak.

I used to have a bumper sticker on my briefcase - " Boeing, my way "

History for me now of course, but nothing will persuade me otherwise, bigotted if you like. Don't care.

What don't you like about it, nothing new, just putting it all together in non-pilot-speak. Don't need the emotional stuff about children, and a few pax, of course, leave that to the other merdia.