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Air Asia Indonesia Lost Contact from Surabaya to Singapore

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Air Asia Indonesia Lost Contact from Surabaya to Singapore

Old 28th Jan 2015, 12:23
  #2641 (permalink)  
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ATTITUDE

surely attitude must be set in addition to the 90% N1. On a Boeing it is safe to use 2-3 degrees nose up in cruise. Must be the same for Airbus
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Old 28th Jan 2015, 12:51
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@Jet Jockey A4

it trims ... to relieve back pressure on the elevator
Yes.

If I'm correct in my assumption as stated above? If so what is the big deal behind this feature on the Airbus?
There is no deal at all.

Assuming the THS is trimming a nose up attitude because you are pulling on the sidestick, wouldn't the reverse also be true
that with a nose down input the THS would trim nose down?
Yes.

Back to AF447 and the THS being full nose up. If the pilot would have just inputed a full nose down on the sidestick, wouldn’t the THS trimmed nose down too?
Yes.

Last edited by jimjim1; 28th Jan 2015 at 12:52. Reason: typo
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Old 28th Jan 2015, 12:53
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Do I understand this feature correctly as a pitch trim that is activated automatically whenever an extended pitch input (up or down) is commanded through the sidestick?
I believe it essentially alters the "THS" (Horiz Stab) to automatically align with the elevators - much as can be done on a 1950s jet fighter So as you say:
In other words if I command a pitch up from the sidestick, it trims in a nose up to relieve back pressure on the elevator not unlike when you fly a conventional aircraft in manual when using the electric trim or basically the same way an aircraft on AP would trim itself?
If so what is the big deal behind this feature on the Airbus?
None whatsoever to you & me - but to the Airbus bashers, it means it was the Airbus' fault that when a pilot held the stick back and flew to a crazily low speed (AF447 & Perpignon), and the aircraft duly trimmed back, it meant when the pilot let go and/or belatedly tried to recover, it was trimmed back in a way that did not assist recovery.

There is the aspect, which is true, that if the aircraft then drops into Direct Law (as happened in Perpignon), the auto-trim stops, so your:
If the pilot would have just inputed a full nose down on the sidestick, wouldn’t the THS trimmed nose down too?
would not happen in this case. In AF447 I don't think it dropped to Direct law, so yes, stick forward and trim forward would have happened...

Assuming you are in cruise at FL370 in an Airbus 320 and all is working properly, you then encounter some mountain wave turbulence or something similar to it, what usually happens at first in an increase in speed with a tendency in an increase in altitude usually compensated by the AP.

In the above scenario in a very conventional aircraft with a basic AP without AT, if left unchecked and if the power is not reduced, the aircraft may overspeed but it should hold altitude. In an aircraft with AT, the FADECs should automatically reduce to a power setting as to avoid overspeed.

However it seems from reading on here that the Airbus might want to pitch up to avoid overspeeding, is this correct? At what point will the Airbus reduce power to keep it within the normal parameters?
To be precise, it does not pitch up to prevent overspeed, but minimise the extent when it has oversped. First the AP disconnects, then further into overspeed it pitches up a small amount (which you can override) until eventually even full forward stick will not counteract it. It does not reduce power - albeit if ATHR is on, that should have selected idle.

The ATHR is not great, and in Mountain Waves it can be almost hilarious as speed oscillates between Min and Max, and Power between full & idle, but out of phase. Disconnect ATHR, set sensible power and leave, and the speed oscillations are far lower

I suspect it would be fair to say that in the well known overspeed events, poor monitoring or inappropriately high cruise speeds may have been factors. Certainly my experience on the line is poor choice of speeds with regard to conditions - any excuse to go faster seems the aim
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Old 28th Jan 2015, 13:02
  #2644 (permalink)  
 
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Do I understand this feature correctly as a pitch trim that is activated automatically whenever an extended pitch input (up or down) is commanded through the sidestick?
Not only for extended input. Any sidestick pitch input commands g´s, or a change in the flight path vector (it needs a normal acceleration to change the flight path vector). Releasing the stick does not mean you return to your previous trimmed pitch, but you maintain the new one. This is achieved by use of the trim.
To oversimplify a little bit, the Airbus Sidestick is the same as the trim switch on some conventional aircraft. Which is exactly what you need for a large transport aircraft. You need the stick to rotate, to level off at cruise altitude, to initiate the descend and to level of for touchdown. In the optimum situation, two of these actions are done via the A/P selector panel, so you basically need the elevator control twice per flight (both times in the nose up direction). So for some pilots that is all they have done with the stick in the recent years...

Back to AF447 and the THS being full nose up. If the pilot would have just inputed a full nose down on the sidestick, wouldn’t the THS trimmed nose down too?
Unless that would have resulted in an overspeed situation, and hence the system would not have allowed it, yes. So maybe it would have required to throttle down or use the speed brake until the system would have accepted this input.
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Old 28th Jan 2015, 13:14
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Originally Posted by NigelOnDraft
None whatsoever to you & me - but to the Airbus bashers, it means it was the Airbus' fault that when a pilot held the stick back and flew to a crazily low speed (AF447 & Perpignon)
The pilots @ Perpignan were pushing their sticks forward once the upset because apparent, not back. They apparently had not seen the "USE MAN TRIM" warning on centre display. The AP had commanded full nose up trim just prior to that, causing the elevator deflection the control sticks could commands to be too small to recover the aircraft before it stalled.

I'm not blaming or bashing anyone or anything, just stating it as it was.

The extreme nose up attitude and then sudden drop toward the ocean AirAsia is tracked to have done is remarkably similar to the Perpignan crash, just higher in altitude.
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Old 28th Jan 2015, 13:19
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Thanks to all for the answers.

Personally I don't see anything wrong with this autotrim function...

Whenever the Airbus pilot inputs a command via the sidestick when flying the aircraft manually, it helps relieve the pressure on the elevator in the same way on a conventional aircraft one would have to use the electric trim (or manual trim) to relieve the pressure and this is not unlike the autotrim feature on aircrafts I flew when the AP is engaged it's just the Airbus takes it a step further and does it when being flown manually.
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Old 28th Jan 2015, 13:22
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This is not Airbus bashing; it's not even FBW bashing, but....

There have been so many threads on here, before and after incidents/accidents, that I am curious about one point. The discussions about what might have happened, what did happen, what the pilot should have done, why they didn't do it, why they did something else or nothing, why they were confused etc. etc. mostly there has been no clear concise answer to these queries; and many AB training captains have been involved in the debate. There have been answers such as " it did exactly what it was supposed to do, but they didn't realise it."
The discussion going on now is how a pilot can take over and fly the a/c in the most basic manner and what its capabilities will be when you do so. And to a non-AB driver it is not clear. I really wonder if you put 100 experienced AB plots in a room, and through these type of questions at them, how many of them would come up with the correct answers. If less then 90 that is very worrying, and why do I expect that to be the case? Am I wrong? And how many would not understand the question?
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Old 28th Jan 2015, 13:22
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Originally Posted by Nigel
To be precise, it does not pitch up to prevent overspeed, but minimise the extent when it has oversped. First the AP disconnects, then further into overspeed it pitches up a small amount (which you can override) until eventually even full forward stick will not counteract it. It does not reduce power - albeit if ATHR is on, that should have selected idle.
Say what?


Back to AF447 and the THS being full nose up. If the pilot would have just inputed a full nose down on the sidestick, wouldn’t the THS trimmed nose down too?
Originally Posted by Nigel
Unless that would have resulted in an overspeed situation, and hence the system would not have allowed it, yes. So maybe it would have required to throttle down or use the speed brake until the system would have accepted this input.
Overspeed? Correct me if I'm wrong, but when the sidestick was pushed forward the speed increased to above 60KIAS and then the Stall Warning came back on. What sort of feedback message does that send?

Autotrim when handflying is bad news; nice and geeky when all is well but precisely what you don't want when things go pear-shaped. Either you're fully flying it ie trimming as well or the AP is in.
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Old 28th Jan 2015, 13:35
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FBW, trim

Just to be absolutely clear, with FBW you can never fly it completely "like" a non-FBW aircraft. There is simply no physical linkage there to fall back to. There is only a wire (wiring) electronically communicating intended control movements to control surface actuators.

Of course in Direct Law the intention is that pilot inputs are directly and immediately communicated as-is to the actuators. (And maybe that is what was meant by "like" above.) But technically it is still a simulation of a direct physical link (something like controlling avatars in video games).

Not that there is anything wrong with that, by any means. Just that it is different. With differing failure modes, for instance.
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Old 28th Jan 2015, 13:41
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Jet Jockey

The AOA Sensors were found to have frozen in place at a relatively flat angle. (due to the plane being washed with a pressure hose, allowing water to get into the workings of the AOA sensors which then froze at altitude)

The crew initiated a near stall moment as part of their flight tests to make sure the AP did it's flight envelope protection job.

Aircraft pitched up as it's supposed to, but due to the stuck AOA sensors, it thought it was still on the level and still slowing, so commanded the nose further up. By the time the crew realised something wrong, the AP had given up, given them control and threw up the "USE MAN TRIM" warning.

As I understand it, with the stabiliser in full nose up trim, and with the low airspeed, the elevators did not have enough authority in the time given to recover the plane before it stalled.

The investigation noted that the pilots were most likely relying on the plane fixing the situation. But when it called quits, it left them with a plane about to fall out of the sky.

Last edited by LiveryMan; 28th Jan 2015 at 14:18.
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Old 28th Jan 2015, 13:50
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I don't fly Airbus, but I find it a bit strange that an aircraft with so many protections built in, will allow the autotrim to trim the stab into a position that you simply can't fly out of.
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Old 28th Jan 2015, 14:00
  #2652 (permalink)  
 
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Mods are quick...

Is there not enough elevator/stab authority in the Airbus to unstall it assuming it was fully stalled by pushing the nose down even if fully trimmed nose up?
In my "normal" aeroplane, I suspect that if stalled with full nose-up trim, I could imagine that I may not be able to get out of it by simply pushing full forward; I suspect that there may not be enough elevator authority, hence the comment about using the Stab trim to get the nose down in the stall recovery procedure. Pushing and trimming, of course, is second-nature to me because that's the way you hand-fly my aeroplane. I would expect that in a full stall, trimming forward (via a convenient pair of buttons on the control column) would also be second-nature.
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Old 28th Jan 2015, 14:15
  #2653 (permalink)  
 
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stay calm?

I am SLF but I have written numerous computer programs containing sanity checks on inputs.

It seems to me there two main ways to make a plane go faster -- nose down or extra thrust. In both cases the speed builds up, doesn't jump 50 or 100 instantly, right? and there are duplicate/triplicate inputs in the knowledge that they may develop faults?

Looks to me they've written it for ideal conditions, assuming the inputs are all true, when the triplications show they know full well they may not be.

So if the nose is more or less level and thrust has not been increased, why on earth should it think it has started to overspeed in the space of 2 or 3 seconds?

Why can't the computer be told to STAY CALM in any situation when an impossibly fast change of speed (or altitude I guess) seems to have occurred? ideally also tell the pilots there are input discrepancies, to get them alert and ready, just in case.
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Old 28th Jan 2015, 14:22
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Graceful Degredation.

After AF447 did Airbus modify the software for loss of speed sensors?

If the aircraft goes from 3 consistent indications to 2, to all different, then why cannot it not provide a warning along the lines of:

"Loss of one speed indicator, if another is lost then the AP will drop , set pitch and power and see you later"

Of course, it may be that you go immediately from 3 to none in which case there is no time for a long speech but even saying "AP dropout due to unreliable speed indications", gives you less surprises.

We must surely have the data to see how long it takes for loss of one speed indicator to all three inconsistent, who knows, it might even provide enough warning to start unloading the trim as well. What does the data say?
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Old 28th Jan 2015, 14:27
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Which Is what a few including myself have been saying:


http://www.pprune.org/8824575-post2010.html


Quoted below:

It certainly didn't help 2 very respectable pilots on D-AXLA A320-232 over Perpignan when THS stayed nose up, with the combination of the AOA sensors freezing and normal law dropping out they were unable to save it with the above aforementioned method (stick hard forward). After reading the report, i was worried by the lack of notification to the pilots, in that moment id expect 'USE MAN PITCH TRIM' to be slapping me round the face. This warning in that instance was possibly the most understated warning.

Why not use a clear indication:

Warning - Normal law FAIL, Law
STATUS NOW Alternate/Direct etc
Cause - AOA/ADIR mismatch(or the like)

Result - USE MAN PITCH TRIM
(?(perhaps added flashing audible warning if stick forward/backward exceeds Xseconds)) or AUTO PITCH TRIM DISABLED!

(In the tragic Perpignan instance above i believe the PITCH TRIM warning dropped off the display shortly after changing flight law.) i stand to be corrected. Which then left the guessing game of 'what did the AP change we haven't worked out yet'.

Last edited by captains_log; 28th Jan 2015 at 19:37.
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Old 28th Jan 2015, 14:29
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Don't Trim Into A Stall

One of the first things I was taught in primary training is, "Don't trim into a stall." Apparently, the AB system will do just that in some circumstances.
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Old 28th Jan 2015, 14:32
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@ Capn Bloggs...

Overspeed? Correct me if I'm wrong, but when the sidestick was pushed forward the speed increased to above 60KIAS and then the Stall Warning came back on. What sort of feedback message does that send?

Autotrim when handflying is bad news; nice and geeky when all is well but precisely what you don't want when things go pear-shaped. Either you're fully flying it ie trimming as well or the AP is in.
Well I think I understand why the Stall Warning came back on when the airspeed increased past 60KIAS and if so it's makes perfect logical sense.

However I think Airbus should have programmed their software better so that the aircraft's system knew they were in the air in a very unusual
circumstance (low speed) but at altitude. Basically I think the system is programmed by logic that if it sees a speed below 60KIAS it is silenced.

I still can't fathom why the crew could not understand (at least initially when the captain wasn't in the cockpit) in which state of flight their aircraft was in.

I don't care if the stall warning was going on and off several times which is a major clue in itself when your airspeed is at or near zero, your attitude is wrong, your bloody VSI is showing a rate of descent near 10,000 feet/minute (or whatever is the max rate is on the Airbus) and with the altimeters showing you that you are losing altitude at a very high rate and speeding towards the surface of the Earth or ocean in this case... All of those clues are a tell, tell that you are in a stall!

Autotrim when hand flying is a bad thing, not necessarily imo!

I see it as an option, a built-in option and I think I know why Airbus included it. I think they decided this in the initial phase of development... Why not have Autotrim when the pilot is flying it manually since we have it (as in most other aircrafts) when the AP is controlling the aircraft. It would be just a nice feature and relieve another action (manual/electrical trimming) from the piloting. I'm sure some of the very old timer said the same thing about autopilots, flight directors, FMS systems etc when they first appeared.

Now not knowing the Airbus, it would perhaps be better if not only a visual warning appeared but also an audio warning like "Autotrim Fail" or "Use/Revert Manual Trim".

Same thing could/should be done with both visual and audio warnings when the aircraft reverts from "Normal Law" to "Alternate or Direct Law".

This would perhaps help the situation in a critical phase of flight.

Last edited by Jet Jockey A4; 28th Jan 2015 at 14:42.
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Old 28th Jan 2015, 14:44
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Question RE inertial data

So if the nose is more or less level and thrust has not been increased, why on earth should it think it has started to overspeed in the space of 2 or 3 seconds?
Seems to me some sort of ongoing comparison between flight computer responding to various sensors (AOA, Airspeed, etc ) and an Inertial reference system would immediately reveal such a difference.

And while one cannot fly for long using an inertial reference system due to lack of true airspeed data, the few seconds with a large delta between the systems could trigger a simple warning. YO dude- Garbage in- Garbage out !!

A body in space cannot change in any direction without some sort of acceleration/deceleration vector as measured by precision accelerometer /gyroscopes. So why isn't such info used ?? In days of yore- it was a simple gyroscope to determine attitude , etc and a pendulum device/equivalent to determine certain vector accelerations.. . .
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Old 28th Jan 2015, 14:52
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Now not knowing the Airbus It would perhaps be better if not only a visual warning appeared but also an audio warning like "Autotrim Fail" or "Use/Revert Manual Trim".
When you go into Direct Law:
  1. USE MAN PITCH TRIM on PFDs
  2. ECAM message "F/CTL DIRECT LAW"
  3. Above message includes "MAN PITCH TRIM" message
  4. ECAM message will have associated audio alert
However, as with any ECAM message, it is prioritised according to what other messages / alerts are being triggered.

One of the first things I was taught in primary training is, "Don't trim into a stall." Apparently, the AB system will do just that in some circumstances.
An Airbus is slightly different to your primary trainer. Firstly, it is not "trimming" as such, but aligning THS and Elevator to retain full elevator control. There are circumstances you need to fly, and retain control at low speed: landing / GPWS / Windshear, and retain control to manoeuvre and counter rapid trim changes (e.g. full power).

So if the nose is more or less level and thrust has not been increased, why on earth should it think it has started to overspeed in the space of 2 or 3 seconds?
Because in the cases where it has "pitched up", AFAIK they were genuine overspeeds. The aircraft does not need to accelerate to go from cruise speed, to significant overspeed.

A body in space cannot change in any direction without some sort of acceleration/deceleration vector as measured by precision accelerometer /gyroscopes
As above, you are misunderstanding basic aerodynamics. Overspeed refers to IAS/M number - nothing to do with KE / Ground Speed.
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Old 28th Jan 2015, 15:13
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@ Smilin_Ed...

One of the first things I was taught in primary training is, "Don't trim into a stall." Apparently, the AB system will do just that in some circumstances.
That is why on all our recurrent training sessions we go through 4 types of stall exercises and they are done all with the AP on so that you do get a nose high trim situation because the AP will trim until the shaker is activated.

Stall exercise #1: 10,000 feet, clean, then slats out only, then slats + flaps at the two takeoff settings, then slats + full flap. One of these will also be done with a continuous 20 degree bank turn.

Stall exercise #2: Low altitude stall in a landing configuration (full flaps), again with the AP on. While levelling off on an approach at the MDA (400' AGL), the trust levers are brought back to idle where they stay until stick shaker/AP disconnect.

Stall exercise #3:
High altitude (high 30s) with a heavy aircraft.

Stall exercise #4:
Takeoff with an engine failure with a climbing turn. The AP is selected on at the proper altitude and a 15 degree bank continuous turn is initiated using the VS mode and set a high rate of climb until stick shaker/AP disconnect and recover.
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