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737-500 missing in Indonesia

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737-500 missing in Indonesia

Old 17th Jan 2021, 17:55
  #301 (permalink)  
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One of the two Singapore investigators joining the team is a flight recorder specialist.
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Old 17th Jan 2021, 17:59
  #302 (permalink)  
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If the aircraft had a history of autothrottle issues, that fact is more likely to have emerged from a study of recent tech logs than from leaked FDR data.
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Old 17th Jan 2021, 23:55
  #303 (permalink)  
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gets my vote, especially as the A/T ‘problems’ are coming to light. Quite possibly with both hands on the control wheel, no thought to apply rudder and the thrust left alone until it was too late. Most likely still in IMC. The CI only managed to recover once in VMC.

the explanation given in the CI006 event was disruptive sleep patterns and not been fully ‘awake’ when the event happened.
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Old 18th Jan 2021, 02:27
  #304 (permalink)  
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For the B737 ATR clutch pack, the underlying problem was occasions where the clutch pack would only pick up a single throttle. The AD issue that altered the ATR computers was to ensure that alert messages and faults would be properly annunciated. The 737 Classic ATR is pretty basic in function, as was the B747 classic etc. It is driving the throttle, which then schedules the engine, so any clutch problem where a level is left behind will end up with an asymmetry.

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Old 18th Jan 2021, 04:27
  #305 (permalink)  
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A/T off

I see no reason that the autothrottle had anything to do with the fast descent and its initiation.
They can be switched off easily, or even just be overpowered by pilots who pull back on the throttles.
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Old 18th Jan 2021, 05:13
  #306 (permalink)  
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Klauss, the issue is that - with cable controlled engines - you can get excessive downstream cable loads that can overcome the capabilities of the autothrottle clutch, allowing one throttle to fail to move when the autothrottle commands it. Not an issue if the flight crew is paying attention - they can easily manually move the throttle to compensate. But if they are not paying attention, you can get a considerable thrust asymmetry. If it gets large enough, it can cause the aircraft to roll - again not an issue if the flight crew is paying attention, but if they aren't and the roll becomes excessive it can lead to a loss of control.
That's what happened on the China Southern crash.
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Old 18th Jan 2021, 08:41
  #307 (permalink)  
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I presently fly B737 Classic's and their knackered old auto-throttle's & MEC's are constantly struggling to match the thrust between each engine. Takeoff using the auto-throttle TOGA mode is particularly good fun, requiring that one dance (albeit without the associated tights or poofery) like Rudolf Nureyev on the rudder pedals whilst the auto-throttle & MEC try to get their collective act together (accordingly we now set takeoff thrust manually / auto-throttle 'off', as it's less problematic)... along with cruise and approach offering similar opportunities to substantially mismatch the thrust levers in order to balance the thrust. But, all in all, it's no big deal and is a problem that's easily dealt with by any pilot whom is competent on multi-engined aircraft (wherein, for our lot, autopilot / auto-throttle / flight-director dependent children of the magenta line need not apply).

Last edited by Old King Coal; 18th Jan 2021 at 08:52.
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Old 18th Jan 2021, 10:38
  #308 (permalink)  
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Quite so, however on a departure with weather complications, with nav tasks, admin tasks, and just having 2 pilots possibly returning to some semblance of currency after the catastrophe of CoViD-19, then a moment of inattention could lead to circumstances where neither pilot is aware of a minor defect, but that leads to a control problem in short order for the APLT. Once the APLT gets to it's servo limit of ailerons (it's 2 axis...) and when the bank angle starts to degrade, then in relatively short order the APLT disconnects and the ailerons flick back to neutral, and the aircraft commences a very rapid roll-off towards the retarded engine. A crew with an awareness of the issue and how it is operating is easily able to recognize the issue and intervene, If for any reason they don't, they are in for a wild ride, and in IMC, it will be a test of their ability to get back into the loop and do a recovery. The simplest protection is to have a hand on a control whenever the system is changing state, but humans are great at reducing effort.

In the Guangdong province loss, the simple expedients to keep in the loop didn't work, and that plane had a bad day.

The roll rate that occurs in this type of set up can be impressive, the aircraft can have considerable yaw from the thrust asymmetry without rudder input. The dihedral with yaw gives a large CL difference which the ailerons are countering along with spoiler rise on the advancing wing. The spoiler and yaw/dihedral increases the stall speed, and substantially increases the abruptness of any stall. In this case, the aircraft doesn't seem to be anywhere near the increased stall speed that high yaw angles result in, but the impact of changing the aileron input is similar to the stall case. Back in the '80s, a military B707 was doing training with the rudder power off, and with one outer engine back at idle. the stall occurred about 45kts higher than it would have been expected, and the roll-off was so violent that the #4 engine was thrown off the wing. The IP on that flight has a couple of thousand hours flying asymmetric tactical ops, and he got caught out by the effect of yaw on a swept wing. The local "World Favorite, we are the greatest airline" head of training, who had been in part involved in the military conversion to the type contended that the captain was a cowboy, and that was about it. The accident report done by guys who had zero time with swept-wing transport aircraft seemed to be oblivious to the fact that they noted that the plane had stalled, yet the stall had occurred some 45kts + higher than the ref speed for the condition. Even without the stall, when the autopilot declares "No Mas", the yaw and the removal of all aileron input will give a roll rate way beyond the normal expected roll rate.
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Old 18th Jan 2021, 11:25
  #309 (permalink)  
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Some years ago in the 737 simulator the instructor failed one throttle clutch motor during idle decsent leading to an ILS. The aircraft was being flown with autopilot and autothrottles engaged. The crew did not notice it because there was nothing to indicate clutch motor failure while the throttles were closed.

It was only when the aircraft was established at 10 miles on the ILS and flaps and gear was extended that power went up to maintain programmed airspeed. But with a clutch motor inoperative only one throttle opened up. The yaw caused by one throttle at idle and the other throttle at 75% N1 was counteracted by the automatic pilot which was flying the ILS. The autopilot used 50% aileron to maintain semi directional control. If the crew noticed this, no words passed between them, although both pilots presumably would have watched with increasing interest; each wondering what the other pilot would do about it.
There was no attempt by either crew member to disengage the automatic pilot which was struggling to maintain the localiser and glide slope. Cultural mores forbade the PM from dropping a hint to the PF who, following company SOP's, still had both hands on his knees unsure what to do.

Suddenly the autopilot disengaged as it had reached beyond its tolerance. The simulator was in simulated IMC so no outside view. The instruments showed the aircraft rolling sharply past 70 degrees angle of bank as all directional control was lost and the aircraft emtered a spiral dive still with one throttle closed and the other at high power. It was only then that the captain who was PF called "Engine failure - checklist." yet still failed to take any physical action.

Astonishingly, both his hands were firmly on his knees at all times. There was nothing wrong with both engines of course. An alert pilot would have earlier seen the offset control wheel and split throttles and taken immediate appropriate action to regain stable flight. The first officer was still diving to find the QRH when the instructor decided to " freeze" the simulator as GPWS warnings sounded and autopilot disengage warning siren sounded.

From the instructor point of view, there was little to be gained by prolonging the embarrassment and cultural loss of face. What shook him was the total stunned horror displayed by the captain as the aircraft was about to crash. The first officer simply stared straight ahead disbelievingly and at no stage did he support the captain with any words of warning. From the first officer's point of view it was the captain's problem - not his. This crew were like robots; totally automation dependent from the time the gear was retracted after takeoff, right to the end. Both pilots were experienced on the 737 but apparently had never seen a thrust lever clutch motor failure in the simulator - a sad indictment on their training. Theirs was total disbelief at what was happening.

Last edited by Centaurus; 18th Jan 2021 at 11:55.
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Old 18th Jan 2021, 11:46
  #310 (permalink)  
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Indeed, A/Ts are probably more responsible than anything else for crews losing the ability to hand fly an aircraft. I put it down to modern A/Ts being just so precise with speed control that the average pilot is afraid to compete.

Suddenly, instead of automation being the slave, it becomes the master. An all too common an occurrence these days.
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Old 18th Jan 2021, 12:36
  #311 (permalink)  
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Old King Coal

Are you following Boeing procedures or are you following your own invented "I have no patience" approach?

If you follow Boeing guidelines you should be just fine with the autothrottle. Set the engines to approximately 40% N1, let them stabilise, and then hit the TOGA button. Too many times you see crews failing to nail a simple request like "approximately 40%", will not wait for the engines to react and push even more forward asking 60-70%. That same "no patience" approach will prevent the pilot from allowing the engines to stabilise a second time, so they go on and hit the TOGA anyway while the engines are reving up with sometimes more than 10% N1 difference. Now that's going to be some fine dancing there. But that's not the autothrottle. That's the pilot's fault not understanding how a HBPR turbofan works.

The same happens when the engines approach target N1 for takeoff. The autothrottle slows down approaching the target N1, then the finetuning begins. When the autothrottle slows down the acceleration, too many pilots are trigger happy to intervene and push the throttles forward leading to overboosts from which now the autothrottle has to correct again.

In cruise the limits are clearly defined and not that big of an issue. There is a max throttle position discrepancy allowed, not an N1 difference. And the autothrottle manages that throttle position discrepancy a lot more/better than many pilots...

And the same happens when coming out of an idle descent. No patience, no idea of how a turbofan works close to idle, and overcorrecting before having to reduce power again.

Between 40%N1 and TOGA/assumed power, the autothrottle should work fine in 98% of the cases. If not, you should talk to your maintenance because these are limitations.

Last edited by BraceBrace; 18th Jan 2021 at 13:36.
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Old 18th Jan 2021, 17:04
  #312 (permalink)  
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This incident makes interesting reading ref. A/T on older B737.


para 2.1.2
Disengagement alerting requirements
The autothrottle warning on G-THOF was typical of its era. Many later generation aircraft incorporate an autothrottle warning, including an audio alert, into an Engine Indication and Crew Alerting System (EICAS). Aircraft in general and automation technology specifically has advanced rapidly in reliability. Pilots familiar with operating older aircraft, which had more variable reliability, are nearing the end of their careers and there is a generation of pilots whose only experience is of operating aircraft with highly reliable automated systems. With this increasing reliability there is concern about flight crew encountering the issues outlined by the CAA paper 10/2004: that of a normally reliable system failing.
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Old 18th Jan 2021, 18:18
  #313 (permalink)  
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Are there no master cautions to advise the crew that the AP is approaching the limit of its control authority? I ask because my (military) type, of 1970s design, had just such a feature. The AP included a low level terrain-following mode and the designers judged it essential to give the pilot some warning of the need to take over heavy control loads lest the aircraft nose-dive straight into the ground upon disconnection. There was also a caution displayed when the HP spool RPMs diverged by more than 15% with autothrottle engaged. Given the severity of the outcomes described above, it would seem that such features might have been invaluable in alerting crews to the crisis about to be unleashed on them. Dumping dangerously out-of-trim forces on the pilot with no prior warning and no fade-out period seems to me very poor system design. Presumably such features are commonplace in contemporary, non-grandfathered designs?

Last edited by Easy Street; 18th Jan 2021 at 18:56.
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Old 18th Jan 2021, 19:24
  #314 (permalink)  
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The problems I have seem in accidents involving auto-throttles on both Boeing and Airbus is the pilots inattention to what is going on with the aircraft as it responds to thrust differentials. This may have been compounded in some cases by over-reliance on the automatics.

I personally witnessed one event in flight where a crew member bumped the throttle on one engine which annunciated and disconnected the AT. This was followed by immediate corrective action and re-engagement of the auto-throttle. The only problem was that after silencing the aural the PNF that bumped the throttle now missed the re-engagement with a fat finger punch. Over a period of time (many minutes) the aircraft responded by losing speed as it tried to regain altitude. It was recovered by the PF when he recognized the change in attitude.
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Old 18th Jan 2021, 19:44
  #315 (permalink)  
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AT has been the cause of accidents before. In those I remember, I have to say I’m not impressed with the actions (or lack of) of the pilots.
But from 10000 ft and not be able to recover anything and just head straight down?
I guess it’s possible (just think of the PIA 320 accident), but I have my doubts.
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Old 18th Jan 2021, 21:12
  #316 (permalink)  
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I guess all I have done is cite history associated with a specific cause , auto throttle. So far I don't yet see a strong link with this accident, maintenance or CVR still not available in this thread
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Old 18th Jan 2021, 21:30
  #317 (permalink)  
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Are there any updates?
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Old 18th Jan 2021, 21:46
  #318 (permalink)  
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Having flown 737's classic for a number of years I can say that A/T failing to move a single thrust lever happens as it occurred to us during the approach. Not a big deal if somebody is actually flying the aircraft. The yoke turning with the AP engaged trying to keep wings level is hard to miss.
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Old 18th Jan 2021, 21:47
  #319 (permalink)  
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I was talking about AT in general as well. This issue with the AT computer mod on the Classic is 20 years old. Hard to think this aircraft was not fixed a long time ago. I’ve flown classics. AT is not as good as on the NG, but it was OK.
Unlike the 320, these throttles move, and it’s hard not to notice a big movement.
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Old 18th Jan 2021, 22:03
  #320 (permalink)  
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Following through on throttles and controls during an approach surely covers all angles of this to have awareness if one was at idle and the other at high thrust?

I personally do so from first stage of flap extension to landing, and during big thrust changes higher up
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