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Old 2nd Jan 2018, 23:51   #321 (permalink)
 
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I've just read your latest release.

You mention the Cockpit Voice Recorder not having any data.........for clarity, they weren't fitted in those days....wasn't mandated in the US until 1967.

There was a flight recorder (FDR in modern terminology) that only records data, and as you mention it had no recoverable information.

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Old 3rd Jan 2018, 00:21   #322 (permalink)
 
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The disengage handle when pulled disconnects the control cable from the boost unit for aileron, rudder or elevator, and simultaneously alters the mechanical advantage. The full range of the control surface is reduced by about half, and the force required by the pilot is about doubled.

Pulling the engage handle reverses the action.

I discuss this in the new chapter currently at https://we.tl/x0L59Bpfmj and it will be in the next release.
I will read with great interest. The schematic I am using for aileron architecture is “Spruematics”, via Lockheed. It shows no disconnect, and the aileron command loop proceeds no further than the boost quadrant. This suggests manual input remains terminus at boost quadrant, and the input is through the power arm, and slave, directly to the final arms, rods, bearings and aileron.

My assumption is that with a pull off of aileron boost, the quadrant shifts to three to one for net nine degree deflection. There is no relative increase or decrease in effort; the power mode is direct, manual is only one third of available deflection in Boost: thirty degrees in boost nine+ in manual...

Do you have the schematic showing this design?

Edit. Aileron in manual is not power limited? All effort at the yokes is translated directly into the system. I compute a possible value of twelve hundred pounds tension is possible. Even more if two athletes are turning the yokes. Potentially enough to sever a cable.
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Old 3rd Jan 2018, 00:38   #323 (permalink)
 
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Design requirements are that in event of ANY sort of boost failure the system can be disengaged. The applicable rule says,OK, the boost unit, faulty or not, had no role to play as to accident causation, because the sole cause was a separated control cable, even though the CAB says "probable" cause.
As I see the design in the schematic, all hydraulic failures are mitigated by manual manipulation, what I do not see is any design feature that prevents a stuck piston or jammed power arm from inhibiting this manual control.

Mechanical failures of the hydraulic system, in other words, are not designed for.

Manual control appears to translate through the cylinder, when pumps are off.
I see no evidence of “cables only” controls around the power arm, or slave.
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Old 3rd Jan 2018, 01:05   #324 (permalink)
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As I see the design in the schematic, all hydraulic failures are mitigated by manual manipulation, what I do not see is any design feature that prevents a stuck piston or jammed power arm from inhibiting this manual control.
Ahh, I see what you're saying. No idea on this end. I'll have to look further.

Frankly, to find possible fault with the boost unit seems to be a stretch.

They had a three-way lottery. The boost unit and normal cabling, the disconnected boost and normal cabling, or the autopilot and boost engaged and broken cabling. Any of those would have saved them. But the autopilot was carded inoperative, and I doubt they had enough time to work through the possibilities. It sounds like they thought their problem was either a bad autopilot or bad boost unit.

They had about 22 seconds from recognizing the problem to impact, and it's asking a bit much for them to evaluate the possibility that the autopilot, though carded inoperative, might function enough to bypass bad cabling and run the boost unit. I have no indication whether the autopilot was actually functional, or whether they knew it.

An uncredited causal factor, as I see it, is that early and low turn. It was, as Cordwainer uncovered, a means to avoid a cumbersome departure double-hand-off. Understandable, but it meant they were in a low and significant turn before they realized they had a problem. As I discuss in my report, that turn and bank also renders a rudder correction largely ineffectual. It was a perfect storm of really bad circumstances lining up. But then, having looked at a number of accidents, this is how crashes tend to happen.
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Old 3rd Jan 2018, 01:19   #325 (permalink)
 
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What I am suggesting, among other things, is that manual control could conceivably overpower, or at least foul, Boost. The boost was found in three degrees right wing down. That is where I believe it jammed whilst manual control by both pilots might have stalled the piston. Routing manual control inputs through even an unpowered hydraulic plumbing seems problematic.

Originally Posted by megan View Post
Design requirements are that in event of ANY sort of boost failure the system can be disengaged. The applicable rule says,OK, the boost unit, faulty or not, had no role to play as to accident causation, because the sole cause was a separated control cable, even though the CAB says "probable" cause.


The reason they say probable cause is because it fits their theory, even though there is no direct evidence of this specific failure... the pertinent equipment was never recovered. I have evidence that the boost unit was articulated at three degrees on impact, a position consistent with the aileron. Assuming one or both pilots were inputting left wing down, the boost did not answer. If unpowered, it wouldn't have mattered, and if powered, it would input left wing down as well. It did neither, during flight.

The report makes a deal about which yoke is “tensioned” at either boost quadrant location specific to each command cable. The yokes are connected, individual tension is not relevant. There was aileron input for left wing down, throughout. The report would have us believe this command did not reach the aileron, because one cable separated. That isn’t the reason. Even if one had separated, the connected yokes would deflect the left aileron upward. Some left roll would have been available, by design. The roll effect would be unusual, as both ailerons would be deflected up. If the right aileron was “stuck” at three degrees, in flight, (it was, the report says so,) the crew would have lessened the left roll input, and a balance would have been reached. An airplane can fly with one active aileron. It needed no study, either, the yokes would behave to control the roll, albeit in unorthodox fashion.

So. No evidence of separated (in flight) aileron cable, but an explanation of jammed Boost cylinder, as above.

But only probable.

Last edited by Concours77; 3rd Jan 2018 at 02:18.
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Old 3rd Jan 2018, 02:53   #326 (permalink)
 
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Last one today, promise.

The Electra will fly just fine with one aileron cable separated. What’s crucial is that tension can be kept on the remaining aileron so the cable won’t foul. Hold the critical aileron into the airstream, if too much, let the airflow blow it down some.

It will not fly at all when the boost quadrant pulley is jammed in place, even with both cables attached.

The boost piston and therefore the boost quadrant were jammed.

“.....we can’t get release.....”
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Old 3rd Jan 2018, 23:25   #327 (permalink)
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Routing manual control inputs through even an unpowered hydraulic plumbing seems problematic.
This is very interesting. I'm buried in learning animation tools for the near term, but I'll find what you're talking about in the manuals and see if I can make sense of it. Thanks.
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Old 4th Jan 2018, 05:51   #328 (permalink)
 
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The Electra will fly just fine with one aileron cable separated
No it will not. In this case the left wing down cable separated, which drove the right wing down because its slack absorber spring relaxed, decreasing the effective length of the control run from the boost unit to the control wheel.
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Even if one had separated, the connected yokes would deflect the left aileron upward. Some left roll would have been available, by design.
That's not the case at all, as I describe above.
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Mechanical failures of the hydraulic system, in other words, are not designed for.
No failure, what ever it may be, is permitted to interfere with aircraft control. Forces will be heavier, that's all. Lockheed had pre-eminent experience with boost controls, the P-38 and Constellation being two. To back up the reliability of the boost unit, no AD (Airworthiness Directive) has ever been issued against the boost unit, which would have been the case had it failed in any manner.

Posted else where. I would say applicable to this thread.
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This is supposed to be a Professional Pilots Rumour Network not a place to go and tell the ATSB how they should be doing their job or attack a fellow pilot that can no longer defend himself.

PPRuNe ceased being professional a long time ago. Twenty years ago when Danny Fyne ran it as a subscribed email service it was the domain of professional aviators. Then it become a BB, was sold and anyone could join. And join they did....

Nowadays PPRuNe is a go-to source for immediate alerting that something has occurred but within 24 hours the Monday-morning forensic keyboard "experts" and wannabees will have taken over, leaving the professional aviators to just look on in dismay. Occasionally there are insightful posts from people with real knowledge but 98% of the time it's just nonsense and sometimes downright disgraceful. Means PPRuNe is a go-to place for the first hour or so after an incident but then "get-the-heck-out-of-here"....
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Old 4th Jan 2018, 14:45   #329 (permalink)
 
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Would like to direct everyone’s attention to page twelve of the report.

In separate paragraphs, (specifically para #3 and para #5), it appears that CAB can’t decide which primary aileron cable separated. Keep going to page thirteen, where the report writer is convinced there are two pairs of aileron cables, not one, and that both pilots’ cables separated in flight.

The description I favor (pilot’s cable failure) would seem to suggest that their situation was dire, but that left wing down was available, in both manual and boosted modes.

The “alternate conclusion by CAB” (failure of co-pilot’s cable) suggests nothing could have saved them. Key words: “in tension”.

From the schematic legend:
The cables to the empennage run in pairs - two sets for the elevators, two sets for the rudder, both designated ‘primary’ in the above image. If either set of cables breaks, the other set would be unaffected. There is also one pair each for the empennage trim tabs. There is only one primary cable pair running to the wing root for the ailerons. An unscrewed turnbuckle on a cable running from the flight deck to the aileron boost pack- age was the cause of the crash. If there had been two sets of cables to the wing, there would have been no crash.

My assumption of the schematic is that any remaining cable remains in tension with the boost quadrant. Something has to retain the cable to each yoke, or there would be insufficient friction (traction) for either yoke to move the cable.

Such a design would allow for deflection of an aileron dependent on the yoke with the remaining tension.

Aileron return to neutral? Blowdown. (Release pressure).

In the case of 706, the co-pilot cable would articulate left aileron up, right aileron down, since the ailerons are directly connected to each other. Co pilot yoke has authority to roll left, but a following Roll right is unavailable, pilot yoke disconnect prevents “pull” on the quadrant.

I can’t believe this was not part of the Lockheed design. I believe it is. The clue? There are two tensioning stations....and a connector rod between yokes.

BTW. My schematic shows the yokes rigged in tandem, with pilots yoke at the end of the cable loop.

Last edited by Concours77; 4th Jan 2018 at 20:58.
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Old 4th Jan 2018, 21:18   #330 (permalink)
 
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If there had been two sets of cables to the wing, there would have been no crash.

If there had been two sets of aileron cables it would have been because Lockheed thought they were critical to flight safety. They didn’t, and that’s because ailerons are not considered critical to flight safety.

Why do the Rudder and Elevator have two pairs? Because they are (critical to flight safety). Losing both ailerons means they fly in trail, and turning can be done by Rudder alone.

Any aileron malfunction can be solved by letting go the command, and waiting for them to land in neutral...

Lockheed doesn’t make blunders, and they almost never make mistakes.

Edit. Then there’s aileron trim, a separate system altogether, which can be used to roll.

Last edited by Concours77; 4th Jan 2018 at 21:31.
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Old 4th Jan 2018, 21:29   #331 (permalink)


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Although there is reference to two cables, the cables actually formed a single loop terminating at each pilot's control yoke. The description of tension in the control cable is with reference to each of the flight yokes. It is this tension that allows movement to be mirrored between the yokes and forces to be distributed to either aileron. Breaking the loop removes aileron control from both yokes. This was a serious design flaw. In hindsight it seems obvious that this single point of failure would be catastrophic, however the cable was over engineered for the anticipated forces it might be expected to experience.

A further section in the report clearly states that in the absence of a safety wire, the turnbuckle fastening would and was demonstrated to unthread in the presence of vibration.
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Old 4th Jan 2018, 21:41   #332 (permalink)
 
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Although there is reference to two cables, the cables actually formed a single loop terminating at each pilot's control yoke. The description of tension in the control cable is with reference to each of the flight yokes. It is this tension that allows movement to be mirrored between the yokes and forces to be distributed to either aileron. Breaking the loop removes aileron control from both yokes. This was a serious design flaw. In hindsight it seems obvious that this single point of failure would be catastrophic, however the cable was over engineered for the anticipated forces it might be expected to experience.

A further section in the report clearly states that in the absence of a safety wire, the turnbuckle fastening would and was demonstrated to unthread in the presence of vibration.
No, there is reference to two pairs of aileron cables in the report. Movement is also mirrored by the large pushrod connecting the yokes, in apposition....

Breaking the loop does not remove aileron Control from both yokes. The cable that is not separated has tension control of its operant aileron....that isn’t sufficient for coordinated turns, but one aileron moving in one direction can control roll in that axis... also, as above, releasing command of both yokes puts the ailerons in trail, a benign condition, and not fatal to control.

Vibration rotation. Note each cable terminal unthreading is tightening its opposite end.

Attach an open end wrench to the end you wish to unscrew. Turn as many turns as you can, likely not many. Let go the wrench, and get a broken wrist. Storing energy in a threaded cable like it is a rubber band is not possible. Tightening the other end can only advance three threads, by design.

Thanks GOULI I value your input, and apologize for my assertive style. I mean no aggression, I wish to learn here.

Last edited by Concours77; 4th Jan 2018 at 21:55.
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Old 5th Jan 2018, 02:16   #333 (permalink)


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At the time of the accident I believe only a single cable loop was used in the aileron circuit. The design was subsequently modified to a dual cable or redundant system in all commercial aircraft thereafter. It was one of those defining moments in aviation history where lessons were learned and fixes put in place immediately afterwards. any engineering drawings or references post the crash will show a dual system that was not fitted to the crash aircraft.

In this particular accident the cable broke and all aileron control was lost to both yokes. The autopilot system was flagged inoperative and the switches that would have enabled it to be activated were not accessible to either pilot. It is not known whether the autopilot system was actually functional or solely deactivated pending maintenance and an update.

The report clearly states that a turnbuckle was observed to unthread and separate during vidration testing on a rig without a safety wire inserted. I agree that a twisted cable in a sheath behaves in a similar manner to a solid rod but you are making the incorrect assumption that both ends of the cable were secured to prevent any axial movement. This is clearly not the case here. The turnbuckle was free to rotate and unthread, effectively severing the cable loop.

Additionally it is mentioned in the report that the loose cable snagged on other parts of the aircfame during testing preventing a return of the ailerons to a neutral position. It cannot be definitively stated whether this happened in the crash, but fact that it happened during testing indicates that this is very likely.

So the ailerons were deflected to initiate a turn, the cable seperated and jammed, leaving the ailerons with a permanent deflection that could not be overcome by rudder inputs. Although the crash investigation team could only prove a deflection of between 3° and 6° from witness marks, I feel it may have been rather more than that prior to impact. The tail and rudder are pretty substantial structures and should have had sufficient authority to level the wings. However the low airspeed during the initial climbing turn must have prevented full rudder authority being available to level the aircraft. Takeoff flaps were still selected when the aircraft crashed and this may have reduced tail and rudder authority slightly by deflecting airflow downward beyond the wing surfaces.

Had the incident happened at 10,000 feet, might the accident have been survivable? Probably not. It seems likely that the aircraft would have entered a spiral dive and impacted the ground before control could be reestablished.

An incident of this type requires immediate intervention and rapid full control inputs, something that airline pilots are trained not to do for the safety and comfort of their passengers. Survivable in a test pilot flight test scenario but not in day to day commecial operations.
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Old 5th Jan 2018, 04:14   #334 (permalink)
 
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Had the incident happened at 10,000 feet, might the accident have been survivable? Probably not
If the crew had knowledge of the crossover speed they could well have possibly, but it's a recently investigated concept following a 737 accident.

Extract from https://www.faa.gov/other_visit/avia...overy_Book.pdf
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2.5.5.4.4 Crossover Speed

Crossover speed is a recently coined term that describes the lateral controllability of an airplane with the rudder at a fixed (up to maximum) deflection. It is the minimum speed (weight and configuration dependent) in a 1-g flight, where maximum aileron/spoiler input (against the stops) is reached and the wings are still level or at an angle to maintain directional control. Any additional rudder input or decrease in speed will result in an unstoppable roll into the direction of the deflected rudder or in an inability to maintain desired heading. Crossover speed is very similar in concept to Vmca, except that instead of being Vmc due to a thrust asymmetry, it is Vmc due to full rudder input. This crossover speed is weight and configuration dependent. However, it is also sensitive to angle of attack. With weight and configuration held constant, the crossover speed will increase with increased angle of attack and will decrease with decreased angle of attack. Thus, in an airplane upset due to rudder deflection with large and increasing bank angle and the nose rapidly falling below the horizon, the input of additional nose-up elevator with already maximum input of aileron/spoilers will only aggravate the situation. The correct action in this case is to unload the airplane to reduce the angle of attack, which will regain aileron/spoiler effectiveness and allow recovery. This action may not be intuitive and will result in a loss of altitude.
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Old 5th Jan 2018, 10:55   #335 (permalink)


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Thank you megan. The 737 rudder hard over incidents were the reverse situation but the principle is the same.
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Old 5th Jan 2018, 16:02   #336 (permalink)
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I've just read your latest release. You mention the Cockpit Voice Recorder not having any data.........for clarity, they weren't fitted in those days....wasn't mandated in the US until 1967.
Don't believe I mentioned a voice recorder. What page are you speaking of?
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Old 5th Jan 2018, 16:23   #337 (permalink)
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The tail and rudder are pretty substantial structures and should have had sufficient authority to level the wings. However the low airspeed during the initial climbing turn must have prevented full rudder authority being available to level the aircraft. ...
An incident of this type requires immediate intervention and rapid full control inputs, something that airline pilots are trained not to do for the safety and comfort of their passengers. Survivable in a test pilot flight test scenario but not in day to day commecial operations.
I discussed some pages back what I call "bevel flying", a sort of poor man's knife-edge flying. For any speed below which knife edge flying is possible, there's some maximum bank angle at which the rudder is sufficient to counter the bank. The closer to that bank angle, the less authority the rudder has.

When right on that angle, it's like balancing on top of an edge, and the phenomenon can roll off either way - increasing bank or decreasing. For an Electra at about 165 kts per my sim runs that angle is about 57 to 60 degrees, somewhere in that neighborhood.

This suggests that left rudder near that angle would be able to counter the bank but it would do so very slowly. As the plane started to level, the relative authority of the rudder would increase.

In my flight sim video I discussed this as if they were simply slow to go to full left rudder. This response at less than the critical 57-60 this gives the same apparent response. Either way, they were well into the zone where rudder authority is degraded by the bank angle. This is why I said the bank was an uncredited culprit in the affair. If they hadn't been already banked when they discovered the problem, I think it would have turned out fine.

As I mentioned a few pages back, modeling this critical angle is probably not possible as part of the sim of the crash, though it's easy to model in free flight. It's like rolling a bowling ball along the crest of a steeply cambered road for quite some distance and having it fall off to hit a specific target. The road crest is an error magnifier and the accuracy drops the longer the ball is on it. But it's much simpler to model a slow command input for left rudder, so that's how I did it.

So I suggest they may have been too slow to call for full left rudder, or they were very close to this critical angle and it took some time for the left rudder to tell.

Last edited by BRDuBois; 5th Jan 2018 at 17:03.
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Old 5th Jan 2018, 17:29   #338 (permalink)
 
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At the time of the accident I believe only a single cable loop was used in the aileron circuit. The design was subsequently modified to a dual cable or redundant system in all commercial aircraft thereafter. It was one of those defining moments in aviation history where lessons were learned and fixes put in place immediately afterwards. any engineering drawings or references post the crash will show a dual system that was not fitted to the crash aircraft.

Fair enough, I am looking for the AD that would have required mitigations on existing aileron control systems, and the requirement for upgraded fasteners and a more rigorous inspection schedule?

In this particular accident the cable broke and all aileron control was lost to both yokes. The autopilot system was flagged inoperative and the switches that would have enabled it to be activated were not accessible to either pilot. It is not known whether the autopilot system was actually functional or solely deactivated pending maintenance and an update.

Actually, it did not “break”, it separated in flight. Did you catch the statement in the report that claims the co-pilot cable parted also?

The report clearly states that a turnbuckle was observed to unthread and separate during vidration testing on a rig without a safety wire inserted. I agree that a twisted cable in a sheath behaves in a similar manner to a solid rod but you are making the incorrect assumption that both ends of the cable were secured to prevent any axial movement. This is clearly not the case here. The turnbuckle was free to rotate and unthread, effectively severing the cable loop.

Axial advance can not occur (unthreading) with the opposite end secured, as I have explained, “winding up” a flexible cable is not possible with one end fixed. I have not postulated that the “turnbuckle” was not free to rotate

Additionally it is mentioned in the report that the loose cable snagged on other parts of the aircfame during testing preventing a return of the ailerons to a neutral position. It cannot be definitively stated whether this happened in the crash, but fact that it happened during testing indicates that this is very likely.

With respect, the part of the pilot’s cable between the column and the turnbuckle if separated, has no effect on the remaining cable. Any bind would have been in “push”, there was no pull available. This work in the lab is not germane. An immediate input of left roll would have worked from either seat.


So the ailerons were deflected to initiate a turn, the cable seperated and jammed, leaving the ailerons with a permanent deflection that could not be overcome by rudder inputs. Although the crash investigation team could only prove a deflection of between 3° and 6° from witness marks, I feel it may have been rather more than that prior to impact. The tail and rudder are pretty substantial structures and should have had sufficient authority to level the wings. However the low airspeed during the initial climbing turn must have prevented full rudder authority being available to level the aircraft. Takeoff flaps were still selected when the aircraft crashed and this may have reduced tail and rudder authority slightly by deflecting airflow downward beyond the wing surfaces.

“The cable separated and jammed”. You are aware that the report states that the co-pilot’s cable also separated in flight? Jammed cables don’t jam the aileron. The aileron back drives the hydraulics, and turns the boost quadrant to neutral. Without aileron command from the yokes connected to the boost quadrant, no jam is possible only the yokes jam, and they did not?

Had the incident happened at 10,000 feet, might the accident have been survivable? Probably not. It seems likely that the aircraft would have entered a spiral dive and impacted the ground before control could be reestablished.



An incident of this type requires immediate intervention and rapid full control inputs, something that airline pilots are trained not to do for the safety and comfort of their passengers. Survivable in a test pilot flight test scenario but not in day to day commecial operations.
You must know that the Captain was busy trying to recover left roll. He said so. Left roll was available, I have demonstrated that. To counter that possibility in the report, CAB came up with “both cables separated” I think they had an idea of what happened, it actually is supported in their own work better than “two cables parted”

I am not here to convince, but to present. I understand it is unwelcome, so I will leave. PM me your data, and I will copy you with my thesis, if you like. Beyond that, I treasure your experience, patience, knowledge and courtesy.

Be well

Bill

Last edited by Concours77; 5th Jan 2018 at 18:07.
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Old 5th Jan 2018, 17:52   #339 (permalink)
 
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An example of CAB’s “certified by signature” logic:

Page twelve para three: “The pilot’s cable disconnected”.....

Page thirteen, para three: “The left wing down cable separated”..... (co-pilot’s cable)
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Old 5th Jan 2018, 18:11   #340 (permalink)
 
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Don't believe I mentioned a voice recorder. What page are you speaking of?
Narrative 38, 4th paragraph down starts "The cockpit voice recorder had no recoverable data."

Probably just a slip, but I wanted other readers to be clear on the recorder type.
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