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Author Topic: It's a talking point. 737 Max series.  (Read 2737 times)

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Offline meharibear

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Re: It's a talking point. 737 Max series.
« Reply #40 on: April 19, 2019, 12:56:27 PM »
A practical example of why fuel is measured by weight was provided by the world record breaking Britannia flight in the 1950s.  They flew non stop and unrefuelled from Prestwick in Scotland to Vancouver in Canada.  The only way they could carry enough fuel for this was to get the fuel company to chill the fuel to get that last drop in!  Dad was the navigator for that flight.  Their record (for the longest unrefuelled flight by a commercial airliner) stood for quite a few years.  Here are the crew on arrival.
Inside every old person is a young person wondering what happened.  (Terry Pratchett)


Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #41 on: April 19, 2019, 13:11:17 PM »
Thank you Peter, your explanation makes perfect sense.  :af :af

Damn - in that case can I have another go at it...

PDR
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Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #42 on: April 19, 2019, 13:38:57 PM »
A practical example of why fuel is measured by weight was provided by the world record breaking Britannia flight in the 1950s.

Another more recent one would be Rutan's un-refuelled round-the-world record flight with his Voyager. If you remember - as the speed rose on the take-off run the wingtips flexed down and scraped along the runway all the way until he eased the stick back for rotation:

<a href="http://www.youtube.com/watch?v=jyRGNcbeS7o" target="_blank">http://www.youtube.com/watch?v=jyRGNcbeS7o</a>

The damage was such that the winglets were only marginally attached and visibly bending over so rather than risk them coming off and causing a control problem at some random moment they increased speed to increase the bending loads to try to snap them off cleanly (it worked).

<a href="http://www.youtube.com/watch?v=M6Kd9zJEucM" target="_blank">http://www.youtube.com/watch?v=M6Kd9zJEucM</a>

But why did the wingtips scrape? The aeroplane was designed with a slightly negatove angle of attack on the ground to prevent premature rotation, and this meant that the wings experienced an increasing additional download from brake release to rotation. The resulting flex had been calculated and measured, and shouldn't have been enough to reach the ground even at full fuel weight. So the night before the flight they wheeled the aeroplane out to the base of the runway and fully fuelled it ready for the next day. Take off was to be shortly after dawn to get the benefit of calm winds and extra air density before the heat of the day. When they went out the next morning they dip-checked the tanks and found there was some spare capacity - they correctly assumed this was due to the fuel cooling overnight (deserts can be chilly places at night) and so without thinking decided to take advantage of this and squeeze in more fuel. They forgot to think about the effect of that extra fuel weight on the wing flex during take off...

Subsequent calculations showed that the extra range from the additional fuel was almost exactly offset by the extra fuel-burn due to the higher drag without the winglets, which was just one of those odd coincidences that happen in engineering!

PDR
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Offline The Saint. (Owen)

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Re: It's a talking point. 737 Max series.
« Reply #43 on: April 19, 2019, 20:50:40 PM »
 ;D ;D ;D
Damn - in that case can I have another go at it...

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Offline meharibear

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Re: It's a talking point. 737 Max series.
« Reply #44 on: April 20, 2019, 12:15:42 PM »
OK - To return to the dreadful 737 - 800 Max.  Here is a very clear explanation of the whole issue.  https://spectrum-ieee-org.cdn.ampproject.org/v/s/spectrum.ieee.org/aerospace/aviation/how-the-boeing-737-max-disaster-looks-to-a-software-developer.amp.html?fbclid=IwAR1u26jS07W_vuneT0wWhC2tJwnu8qQXD108Pw2UhreaGHjzaJGinzm-4go&amp_js_v=0.1#referrer=https%3A%2F%2Fwww.google.com&amp_tf=From%20%251%24s&ampshare=https%3A%2F%2Fspectrum.ieee.org%2Faerospace%2Faviation%2Fhow-the-boeing-737-max-disaster-looks-to-a-software-developer

When I was concerned with such things, the rules said basically that you needed at least triplicated systems which talk to each other so that in the event of any disagreement the "different readings" would be ignored  Boeing seem to have unilaterally decided that two totally independent systems can do the job.  Do they have no independent system safety analysts left?
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Offline SteveBB

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Re: It's a talking point. 737 Max series.
« Reply #45 on: April 21, 2019, 06:08:47 AM »
Very good piece. Also pretty damning.
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Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #46 on: April 21, 2019, 07:57:18 AM »
It may be interesting, but it contains lots of very basic errors and things that are simply not true. Fregsample:

Quote
The solution was to extend the engine up and well in front of the wing. However, doing so also meant that the centerline of the engine’s thrust changed. Now, when the pilots applied power to the engine, the aircraft would have a significant propensity to “pitch up,” or raise its nose.

The engine thrustlines are below the rest of the aeroplane and so below the centre of drag. Therefore raising the thrustlines upwards would REDUCE the pitch-up with power, not increase it.

Quote
I’ll say it again: In the 737 Max, the engine nacelles themselves can, at high angles of attack, work as a wing and produce lift. And the lift they produce is well ahead of the wing’s center of lift, meaning the nacelles will cause the 737 Max at a high angle of attack to go to a higher angle of attack. This is aerodynamic malpractice of the worst kind.

This is simply untrue - if it was true then why don't aeroplanes with rear-mounted engines (DC9/MD90 family, Embraer 145, virtually all bizjets) suffer a massive nose DOWN pitch with power? Wing-mounted jets are almost invariably mounted forward of the wing ever since the original B707 because it allows the use of the engines as mass dampers to reduce the torsional stiffness requirements of the wing structure (with a large weight saving).

Quote
Pitch changes with increasing angle of attack, however, are quite another thing. An airplane approaching an aerodynamic stall cannot, under any circumstances, have a tendency to go further into the stall. This is called “dynamic instability,” and the only airplanes that exhibit that characteristic—fighter jets—are also fitted with ejection seats.

This is just plain twaddle.

Quote
But it’s also important that the pilots get physical feedback about what is going on. In the old days, when cables connected the pilot’s controls to the flying surfaces, you had to pull up, hard, if the airplane was trimmed to descend. You had to push, hard, if the airplane was trimmed to ascend. With computer oversight there is a loss of natural sense in the controls. In the 737 Max, there is no real “natural feel.”

Again, this is factually incorrect. The 737max familiy have as much "natural feel" as other similar aeroplanes. In fact that's actually the REAL cause of the MCAS problem - the certification requirement for that "feel" (in terms of "stickforce per G" and "stickforce per alpha") which didn't meet the regulations. The aeroplane isn't unstable, it just has a stickforce gradient that doesn't meet the rules.

Quote
When the flight computer trims the airplane to descend, because the MCAS system thinks it’s about to stall, a set of motors and jacks push the pilot’s control columns forward. It turns out that the flight management computer can put a lot of force into that column—indeed, so much force that a human pilot can quickly become exhausted trying to pull the column back, trying to tell the computer that this really, really should not be happening.

Again, this suffers from some shortfalls of fact. The MCAS is not part of the "flight computer", but is a completely separate and independent system. That's kinda the whole f*****g point here. If it HAD been implemented as an integrated part of the flight control system it would have inherently had  the required redundancy and checks of reasonableness which would have prevented its misbehaviour when a sensor was damaged. MCAS is a separate and very simple system which takes data directly from one AoA vane act acts directly on the tailplane trim motors. The point is that Boeing fudged the FMECA numbers such that its criticality looked benign enough for such a crude system to be acceptable, when in reality it isn't.

All the rest of the article is based on flawed assumptions built on these fundamental errors of fact and understanding. This sort of thing makes me cross. There is more than enough authoritative information on this even in the public domain that would only have taken a few minutes of googling to find, but uniformed grandstanding arseholes like this guy keep filling up the internet with twaddle like this. He should stick to doing things he's good at - I'm guessing that would be something like writing software for TSB's on-line banking or similar...

PDR
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Offline itsme

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Re: It's a talking point. 737 Max series.
« Reply #47 on: April 21, 2019, 08:37:37 AM »
"writing software for TSB's on-line banking or similar." Dont get me started.... >:( >:( >:(

Offline Michael_Rolls

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Re: It's a talking point. 737 Max series.
« Reply #48 on: April 21, 2019, 17:38:12 PM »
"writing software for TSB's on-line banking or similar." Dont get me started.... >:( >:( >:(
Nor me!
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Offline SteveBB

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Re: It's a talking point. 737 Max series.
« Reply #49 on: April 21, 2019, 18:38:17 PM »
It may be interesting, but it contains lots of very basic errors and things that are simply not true. Fregsample:

The engine thrustlines are below the rest of the aeroplane and so below the centre of drag. Therefore raising the thrustlines upwards would REDUCE the pitch-up with power, not increase it.

This is simply untrue - if it was true then why don't aeroplanes with rear-mounted engines (DC9/MD90 family, Embraer 145, virtually all bizjets) suffer a massive nose DOWN pitch with power? Wing-mounted jets are almost invariably mounted forward of the wing ever since the original B707 because it allows the use of the engines as mass dampers to reduce the torsional stiffness requirements of the wing structure (with a large weight saving).

This is just plain twaddle.

Again, this is factually incorrect. The 737max familiy have as much "natural feel" as other similar aeroplanes. In fact that's actually the REAL cause of the MCAS problem - the certification requirement for that "feel" (in terms of "stickforce per G" and "stickforce per alpha") which didn't meet the regulations. The aeroplane isn't unstable, it just has a stickforce gradient that doesn't meet the rules.

Again, this suffers from some shortfalls of fact. The MCAS is not part of the "flight computer", but is a completely separate and independent system. That's kinda the whole f*****g point here. If it HAD been implemented as an integrated part of the flight control system it would have inherently had  the required redundancy and checks of reasonableness which would have prevented its misbehaviour when a sensor was damaged. MCAS is a separate and very simple system which takes data directly from one AoA vane act acts directly on the tailplane trim motors. The point is that Boeing fudged the FMECA numbers such that its criticality looked benign enough for such a crude system to be acceptable, when in reality it isn't.

All the rest of the article is based on flawed assumptions built on these fundamental errors of fact and understanding. This sort of thing makes me cross. There is more than enough authoritative information on this even in the public domain that would only have taken a few minutes of googling to find, but uniformed grandstanding arseholes like this guy keep filling up the internet with twaddle like this. He should stick to doing things he's good at - I'm guessing that would be something like writing software for TSB's on-line banking or similar...

PDR

I wish you'd say what you really mean Pete.

Though I confess when he mentioned the engine thrust lines and the effects on pitch I thought it wrong, and came to the same conclusion as you cited, re: rear engined airliners. Not only but also, if you raise an engine relative to the centreline (longitudily) it would reduce the pitch until it was above the centreline when it would induce a downward pitch (all other things being equal), and the thrust line was parallel to the centreline, no?

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Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #50 on: April 21, 2019, 19:06:21 PM »
Tell your friend I think that's what I actually said in my second sentence...

:ev

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Offline meharibear

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Re: It's a talking point. 737 Max series.
« Reply #51 on: April 21, 2019, 23:30:13 PM »
As I understand it, looking at the drawings, the issue start from the fact that in order to keep the intake sufficiently far off the runway to be acceptable from a FOD point of view they raised it (the intake).  However, they could not raise the exhaust, thus the engine on the Max8 aircraft now has a significant UP Thrust on what is still an aircraft with the engines mounted below the centre of drag.  Thus opening the throttle induces pitch up to a far greater degree than previously.
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Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #52 on: April 22, 2019, 08:03:36 AM »
Given that the exhaust and the fan are co-axial I'm not sure how that could be possible. They certainly changed the shape of the upper cowl compared to the previous models to improve the flow around the wing/nacelle/pylon interface (moving some engine accessories form the sides to the top of the engine to take better advantage of the resulting space), but the thrust lines are still in the same place. The core objective of the whole exercise was to shave as much drag off the aeroplane as possible, and an offset thrust line would result in significantly higher trim drag, negating all that drag reduction. Again, the authoritative sources indicate that the actual issue the MCAS was to address wasn't a negative stability - it was a low stickforce gradient that didn't meet the regs.

PDR
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Offline itsme

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Re: It's a talking point. 737 Max series.
« Reply #53 on: April 22, 2019, 09:11:18 AM »
I am still a bit bothered about the idea of an airliner which decides for itself the best way to land, and if it goes wrong and the pilot does the gung ho "going to manual overide" he is most likely going to plant it vertically in to Mother Earth. I think I prefer the old 747 idea that 'any idiot can land it'...maybe thats a fallacy, but you see my point? For instance US 1549 dead sticking in to the Hudson.

Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #54 on: April 22, 2019, 09:55:34 AM »
I am still a bit bothered about the idea of an airliner which decides for itself the best way to land, and if it goes wrong and the pilot does the gung ho "going to manual overide" he is most likely going to plant it vertically in to Mother Earth.

Umm...not sure where you got that from - unless IU'm misunderstanding you it's not actually true.

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Offline itsme

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Re: It's a talking point. 737 Max series.
« Reply #55 on: April 22, 2019, 18:50:48 PM »
Umm...not sure where you got that from - unless IU'm misunderstanding you it's not actually true.

PDR
its sort of how it was described. CAN the pilot take over control, and is it going to be a perfectly safe landing?

Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #56 on: April 22, 2019, 20:53:26 PM »
Of course the pilot can disengage the autopilot and take over (there's a switch in the control yoke which makes it very easy) and yes, it would probably end well. The only circumstances in which it would be a bad idea would be the very rare full Cat 3a auto-land - this is only because in a full Cat 3a the pilot will not be able to see the runway to keep straight. There are no airliners in service which can't be flown manually if desired. The point about the 737max's MCAS is that it is NOT part of the flight control system OR part of the autopilot. As such the crew weren't necessarily aware that it was operating, and to switch it off required switching off the elevator trim motors (not an intuitive thing to do) rather than disengaging the autopilot or failing a mode in the flight control system to force it into a different mode (which are the two things that pilots are used to doing as a matter of course.

MCAS triggering is not common - indeed I have heard it said that there have only been two occasions when it has triggered on an in-service aeroplane. On both occasions it resulted in a catastrophe. I suspect that's why Boeing are bricking it right now.

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Re: It's a talking point. 737 Max series.
« Reply #57 on: April 23, 2019, 07:11:46 AM »
its sort of how it was described. CAN the pilot take over control, and is it going to be a perfectly safe landing?

 They must be able to take full control, coming back from Naples Friday the Ryan air captain turned the ignition off and pulled out the key 10 metres before touchdown,,  :''
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Offline itsme

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Re: It's a talking point. 737 Max series.
« Reply #58 on: April 23, 2019, 08:09:51 AM »
Of course the pilot can disengage the autopilot and take over (there's a switch in the control yoke which makes it very easy) and yes, it would probably end well. The only circumstances in which it would be a bad idea would be the very rare full Cat 3a auto-land - this is only because in a full Cat 3a the pilot will not be able to see the runway to keep straight. There are no airliners in service which can't be flown manually if desired. The point about the 737max's MCAS is that it is NOT part of the flight control system OR part of the autopilot. As such the crew weren't necessarily aware that it was operating, and to switch it off required switching off the elevator trim motors (not an intuitive thing to do) rather than disengaging the autopilot or failing a mode in the flight control system to force it into a different mode (which are the two things that pilots are used to doing as a matter of course.

MCAS triggering is not common - indeed I have heard it said that there have only been two occasions when it has triggered on an in-service aeroplane. On both occasions it resulted in a catastrophe. I suspect that's why Boeing are bricking it right now.

PDR
so its almost a separate auto system that simply needs a big flashing light saying 'MCAS Engaged' and big red button marked 'Disengage MCAS'...if it is indeed needed, from what you are saying, it is a superfluous bit of technology. Can I suggest my personal KISS system to them?

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Re: It's a talking point. 737 Max series.
« Reply #59 on: April 23, 2019, 10:57:31 AM »
It is needed to meet the regulations*. Wiuthout it the aeroplane couldn't be certified. If they gave it a waiver against that regulation the other aircraft manufacturers would sue, and even if they didn't they would demand similar relaxations of regulations for their aircraft. That would be an inclined plane with a very low coefficient of friction.

PDR

* for stickforce gradients
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Offline FrankS

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Re: It's a talking point. 737 Max series.
« Reply #60 on: April 23, 2019, 18:33:22 PM »
Plus if it had markedly different handling characteristics to the existing 737s then the conversion course would have been more than a 30 min IPad session, which wouldn't be a good selling point to airlines , especially big 737 users.

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Re: It's a talking point. 737 Max series.
« Reply #61 on: April 23, 2019, 21:03:33 PM »
Plus if it had markedly different handling characteristics to the existing 737s then the conversion course would have been more than a 30 min IPad session, which wouldn't be a good selling point to airlines , especially big 737 users.
not filling me with confidence here. Any DC3s still operating?

Offline PDR

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Re: It's a talking point. 737 Max series.
« Reply #62 on: April 23, 2019, 21:13:02 PM »
That's ironic - the DC3 has a far worse accident record than even the 737max8!

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Offline Albert

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Re: It's a talking point. 737 Max series.
« Reply #63 on: April 24, 2019, 06:03:24 AM »
I spent my last 15 years of a 45 year engineering career writing flight manuals for Airbus. So I'm keeping quiet.😊

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Re: It's a talking point. 737 Max series.
« Reply #64 on: April 24, 2019, 07:04:48 AM »
I spent my last 15 years of a 45 year engineering career writing flight manuals for Airbus. So I'm keeping quiet.😊
you missed a bit!

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Re: It's a talking point. 737 Max series.
« Reply #65 on: April 24, 2019, 07:39:07 AM »
That's ironic - the DC3 has a far worse accident record than even the 737max8!

PDR

 So that explains this then,,, :''
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Re: It's a talking point. 737 Max series.
« Reply #66 on: April 24, 2019, 08:25:29 AM »
only time I would jump out of one was if it was on fire! Over 10,000 Daks were built, and still some in service 77 years later....and no autopilot...

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Re: It's a talking point. 737 Max series.
« Reply #67 on: April 24, 2019, 11:23:09 AM »
Of course the DC-3 has an autopilot, as did all transport and bomber aircraft of that era.

But if you look at their accident records, the DC3 accident list on Wiki has separate pages for each year - there are literally hundreds of them whereas in nearly 50 years of 737 operation the accidents (fatal and non-fatal, including hijackings) all fit on the one page.

Both 737 and DC3/C47 were built in similar numbers (roughly 11,000 of each) but the DC3s flew most of their lives at low utilisation, while the 737s generally average a minimum of 4 flying hrs/day (even allowing for maintenance downtime). The safety records of the two aren't in the same league - this idea that the ancient aeroplane was somehow safer is completely illusory.

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Re: It's a talking point. 737 Max series.
« Reply #68 on: April 24, 2019, 11:33:30 AM »
No, I dont imagine it is. Just a joke point, really. I would expect modern aircraft to be a quantum leap ahead in safety. Mind you, a lot of Daks were SHOT down.. Maybe we should compare with the 747-400 with, I believe, one fatal crash in the passenger version, (down to pilot error). Thats in 30 years.

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Re: It's a talking point. 737 Max series.
« Reply #69 on: April 27, 2019, 09:58:03 AM »

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Re: It's a talking point. 737 Max series.
« Reply #70 on: April 27, 2019, 11:22:59 AM »
Not the only one who isn't in the plane,,,
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Re: It's a talking point. 737 Max series.
« Reply #71 on: April 27, 2019, 19:31:24 PM »
No, I dont imagine it is. Just a joke point, really. I would expect modern aircraft to be a quantum leap ahead in safety. Mind you, a lot of Daks were SHOT down.. Maybe we should compare with the 747-400 with, I believe, one fatal crash in the passenger version, (down to pilot error). Thats in 30 years.

Also factor in the reliability aspects, the multiple redundancy systems, and the generally better built modern aircraft.
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Re: It's a talking point. 737 Max series.
« Reply #74 on: May 08, 2019, 06:35:00 AM »
Hardly fills one with confidence - a change to the old mantra of 'If it ain't Boeing I ain't going' to 'If it's Boeing, I ain't going.' Just looking at that item Boeing's behaviour seems appalling.
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Re: It's a talking point. 737 Max series.
« Reply #75 on: May 08, 2019, 08:23:52 AM »
It's a lot of the big companies, remember the pitot tubes on the Airbuses, and fuel tanks on a couple
of American built cars that blew up when hit in the rear,,,  :-X
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Re: It's a talking point. 737 Max series.
« Reply #76 on: May 08, 2019, 11:57:20 AM »
It's a lot of the big companies, remember the pitot tubes on the Airbuses,

I assume you're refering to AF447. There was nothing "wrong" with the pitot tube design or manufacture. The aeroplane suffered a pitot heat failure (to which it was no more or less prone than any other airliner) causing inconsistent readings between the multiple probes. The aircraft systems reacted correctly by sounding an alarm and disengaging the autopilot. The events after that were simply due to the crew mishandling the aircraft.

The car example isn't a couple, it's one - the Ford Pinto. This became such a famous case, and it cost Ford so much in damages, that the US car industry changed its behaviour in that regard (although they came close to trying it again withe the Explorer Cruise Control issue a few years back).

But these are exceptions rather than a general behavioural style IMHO.

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