Boeing woes

[krus_aragon]

Why? Because with two data sources there is no way of knowing which one is wrong if one or the other fails. The system was designed to alert the pilots to a disagreement between the two sensors and allow them to decide what to do next. (i.e following standard procedures to neutralise the fault).

My understanding (from researching this earlier this year) is that the Angle of Attack Disagree indicator was an optional extra with a significant price tag, and that at least one of the two crashed planes weren't fitted with it. (If it was part of the base package, it would have had greater implications for the aircraft certification process, and increased airlines' expense in having to retrain their exisiting 737 pilots. The airlines are the paying customers, so their wishes naturally influence the design process.)

 

[TrafficEng]

I think this gets to the nub of the issue. A perfect pilot in perfect circumstances might have been able to recover the situation, but if the pilot is less experienced or less trained, and something else is causing a distraction? Do you blame the pilot or the company? Particularly pertinent is the issue that Boeing were trying to avoid requiring the pilots to take extra simulator training, which might have given them that extra knowledge and confidence about how and when to intervene if the system let them down.

Yes, I agree it is the nub of the issue. The Sullenberger quote is spot on.

But what has been lost in all the discussion about MCAS is that the Lion Air crew did not adequately perform the first basic tasks (that have absolutely nothing to do with MCAS) in response to a condition that can happen on any 737 at any time. Those tasks should be instinctive and recalled from memory by any pilot qualified to fly any 737, as well as many other models of aircraft.

That isn't to absolve Boeing of blame, but simply to point out the fact that things were going wrong long before MCAS came into the picture. Those errors were contributory to the final outcome - not my words, but what the independent accident investigation report concludes.

 

[krus_aragon]

The impression I'm getting from this thread is:

The pilots were at fault
Boeing did nothing wrong
Other planes crash (but are not reported in the media)
The air transport industry as a whole is to blame

That's not the angle I took when I used this MCAS issue as a case study for engineering students looking at security and safety issues for embedded systems. I portrayed it more as a conflict between good engineering practice and external influences (expressed via management). These influences included the continued development of a legacy aircraft frame, a need to get a more fuel-efficient model to market to compete with Airbus' latest offering, and a desire to deliver a "flies-just-like-the-last-one" model to please their customers.

My main teaching points for the students were "what you do can be a matter of life and death, so give it proper attention", and "be willing to stand up against external influences when safety is an issue".

 

[GRALISTAIR]

My main teaching points for the students were "what you do can be a matter of life and death, so give it proper attention", and "be willing to stand up against external influences when safety is an issue".

excellent well said.

 

[JonasB]

The raw rate of accidents isn't really a helpful measure given the small numbers involved. What conclusion can you draw if a high rate of accidents happen with a particular model, but the nature and cause of the accidents varies widely?

The approach is also flawed if you compare (say) an A330/A340 with (say) a 737 when the role of the aircraft is very different. Given take off and landing is typically higher risk than cruising you might expect a higher rate of accidents in short-haul aircraft that spend a greater percentage of their lives taking off and landing than long-haul aircraft do.

The specific issue with the Max was that two accidents happened with apparently the same basic cause, and the mitigation steps introduced after the first accident appeared to be ineffective at preventing the second one.

It was you who brought up the A330, but yes it is not really comparable. So let's compare the 737MAX to the A320neo instead. The neo has been in service since january 2016, almost four years and around 1100 are built so far. And during that time it has been involved in 0 fatal accidents. Even the 737NG and and A320ceo have accident rates a lot lower than the MAX.

That post (and this one) wasn't trying to claim that other aircraft don't crash. I was just trying to point out that they do not crash at nowhere the same rate (at least if we exclude aircraft that have been built in very small numbers, like Concorde).

 

[TrafficEng]

It was you who brought up the A330, but yes it is not really comparable.

Yes, I did. For a specific purpose which I clearly stated.

And that wasn't to make a comparison of accident rates.

 

[DaleCooper]

Yes, I did. For a specific purpose which I clearly stated.

And that wasn't to make a comparison of accident rates.

I suspect a lot of readers will have understood you to be making just that comparison, as I did. Perhaps your posts are not clearly conveying the meaning you intended and because of that, like me, readers are misinterpreting them. Remember most of us are not experts in this field.

 

[JohnMcL7]

The impression I'm getting from this thread is:

The pilots were at fault
Boeing did nothing wrong
Other planes crash (but are not reported in the media)
The air transport industry as a whole is to blame

I noticed the same and quite shocked at some of the responses here which are certainly very in line with Boeing's own response but go against just about everything I've read and seen on the issue.

 

[MidlandsChap]

The 737 Max8 is inherently unstable due to the forward positioning of the engines. It's like a 220 seat Typhoon.

I have not heard about the revised engine location causing "instability", indeed if any commercial plane were inherently unstable I doubt they would have been passed safe for use. My understanding is that the engine positioning caused the plane to pitch upwards. Boeing installed software (MCAS) that automatically pushed the plane back down if the plane was climing to too high of an angle. The problem was the Max was marketed as being very similar to the existing 737s and MCAS was not well known about and pilots only got a short amount of training which supposedly didnt even mention MCAS.

 

[LOL The Irony]

What I don't get, is why didn't Boeing decide to do a chopped down B757 MAX instead, the 757s newer design and greater height would mean that it would have less issues with the bigger Leap engines?

757's use the same airframe as the 367-80, the 707, the 727 and the 737. It just has taller landing gear and a different nose design. So yes, it would help but it's still an ancient airframe. A 65 year old airframe.

On another note, Airbus are certainly showing Boeing how it's done.

 

[Grumpy Git]

Boeing are just being cheapskates in an attempt to undermine Airbus (and successfully looking at order numbers until planes started falling out of the sky).

 

[LOL The Irony]

Just a reminder that Airbus intend to replace the A320 family with a clean sheet aircraft in the 2030's. Boeing were managing this decade for roll out of such an aircraft at best and it looks like at least the middle of the next decade before a roll out occurs. Only problem if the MAX can't fly again, what replaces it? The A320 family order books are full up for the next few years.

 

[krus_aragon]

I have not heard about the revised engine location causing "instability", indeed if any commercial plane were inherently unstable I doubt they would have been passed safe for use. My understanding is that the engine positioning caused the plane to pitch upwards. Boeing installed software (MCAS) that automatically pushed the plane back down if the plane was climing to too high of an angle. The problem was the Max was marketed as being very similar to the existing 737s and MCAS was not well known about and pilots only got a short amount of training which supposedly didnt even mention MCAS.

It's two sides of the same coin. Moving the (larger) engines forward from under the wing affected/changed the stability of the aircraft. The most relevant change is that at low speed and at a high angle-of-attack, the new dynamics of the aircraft would cause it to continue pitching upwards, potentially resulting in a stall. (This degree of AoA was more than would be encountered in normal flight, and there wasn't a danger with the old under-wing engines anyway.)

MCAS was designed to stop the pilot pulling up into a very high Angle-of-Attack by adjusting the trim to pitch the nose back down, but the fact that each MCAS system only had one AoA sensor means that in two instances the system started pitching down even when the plane was level, and "corrected" the plane back to the ground.

(As you described, there was a desire to keep the new plane as similar as possible to the previous model, so a lot of these changes were glossed over.)

 

[furnessvale]

Boeing are just being cheapskates in an attempt to undermine Airbus (and successfully looking at order numbers until planes started falling out of the sky).

I wonder how many of those orders for Boeing are provisional and perhaps will transfer to Airbus?

 

[AM9]

I wonder how many of those orders for Boeing are provisional and perhaps will transfer to Airbus?

Unless Boeing can get a fully certificated aircraft* available to deliver in a timeframe that meets the customers' declared deployment programme, I would imagine that Boeing would be liable for liquidated damages for non-performance of the contract. By inference, the customer also would be freed from all obligations to accept any deliveries of 737 MAXs.
* in the case of a non-US customer (or even a US customer intending to deploy the aircraft flying into non-US airspace), that would probably mean certification signed-off by non-US air safety administrations. That is the killer for Boeing, as without universal approval of the aircraft's safety, it is a commercial failure.

 

[edwin_m]

It's two sides of the same coin. Moving the (larger) engines forward from under the wing affected/changed the stability of the aircraft. The most relevant change is that at low speed and at a high angle-of-attack, the new dynamics of the aircraft would cause it to continue pitching upwards, potentially resulting in a stall. (This degree of AoA was more than would be encountered in normal flight, and there wasn't a danger with the old under-wing engines anyway.)

MCAS was designed to stop the pilot pulling up into a very high Angle-of-Attack by adjusting the trim to pitch the nose back down, but the fact that each MCAS system only had one AoA sensor means that in two instances the system started pitching down even when the plane was level, and "corrected" the plane back to the ground.

(As you described, there was a desire to keep the new plane as similar as possible to the previous model, so a lot of these changes were glossed over.)

In other words the handling was different, and the instinctive response of a pilot familiar with the previous versions of the 737 would potentially make the situation worse. That doesn't make it intrinsically unstable. An intrinsically unstable aircraft is like a pencil balanced on its point - the tiniest disturbance creates an effect that increases that disturbance, so to keep flying it constantly needs very rapid control inputs which can only be provided by an automated control system linked to sensors. With a stable aircraft (as in any other stable system) the response to such a disturbance would tend to push back towards the equilibrium position.

Your comment also raises another worry for me. If MCAS had failed and the pilot either wasn't aware of the failure or wasn't aware of MCAS at all, then if it got into that low speed high AoA situation could the pilot inadvertently put the MAX into a dangerous stall?

 

[AM9]

In other words the handling was different, and the instinctive response of a pilot familiar with the previous versions of the 737 would potentially make the situation worse. That doesn't make it intrinsically unstable. An intrinsically unstable aircraft is like a pencil balanced on its point - the tiniest disturbance creates an effect that increases that disturbance, so to keep flying it constantly needs very rapid control inputs which can only be provided by an automated control system linked to sensors. With a stable aircraft (as in any other stable system) the response to such a disturbance would tend to push back towards the equilibrium position.

Your comment also raises another worry for me. If MCAS had failed and the pilot either wasn't aware of the failure or wasn't aware of MCAS at all, then if it got into that low speed high AoA situation could the pilot inadvertently put the MAX into a dangerous stall?

I suppose 'unstable' is a relative term here. The previous versions of the 737s, like most conventional commercial planes in the air, were broadly neutral in terms of AoA stability around the wing spar, once loaded correctly and trimmed. An excessive AoA approaching a stall could be corrected by lowering the nose and then accelerating. By putting larger, higher thrust engines on the MAX range, that were further forward than the natural C of G, the thrust moment now rotates the aircraft into a steeper attitude, which if unchecked would cause a stall. Pilots are trained to respond to stalls, where they would normally expect a normally neutral balance around the spar line. If the MCAS was trying to give the plane neutral characteristics by altering the trim, such a system would be fighting the pilots natural actions if the thrust was altered.

 

[edwin_m]

By putting larger, higher thrust engines on the MAX range, that were further forward than the natural C of G, the thrust moment now rotates the aircraft into a steeper attitude, which if unchecked would cause a stall.

Does the position fore and aft make any difference? If the thrust and drag are both parallel to the direction of flight then the moment depends only on the magnitude of these forces and the distance between their effective positions, which is measured perpendicularly and therefore doesn't depend on where along the parallel line the force actually acts.

Also have they actually increased the thrust? I understood the larger engine was to reduce fuel consumption and I'm not sure why it would need more thrust except to overcome the extra drag from the larger engines.

 

[DaleCooper]

Does the position fore and aft make any difference? If the thrust and drag are both parallel to the direction of flight then the moment depends only on the magnitude of these forces and the distance between their effective positions, which is measured perpendicularly and therefore doesn't depend on where along the parallel line the force actually acts.

Also have they actually increased the thrust? I understood the larger engine was to reduce fuel consumption and I'm not sure why it would need more thrust except to overcome the extra drag from the larger engines.

If either or both the drag and thrust do not act through the CofG they will tend to produce a rotation about the CofG.

 

[cjmillsnun]

The 737 Max8 is inherently unstable due to the forward positioning of the engines. It's like a 220 seat Typhoon.

No that’s not true at all. the Max 8 is no less unstable than any commercial airliner.

The reason MCAS was fitted is to make the plane feel like a 737NG so that pilots could fly a Max with no type recertification.

With the engines in a different place the plane will feel different to a 737NG (not unstable, just different). MCAS mitigates this difference. That’s it.

 

[edwin_m]

If either or both the drag and thrust do not act through the CofG they will tend to produce a rotation about the CofG.

Indeed so. But the size of this effect depends on how far the effective positions of these forces are from the CofG measured perpendicular to the lines of the force. Moving the engine forward along the axis of thrust doesn't make any difference, directly at least. Moving them upwards ought to reduce the rotation effect, but previous posts mention this is outweighed by various aerodynamic effects.

 

[DaleCooper]

Indeed so. But the size of this effect depends on how far the effective positions of these forces are from the CofG measured perpendicular to the lines of the force. Moving the engine forward along the axis of thrust doesn't make any difference, directly at least. Moving them upwards ought to reduce the rotation effect, but previous posts mention this is outweighed by various aerodynamic effects.

I've read your post again and I see I misunderstood it the first time, sorry.

 

[AM9]

The 737 Max8 is inherently unstable due to the forward positioning of the engines. It's like a 220 seat Typhoon.

Not at all. The Typhoon has been deliberately made unstable to the extent that it cannot be flown manually. The CofG is way back down the delta wing structure meaning that the canard wings have a very responsive control over the aircraft attitude but there is no chance of a stable position of the control surfaces that would enable even the best pilot a chance of keeping the thing in the air without autonomous flight control electronics.
There are four flight control computers on a Typhoon where each decision is based on a poll of those engaged in the manoeuvre. To reinforce the reliability of the four computers and minimise any common mode or pattern failures, each one has a different architcture, operating system and different development teams for the flight software.
The MAX8s have a bodged engine upgrade where the changes to the aircraft's flight characteristics were enough to require additional pilot training, but the manufacturer pretended that a software hack made it behave the same as the old craft (that wasn't properly engineered and failed catastrophically at least twice in service).

 

[Grumpy Git]

Can I take my tongue out of my cheek now?