Somebody's not paying the "government inspectors" enough "bonus money" to get the planes passed again.
Good thing we have IQ45's people on the job - keeping people saf.......er, rich!

Determining When to Shield Aircraft Wiring

Shielded wiringA common question when it comes to designing an aircraft’s electrical wiring interconnect system (EWIS) is which of the cables should be shielded? There are several standards, such as MIL-STD-461, Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, that provide an excellent basis for topics and testing that should be considered. But for those who aren’t interested in reading a 280-page standard, this article reviews several shielding considerations that should be made before making particular design choices.

Why Shield?

The quick and simple answer is to limit the impact of electromagnetic interference (EMI) on system performance. But just because EMI is a concern doesn’t mean that shielding should be used on the wiring for every system. The consequences of selecting a shielded cable construction include increased bend radius, increased weight, and increased installation/repair time.

There are a couple of options when protecting systems from EMI and implementing shielding. These options include: shielding only the signal wires, shielding only the power wires, and shielding both. Each of these is reviewed here.
Shield Signal Wires

The starting point for many engineers and EWIS designers is to limit the potential EMI impact on signal cables. After all, these cables, which send signals with low voltages (often less than 10V) at high data speeds, can be negatively impacted by the electrical noise of aircraft equipment. So, protecting the integrity of signals thus improves the confidence in the data integrity.

How far away does your wire/cable need to be to avoid the impacts of EMI? This is a difficult question to answer due to the many factors impacting EMI. Shielding can be an easier solution.

Of course, once the signal wires are identified for EMI protection/shielding, the question is whether the individual wires/pairs should also be shielded if inside a multiconductor cable. This decision is dependent on the data rate, as high frequencies generate more EMI and are more susceptible to EMI. With shielding a cable and the internal pairs, the outer shielding can be a generalized shield and the internal shield can be optimized to shield the frequencies likely to be carried by the internal wires.

The benefits of shielding the signal wires:

Transmitted data for each system is isolated
If improved EMI protection is needed, it can be implemented system by system
Conceptually easy to address and plenty of available shielded cable options

Shield Power Wires

Another perspective is to shield the power wires. A typical aircraft design will have a larger percentage of signal wires than power wires, so it can be easier to shield the power wires.

When considering the latest trends in electrical power systems, there are several systems that gain equipment control with the use of pulse width modulation and wide frequency power generation. Each of these systems offer power and control benefits, but create new challenges for EMI. For example, the use of pulse width modulated power with high-frequency current changes (dI/dt) can generate strong electromagnetic flux (EMF) and impact nearby signal wires.

The benefits of shielding the power wires:

Reduce the EMI sources throughout the aircraft
Possible overall aircraft EMI reduction, thus reducing noise for sensitive electronic equipment
Additional chafe protection for power cables

Shield Both

Another option is to shield both, which provides the benefits of both; however, this option comes with significant weight penalty. Solutions for reducing the overall weight burden of shielding exist, but there will always be a financial and weight cost for shielding. The difficulty with implementing this solution is that many of the harnesses become part of line replaceable units (LRUs), which aren’t designed for field maintenance of just the harness. Naturally, the argument here is: If you can increase the reliability of a wire harness by a factor of two (as an example, no data to support), would you be willing to remove the field serviceable capability?

The use of shielding to protect wiring and signals has been around for generations, but the importance of protecting signal integrity is more important than ever in modern aircraft designs. There are multiple strategies for accomplishing this, and the implementation of shielding is entirely dependent on the performance requirement and the operation frequencies of the signal equipment.

In the end, it comes down to what design choice can create the safest EWIS design capable of completing the prescribed task.

They don't say where this problem is located. Is it aft of the pressure bulkhead?

Is it an EMI issue or friction. Is it close to a sensor where a new wire bundle could be easily replaced?

I have never seen a wire bundle run the entire length of an aircraft without some sort of disconnects, either in the electronics bay or at a structural point. Have you? Maybe things have changed since we built the prototype DC-9 Series 80.

It still sounds like a lot of panel pulling to add spacing, insulation or a new loom entirely.

Since the line is closed for a while they can build your solution into new craft easily. Get busy wire assembly shop.

I would like more detailed info.

As for MCAS, if dual redundancy was not built in, a software fix is BS! They need to add the second AOA input before that would work.

I also wonder why they couldn't just pop the breaker. I think the stall warning and stick shaker would be separate from the automatic MCAS and still function.

I miss avionics.

Unfortunately, there isn't a lot more detail of that exact issue- but there IS more information here:

https://www.nytimes.com/2020/01/05/business/boeing-737-max.html

Enjoy.

I guess I was on the right track.

Now what's your opinion on software and MCAS. I call BS!

Also, you do NOT modify or change anything on a production line without submitting an ECR, have an engineer come down to look,
change the build paper AND sell it to the FAA inspector.

Everything, no matter how small has consequences.

I was a final systems checkout tech at McDonnell Douglas in Long Beach, (RIP). Our paperwork was checked down to every comma and period.

Is this how Boeing builds their crew capsules? See Apollo 1.

Final approval if it doesn't fail?

It looks like you need to disco the AP to even touch it.

Nothing a pilot is going to do. Buttons first, oops, too late to figure this out.

I forget who wrote the tech manuals for Douglas, but he was the master of breaking every system down and showing the interconnects.

These were the best ever.

Boeing can't even get good camera shot of their launches. I just watched Space X nail another one!

Be afraid Boeing! Without the government largess, you would be toast!

I woke up last night and remembered who wrote the best tech books in the business.

Larry Lamm: http://wikibin.org/index2.php?option=com_content&do_pdf=1&id=35100

His breakdowns make problems like this easy to diagnose.
We would have noticed the lack of dual redundancy due to his drawings.
It's worth a read.

and the books where very easy to understand.................

What I liked about the B737 was that we could go into the CDU's and find all of our ATA's to perform flight checks on what we were working on and perform pre-flight checks if we had write-ups............................man do I miss working on planes.......................absolutely loved aviation.........................

Thanx's for the information.....................

before asking for a refund? This could sink Boeing