FlyerTalk Forums - View Single Post - Air Canada Selects Boeing 737 MAX to Renew Mainline Narrowbody Fleet

Jan 4, 2020 | 7:52 pm

#3589

bimmerdriver

Join Date: Aug 2012

Location: YVR

Programs: AC E50K, NEXUS

Posts: 648

Quote:

Originally Posted by canopus27

You & I have debated this issue in the past, so against my better judgment I'll give it one last shot. Yes, I'm including references for you.

You consistently understate the significant difference in the role that the FCS has in the 737 family, and the much more significant role that it plays in fly by wire aircraft such as the 777, 787, and the 3xx Airbus family.

In the FBW cases (B777, B787, A3xx), you are correct that the FCS is required in all normal situations. As a result, the entire system (hardware and software) is designated as safety critical. ref: https://www.researchgate.net/publica...ontrols_system

In contrast, all (previous) versions of the 737 had "classical" control systems. There were either physical cables from the cockpit to the control surfaces, or electrical or hydraulic equivalents. No computer is involved in the movement of the flight surfaces. That's why the FAA didn't treat the 737 MAX FCS as a safety critical system. ref: https://www.wsj.com/articles/faa-saw...sk-11557831723

You seem to like Bjorn, so you should read his series on flight control systems: https://leehamnews.com/2019/07/26/bj...ectrical-wire/

The whole problem, from the outset, is that the 737 was never designed as a FBW system, and so the entire FCS infrastructure (hardware and software) was never build with the required levels of safety, testing, and redundancy. That doesn't mean it couldn't be done. It just means that it wasn't done in the 737 family - because it wasn't necessary to do so.

And then along came the MAX, with the MCAS - a system that all of a sudden granted full control over a critical flight control surface, to a computer. Specifically, to a computer that was never designed nor tested as a safety critical system.

Of course it's possible to turn the MCAS into a safety critical system (and I believe that's what Boeing are now attempting to do). But that change will require far more than software changes - it will require hardware changes (for example, leveraging two computers in parallel, ref: https://fortune.com/2019/08/02/boein...trol-computer/), and it will require a ton of additional testing and certfication.

For you to gloss over those facts is disingenuous at best.

The fact that the 737 family doesn't have a full FBW flight control system is not the issue. The issue is that the original implementation of MCAS was flawed because it did not take advantage of the redundant AoA sensors, among numerous other things. MCAS is a software function within the redundant flight control computers, which have full authority over the control surfaces. Boeing is not adding a second flight control computer to fix MCAS as your reference claims. All 737s including the MAXes already have redundant flight control computers. Here is a reference: Flight Control Computer (FCC). The "ton of additional testing and certification" is exactly what's taking place as we speak.