4 ms·
> they can lock out the bad one as an input to the MCAS The 737 MAX didn't support that as delivered. Even with the warning (which some US airlines paid for),
by Someone1234 6y ago
> they can lock out the bad one as an input to the MCAS
The 737 MAX didn't support that as delivered.
Even with the warning (which some US airlines paid for), you couldn't "lock out the bad one" even assuming you knew which one that was (you don't). All you could do is disable the electronic horizontal stabilizer completely, so that MCAS couldn't command it into a dangerous and unrecoverable state.
MCAS has been fixed, in the sense that it won't command continuously bad trim inputs until the aircraft is unrecoverable when bad AOA data is provided, but ultimately the aircraft either needs to be designed around trustworthy AOA data (i.e. triple) or you untrustworthy (i.e. double, even with the warning). Half measures are exactly how we got to this point, with two crashes.
Both answers are actually acceptable. A lot of completely safe aircraft have untrustworthy AOA inputs, the key there though is that automated systems are designed around that assumption. MCAS had too much flight authority to be linked to untrustworthy inputs.
- WatchDog 6y ago> Even with the warning (which some US airlines paid for) Are you saying that a bad input warning was an optional(paid) extra?
- mitchellgoffpc 6y agoYep. The "AOA sensor disagreement" light is built into every cockpit, but it won't turn on unless the airline purchased a specific premium add-on. Boeing claims they initially didn't realize the light was bundled as an add-on, and then once they did realize, they decided it wasn't critical to the airplane's safety and just kept selling it as an add-on, which is... kind of alarming, to put it mildly. https://www.nytimes.com/2019/05/05/business/boeing-737-max-warning-light.html https://www.nytimes.com/2019/05/05/business/boeing-737-max-w...
- AnssiH 6y ago> The "AOA sensor disagreement" light is built into every cockpit, While technically true, that sounds a bit misleading considering that the AOA DISAGREE warning "light" is just a text indicator on the primary flight display: https://www.boeing.com/resources/boeingdotcom/commercial/737max/assets/images/737-flightdeck-1242x272.png https://www.boeing.com/resources/boeingdotcom/commercial/737...
- mitchellgoffpc 6y agoShoot you’re right, I read “warning light” and just assumed. Thanks for the correction! On the other hand, it’s even weirder that Boeing didn’t bother fixing it if it was just a software patch...
- WatchDog 6y agoWow, that is remarkable.
- LoSboccacc 6y ago> Half measures are exactly how we got to this point also they increased the maximum stabilizer angles mcas could command, the review/certification was done with a 0.6 cap (effective but not overwhelming) and on production it was increased on a whopping 2.5 degrees. it was also meant to be using vertical acceleration to understand whether the plane was actually stalling, but that trigger was removed it was also supposed to operate slowly enough to let people catch up with its operation and be able to disconnect it in case of a runaway trim, but the increased angle required to move the stabilizer faster I don't know the exact English term for this kind of iterative failure of people communicating changes to each other assuming they are both fixing a problem, instead making it worse, but it's not just like they were doing half measures, they were each tuning their systems in silos, without considering cross system functionality from each system behavioral changes
- salawat 6y agoImpedance mismatch I believe they call that in the Electrical Engineering world. If you don't match up your output characteristics to the specified input characteristics of the next circuit element, you're hosed.