Timeline for Why does the KC-46 use two sensors for MCAS when the 737 Max doesn’t?
Current License: CC BY-SA 4.0
14 events
| when toggle format | what | by | license | comment | |
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| Jul 19, 2022 at 16:08 | comment | added | Pete P. | With one sensor feeding a side, when discrepant, backup is the other side, pilot arbitrates which one is valid by comparing with standby ASI, altimeter. AoA value is not critical to pilots or autopilot—unlike Airbus, the autopilot doesn't override pilots inputs, so automated source select to maintain availability & reliability of params not necessary. MCAS operation is not safety-critcal. On 767 KC-46 MCAS used with refueling operations, critical enough to require 2nd sensor (for validity monitoring) but not 3rd (reliable redundancy). | |
| Jul 19, 2022 at 13:51 | comment | added | Pete P. | The number of sensors used to provide a parameter is driven by the criticality of the function that uses the parameter as well as the operational philosophy behind the architecture of involved systems, which for the 737 is "same-side only" for sensors, computers, displays and controls, to facilitate fault isolation and resolution. Thus the left display & FCC get airspeed, altitude, AOA from ONE resp. sensor on the left, the left SMYD computer gets AOA from L sensor & drives L control column stick shaker. Ditto for right. Exception: only one FCC can be active & operate both flight controls. | |
| Dec 1, 2021 at 3:53 | answer | added | MacGuffin | timeline score: 1 | |
| May 14, 2021 at 0:00 | history | tweeted | twitter.com/StackAviation/status/1392993071252582415 | ||
| Feb 15, 2020 at 9:23 | comment | added | Koyovis | @Firefighter1 "So if Boeing changed the computer, and opened it up to read two sensors at once, the plane now requires additional training? ". Yes indeed. Upon a discrepancy in outputs the system will warn the pilots and switch off, since there iss no way to tell which one is wrong in case of 2 systems. The flight crew will then need to be trained in handling the aeroplane in the circumstance that the system was designed to deal with. | |
| Feb 14, 2020 at 19:33 | vote | accept | George Clooney In a Mooney | ||
| Feb 14, 2020 at 13:11 | comment | added | Mike Sowsun | No, MCAS does not read Data from the flying pilot side. MCAS gets AOA data from either FCC 1 or FCC 2 and this is automatically alternated for each flight. FCC 1 uses the left AOA and FCC 2 uses the right AOA. | |
| Feb 14, 2020 at 11:31 | answer | added | Koyovis | timeline score: 13 | |
| Feb 14, 2020 at 11:25 | comment | added | MSalters | @Firefighter1: You then need to compare them. And since you only have two, in case of conflict you need a pilot procedure in case the readings differ too much. Can't vote with just two sensors. | |
| Feb 14, 2020 at 10:59 | comment | added | George Clooney In a Mooney | @MSalters So if Boeing changed the computer, and opened it up to read two sensors at once, the plane now requires additional training? | |
| Feb 14, 2020 at 10:47 | comment | added | Dohn Joe | @Jpe61 redundancy is very important for the military. Furthermore, I believe that the military is in a much better position to gain insight into a plane's design; and if they see a need, to demand changes. MCAS was deliberately designed by Boing so that the MAX falls under the 737's existing certification. If the 737 MAX fulfills all existing regulation, on what grounds could the FAA demand changes to the MCAS? | |
| Feb 14, 2020 at 10:45 | comment | added | MSalters | Also a guess: There was no pressure to carry over a type rating for pilots of a previous model. | |
| Feb 14, 2020 at 10:23 | comment | added | Jpe61 | Regarding Boeings decision to use single sensor data for MCAS, words that come to mind are stupid and reckless. The Air Force was most likely smart enough to demand a system with some basic redundancy. | |
| Feb 14, 2020 at 9:34 | history | asked | George Clooney In a Mooney | CC BY-SA 4.0 |