Originally Posted by
yanrair
Alf
I don’t think runaway stab is avoidable on any type? Certainly not on any 737 or even 707 for that matter.
Any electrical input can trigger the motor and if that input, false or otherwise , doesn’t stop, the stab keeps running.Manual elec. trim, autopilot trim, Mach trim, STS trim or MCAS trim all have
This is easy, well known, and is regularly done in the automation industry.. And on real fly by wire aircraft like the Falcon, except that the Falcon had a flawed design/risk analysis :-(
One method is to use an AC motor powered from DC via. an 3 phase DC/AC converter. The DC/AC converter need to perform an accurate PWM modulation to generate an rotating magnetic field. Anny fault like a shorted transistor would generate no field or a DC field blocking the rotation. (and burn a fuse)
The system need at least two redundant control paths with diagnostics checking for agreement between the two channels, this information can be communicated either via. communication protocols, or via. discrete signals.
The two control channels typically disconnect the High and Low side gate drivers, and signal to the DSP/MCU that it should not generate an PWM field.
https://en.wikipedia.org/wiki/Safe_Torque_Off
https://www.sdsdrives.com/downloads/...ick_guides.pdf
Falcon Runaway Trim
The biggest risk is if the designers fail to properly analyze all failure modes..
If we compare the B737 MAX with the Falcon, then both aircraft had incorrect trim movement.
In the case of the Falcon it was a matter of fixing and re-qualifying some HW and SW since the architecture of the system was sound.
In the case if the B737 MAX it is much more difficult.