4 ms·
Im gonna re-explain what you’ve written in language I’m more familiar with to check my understanding, and hopefully you can correct any errors I make. As I und
by avianlyric 5y ago
Im gonna re-explain what you’ve written in language I’m more familiar with to check my understanding, and hopefully you can correct any errors I make.
As I understand it the A330 has three primary flight computers, all observing the same inputs (which might come from different physical sensors monitoring the same thing?) and producing outputs, also know as “orders” for other systems in the plan, like actuators.
Of these three computers, one will act as the primary command computer (COM), one as a monitoring computer (MON), the third is spare and normally ignored. Only orders from the COM machine is sent to downstream systems like actuators.
There’s a separate watchdog system that monitors the outputs (orders) of both the COM and MON, and if their order values diverge by too much for too long, it shuts down the COM computer and passes control to the MON computer and the spare. As part of this process one of those two computers becomes COM the other MON. I assume the size of the divergence determines how long they can diverge for. A large divergence is only allowable for a very short period of time, and a small divergence is allowed for longer.
In addition to all of this, the computers have different operating modes which changes how they respond to inputs. In this case the relevant states are “normal law” for in the air, and “ground law” for on the ground. One things that’s different between these states is how tightly coupled rudder inputs from the pilot are to rudder orders to the actuator. In the air the rudder is less tightly coupled than on the ground. E.g. commanding full left rudder in the air results in less physical movement of the rudder than the same command on the ground.
When the plane lands, detected by a pressure sensor in the landing gear, the computers transition from “normal law” to “ground law”. Which for some outputs, like the rudder, might result in a step change in outputs (orders) from the computers.
So in this specific scenario what happened is that the flight computers for some reason didn’t transition between “normal law” and “ground law” simultaneously (or close enough to simultaneously). So the COM computer significantly changed its rudder output as a result of changing law, but the MON computer didn’t, because it hadn’t changed law yet (the inverse of this is also possible). As a result they were producing very large differences in rudder orders, resulting in the monitoring watchdog killing the COM computer and failing over to the spare. Where the above situation happened a second time, resulting in all computers being shut down.
Is all of the above correct?
All of this does make me wonder, if changing law can result in computer outputs quickly changing, doesn’t that make law changes inherently dangerous? If you’re a pilot landing a plane applying significant rudder inputs, doesn’t the above me that those inputs will have a vastly different effect once the wheels touch the runway?
- strogonoff 5y ago> If you’re a pilot landing a plane applying significant rudder inputs, doesn’t the above me that those inputs will have a vastly different effect once the wheels touch the runway? IANAP so hopefully someone will correct me, but from my understanding in this case the answer would be “yes, they will have a different effect, as they should”. Rudder is used in crosswind landings to maintain runway alignment while still in air; once enough weight is on wheels and speed is low enough rudder quickly loses its effectiveness and control shifts to wheel steering and brakes. That said, I suspect it doesn’t help that flight computer sort of has a binary context flag (either we are in the air or on the ground), it might have simplified some of the business logic but does not seem to map well to reality at a crucial moment. If imagined in slow motion, the system doesn’t just flip a state but goes through a spectrum.
- lisper 5y ago[Private pilot here] > doesn’t that make law changes inherently dangerous? Well, yeah, but landing is inherently dangerous for the exact same reason: the system dynamics change suddenly when the wheels touch the ground. That happens (obviously) as a consequence of the laws of physics whether or not you have a computer in the loop. So this is a risk that just goes with the territory.
- marcodiego 5y ago> So in this specific scenario what happened is that the flight computers for some reason didn’t transition between “normal law” and “ground law” simultaneously (or close enough to simultaneously). So the COM computer significantly changed its rudder output as a result of changing law, but the MON computer didn’t, because it hadn’t changed law yet (the inverse of this is also possible). As a result they were producing very large differences in rudder orders, resulting in the monitoring watchdog killing the COM computer and failing over to the spare. Where the above situation happened a second time, resulting in all computers being shut down. Possible solution: always designate COM/MON computers which agree on the mode: flight or ground. Only disable a primary COM computer if it disagrees with the MON computer while both are running on the same mode. > There’s a separate watchdog system that monitors the outputs (orders) of both the COM and MON, and if their order values diverge by too much for too long, it shuts down the COM computer and passes control to the MON computer and the spare. So, if the MON computer is faulty it will always disable the 3 computers?