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On a recent flight the captain made an announcement at the gate that they need to reboot the computer as somethings aren't working, and rebooting usually solves the problem.
After posting this on twitter I got a replying saying that most planes have 3 computers and rebooting during flight is common.
What are the reasons for reboot? For me it sounds like a software/computer that isn't tested thoroughly instead.
Today's modern airplanes have many computers, and sometimes things get out of sync and don't communicate properly. During the preflight checks, we check each system according to the recommendations of the manufacturer, and occasionally one of the checks will not pass correctly. Depending on the system, when a check fails, rebooting the computer may be an option to see if it fixes it (similar to rebooting your home computer). Other times, the issue is the communication protocol between two (or more) different computers. Often times in this case, the entire airplane must be powered down in order to reset them all at the same time. Since these tests aren't performed in the air, the problem usually doesn't crop up there.
In the vast majority of cases it isn't a safety critical item that we are dealing with (where the software testing is more thorough), but rather a monitoring system. The safety critical items (like flight control software, etc.) will have multiple redundant systems and often have different versions of software on at least one of the computers "just in case".
Generally the flight crew won't tell the passengers when they need to "reboot" a computer unless we have to power down the entire airplane. That's very noticeable and will cause some people to worry so we like to give you a heads up before all of the lights go out and the air conditioning system shuts down. That of course is up to the discretion of the flight crew though.
This was probably the captain trying to be funny and use relatable terms for what was going on. In the EMB-145 occasionally the ACARS would stop working and you could re-initialize that during flight. The actual computer systems, however, could be reset but this is something you'd only do on the ground as it either involved powering down the aircraft or getting MX out to open some panels and pull breakers / flip switches.
The few things that could be reset in the air by pulling and resetting a breaker were things like FMS, ACARS and the autopilot, which we would do when directed by the QRH or MX and perhaps this is what the captain was referring to.
Airliners are more complex, so I suppose it's possible. On light aircraft, I've never heard of anyone having to reboot the computer (glass panel systems). Sometimes we have to reboot our radios, and I've had to reboot FADEC (engine control) computers before though.
Is rebooting the computer normal before/during flights?
It is wrong to write about "the computer" because a modern airliner probably contains hundreds or thousands of microprocessors and microcontrollers.
For example, the seat-back information and entertainment systems almost certainly each contain a microprocessor running some embedded operating system. Each of the items of avionics equipment will contain several microprocessors, probably running real-time operating systems that are certified for use in aircraft.
Critical avionics systems have the ability to reset themselves if they detect problems - for example they will have hardware-based watchdog timers that will restart a processor or system that isn't showing signs of running it's software normally.
Unlike a typical desktop computer, I'm pretty certain that critical systems on board an airliner are designed to handle multiple failures and keep working, in a degraded or fall-back mode if necessary.
most planes have 3 computers
As I wrote above, large commercial airliners will likely have hundreds or thousands of microprocessors and microcontrollers.
The "3 computer" idea comes from the use of redundancy in each critical system. Each subsystem may have two, three or more independently created systems with different hardware and with software written by different teams, each doing the same job. An arbitration system compares the outputs and if one of three disagrees, the other two get to determine what happens.
No, it's not normal part of the pre-flight checklist for the pilot to walk round to the rear of the aircraft, flip a big red switch off and on again and wait for a start-up jingle.
If the in-flight movie system isn't working, I'd expect someone will try turning it off and on again.
If there is a problem with one of the many independent systems in the cockpit, I'd guess it's plausible, in some cases, the pilot or engineer's checklists for the problem include some kind of reset operation.
