Is there any in-flight built-in-test in the airplanes separate from the in-flight monitoring?
The thing that separates a BIT/BITE (E=Equipment) from continuous fault monitoring is that the continuous fault monitoring simply responds to exceedances of various paramters or values outside of specified limits in real time as they happen (say where an analog voltage signal of a device drops below a minimum of X or exceeds a maximum of Y), whereas a BIT/BITE test is a specific routine where simulated fault signals are injected into a circuit to see if a fault monitor is doing its job.
BIT or BITE tests of electronic controllers can be categorized into three types:
- Start Up BIT: Most common. A test routine automatically run on power up of the controller.
- Initiated BIT: Controllers may have a manually initiated BIT/BITE that can be run any time, but this is usually triggered by a button on the controller itself in the avionics compartment, or through a maintenance access portal in the aircraft maintenance diagnostic system, and usually doesn't have a flight crew initiated facility (but may in some cases). These are normally intended for troubleshooting.
- Continuous BIT: A controller may run an internal self test routine on a continuous cycle while powered up, where a controller has fault detection circuits that can't be allowed to have dormant failures, even for the duration of the flight.
Yes, in-operation BITs exist.
For example some older analog fuel flow meters have two BITs, one for zero flow and one for nominal flow. The zero flow BIT will move the needle up to a non-zero flow value, while the nominal flow one will move them down to a different low one. This way the instrument can be tested in any condition without risk that the results of the BIT could be misinterpreted.
On modern complex aircraft systems, each LRU (Line Replaceable Unit) will be conducting a number of built in tests on itself continuously as well as checks that only occur when the unit is powered on
For example, the power supply voltages will be monitored for too high/too low voltages, memory devices will be checked for read/write faults, interfaces with other systems will be checked for a frequent stream of data, data buses may perform a loopback test of themselves to check that it is working etc etc.
Any faults may be reported to a centralised maintainance/fault system which will determine whether the pilots need to be informed of the fault.