FAA orders B787 operators to reboot the aircraft within every 51 days.
Boeing 787s must be turned off and on every 51 days to prevent ‘misleading data’ being shown to pilots
US air safety personnel call it ‘potentially catastrophic’ if the reboot directive not implemented
The FAA has ordered Boeing 787 operators to switch their aircraft off and on every 51 days to prevent what it called “several potentially catastrophic failure scenarios” – including the crashing of onboard network switches.
The airworthiness directive, due to be enforced from later this month, orders airlines to power-cycle their B787s before the aircraft reaches the specified days of continuous power-on operation.
The power cycling is needed to prevent stale data from populating the aircraft’s systems, a problem that has occurred on different 787 systems in the past.
According to the directive itself, if the aircraft is powered on for more than 51 days this can lead to “display of misleading data” to the pilots, with that data including airspeed, attitude, altitude and engine operating indications. On top of all that, the stall warning horn and overspeed horn also stop working.
This alarming-sounding situation comes about because, for reasons the directive did not go into, the 787’s common core system (CCS) – a Wind River VxWorks real-time OS product, at heart – stops filtering out stale data from key flight control displays. That stale data-monitoring function going down in turn “could lead to undetected or unannunciated loss of common data network (CDN) message age validation, combined with a CDN switch failure”.
Solving the problem is simple: power the aircraft down completely before reaching 51 days. It is usual for commercial airliners to spend weeks or more continuously powered on as crews change at airports, or ground power is plugged in overnight while cleaners and maintainers do their thing.
The CDN is a Boeing avionics term for the 787’s internal Ethernet-based network. It is built to a slightly more stringent aviation-specific standard than common-or-garden Ethernet, that standard being called ARINC 664.
A previous software bug forced airlines to power down their 787s every 248 days for fear that electrical generators could shut down in flight.
Airbus suffers from similar issues with its A350, with a relatively recent but since-patched bug forcing power cycles every 149 hours.
A 787 stale data problem in years gone by saw superseded backup flight plans persisting in standby navigation computers, and activating occasionally. Activation caused the autopilot to wrongly decide it was halfway through flying a previous journey – and manoeuvre to regain the “correct” flight path. Another symptom was for the flight management system to simply go blank and freeze, triggered by selection of a standard arrival path (STAR) with exactly 14 waypoints – such as the BIMPA 4U approach to Poland’s rather busy Warsaw Airport. The Polish air safety regulator published this mildly alarming finding in 2016 [2-page PDF, in Polish].
This was fixed through a software update, as the US Federal Aviation Administration reiterated last year. In addition, Warsaw’s BIMPA 4U approach has since been superseded.