The US Federal Aviation Administration 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) 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".

Boeing 787 software bug can shut down planes' generators IN FLIGHT READ MORE

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. More about ARINC 664 can be read here.

Airline pilots were sanguine about the implications of the failures when El Reg asked a handful about the directive. One told us: "Loss of airspeed data combined with engine instrument malfunctions isn't unheard of," adding that there wasn't really enough information in the doc to decide whether or not the described failure would be truly catastrophic. Besides, he said, the backup speed and attitude instruments are – for obvious reasons – completely separate from the main displays.

Another mused that loss of engine indications would make it harder to adopt the fallback drill of setting a known pitch and engine power (see sidenote) setting that guarantees safe straight-and-level flight while the pilots consult checklists and manuals to find a fix.

Sidenote Pitch and power is a simple concept. If you have the throttles, say, three-quarters open and the nose of the aeroplane is pointing a few degrees above the horizon, chances are you're probably flying straight and level at a safe speed. Training manuals normally contain a number of precise pitch and power settings (they vary between aeroplane types) so if display systems start failing, pilots can fall back to these with confidence.

A third commented, tongue firmly in cheek: "Anything like that with the aircraft is unhealthy!"

A previous software bug forced airlines to power down their 787s every 248 days for fear 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.

Persistent or unfiltered stale data is a known 787 problem. In 2014 a Japan Airlines 787 caught fire because of the (entirely separate, and since fixed) lithium-ion battery problem. Investigators realised the black boxes had been recording false information, hampering their task, because they were falsely accepting stale old data as up-to-the-second real inputs.

More seriously, another 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.

The Register asked Boeing to comment. ®

Editor's note: An earlier version of this article mentioned the Boeing 787 CCS uses a Wind River VxWorks real-time OS product at its heart. While this is true, Wind River has been in touch to remind us "the CCS is made up of 80 to 100 applications," as well as VxWorks, and said the bug described in this article is not the fault of its operating system.

"The functions of VxWorks have nothing to do with the data issue you are highlighting in the 787," a spokesperson added. We are happy to clarify our coverage.