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Reboot your Boeing 787 every 248 days

  • 02 May 2015
  • Written by 
  • Published in Space

Or you might experience "loss of control."

One might hope that an aircraft isn't powered up continuously for 248 days (a little over 8 months), but if it is, and that aircraft is a Boeing 787, it is likely to lose all AC power.

Today, the US Federal Aviation Administration (FAA) issued an Airworthiness Directive (AD) as follows:

We are adopting a new airworthiness directive (AD) for all The Boeing Company Model 787 airplanes.  This AD requires a repetitive maintenance task for electrical power deactivation on Model 787 airplanes.  This AD was prompted by the determination that a Model 787 airplane that has been powered continuously for 248 days can lose all alternating current (AC) electrical power due to the generator control units (GCUs) simultaneously going into failsafe mode.  This condition is caused by a software counter internal to the GCUs that will overflow after 248 days of continuous power.  We are issuing this AD to prevent loss of all AC electrical power, which could result in loss of control of the airplane.

Applying finger to calculator, it seems that Boeing is counting centi-seconds in a 32 bit signed Integer.


The AD goes further to add, "If the four main GCUs (associated with the engine mounted generators) were powered up at the same time, after 248 days of continuous power, all four GCUs will go into failsafe mode at the same time, resulting in a loss of all AC electrical power regardless of flight phase."

That's hardly "failsafe," surely.  However, the required action by all operators is as follows:

At the latest of the times specified in paragraphs (g)(1), (g)(2), and (g)(3) of this AD, accomplish electrical power deactivation on the airplane, in accordance with step 2) in ''DESIRED ACTION'' of Boeing Multi Operator Message MOM-MOM-15-0248-01B, dated April 19, 2015; or Boeing Multi Operator Message MOM-MOM-15-0248-01B(R1), dated April 20, 2015. The main and auxiliary power unit (APU) batteries do not need to be disconnected when performing the electrical power deactivation. Repeat the electrical power deactivation thereafter at intervals not to exceed 120 days.

(1) Within 120 days after the last electrical power deactivation in accordance with step 2) in ''DESIRED ACTION'' of Boeing Multi Operator Message MOM-MOM-15-0248-01B, dated April 19, 2015; or Boeing Multi Operator Message MOM-MOM-15-0248-01B(R1), dated April 20, 2015.

(2) Within 120 days after the date of issuance of the original certificate of airworthiness or the date of issuance of the original export certificate of airworthiness.

(3) Within 7 days after the effective date of this AD.

The issue was discovered a couple of weeks ago by Boeing's engineers during lab testing and Boeing has announced that its software developers are working on a permanent solution (probably a 64-bit Integer!).  Once delivered, this update ought to override the directive.

Before boarding one's next flight on a B787, it might be prudent to ask, "so, when was the last time this thing was re-booted?"

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