Saturday, April 04, 2020

Meet the new bug. Same as the old bug.

The Guardian in the U.K. reports on an FAA Airworthiness Directive that requires Boeing 787 aircraft be power cycled every fifty-one days to prevent "several potentially catastrophic failure scenarios".

From the AD:
The FAA is adopting a new airworthiness directive (AD) for all The Boeing Company Model 787–8, 787–9, and 787–10 airplanes. This AD requires repetitive cycling of the airplane electrical power. This AD was prompted by a report that the stale-data monitoring function of the common core system (CCS) may be lost when continuously powered on for 51 days. This could lead to undetected or unannunciated loss of common data network (CDN) message age validation, combined with a CDN switch failure. The FAA is issuing this AD to address the unsafe condition on these products.  
My friend and colleague Doug Young, who knows a thing or two about aircraft and avionics, brought this to my attention. I find fifty-one to be an interesting number, because unlike a lot of this class of bugs, it doesn't map quite so obviously into some kind of power-of-two/number-of-bits/frequency issue.

I tried to talk Doug into it being related to the number of bits (nineteen) in the data field of an ARINC 429 message (A429 being a common avionics bus that he and I have worked with), but even I thought it was a bit of a stretch, since it would require some system clock maintaining a frequency of 10000 ticks per day, causing the data field to overflow after 52.4287 days.

Both Doug and I independently arrived at the possibility of a uint32_t value in units of milliseconds, but that overflows after 49.71 days, a discrepancy that I find makes it unlikely.

As I've mentioned before, I ran into stuff like this all the time in my Bell Labs telecommunications days. Occasionally - alas - it was in code I wrote, definitely making for a learning experience.

Oh, and by the way, just last month HP announced yet another firmware bug in which some of their disk drives stop working after 40,000 hours of operation, also not an obvious power-of-two issue.

We will get fooled again.

Cabin Fever

Gotta wonder if the current COVID-19 craziness has pointed out a big problem in the tiny house movement. Just like those microapartments you read about in ginormous (and expensive) cities like Tokyo and New York did explicitly, the tiny house movement may have implicitly assumed that the residents won't actually spend all that much time in their abodes.

The Spousal Unit and I are really fortunate: a four bedroom tri-level in a semi-rural neighborhood near Denver Colorado. But even then, we get our share of cabin fever. If we were stuck together in a tiny house, we'd be posting photographs of each other holding the daily newspaper just to prove we hadn't killed each other.

(I'm not the only one thinking about this: "Homes Actually Need To Be Practical Now" and "Trapped at Home With People You Met On Craigslist", both in my favorite magazine, The Atlantic.)

Tuesday, March 31, 2020


You’ve heard the maxim that it’s far far more expensive to acquire a new customer than keep an existing one? Part of this is habit. Humans are for the most part creatures of habit. To keep an existing customer, you just need to keep feeding their habit (e.g. a latte every morning at the local Starbucks drive through), and not give them a reason to change it (e.g. keep botching their order). But to get a new customer, you have to get them to change their habits. You have to convince them to do your thing in place of whatever thing they've been doing. That’s a big deal.

I wonder how many of us are going to have our habits rewired due to all of this Coronavirus craziness. Sure, I used to go to Starbucks around 0600 every morning and read for an hour or two. Maybe when all this is over, I realize I can save a lot of calories and money by just making a cup of coffee at home - like I’ve been doing for the past several weeks... or months.

Yeah, I used to go to the gym six times a week. But maybe I can save a lot of time and effort by staying with doing those core floor exercises on my mat in the living room three times a week, and taking a long walk outside for cardio the other three times.

Maybe I learn an important lesson: I don’t really need Starbucks four dollar lattes, or an expensive gym membership.

Could be a lot of places are going to find out a lot of their regulars aren’t coming back.

Other ideas:

Businesses are slow to rehire, acting conservatively in the face of an uncertain future (H/T to Demian Neidetcher).

Businesses find their old employees have made other plans, so they are forced to hire new people who must be retrained, preventing them from achieving their prior level of service, at least anytime soon.

Businesses don't survive this, forcing us to make other accommodations anyway (H/T to Kelly Dixon).

I think it's going to be a substantially different world at the far end of all of this; not just politically and medically, but professionally and personally.