Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
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Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#2tl;dr: 51 days is the wraparound point of a signed 6-byte counter running at 33 MHz, used to invalidate stale data from instruments.
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#3tl;dr: 51 days is the wraparound point of a signed 6-byte counter running at 33 MHz, used to invalidate stale data from instruments.
This assumes there is no margin of error baked into the 51 day rule, which surprises me.
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#4Fascinating analysis. I know planes get used a lot, but I'm surprised that they go for such a long time without ever being powered down.
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#5Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#6This is a really good analysis of the issue from the just the verbage from FAA. Well done.
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#7For example, let’s imagine that the timestamp set by the transmitting ES is close to its wrap-around value. After performing the required calculation, the receiving ES obtains a timestamp that has already wrapped-around, so it would look like the message had been received before it was actually sent.
Isn't it surprising that modulo arithmetic, as already employed successfully in TCP sequence numbers and the like, still seems to be incorrectly implemented today? What's more disappointing is seeing all the other incredible systemic complexity they've added, and yet the plane appears to have no mechanical backup instruments?
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#8Fascinating analysis. I know planes get used a lot, but I'm surprised that they go for such a long time without ever being powered down.
51 days seems to be approximately how often my mac dies in kernel panic or starting to be bugged by persistent software problems that go away with a restart.
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#9[deleted]
Re: Reverse Engineer’s Perspective on the Boeing 787 ‘51 days’ Directive
#10For example, let’s imagine that the timestamp set by the transmitting ES is close to its wrap-around value. After performing the required calculation, the receiving ES obtains a timestamp that has already wrapped-around, so it would look like the message had been received before it was actually sent. Isn't it surprising that modulo arithmetic, as already employed successfully in TCP sequence numbers and the like, sti…
To address the second part:
> and yet the plane appears to have no mechanical backup instruments[?]
This is unlikely in a modern aircraft because mechanical instruments to back up e.g., the artificial horizon / attitude indicator or directional gyro (DG) / heading indicator are:
1) Mechanically complex - the attitude indicator and DG make use of gyroscopes which rotate at up to 24,000 RPM along with other mechanisms. They are typically powered by vacuum or electric motors which consume relatively more power (or require vacuum lines and a vacuum pump)
2) Expensive to maintain - see (1) - they need to be serviced somewhat regularly
(3) Heavier than their solid-state counterparts
(4) Have [dramatically] different failure modes - instead of a display going dark, a DG will slowly drift as the gyroscope precesses, giving erroneous values. Same with the artificial horizon. This can lead to catastrophic results under instrument meteorologicalconditions (IMC) where the pilots rely solely on instruments to maintain essential things such as heading and level flight.
(5) Because of (4) they require additional redundancy to ensure instruments can be cross-checked with one another. This compounds (2) and (3)