On the morning of Jan. 26, as two Alaska Airlines flights from Seattle to Hawaii lifted off six minutes apart, the pilots each felt a slight bump and the flight attendants at the back of the cabin heard a scraping noise.

As the noses of both Boeing 737s lifted skyward on takeoff, their tails had scraped the runway.

Caused by an update to the software that calculates thrust and speed settings for takeoff:

That morning, a software bug in an update to the DynamicSource tool caused it to provide seriously undervalued weights for the airplanes.

A concurrency bug?

Peyton added that even though the update to the DynamicSource software had been tested over an extended period, the bug was missed because it only presented when many aircraft at the same time were using the system.

Good procedures overall for noticing, stopping all airplanes, and fixing the system.

The dilemma of safety-critical systems: update the system and suffer new bugs or never update and end up with an and old barely usable system prone to mistakes.