Timelines help evaluate the strength of assertions by interested parties, such as "Event A (his mistake) caused Event B (my crash)." Did A really happen before B?
Timelines are only part of the picture, of course. Even if investigators confirm that Event A did happen before B, as claimed, and happened to the same machine, where's the proof that the first event led to the other?
After most disasters, investigation turns up multiple causes -- errors and malfunctions -- that linked up over time, building to a system fracture. Knowing all the precursor events is important in preparing cautionary reports later. Here's a lessons-learned summary on the Piper Alpha rig blast, 20 years on.
Let's say evidence shows that the people in charge knew of a certain trouble report, and also knew that it put some of the mission goals in jeopardy. If they ordered that the mission proceed anyway, that tells us something important about the day-to-day priorities. The message: "It's safe enough for what I need. Get the job done, and then take the machine into the shop."
That's the thinking that apparently led to the loss in 1968 of the nuclear submarine USS Scorpion and all her crew. The sub was supposed to get a safety overhaul in 1967, but Navy facilities were backed up so the sub went off to sea without the most important fixes indicated by the 1963 loss of the sub Thresher. The exact cause of Scorpion's loss has never been settled officially, but we do know from books like Silent Steel that it imploded as it went down, launching the aft end of the sub into the center of the pressure hull.
Timelines can take on a non-linear shape when the center of interest is moving. Here's one for the ValuJet 592 crash, from the NTSB report. It's a two-dimensional timeline of the DC-9's final moments, superimposed on an overhead view of the area.
Here's the same timeline with the third dimension added, reflecting changes in altitude.
A separate timeline would be needed to show the earlier string of errors and oversights, particularly those at ValuJet's contractor, SabreTech. This entirely avoidable crash (due to the ignition of boxes of old oxygen canisters carried in the cargo compartment) claimed all 110 souls.
From the NTSB's report on the inflight breakup of TWA Flight 800, here's a different kind of timeline that conveys how the B-747 airframe broke up into three sections, each taking its own trajectory.
To be useful in unraveling the root causes, all events on a timeline must share a common time base. It may seem easy -- "Just look at the clock!" -- but sequencing evidence from multiple sources can take a great deal of work.
It used to be even harder. After Titanic sunk, landlubbing readers of the British and American findings may have been surprised to learn the logs provided by the vessels did not share a common time base.
In other words, an event that all the ships experienced simultaneously -- a specific CQD from Titanic's wireless operators -- wouldn't have been entered in the ships' log books as having happened at the same hour and minute, even if all ships were in the same time zone. The ships (including Titanic) reset their clocks to local noon each day, and local noon changes as ships go 'round the globe.
See this paper on Encyclopedia Titanica, "Titanic's Time Enigmas," for a detailed analysis of all the time reports (including the thorough logs kept by Cape Race) and a conclusion on what happened when.
Long before the Internet and cellphones linked us to atomic clocks, the best I could do when setting our house clocks was to tune to a WWV on our shortwaveradio. WWV counted down to the correct time each minute.
Synchronization problems can still arise. Some eyewitnesses might not have noted the time at all. Land line phone calls aren't time stamped, unless the caller happens to phoning 911 or somebody on a cellphone.
If a witness with a 1990's-era video camera records a view of something falling off a plane at an air show, the time stamped on the tape won't be as accurate as if the witness used the camera on her smartphone instead. (That's assuming the user didn't try to set the time herself. By default, cellphones draw on network time to stamp phone calls and text messages; in that respect, we're all carrying around atomic clocks.)
So this reminder to drivers who are tempted to correspond while at the wheel, defying common sense and repeated warnings from the National Transportation Safety Board: after an accident, the police can use network records to prove you were texting at the time of the crash. So don't do it!