You'd think confirming that the piece of airplane that washed ashore on Réunion came from Flight MH370 would be great news. If nothing else, it proves investigators have been spending their time and money looking in more or less the right place. And it provides a pretty good idea of where they ought to be hunting for more clues, so they can puzzle out where and why the Boeing 777 went down, right?

Not exactly.

On Wednesday, the Australian Transportation Safety Bureau (ATSB), which is leading the hunt for MH370, published an updated map (above) showing its latest, best guess of where it expects to find debris from the Malaysia Airlines jet, which disappeared March 8, 2014. After reviewing calculations based on the new evidence, the bureau says it is "satisfied" that finding wreckage on Réunion "is consistent with the current underwater search area in the southern Indian Ocean."

That last part is crucial, because floating debris is unlikely to reveal much about what happened to the plane. If you find pieces that have been burned, or have chemical traces indicating an explosion, you've got something. But to really know what happened, investigators must find the plane's black boxes—the cockpit voice recorder and flight data recorder—which would have sunk soon after hitting the water.

Backtracking

Based on an approximation of how things move around the Indian Ocean, the flaperon's arrival on Réunion indicates authorities were right to base the search along what's called the "seventh arc"—the black line on the map that defines where the plane could have gone down, based on its last known position.

The map indicates where debris dropped in the water along that arc in March 2014 might be now. The different color dots represent different leeway factors—essentially, how much the effect of wind on movement, compared to water currents. A sailboat has a big leeway factor, something barely sticking out of the water has a small one. The ATSB modeled pieces with a variety of leeway factors, because different pieces of wreckage would react to the wind in different ways.

One of those dots is on Réunion! So the model's correct, and the team's been searching for the black boxes in the right location, right?

There's so much error in estimating where it came from because of the currents and the time period, that really it's just gonna tell you it hit the ocean somewhere. Colleen Keller, senior analyst at Metron

Kind of. The problem, says Colleen Keller, is that arc represents so big a starting point, it's effectively useless. "They're saying, it's somewhere in this area, but we have no bias in any particular part of the arc." Keller is a senior analyst at consulting company Metron and helped in the successful two-year search for the wreckage of Air France 447, which crashed off the coast of Brazil in 2009. Metron unsuccessfully competed for a contract to help in the search for MH370.

Then you have to throw in the fact that our models for how the oceans move are far from perfect. Keller points out that during the search for Air France 447, three agencies simulated where the plane may have gone down, based on wreckage found after five days. They produced three different sets of results. Here, you're dealing with 500 days of drift.

All of which should temper hopes that finding one piece of wreckage—or even many more—will eventually lead to the crash site, and the all-important black boxes.

"We want to say this is great, this is a clue," Keller says. "Unfortunately, there's so much error in estimating where it came from because of the currents and the time period, that really it's literally just gonna tell you it hit the ocean somewhere. Probably somewhere to the east."

Well, at least we know the plane's not hiding in Kazakhstan.