There was a French plane that dissapeared near Brazil a few years ago, took 2 years to find it.
Outcome could be very different this time.
- First of all, they found signs of the Air France crash site within hours and were recovering bodies within 5 days, which allowed them to deduce the approximate locale of the crash site.
- Secondly, the were able to narrow down the location of the ultrasonic transponder in the data flight recorders from sonar data collected near the crash site by the nuclear sub Émeraude. It took almost a year of analyzing the sonar data to determine the location. By the time this was done, the transponders were long dead.
- Using the 1-year old data, they started a search on the ocean floor in the vicinity with robot submersibles using sidescan sonar and within a week located the wreckage 12,000 feet down on the ocean floor.
The difference here is first there is no wreckage as of yet to narrow down the site, and second, once the transponders die (they say in about 20 days), it will be extremely difficult to know where to even begin looking.
http://en.wikipedia.org/wiki/Air_France_Flight_447