Internet theories on what could have happened to Malaysia Airlines Flight 370 are ubiquitous. The Post's Joel Achenbach explains what some of them mean. (Gillian Brockell and Davin Coburn/The Washington Post)

A simple computer upgrade that Malaysia Airlines decided not to purchase would have provided critical information to help find the airliner that disappeared 12 days ago.

The upgrade, which wholesales for about $10 per flight, would have provided investigators with the direction, speed and altitude of Malaysia Airlines Flight 370 even after other communications from the plane went dark, said a satellite industry official familiar with the equipment.

Data from a similar computer upgrade allowed investigators in the crash of an Air France jetliner in 2009 to quickly narrow their search area to a radius of about 40 miles in the Atlantic Ocean, and in five days, they found floating evidence of the crash.

The ocean search for the missing Malaysian flight now covers a vast expanse of water, about 2.24 million square nautical miles of the Indian Ocean from the west coast of Malaysia to the waters off Perth, Australia.

“We’ve got to hope for a break,” said Dave Gallo, who directed the search by the Woods Hole Oceanographic Institution that found the Air France plane. “Someone’s got to find on the surface some bit of that plane floating.”

The story of the missing Malaysian plane, with 239 passengers and crew members on board, has drawn the public into the arcane nuances of communications between aircraft and ground bases.

The new information indicates that had the upgrade for a system called Swift been installed, it would have continued to send flight data by satellite even after signals from the plane’s transponder and Aircraft Communications Addressing and Reporting System (ACARS) went dead.

Investigators say they think those two systems were shut down by a pilot or hijackers in the cockpit before the plane flew on for seven more hours.

The satellite industry official, who spoke on the condition of anonymity because of the sensitivity of the investigation, likened the Swift system to a cellphone that sends data to a satellite. He described ACARS as akin to an app for a mobile phone.

Had the Swift system been upgraded to include the full package of applications, it could have sent information on engine performance, fuel consumption, speed, altitude and direction, regardless of whether the transponder and ACARS were working, he said.

“When ACARS is turned off, Swift continues on,” he said. “If you configure Swift to track engine data, that data will be streamed off the plane. It continues to be powered up while the aircraft is powered up.”

Many major airlines use the full package of Swift options. The detail it provides is mandated under international aviation guidelines for airlines that ply the busy North Atlantic corridor between the United States and Europe. There are no such requirements elsewhere in the world, the industry official said.

In addition to sending information to the airline, Swift also can be programmed to send data to the manufacturer — usually Boeing or Airbus — and the engine maker — usually Rolls-Royce or Pratt & Whitney.

“It’s a choice of what you do with your aircraft,” the satellite industry official said. “When you get your plane from Boeing, you can get an engine management app, a route management app, or you might decide that you want the bare minimum. There isn’t a mandated requirement.”

The application wholesales for about $10 per flight, but airlines pay a higher retail fee. Some airlines have decided they do not want to pay the higher cost for an information stream that they deem unnecessary except under the most extreme circumstances.

"The need for SWIFT has never been mandated and all our aircraft have what is called the Aero H SATCOM communications systems,” Malaysia Airlines said in a statement. “This installation is sufficient to meet all of MAS’s operational requirements and at the same time meets all international requirements that enable us to fly international airways.”

 “The statement that this $10 per flight upgrade will provide direction, speed and altitude in the event that the communications were deliberately shutoff from the aircraft is untrue,” the airline stated.

Asked why an airline might choose not to buy an application that sells for a relatively modest cost, the official said: “Every pound on an aircraft is fuel consumed. As in all matters, it always comes down to cost.”

Rather than stream those data, he said, some airlines choose to download it onto a USB stick once the plane lands.

Because Malaysia Airlines went with the cheaper option, he said, “there was not an awful lot that was captured.”

With the transponder and ACARS not operating, the satellite tried to contact the missing flight hourly.

He said the satellite “sends out an automated ping to say, ‘Are you there?’ and the machine-to-machine response is, ‘Yes, I am.’ ”

That “yes, I am” response is how investigators determined that the plane flew on for several hours.

“On the Air France flight, they used the satellite network to bring the [additional] information off and narrow the search,” he said.

With all the on-board systems working — the transponder, ACARS and Swift with all available applications — the search for Air France 447 narrowed quickly.

“We had a last-known position, and we knew that after the last-known position there were four bursts of ACARS transmissions, and then they stopped abruptly,” Gallo said. “So the decision was made that the plane was down four minutes after the initial event.”

Once debris was found, investigators had to calculate how far it had drifted in the five days since the plane crash.

“We thought five days was a long time for that tracking,” Gallo said, reflecting on the number of days since the Malaysian flight disappeared.

It took almost two years — from June 2009 to May 2011 — before the wreckage of the Air France flight was found on a plateau more than 11,000 feet deep in the Atlantic off the coast of Brazil. With no hint of where the Malaysian plane might have gone down, Gallo said that perhaps the only hope it will be found rests on the discovery of some floating debris.

“It’s a big chunk of the Indian Ocean, almost the size of the North Atlantic,” Gallo said. “If [the plane] is in the ocean, it can range from the flat sediments of the Bay of Bengal, two miles-plus deep, and the further south you get, west of Perth is one of the most in­cred­ibly complicated underwater terrains on the planet.”

Although there are no industry-wide standards for aviation, in the maritime industry all ships must carry an advanced tracking system known as the Long Range Information and Tracking system, which enables satellites to follow the movements of vessels as they traverse the globe. If a ship goes down, rescue crews will know which area to concentrate their search.

“The U.S. Coast Guard came to us and we introduced the system. There really wasn’t anything before that,” said Andrew Winbow, director of the safety division for the International Maritime Organization in London.

Julie Tate, Alice Crites and Jessica Schulberg contributed to this report.