(CNN) -- When authorities confirmed last month that four "pings" heard in the southern Indian Ocean had nothing to do with Malaysia Airlines Flight 370, it was a devastating blow for those involved in the investigation, the families of those on board the missing jet, and the countless number of people around the world who had become captivated by the mystery surrounding the plane's disappearance.
What was described as "the most promising lead" in the search had proved fruitless.
The investigation into the ill-fated flight is already the most expensive in aviation history. Malaysia has spent $8.6 million so far, Australia is expecting to spend around $84 million, and other countries involved in the search have reportedly set aside sizable sums. Meanwhile, families of the missing passengers are working to raise $5 million to encourage anyone with information about the plane's whereabouts to come forward.
Sunday marked 100 days since the Boeing 777 disappeared. To the frustration and disappointment of many, no tangible evidence has been found. How long will authorities keep working to solve this expensive mystery and what are their reasons for doing so? CNN speaks to aviation experts for their views.
Aviation safety and security
Mary Schiavo, a former inspector general for the U.S. Department of Transportation, says there's a tremendous need to find the plane, particularly if mechanical failure, and not foul play, is to blame.
"The longer time that goes on, the more it appears it was not terrorism, hijacking, sabotage, (or) suicide, and it does appear that something else happened -- something mechanical, some kind of a catastrophic failure, an explosion, something that debilitated the persons on board; and they really need to solve that mystery because until we solve it we can't improve air safety," Schiavo says.
Some improvements have already begun. Malaysia Airlines has changed its cockpit regulations. International aviation bodies such as the International Air Transport Association (IATA) have called on the aviation industry to change aircraft tracking systems.
But convincing the industry to implement safety measures before an accident happens remains a challenge, Schiavo adds.
"In the United States and in many other countries, we legislate by counting bodies. We don't make something the law until someone has died, and that's just awful and unacceptable but that's the way our regulations work," she says.
Maintaining the political will to implement the changes is part of the problem, said David Soucie, a former safety inspector at the U.S. Federal Aviation Administration and author of "Why Planes Crash." Findings can come years after an accident by which time the sense of urgency has subsided.
"Knowing what happened and knowing what could be done to improve a crash investigation, while that's extremely important, often civil aviation authorities make a particular recommendation and it falls on deaf ears because our memories are so short," says Soucie.
In the case of Air France Flight 447, which crashed en route to Paris in June 2009 with 228 people on board, the plane's black boxes weren't recovered until May 2011. French authorities made a series of recommendations, including that the flight recorders' pinger duration should be increased from 30 days to 90 days, but by that time the motivation to do something had dwindled, Soucie says. Since then, some carriers have implemented those adjustments voluntarily but they haven't become mandatory across the industry. Still, the accident did prompt changes in pilot training and Airbus tweaked a key cockpit sensor.
"I'm worried about that with this accident as well -- that the longer we take to find it, the less passionate people are about doing something about what it is that we find," Soucie says.
David Gallo, an oceanographer and Director of Special Projects at the Woods Hole Oceanographic Institution, was involved in the search for Air France Flight 447. He says it's critical to find the wreck soon to prevent a similar accident occurring again, and that the amount of money being set aside for the search is trivial in this context.
"If it happened at one time it can happen again," says Gallo. "We can (find out what happened) and I'm confident that we will," he adds.
But it requires certain things: "It takes the right technology because it's at the bottom of the ocean which is deep and vast. It takes the right team because at the depths where the plane may be -- at some of the greatest depths of the sea -- the club is very small of people that know how to go to those depths and work. Most importantly, it takes the right operational plan. And then on top of that, it takes a bit of luck and a lot of prayer," he says.
Under an international pact between aviation nations, there's a pecking order of who takes charge of a crash investigation. At the top in this case is Malaysia, but Malaysia is allowed, and in fact encouraged, to bring in other countries or hand off the task to other countries that can do the job. In early April, Australia accepted an invitation from Malaysia to lead the search for the missing aircraft and participate in the investigation as an accredited representative.
"I think it was very sensible that (Malaysia) did that. I really think that Australia is doing all that they can... and it's a very big and expensive task," says Schiavo.
But there's a precedent for that kind of expenditure, she adds. The two-year search for Air France Flight 447 cost around $40 million, according to French authorities, while the investigation into the Trans World Airline 800 crash into the Atlantic in 1996 cost in excess of $50 million.
Although Malaysia has delegated the responsibility for the search to Australia, it is still responsible for the accident investigation. That's something that concerns Soucie.
"Once the aircraft is found -- if it is -- at that point the Australians will be pretty much done with their part and the Malaysians will conduct the investigation, which concerns me a great deal because there are going to be a lot of unknowns, even when we have the aircraft in hand. What we're really trying to do is get to the bottom of what happened, and that doesn't come from the search; that comes from the investigation following the search," Soucie says.
Closure for families
More than three months since they last saw their loved ones, the families of those on board MH370 still live in a state of limbo.
"For us this is frustrating, and befuddling, and at the end of the day I go to sleep disappointed, but for the families it's a nightmare that never ends," says David Gallo. "It's not something that they can just turn off for a while. I speak to several of them on a fairly routine basis and it's just absolutely horrible what they're going through."
Beyond the emotional closure that would come from finding out what happened to the flight, there's the question of compensation.
"When this plane fell out of the sky, it fell into the murky world of airline liability insurance litigation," says Schiavo. Officially, no one involved in the crash has been declared dead. Malaysia Airlines has begun to give families an initial compensation payment of $50,000 with the final amount to be determined "when the issue of the tragedy" is over, Malaysian authorities said last week.
But Schiavo says this is a fraction of the $176,000 carriers are liable to pay under an international aviation treaty, even before any evidence is found. Under the Montreal Convention, carriers are automatically liable to their passengers unless the airline can prove somebody else is responsible -- which Schiavo believes won't be possible in this case, so they're going to have to pay.
"The fact that they're telling people they're going to give them $50,000 is really an outrage because the treaty itself sets forth the initial automatic amount they are responsible for, without any evidence whatsoever," Schiavo says. The aviation lawyer calls the airline's payment outrageous but "they're not alone....carriers always wait to be sued," she says.
Lack of trust?
As new leads have emerged and been discredited, the public's faith in authorities has waned, says Soucie.
"I think that trust in the investigation has faltered, but I hope that people would understand that when there's little or no information at all, there's not a whole lot (authorities) can do," he said.
All clues, no matter how trivial must be examined. For instance, one of the latest leads is an underwater sound that could possibly be related to the final moments of the missing plane. A scientist assisting the investigation into the noise admits it's a long shot: "We consider it's extremely unlikely," says Curtin University's Alec Duncan, but "the chance of (the sound) being related to MH370 is not zero."
Sarah Bajc, whose husband Paul was on the flight, says that she and other families are "beyond frustrated," at this point.
"We're just totally fed up," Bajc said in a recent interview with CNN's Ashleigh Banfield. "The authorities have completely failed the families in this case. They've failed the flying public. The authorities have allowed a jumbo jet to just disappear and to stay missing, and that's just not acceptable."
Angus Houston, who heads the Australian search efforts understands their frustration. "I think the families are in a very difficult position at the moment. They've got no closure. They want to find the aircraft and they will do whatever it takes to find the aircraft... If I were in their shoes, I'd probably feel exactly the same way," Houston told CNN's Andrew Stevens.
When investigation ends, quest for answers will continue
The search is taking place in very deep and vast waters, but David Gallo believes it may not be so long before the plane is found, and experts mostly agree that attention is focused on the right area.
But if the wreck isn't located, funds will be exhausted at some point and authorities will have to end their investigation. They will be required by the International Civil Aviation Organization to make their work public, even if there is no cause or concrete findings, says Schiavo.
"At that point I suspect that various groups and organizations and the families themselves will then take that information and try to keep searching. It will remain to be seen whether (the authorities) will be truly open and make all of (their data) public," she says.
As officials prepare for the next stage of the search, David Gallo stresses the need to find the aircraft as soon as possible to obtain any evidence before it's worn away by the action of the sea.
"We have to understand what happened for the sake of the families of those passengers, for the sake of the flying public, for the tens of thousands of people that fly every day, and for the sake of the aircraft industry," he says.
"We need to find that plane, because the plane itself may be a mobile crime scene. The only witnesses to what happened that night are those black boxes that are inside the plane."
CNN's David Molko contributed to this report.