There are still more questions than answers about an Ethiopian Airlines plane that crashed shortly after taking off for Nairobi on Sunday, killing all 157 people on board. Ethiopian authorities said Wednesday that they will send the flight recorders recovered from the plane to an as-yet-unspecified European country for analysis.

Meanwhile, dozens of countries have grounded the Boeing 737 Max 8, the plane involved in Sunday's crash and another five months ago that killed 189 people off of Indonesia. Canada on Wednesday became the latest country to say the plane can't fly in its airspace.

The U.S. Federal Aviation Administration continues to back the plane's airworthiness and U.S.-based Boeing maintains it has no reason to pull the hot-selling jet from the skies.

Here are some questions and answers about the crash, the plane and the investigation:

Q. WHAT ARE FLIGHT RECORDERS AND WHY ARE THEY SO IMPORTANT?

A. The FAA requires every large commercial aircraft to have a cockpit voice recorder and a flight data recorder, known as "black boxes." The recorders, which can withstand temperatures of 1,100 degrees Celsius and water depths of 20,000 feet, collect information about a flight.

Voice recordings pick up the flight crew's voices, as well as other sounds inside the cockpit. Information from the data recorder can generate a computer animated video reconstruction of the flight. Investigators can then visualize the airplane's altitude, instrument readings, power settings and other details of the flight to help with the investigation, according to the National Transportation Safety Board. Some experts have said the search for answers about what caused the crash could take months.

Q. WHO HAS GROUNDED THE PLANES?

A. More than 40 countries including the entire European Union and Canada have suspended flights by the plane or barred it from their airspace. China ordered its airlines to ground the planes — they had 96 Max 8 jets in service, more than one-fourth of the approximately 370 Max jets in circulation.

Q. WHY IS THE FAA HOLDING OUT?

A. The FAA prides itself on acting based on facts and is cautioning against comparing the Ethiopian Airlines crash with the one off of Indonesia or assuming that they are related.

"External reports are drawing similarities" between the crashes, the agency said in a statement. "However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

The agency has been criticized for its inaction. Rep. Peter DeFazio, D-Ore., the chairman of the House Transportation and Infrastructure Committee, said that he is concerned that international aviation regulators are providing more certainty to the flying public than the FAA. Critics have also said the FAA is too cozy with the industry that it is supposed to regulate.

Q. WHAT IS BOEING DOING IN RESPONSE TO THE CRASHES?

A. While defending the Max as safe, the company promises to upgrade some flight-control software "in the coming weeks."

Boeing began working on the changes shortly after the Lion Air crash. It is tweaking a system designed to prevent an aerodynamic stall if sensors detect that the plane's nose is pointed too high and its speed is too slow.

Officials at Lion Air in Indonesia said sensors on their plane produced erroneous information on its last four flights, triggering an automatic nose-down command which the pilots were unable to overcome. The plane plunged into the sea.

A Boeing spokesman said once updated software is installed, the system will rely on data from more than one sensor to trigger a nose-down command. Also, the system won't repeatedly push the nose down, and it will reduce the magnitude of the change, he said. There will also be more training for pilots.

Q. HAVE THERE BEEN COMPLAINTS ABOUT THE PLANES?

A. Airline pilots on at least two U.S. flights reported that an automated system seemed to cause their Boeing 737 Max planes to tilt down suddenly. The pilots said that soon after engaging the autopilot on Boeing 737 Max 8 planes, the nose tilted down sharply. In both cases, they recovered quickly after disconnecting the autopilot.

As described by the pilots, the problem did not appear related to a new automated anti-stall system that is suspected of contributing to the Lion Air crash in Indonesia.

The pilot reports were filed last year in a data base compiled by NASA. They are voluntary safety reports and do not publicly reveal the names of pilots, the airlines or the location of the incidents. It was unclear whether the accounts led to any actions by the FAA or the pilots' airlines.

Q. ARE AIRLINE EMPLOYEES AND PASSENGERS WORRIED?

A. Patrick Smith, a Boeing 767 pilot who writes a column called "Ask the Pilot," says passengers ask him if the 737 Max is safe. He tells them it is, and he hasn't heard of any pilots who worry about flying the plane.

"We have two accidents, we somewhat understand one, and we don't know what happened in the second case at all," Smith says. "It's just too early to be jumping to the conclusion of the plane being defective to the point that it's unsafe."

Others don't want to take any chances. The Association of Professional Flight Attendants, which represents more than 26,000 flight attendants at American Airlines, called on CEO Doug Parker to "strongly consider grounding these planes until an investigation can be performed."

Q. WHAT IF I WANT TO SWITCH MY FLIGHT IF I'M ON A MAX?

A. You can do it, but the ease — and possible cost — will differ by airline. In the U.S., Southwest and American fly Max 8s, and United flies larger Max 9s.

Southwest doesn't charge ticket-change fees, making it easy for customers to book a different flight. United and American typically add a $200 change fee.

To find out whether you're booked on a 737 Max, you can check out apps like FlightAware and Flightradar24 that include the aircraft type. When buying a ticket on American's website, it's under "details." On Southwest, you have to click on the flight number.

"The average person wouldn't be able to find the model on Southwest," says George Hobica, founder of airfarewatchdog.com.