SEATTLE, WA - MARCH 22: A Boeing 737 MAX 9 test plane is pictured at Boeing Field on March 22, 2019 in Seattle, Washington. 737 MAX airplanes have been ground by multiple aviation authorities after two two 737 MAX 8 airplanes crashed within five months of each other. (Photo by Stephen Brashear/Getty Images)
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SEATTLE, WA - MARCH 22: A Boeing 737 MAX 9 test plane is pictured at Boeing Field on March 22, 2019 in Seattle, Washington. 737 MAX airplanes have been ground by multiple aviation authorities after two two 737 MAX 8 airplanes crashed within five months of each other. (Photo by Stephen Brashear/Getty Images)
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VICTORVILLE, CA - MARCH 27:  A number of Southwest Airlines Boeing 737 MAX aircraft are parked at Southern California Logistics Airport on March 27, 2019 in Victorville, California. Southwest Airlines is waiting out a global grounding of MAX 8 and MAX 9 aircraft at the airport.  (Photo by Mario Tama/Getty Images)
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RENTON, WA - MARCH 11: The Boeing 737-8 is pictured on a mural on the side of the Boeing Renton Factory on March 11, 2019 in Renton, Washington. Two of the aerospace company's newest model airliners have crashed in less than six months. (Photo by Stephen Brashear/Getty Images)
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RENTON, WA - MARCH 11: The Boeing 737-8 is pictured on a mural on the side of the Boeing Renton Factory on March 11, 2019 in Renton, Washington. Two of the aerospace company's newest model airliners have crashed in less than six months. (Photo by Stephen Brashear/Getty Images)
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People stand near collected debris at the crash site of Ethiopia Airlines near Bishoftu, a town some 60 kilometres southeast of Addis Ababa, Ethiopia, on March 11, 2019. - An Ethiopian Airlines Boeing 737 crashed on March 10 morning en route from Addis Ababa to Nairobi with 149 passengers and eight crew believed to be on board, Ethiopian Airlines said. (Photo by Michael TEWELDE / AFP)        (Photo credit should read MICHAEL TEWELDE/AFP/Getty Images)
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People stand near collected debris at the crash site of Ethiopia Airlines near Bishoftu, a town some 60 kilometres southeast of Addis Ababa, Ethiopia, on March 11, 2019. - An Ethiopian Airlines Boeing 737 crashed on March 10 morning en route from Addis Ababa to Nairobi with 149 passengers and eight crew believed to be on board, Ethiopian Airlines said. (Photo by Michael TEWELDE / AFP) (Photo credit should read MICHAEL TEWELDE/AFP/Getty Images)
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People walk past a part of the wreckage at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, southeast of Addis Ababa, Ethiopia March 10, 2019. REUTERS/Tiksa Negeri
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RENTON, WA - MARCH 14: Boeing 737 MAX airplanes, including the 737 MAX 9 test plane (L), are seen at Renton Municipal Airport, on March 14, 2019 in Renton, Washington. The 737 MAX, Boeing's newest model, has been been grounded by aviation authorities throughout the world after the crash of an Ethiopian Airlines 737 MAX 8 on March 10.  (Photo by Stephen Brashear/Getty Images)
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RENTON, WA - MARCH 14: Boeing 737 MAX airplanes, including the 737 MAX 9 test plane (L), are seen at Renton Municipal Airport, on March 14, 2019 in Renton, Washington. The 737 MAX, Boeing's newest model, has been been grounded by aviation authorities throughout the world after the crash of an Ethiopian Airlines 737 MAX 8 on March 10. (Photo by Stephen Brashear/Getty Images)
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A grounded Lion Air Boeing Co. 737 Max 8 aircraft sits on the tarmac at terminal 1 of Soekarno-Hatta International Airport in Cenkareng, Indonesia, on Tuesday, March 15, 2019. Sundays loss of an Ethiopian Airlines Boeing 737, in which 157 people died, bore similarities to the Oct. 29 crash of another Boeing 737 Max plane, operated by Indonesias Lion Air, stoking concern that a feature meant to make the upgraded Max safer than earlier planes has actually made it harder to fly. Photographer: Dimas Ardian/Bloomberg via Getty Images
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A grounded Lion Air Boeing Co. 737 Max 8 aircraft sits on the tarmac at terminal 1 of Soekarno-Hatta International Airport in Cenkareng, Indonesia, on Tuesday, March 15, 2019. Sundays loss of an Ethiopian Airlines Boeing 737, in which 157 people died, bore similarities to the Oct. 29 crash of another Boeing 737 Max plane, operated by Indonesias Lion Air, stoking concern that a feature meant to make the upgraded Max safer than earlier planes has actually made it harder to fly. Photographer: Dimas Ardian/Bloomberg via Getty Images
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Rescuers work at the scene of an Ethiopian Airlines flight crash near Bishoftu, or Debre Zeit, south of Addis Ababa,  Ethiopia, Monday, March 11, 2019. A spokesman says Ethiopian Airlines has grounded all its Boeing 737 Max 8 aircraft as a safety precaution, following the crash of one of its planes in which 157 people were killed. (AP Photo/Mulugeta Ayene)
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Rescuers work at the scene of an Ethiopian Airlines flight crash near Bishoftu, or Debre Zeit, south of Addis Ababa, Ethiopia, Monday, March 11, 2019. A spokesman says Ethiopian Airlines has grounded all its Boeing 737 Max 8 aircraft as a safety precaution, following the crash of one of its planes in which 157 people were killed. (AP Photo/Mulugeta Ayene)
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A majority of customers who ordered 737 Max planes have now had a chance to test a fix for software that has been linked to two deadly crashes, Boeing CEO Dennis Muilenburg said.

About two-thirds of the more than 50 customers from various airlines have been able to test the software patch using a flight simulator, Muilenburg said Thursday. The software has been tested by pilots and airline leaders on 96 flights for a total of 159 flying hours.

Muilenburg did not say when Boeing (BA) 737 Max planes may begin flying again. They have been grounded worldwide since mid-March, after two crashes in about five months killed 346 people.

The Boeing chief, speaking at a George W. Bush Presidential Center leadership forum in Dallas, said the last few weeks have been the most “heart wrenching” of his career.