The aircraft are so similar, the sales line went, the airlines could avoid extensive and expensive training for pilots flying earlier versions of 737.
At the Paris Air Show in 2017, Boeings showed 737 MAX pilot pilot time, Ed Wilson, highlighted the similarities between 737 MAX 9 and the previous series known as Next Generation or NG. At that time, the video of the demonstration of the Airport FlightGlobal was shown.
"The aircraft is configured to be very common with NG," says Wilson, now retired, "and so a pilot can enter here and find everything he can, just as he can in NG."
" FAA approved this for two and a half hours of computer-based training for the transition between the two aircraft, "Wilson continued. "All overhead panel switches are the same. The only minor difference is that due to the change in the display, some of the center console's objects are moved here on the forward console."
What Wilson did not mention in the video is a new computerized control system, Maneuvering Characteristics Augmentation System or MCAS, which seems to be at the center of investigations on two new crashes with 737 MAX 8.
After a 737 MAX flying by Lion Air crashed at the end of October, the airline union's union said that MCAS was not revealed in the aircraft's manual. CNN has not independently looked at the flight manual.
The FAA said Friday that before the flight Lion Air had the knowledge to handle the plane pitching down, even though they did not know the MCAS system was the cause.
"While the Boeing 737 MAX training requirements do not address the MCAS by name, the requirements contain the knowledge to handle an MCAS event," the agency said.
After the Lion Air crash, the handbook was "revised to reinforce flight crew how to recognize and respond to indefinite stabilizer trim motion and MCAS events," the FAA said.
After the Lion Air crash, Boeing sent a statement saying that manual "already describes an existing procedure to safely handle the unlikely event of erroneous data coming from an AOA sensor," entering the MCAS system, but indicated not specifically if the MCAS system was part of the manual.
The MCAS system worked when auto-pilot functions were turned off and pilots thought they had control over the plane, according to pilots talking to CNN.
A preliminary survey report in the Lion Air crash showed that the pilots struggled to stabilize the plane, which repeatedly held down as the MCAS system received incorrect data. After a 737 MAX crash in Ethiopia earlier this month, the Federal Aviation Administration data said "a bit of similarity between the two accidents" and founded the plane for further information and a fix.
In his MAX marketing material, Boeing highlighted the similarities between the two series and called the transition a "simple bill".
"The common conditions of the aircraft with earlier 737 models will allow easy integration into existing 737 fleets," the company wrote in its quarterly magazine, 201
Boeing's website said, "Millions of dollars will be saved because of its commonality with Next Generation 737," including maintenance and "broad availability of 737 pilots."
"737 Flight Crews will feel at home in MAX" reads the site.
The Allied Pilots Association said after the FAA's decision to base the MAX Navy that it conforms to the decision, but "remains safe in the Boeing 737 Max and in our members' ability to safely fly it."
The union had previously said it first learned the MCAS system when Boeing issued a safety directive after the Lion Air crash.
Pilots at American Airlines, represented by APA and Southwest Airlines, represented by another union, moved from older versions of 737 to MAX by taking an online course lasting between 56 minutes and three hours, according to union exponents.
That training contained some differences between series two, but did not explain MCAS, they said.
"MCAS was installed in the aircraft and Boeing did not reveal it to the pilots," Mike Trevino, a spokesman for the Southwest Airlines Pilots Association, told CNN.
CNN's Audrey Ash contributed to this report.