It turns out pilots did file a stream of written complaints with the U.S. federal government about the dangers they faced flying the new Boeing 737 MAX 8.

Two of these planes crashed shortly after take-off within five months of each other, a puzzling event some experts now believe is due to a fatal design flaw in the aircraft.

Months before the second crash of a MAX 8 in Ethiopia on March 11, concerned pilots repeatedly wrote to the U.S. Federal Aviation Administration (FAA) through NASA about safety concerns relating to the Max 8, said a report by The Dallas Morning News.

The report said one captain described the MAX 8 flight manual as "inadequate and almost criminally insufficient" several months before the Ethiopian Air crash that killed 157 people.
“The fact that this airplane requires such jury rigging to fly is a red flag,” wrote this captain. “Now we know the systems employed are error-prone — even if the pilots aren't sure what those systems are, what redundancies are in place and failure modes. I am left to wonder: what else don't I know?"

His complaint confirms a story published by Aviation Week in November 2018 saying the 737 MAX flight crew operations manual does not mention MCAS. Boeing said it "decided against disclosing more details to cockpit crews due to concerns about inundating average pilots with too much information.”

https://www.ibtimes.com/boeing-737-max-pilots-complained-about-safety-flaw-months-ethiopian-airlines-crash-2774769