The comments by Forkner, who has since left Boeing, were among those pinpointed by U.S. lawmakers in hearings in Washington as evidence that Boeing knew about problems with flight control software before two crashes of its 737 MAX aircraft in October 2018 and March 2019 killed 346 people.
Experts said the preliminary document left unanswered questions, which are expected to be addressed in the interim report, on why the plane did not respond to pilot and automatic commands including two final nose up instructions from the pilots about 30 seconds before the crash. On Tuesday the company had said debris was found in "several" planes but it did not give a precise number. "This is unacceptable and won't be tolerated on any Boeing aircraft when it's delivered to the customer", Boeing said in an emailed statement. Boeing is testing updated flight control software that will replace a system that has been implicated as a cause of the crashes.
Boeing is hoping to have the MAX approved for flight by mid-summer.
This week, an internal memo seen by Reuters showed that Boeing found debris that could pose potential safety risks in the fuel tanks of several 737 MAX aircraft in storage and waiting to be delivered to airlines.
Max airplanes were grounded worldwide last March.
MCAS was created to activate only when the plane was making sharp turns at high speeds.
In the messages from November 2016, Forkner seems to note that MCAS was triggering at low speeds.
Federal prosecutors issued a subpoena to Forkner past year for documents related to the plane's development, and Forkner reportedly invoked the Fifth Amendment right against self-incrimination.
Forkner never told the FAA group in charge of training that a change had been made to the software, The New York Times reported a year ago. Two months after his experience in the flight simulator, Forkner emailed FAA officials to ask that they remove mention of the software from official training materials because he said it was so unlikely that it would ever trigger in normal circumstances.
Earlier in 2016, Forkner asked the FAA for permission to remove mentions of the MCAS from the pilot manual for the 737 Max, arguing that it would activate only in rare cases and was created to run in the background.