(Adds details from hearing, quote from NTSB chief, family statements, background)
WASHINGTON, Sept 12 (Reuters) - The chairman of the U.S. National Transportation Safety Board (NTSB) said on Tuesday "operational limitations" in the Tesla Model S played a "major role" in a May 2016 crash that killed a driver using the vehicle's semi-autonomous "Autopilot" system.
Reuters reported on Monday that the NTSB is expected to find that the system was a contributing factor because it allows drivers to avoid steering or watching the road for lengthy periods of time.
The NTSB is also expected to find that Tesla Inc could have taken additional steps to prevent the system's misuse and will fault the driver for not paying attention.
"Today's automation systems augment, rather than replace human drivers. Drivers must always be prepared to take the wheel or apply the brakes," NTSB Chairman Robert Sumalt said.
The NTSB said the Autopilot system operated as designed but did not do enough to ensure drivers paid adequate attention. The safety board noted that on some roads, drivers could use Autopilot at up to 90 miles per hour.
The system could not reliably detect cross traffic and did little to constrain the use of autopilot to roadways for which it was designed, the board said.
Monitoring driver attention by measuring the drivers touching of the steering wheel was a poor surrogate for monitored driving engagement.
Tesla said in June 2016 that Autopilot "is not perfect and still requires the driver to remain alert."
Joshua Brown, a 40-year-old Ohio man, was killed near Williston, Florida, when his Model S collided with a truck while it was engaged in the "Autopilot" mode.
At a public hearing on the crash on Tuesday, NTSB said the truck driver and the Tesla driver "had at least 10 seconds to observe and respond to each other."
On Monday, Brown's family said the car was not to blame for the crash.
"We heard numerous times that the car killed our son. That is simply not the case," the family's statement said. "There was a small window of time when neither Joshua nor the Tesla features noticed the truck making the left-hand turn in front of the car."
"People die every day in car accidents," the statement said. "Change always comes with risks, and zero tolerance for deaths would totally stop innovation and improvements."
A spokeswoman for Tesla and a lawyer for the family, Jack Landskroner, have declined to say if the automaker had reached a legal settlement with the Brown family. (Editing by Bernadette Baum)