WASHINGTON — Ruling that airlines have broad immunity from lawsuits under a post-9/11 security law, the Supreme Court on Monday threw out a $1.4 million defamation judgment awarded to a pilot who was reported by his employer as mentally unstable and potentially armed.

The court was unanimous in holding that a law aimed at encouraging reports of possible security threats to the Transportation Security Administration shields airlines from defamation claims when the reports are substantially true.

‘‘Congress wanted to ensure that air carriers and their employees would not hesitate to provide the TSA with information it needed,’’ Justice Sonia Sotomayor wrote for the court. Airlines may be held liable only for recklessly false reports, she said. Applying that reasoning to the case of veteran pilot William Hoeper, the justices voted, 6 to 3, to overturn a Colorado jury’s verdict against Air Wisconsin, Hoeper’s former employer.


The case stems from Hoeper’s final, failed attempt to win airline approval to fly a new aircraft. The session ended with an angry exchange between Hoeper and another employee at a Virginia training facility.

Later that day, Hoeper was a passenger on a United Airlines flight home to Denver that was ordered to return to its gate after Air Wisconsin called TSA with its report of Hoeper as a potential threat. He was removed from the plane by armed police officers and asked about his gun — properly locked up at home — while his luggage was emptied on the jet bridge.

TSA eventually determined Hoeper was not a threat, but he said he was so embarrassed by the incident that he took a later flight rather than reboard the delayed plane that was still sitting at the gate.

In other matters Monday:

■  The Supreme Court ruled that a dealer convicted of selling drugs to someone who dies from an overdose may not be sentenced to a mandatory 20 years in prison without proof that the death resulted from the use of the drugs.


■  The high court said steelworkers do not have to be paid for time they spend putting on and taking off protective gear they wear on the job.

■  The court declined to reverse the conviction of former HealthSouth chief executive Richard Scrushy on bribery and fraud charges.