In December, about a minute after a United Airlines flight departed from Kahului Airport in Maui, the plane, a Boeing 777-200, entered a steep dive and came within 775 feet of the surface of the Pacific Ocean, according to flight data.
The plane had initially ascended to an altitude of 2,200 feet before suddenly falling at a rate of nearly 8,600 feet per minute, according to The Air Current, which reported on the episode on Sunday after analyzing data from the Dec. 18 flight.
The flight, which did not appear to have been widely discussed on social media, departed Maui the same day that 36 people were hurt, 11 of them seriously, when a Hawaiian Airlines flight to Honolulu from Phoenix was rocked by severe turbulence.
Joshua Freed, a United Airlines spokesman, declined to say how many passengers were aboard United Flight 1722 when it took off from Maui at 2:49 p.m. local time on Dec. 18. The Boeing 777-200 can carry 312 passengers.
Mr. Freed said on Monday that the plane did not have any mechanical issues but declined to comment on what may have caused the plane to plummet.
The pilots filed the “appropriate safety report” after the plane arrived safely at its destination, San Francisco, Mr. Freed said.
“Safety remains our highest priority,” Mr. Freed said in a statement, adding that United Airlines had “closely coordinated” with the Federal Aviation Administration and the Air Line Pilots Association to investigate the matter.
The pilots, who have about 25,000 hours of combined flying time, are receiving additional training, Mr. Freed said. He declined to elaborate on what kind of training the pilots are receiving.
Michael McCormick, a former F.A.A. control tower operator and an assistant professor of aviation science at Embry-Riddle University, called this amount of flight experience “average” for experienced flight crews. He also said that the nosedive was “very atypical and very serious.”
An F.A.A. spokesman said the pilots had voluntarily reported the incident to the agency’s Aviation Safety Action Program. These reports are confidential, the spokesman said, adding that the agency reviews submissions and then “directs any needed changes.”
The Air Line Pilots Association could not immediately be reached for comment on Monday.
Experts said that while the episode was concerning, it highlighted the key role that the Aviation Safety Action Program plays in ensuring safer skies.
Ross Feinstein, a former spokesman for both the Transportation Security Administration and American Airlines, emphasized that without this program, the episode might never have been disclosed to officials at all.
“And the reason that ASAP has been successful is that it encourages voluntary reporting,” Mr. Feinstein said. “Safety issues through this program are usually resolved through corrective action rather than through punishment or discipline and, as a result, it continues to enhance aviation safety.
United Airlines was not required to report the episode to the National Transportation Safety Board, Mr. Freed said. The investigative agency must be notified in cases of aircraft collisions, runway incursions and mechanical failures, according to federal law.
An N.T.S.B. representative said that the agency had learned of the episode involving United Flight 1722 through media reports and that it was not investigating the incident.
