A Boeing Co. 737 freighter ditched in Hawaiian coastal waters after experiencing engine trouble early Friday, and both pilots were safe after a dramatic pre-dawn rescue by the U.S. Coast Guard.

Transair Flight 810’s crew tried to return to Honolulu but were forced to make an emergency landing, according to the Federal Aviation Administration. U.S. safety investigators immediately began gathering information for a formal probe of the incident.

The plane was a 46-year-old version of Boeing’s workhorse twin-engine 737, long predating the new Max passenger model that was ordered grounded for 20 months after two fatal crashes. The emergency landing off Hawaii would be the second major incident this year of an older 737. In January, a Sriwijaya Air passenger flight crashed off the coast of Indonesia, killing all 62 people aboard the 737-500.

Boeing said it was aware of the reports out of Hawaii and was in contact with safety investigators. The U.S. National Transportation Safety Board said it was sending a team of seven investigators to the accident site off Oahu.

The U.S. planemaker’s 737-200 jets are powered by Pratt & Whitney engines. Pratt, a division of Raytheon Technologies Corp., didn’t immediately comment.

Boeing fell 1.1% to $237.09 at 11:39 a.m. in New York. Raytheon climbed less than 1% to $86.48.

Flight 810, branded as Transair and operated by Rhoades Aviation Inc., took off for Kahului a little after 1:30 a.m. local time and climbed to around 2,100 feet (640 meters), according to a track posted by Flightradar24.com. It made a right turn over the ocean and circled back toward the airport, ditching in the water about 11 minutes later.

The track provided by FlightRadar24 doesn’t show the normally smooth altitude and speeds of a typical flight, suggesting the pilots may have been struggling to control the plane.

Engine failures that lead to the loss of a jet are extremely rare. The 737, like all twin-engine aircraft, is designed to fly on a single engine if one has a malfunction, and maintenance programs are designed to ensure that the same issue doesn’t occur on both engines at the same time.

Sully Flight

While it’s not clear what happened with the Transair plane, at least one possibility is a twin-engine failure. Among the potential causes would be a maintenance error on both powerplants. In 2009, a US Airways jet plowed into a flock of birds in New York—the plane that pilot Chesley “Sully” Sullenberger famously landed on the Hudson River.

Jets are designed to be able to ditch in water and float for a period of time, according to federal regulations.

Pratt’s JT8D engines power the 737-200, and more than 14,000 of the low-bypass turbofans have been produced since first entering service on the Boeing 727 in 1964, according to the engine maker’s website. About roughly 2,400 are still in use.

Transair began operations in 1982 and says it provides the longest-running all-cargo service in Hawaii. It specializes in inter-island transport between all major Hawaiian destinations, according to its website.

Representatives of the cargo carrier didn’t immediately return requests for comment before normal business hours in Honolulu.