Northwest Airlines Flight 710 was a scheduled flight between Minneapolis, Minnesota and Miami, Florida, with a stop in Chicago. On March 17, 1960, the six-month-old Lockheed L-188 Electra operating the flight broke up in midair in southern Indiana, near Cannelton, killing all 63 people on board. After unexpectedly encountering clear-air turbulence at , the aircraft's right wing and part of the left wing separated, causing the fuselage to plummet and impact the ground at a nearly 90-degree angle, leaving a deep crater. Portions of the wings landed up to away.
The in-flight breakup of the Electra closely resembled the September1959 crash of Braniff International Airways Flight 542 near Buffalo, Texas, which killed all 34 occupants. That flight was also operated by a nearly new Electra. In that crash, the left wing separated and landed about one mile (two kilometers) from the main wreckage. Investigators were unable to determine the cause at the time, but the similarities between the two crashes led the Federal Aviation Agency to place flight restrictions on the Electra pending identification of the cause, and to order Lockheed to reevaluate the aircraft's structural integrity and demonstrate its airworthiness. The subsequent investigation, involving more than 250 engineers and technicians, determined that when an Electra with damage to the mounting structures of one of its outboard engines operated at high speeds or in turbulence, a destructive phenomenon called whirl mode wing flutter could occur, leading to wing failure.
After identifying the cause of the failures, Lockheed launched a program to design structural modifications to prevent whirl mode wing flutter and to retrofit all Electras already in service. These changes proved effective, and modifications to the fleet were completed on July 5, 1961.
Background Northwest Airlines Flight 710 was a regularly scheduled flight between Minneapolis, Minnesota, and Miami, Florida, with a scheduled stop in Chicago. On March 17, 1960, the flight was operated by a Lockheed L-188C Electra aircraft. At 12:51p.m. Central Standard Time, the flight left Wold-Chamberlain Airport in Minneapolis with 69 passengers. It arrived at Midway Airport in Chicago after an uneventful flight, and 51 of the passengers departed the plane. Another 39 new passengers boarded while the crew prepared for the flight to Miami and the ground crew refueled the aircraft. Flight 710 took off from Midway Airport at 2:38p.m. with 57 passengers and a crew of 6.
At 3:13p.m., as the flight passed a navigational waypoint at Scotland, Indiana, the crew made a routine report to the Indianapolis Air Route Traffic Control (ARTC) Center with the aircraft's position and altitude at . The crew estimated that the flight would reach its next check point at Bowling Green, Kentucky, in twenty minutes, and reported that the flight was proceeding normally. Indianapolis controllers told them to contact the Memphis, Tennessee, ARTC in fifteen minutes on that center's radio frequency.
In the area, the weather at the plane's altitude was clear, with unrestricted visibility. Below it, there were two layers of broken to overcast clouds; cloud tops in the upper layer extended up to altitude. An area of low pressure centered over the Lower Peninsula of Michigan and a low pressure trough extended southward along the Illinois-Indiana border. At the same time, a ridge of high pressure extended from the southern plains to the northeast, from Arkansas, through central Kentucky and into southern Ohio.
A different Electra flight that had passed through the same area at 2:30p.m. that day reported that it had encountered severe airspeed fluctuations and turbulence in the area. Two U.S. Air Force B-57s operating near Louisville, Kentucky, reported moderate to severe clear-air turbulence in the area at high altitudes between 3:00 and 3:30, with one of them slowing down and descending from between 20,000 and 25,000 feet to 15,000 feet to escape the turbulence. A DC-7 passing through the area reported severe clear-air turbulence at 19,000 feet. Before leaving Chicago, the pilots of Flight 710 had been informed about the weather conditions in the area, but the weather reports did not mention anything about clear-air turbulence along the flight's path.
Accident
At about 3:20p.m., Flight 710 was in level flight at when the aircraft's right wing separated from the fuselage. The upper portion of the wing near the fuselage failed first, followed by the rest of the right wing and its engines over the next six to ten seconds. Portions of the outer left wing and its outboard engine also broke away. The loss of one wing and the partial loss of the other caused the aircraft to turn upside-down in the air.
Despite the aircraft losing its wings, its forward momentum carried it for more than before it began to arc downward toward the ground. During the descent, at least one of the engines on the remaining left wing continued to run at full power, which accelerated the aircraft into the dive. The plane struck farmland about east of Cannelton, Indiana, in a near-vertical, nose-first attitude at a speed of at least . Upon impact, the pressurized fuselage ruptured, and the rapid decompression flung debris and human remains over several hundred yards with a thunderous blast. Most of the fuselage buried itself in the frozen, snow-covered ground, leaving a crater about deep and wide.
Eyewitnesses on the ground reported seeing the aircraft fly overhead and seeing two puffs of white smoke come from the aircraft, followed by a large cloud of dark smoke within a few seconds. They heard two loud explosions, then saw a large piece of the aircraft fall off and burst into flames, followed by additional flaming objects falling from the plane. They saw the fuselage arcing downward, and when that struck the ground they heard what sounded like a huge explosion that rattled windows for miles around. Pilots of six United States Air Force aircraft that were conducting refueling operations nearby at spotted a dark smoke trail that disappeared into the cloud layer below, with a second horizontal plume extending a considerable distance from the main smoke cloud.
The crash was the third involving a Lockheed L-188 Electra since the type entered commercial service just over a year earlier. It was the first accident for Northwest Airlines since the April 1956 crash of Northwest Orient Airlines Flight 2, in which a Boeing 377 Stratocruiser crashed into Puget Sound, killing five people.
Aftermath
The main part of the aircraft struck the ground near Cannelton, Indiana, about a mile (two kilometers) north of the Ohio River and about southwest of Louisville, Kentucky. It buried itself in the ground, leaving a deep crater, throwing large chunks of earth up to away, and scattering small pieces of debris in the area. Outside the crater, only small pieces of the aircraft could be found, but small strips of metal from the aircraft, clothing, personal effects of the passengers, and unrecognizable body parts were scattered over a area. Inside the crater, only a few pieces of wreckage were visible, and smoke from a smoldering fire obscured views.
A badly damaged, section of the aircraft's right wing landed in a barley field a little over two miles (five kilometers) from the crater, with its inboard engine still attached. The outer engines from the left and right wings and their propellers landed within of the right wing. The remaining parts of the left and right wings and portions of the engine structures were scattered over an area of about one mile (two kilometers) wide and seven miles (eleven kilometers) long. Some portions of a wing were recovered from a lake four miles (six kilometers) from the crater.
Witnesses originally called in reports that two aircraft had crashed after a mid-air collision, but they had mistaken the wing of Flight 710 as a second aircraft. Officers from the Indiana State Police and local police agencies arrived at the crater site and began searching for victims. They conducted a search in the field and the dense woods in the area, collecting clothing, personal belongings, and mail scattered around the area. A bus full of school children that had come across the accident scene a few minutes after the crash helped recover hundreds of pieces of mail. The Perry County Coroner and an assistant arrived, but they said the victims were so badly shattered that recovery was not immediately possible. Curious onlookers also arrived at the scene, so officers set up posts throughout the area to keep spectators away and blocked the roads that led to the crash scene. State police brought in portable generators and floodlights and officers continued the search into the night.
In Miami, where the plane had been scheduled to arrive at 6:21p.m. Eastern Standard Time, nobody told the friends and relatives of the passengers of the flight about the crash. After waiting nearly an hour after the scheduled arrival time, the airline posted a sign that said that Flight 710 had turned around because of a storm and that the flight had been cancelled. The husband of one of the passengers aboard the flight called a local newspaper for more information and was told about the crash, and he passed the information on to the other people waiting at the airport. At 7:45, airline representatives called the relatives into a private office where they explained what had happened.
An inspection team consisting of three investigators from the Civil Aeronautics Board (CAB) and eight men from Northwest Airlines arrived late in the day at Dress Memorial Airport in nearby Evansville, Indiana. At 11:30p.m. they left the airport in a caravan escorted by the State Police to visit the crash site. By the next day, they had been joined by additional investigators from the CAB, Northwest Airlines, Allison Engine Company, and the Federal Bureau of Investigation (FBI). A team of 44 soldiers from the Indiana National Guard and five private security agents arrived to help secure the area and to prevent onlookers from approaching closer than three miles (five kilometers) from the crash site or taking any of wreckage as souvenirs. Red Cross and Salvation Army workers set up portable kitchens in Cannelton to provide meals for the investigators and to provide assistance to any relatives of the victims who might arrive to try to claim the bodies of the deceased passengers. Postal inspectors arrived to search for the contents of the five sacks of mail that the plane had been carrying. They were eventually able to recover of the that was on the plane, but much of what they recovered was too damaged to read the addresses. Elected officials also came to the site to view the site first-hand.
The day after the crash, bright sunshine started melting the snow on the ground, and the edges of the crater began to cave in when searchers walked too close to it. Because of the cave-ins, the CAB cancelled its plans for searchers to use picks and shovels to dig into the crater to search for wreckage and victims. After a meeting of federal officials and Northwest Airlines officials, they decided to employ steam shovels to scoop out the wreckage from the crater. Grey smoke continued billowing from the crater. The Indiana Department of Transportation brought in heavy equipment including bulldozers, trucks, and a dragline excavator.