Pretend you’re the pilot of a large jetliner. You’ve completed pre-flight checks, both inside and outside, and are ready for takeoff. As you climb, the plane begins to vibrate and then pitch to one side. The number two engine then separates and you are faced with a decision – jettison the remaining fuel on the aircraft or make a heavy landing with fuel on board. While engine separations are not frequent occurrences of air travel, they can have tragic consequences for both the plane and the surrounding area. During the course of this blog, we’ll review two key cases involving such incidents.
In May 1979 a McDonnell Douglas DC-10 (Flight 191) was making a regularly scheduled passenger flight from O’Hare International Airport in Chicago to Los Angeles International Airport. Moments after takeoff, the aircraft plummeted downward, killing all 258 passengers along with the crew of thirteen and two on the runway. A subsequent investigation by the FAA revealed the number one engine separated from the left wing, flipping over the top and then landing on the runway. During the separation from the wing, the engine severed several hydraulic lines which locked the leading edge wing slats into place, as well as damaging a three-foot section of the wing. As the plane began to climb, it experienced a state of uncontrolled aerodynamics, in which the left wing provided minimal lift compared to that of the right wing while the engine was at full throttle. This condition caused the aircraft to roll abruptly to the left, reaching a bank angle of 112 degrees before crashing.
While the cause of the DC-10 engine loss was later determined to be due to a damaged pylon structure connecting the engine to the wing, several other factors also played a role in the crash. The hydraulic system powered by engine number one actually failed but ran from motor pumps connecting it to the engine three systems. While hydraulic system three was also damaged, it continued to provide pressure until the crash in spite of leaking fluid. Electrical problems were also a factor in the crash of Flight 191. The number one electrical bus, attached to the number one engine, failed, resulting in several electrical systems going offline including the flight captain’s instruments, stick shaker and wing slat sensors. As a result of the partial electrical failure, the flight crew only received a warning about the number one engine failure – not its loss. Though the crew had a closed circuit television screen behind the pilot from which to view the passenger compartments, it too was subject to the loss of power from the engine. After the Flight 191 incident and three other DC-10 crashes during the 1970s, a number of major airlines began to phase out the DC-10 in the early 1980s in favor of newer and more fuel- efficient jetliners such as the Boeing 757 and 767. While the phaseout had more emphasis on fuel efficiency, the safety of the aircraft cast a cloud over its service.
The DC-10 wasn’t the only wide-body jet to experience engine separation. In October 1992 an El Al Israeli Airlines Boeing 747-200 cargo plane (Flight 1862) with three crew members and one passenger on board, began a flight from John F. Kennedy Airport , New York to Ben Gurion International Airport, Tel Aviv with an intermediate stop at Schiphol Airport, Amsterdam. Weather conditions were favorable at the time of departure with all pre-flight checks performed, with no defects found. About ten minutes out of Schiphol, the flight data recorder indicated both engines 3 and 4 and their connecting struts had left the aircraft. The co-pilot transmitted an emergency call to Schiphol, requesting a return to the airport. However, the aircraft could not make a straight-in approach, due to both altitude and proximity to the airport. Therefore, the air traffic controller had to vector the El Al plane back to the airport by flying a pattern of descending circles to lower the altitude for a final approach. About five minutes into the flight pattern, the flight crew informed the controller of the loss of engines three and four and were beginning to experience flap control problems. The controller directed a new heading to the flight crew, but noticed the plane was taking 30 seconds to change headings. About three minutes later, the flight crew informed air traffic control they were receiving audible warnings indicating a lack of control and low ground proximity. Approximately twenty-five seconds later, the aircraft crashed into an eleven-story apartment building, about seven miles from Schiphol Airport.
Both number 3 and number 4 engine struts were recovered from Naarden Harbour, just east of Amsterdam with both engines attached to the struts. Remaining parts of the aircraft were located within a thousand foot radius of the impact. From an analysis of the parts and their placement, investigators were able to determine the number 3 engine separated first, traveling in an outboard direction, striking engine 4 and causing it and the supporting strut to separate from the plane. The engine struts or pylons are designed as two-cell torque boxes absorbing vertical, horizontal and torsional thrust loads to the wing, acting as an aerial shock absorber. The Boeing 747 pylon was supported internally by five fuse pins, which provide enough strength to hold the pylons in place with the exception of extreme loads, in which the pins fail, allowing the engine to break away without damaging the wing fuel tanks. This philosophy was adopted by Boeing from experiences with the earlier 707 and 727 models, in which a number of incidents of both in-ground and mid-air engine separations occurred. The crash of the El Al jetliner was attributed to a failure of a center fuse pin in the number 3 engine strut. The pin cracked due to metal fatigue and was a bottle bore design. The FAA issued a directive in 1979 requiring airlines to conduct inspections of the fuse pins every 2,500 flight hours as the bottle design was prone to fail at that point. The El Al 747 was one of a few aircraft which had not replaced their bottle pin units. As a result of the El Al crash and two other 747 crashes, the FAA mandated a retrofit of all Boeing 747 wing struts in 1995. The new strut design offered increased protection in the event of an engine separation, while still using fuse pins to protect the wing tank from damage during ground impact.
As the two previous cases indicate, engine separations may result from a number of problems. Sometimes it’s a matter of faulty parts, while lack of proper maintenance plays a role in others. The overall design of the aircraft itself may be a factor. However, the safe operation of an aircraft requires a continual interplay of aviators, air controllers, engineers and the flying public to promote flight safety.