When properly deployed, slats change the airflow around the wing and allow it to continue producing lift at angles and speeds that would otherwise increase the risk of a stall. Their position is carefully controlled because the aircraft's performance depends on the configuration of the wing.
On Flight 191, the separation of the left engine and its pylon damaged hydraulic lines and control-related components. The investigation found that the outboard leading-edge slats on the left wing retracted unexpectedly.
The right wing did not experience the same change. This created an aerodynamic imbalance: the two wings were no longer producing lift under equivalent conditions.
The difference became critical as the aircraft climbed and its airspeed decreased. The left wing, with its slats retracted, reached a stall condition while the other wing continued to produce comparatively more lift. The resulting imbalance caused the aircraft to roll sharply to the left.
Why the Loss of Warning Systems Mattered
The engine separation also interrupted electrical power to systems associated with the left engine. Among the affected equipment were the captain's flight-director instrument, stall-warning equipment, and the slat-disagreement warning system.
These were not minor inconveniences. Warning systems are designed to provide crews with information that may be difficult or impossible to observe directly from the cockpit. Pilots cannot look through the cabin windows and inspect the full position of the wings while simultaneously controlling the aircraft.
In this case, the pilots did not receive the expected indications of the dangerous slat configuration and impending stall. The situation was made even more difficult because the event initially resembled an engine failure during takeoff—an emergency that flight crews train to handle.
The crew had to respond based on the information available to them, while the aircraft's behavior was being affected by damage they could not fully see. The official investigation concluded that the loss of the warning systems left the crew with inadequate opportunity to recognize and prevent the developing stall.
It is important to understand this point before judging the actions taken in the cockpit. Investigators examined the crew's response in the context of the information available, the procedures in force at the time, and the extremely limited time between the mechanical failure and impact.
Flight 191's tragedy was not simply that an engine came loose. It was that the separation triggered several linked failures at once, leaving the aircraft in a configuration that was extraordinarily difficult to recover from at such low altitude.
The Maintenance Work Before the Flight
One of the most important findings in the investigation concerned work performed on the aircraft before the accident. The left engine and its pylon—the structural assembly connecting the engine to the wing—had been removed and reinstalled during maintenance.
The work was associated with service actions involving the DC-10's engine pylon attachment area. The aircraft's maintenance history became central to understanding why the assembly separated during takeoff.
An engine pylon must support enormous loads. It holds the engine in position while the aircraft experiences vibration, thrust, aerodynamic forces, and changes in loading throughout flight. The attachment structure is engineered to withstand those forces, but it depends on its components remaining within specified tolerances and free from damaging cracks or deformation.
Removing and reinstalling such a heavy assembly requires precise handling. If the engine and pylon are moved in a way that places unintended loads on attachment points, parts that appear intact can sustain structural damage.
Investigators found that the pylon had suffered damage associated with improper maintenance procedures. The critical structure had been weakened, and the assembly ultimately failed under the forces experienced during takeoff.
Why the Removal Method Became So Important
The investigation examined how the engine and pylon had been handled during the maintenance process. The aircraft manufacturer’s procedures called for the engine and pylon to be removed separately, but the maintenance operation involved removing and reinstalling them together.
The method used to handle the combined assembly involved a forklift. The investigation identified problems with using that equipment to perform the precise movements required to align the heavy structure with the wing attachment points.
The clearances between the pylon and wing structure were tight. A small unwanted movement could cause contact where no contact was intended, creating damage that might not be immediately obvious.
The issue was not simply whether the engine could be lifted or moved. It was whether the operation could be performed without placing damaging forces on the attachment structure.
The investigation determined that damage caused during the maintenance process weakened the pylon structure. Under the loads of takeoff, the damaged structure failed, allowing the engine and pylon assembly to separate from the aircraft.
This finding was especially significant because maintenance is intended to preserve airworthiness. A task undertaken to keep an aircraft safe should not introduce a hidden structural defect that remains undetected until the aircraft is under the stress of flight.
Why a Small Crack Can Have Enormous Consequences
Large aircraft structures are engineered with carefully calculated load paths. Forces are transferred through components and attachment points so that the aircraft can withstand the stresses of normal operation.
A crack in a critical structural component can reduce its ability to carry those loads. If the crack grows or the remaining material becomes too weak, the component may fail when the structure is subjected to sufficient force.
The pylon failure on Flight 191 demonstrated how damage that began during maintenance could remain hidden until a critical moment. The assembly had been reinstalled, and the aircraft subsequently entered service. The defect did not reveal itself during an ordinary ground inspection in a way that prevented the flight from departing.
During takeoff, however, the engine and pylon were subjected to substantial forces. The weakened structure could no longer withstand them.