The engine assembly did not simply fall away without affecting anything else. Its separation damaged systems in the left wing area, including hydraulic lines and electrical components. Those failures changed the behavior of the aircraft and removed important warning indications that the pilots would normally use to recognize an unsafe condition.
To understand the significance, it helps to look at two parts of an airplane that passengers rarely think about: leading-edge slats and stall-warning systems.
What Are Leading-Edge Slats?
Leading-edge slats are movable aerodynamic surfaces fitted to the front edge of a wing. They help the wing generate lift at lower speeds, particularly during takeoff and landing.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.
