Investigation
The National Transportation Safety Board was responsible for investigating the accident. The safety board was assigned to determine why the engine separated from the airplane and why the airplane was unable to remain airborne on its two remaining engines.
The loss of the engine by itself should not have been enough to cause the accident; Flight 191 should have been capable of returning to the airport using its remaining two engines. However, unlike other aircraft designs, the DC-10 did not include a separate mechanism to lock the extended leading edge slats in place, relying instead solely on the hydraulic pressure within the system. In response to the accident, slat relief valves were mandated to prevent slat retraction in case of hydraulic line damage.
Wind tunnel and flight simulator tests were conducted to help to understand the trajectory of flight 191 after the engine detached and the left wing slats retracted. Those tests established that the damage to the wing's leading edge and retraction of the slats increased the stall speed of the left wing from 124 knots to 159 knots.
The DC-10 incorporates two warning devices which might have alerted the pilots to the impending stall: the slat disagreement warning light, which should have illuminated after the uncommanded retraction of the slats, and the stick shaker on the captain's control column, which activates close to the stall speed. Unfortunately, both of these warning devices were powered by an electric generator driven by the number one engine; following the loss of that engine, both systems became inoperative. The first officer's control column was not equipped with a stick shaker; the device was offered by McDonnell Douglas as an option for the first officer, but American Airlines chose not to have it installed on its DC-10 fleet. Stick shakers for both pilots became mandatory in response to this accident.
Read more about this topic: American Airlines Flight 191