mount also failed from the resulting stresses. Inspection of the DC-10 fleets of the three airlines revealed that while United's hoist approach seemed harmless, several DC-10s at both American and Continental already had fatigue cracking and bending damage to their pylon mounts caused by similar maintenance procedures. The field service representative from McDonnell-Douglas stated the company would "not encourage this procedure due to the element of risk" and had so advised American Airlines. McDonnell-Douglas, however, "does not have the authority to either approve or disapprove the maintenance procedures of its customers." Inadequate speed The NTSB determined that the loss of one engine and the asymmetrical drag caused by damage to the wing's leading edge should not have been enough to cause the pilots to lose control of their aircraft; the aircraft should have been capable of returning to the airport using its remaining two engines. The NTSB thus examined the effects that the engine's separation would have on the aircraft's flight control, hydraulic, electrical, and instrumentation systems. Unlike other aircraft designs, the DC-10 was not equipped with a separate mechanism that would lock the extended leading-edge slats into place, relying instead solely on the hydraulic pressure within the system. The NTSB determined that the engine tore through hydraulic lines as it separated from the DC-10's wing, causing a loss of hydraulic pressure; airflow over the wings forced the left wing slats to retract, which caused a stall over the left wing. In response to the accident, slat relief valves were mandated to prevent slat retraction in case of hydraulic line damage. The wreckage was too severely fragmented to determine the exact position of the rudders, elevators, flaps, and slats before impact. An examination of eyewitness photographs showed only that the right wing slats were fully extended as the crew tried unsuccessfully to correct their steep roll. The position of the left wing slats could not be determined from the blurry color photographs, so they were sent to a laboratory in Palo Alto, California, for digital analysis, a process that was pushing the limits of 1970s technology and necessitated large, complicated, and expensive equipment. The photographs were reduced to black-and-white, which made distinguishing the slats from the wing itself possible, thus proving that they were retracted. In addition, the aircraft's tail section was verified to be undamaged, and the landing gear was down. Wind-tunnel and flight-simulator tests were conducted to help