American Airlines Flight 191

The most deadly aircraft accident on U.S. soil was American Airlines flight 191.  This aircraft involved in this accident was a McDonnell Douglas DC-10.  This flight brought a lot of negative attention to the DC-10.

The accident occurred on 25 May 1979 while leaving O'Hare International Airport, Chicago, Illinois.  It happened only moments after takeoff.  The aircraft was accelerating down the runway fine, but at the moment of takeoff rotation the number one engine and pylon separated from the left wing and flew up and over the aircraft, landing behind on the runway.  The aircraft immediately lost hydraulic and electrical power supplied by the number one engine.  At the time all the crew knew is that they lost power in the first engine.  They immediately followed emergency procedures by reducing power on the other 2 engines and preparing for emergency landing.  A pilot on the ground was noted for saying that initially the aircraft seemed to climb normally.  Then shortly after the aircraft rolled rapidly to the left and pitched down into a field.

Ideally, completely losing one engine would not be enough to bring the aircraft down.  Unfortunately when the crew lost the number one engine they lost number one hydraulic power and number one electrical power.  Upon takeoff the aircraft had slats deployed as would have been normal for this stage of flight to create lift.  Each wing's set of slats where controlled by their respective hydraulic system.  When the aircraft lost number one hydraulic system the slats on the left wing lost hydraulic pressure on them so the force from the air moving across the left wing caused the left side slats to retract.  This effectively caused the left wing to go into an aerodynamic stall while the right wing maintained lift.  This effectively caused the aircraft to roll hard left (NTSB, 1986, p. ii).  

When the aircraft lost number one electrical power the crew lost power to vital systems like the stick shaker and the slat disagree sensors (NTSB, 1986, p. 23).  The stick shaker is a system that literally shakes the pilots flight stick in order to let him know that the aircraft is going into a stall.  The slat disagree sensors are sensors that let the pilot know if the left and right wing slats are not in the same position.  This is obviously vital for a stage of flight as important as takeoff.  So as you can see, the crew had a major problem and no way to know about it.

At the time the DC-10 didn't have any sort of slat locking system to keep the slats deployed under air load like most modern aircraft have.  Because of this accident, slat relief valves were installed on all DC-10 aircraft (NTSB, 1986, p. 43).

All these things still don't answer the big question here in this accident and that is why did the number one engine separate from the wing in the first place?  For engine change procedures McDonnell-Douglas required that the engine be first removed from the pylon and then have the pylon removed from the aircraft.  To save time, American Airlines developed a technique where they removed the engine and the pylon whole while supported by a forklift (NTSB, 1986, p. 26).  This method saved a lot of time but it was difficult to keep the engine from moving.  Several weeks before the accident, the umber one engine was reported to have been changed.  The engine had shifted and damaged the mounts, but nothing was done when the damage was noticed.  After weeks of flying the mounts had developed growing fatigue cracks until the mount completely failed on takeoff.

This flight along with several other DC-10 crashes gave the aircraft a lot of negative attention.  So much so that after this flight the FAA pulled the flying certification and essentially ground the aircraft until further investigation was done.


NTSB. (1986). Aircraft accident report NTSB-AAR-79-17. Retrieved from http://libraryonline.erau.edu/online-full-text/ntsb/aircraft-accident-reports/AAR79-17.pdf