- American Airlines DC-10 at Chicago
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- American Airlines DC-10 at Chicago
Photo of American Airlines Flight 191, DC10
Photo of Chicago DC10 crash site
American Airlines Flight 191, a DC-10, experienced a number 1 (left wing) engine/strut failure, and subsequent engine and strut separation, on takeoff from Chicago, O'Hare Airport on May 25, 1979. During rotation, the left engine and strut assembly, and about three feet of wing leading edge, separated from the wing as a single unit, went over the top of the wing, and fell to the runway. The takeoff continued, the pilots assuming that they had experienced an engine failure, unaware that the left engine and pylon assembly had separated and fallen off the airplane. Compounding the developing problems, the pylon separation also severed hydraulic lines, causing a loss of associated hydraulic pressure and retraction of all slats outboard of the left engine.
The separation of the left engine also caused the loss of electrical power provided by that engine's electrical generator. This resulted in the loss of many aircraft systems and instruments, including the captain's flight instruments, the left stall warning computer, the stickshaker (stall warning) motor, number 1 engine instruments, the slat disagree warning system, and parts of the flight control indicating system.
The airplane climbed to approximately 300 feet with wings level, then began to turn and roll left, with bank angle increasing to approximately 90 degrees. During the roll, the nose also pitched down, and the airplane began to descend. The airplane continued to roll (112 degrees of roll at impact) and crashed in a trailer park near the airport. All 271 passengers and flight crew and two persons on the ground were killed. Additionally, two persons on the ground received serious injuries.
Approximately one second prior to liftoff, the left engine and strut assembly separated from the airplane. The airplane had accelerated through the engine-out climb speed, V2, a speed normally achieved at approximately 35 feet above the ground, while still on the ground, and lifted off at V2 + 6 knots. The airplane lifted off slightly left wing down, and was corrected back to wings-level flight via appropriate flight control inputs. The airplane continued to climb wings level, and about nine seconds after liftoff, the airplane had accelerated to 172 knots and reached 140 feet of altitude. As the climb continued, the airplane began to decelerate at a rate of about one knot per second, and at 20 seconds after liftoff, and an altitude of 325 feet, airspeed had been reduced to 159 knots. At this point, the airplane began to roll to the left, countered by rudder and aileron inputs. The airplane continued to roll until impact, 31 seconds after liftoff, and in a 112-degree left roll, and 21-degree nose down pitch attitude.
Click on this link to view an aerial photograph of O'Hare Airport and surrounding areas. The crash site is in the upper right portion of the photograph, between the green area north of the runway and the fuel tank farm at upper center.
Photo of Chicago DC10 crash
The NTSB concluded that the airplane was being flown in accordance with American Airline's engine failure procedures. Immediately after liftoff, a pitch attitude of 14 degrees was attained and maintained, indicating use of the flight director for pitch guidance. It was also subsequently determined that the captain's flight director was inoperative, implying that the first officer was flying the takeoff. American Airlines procedures required that, following an engine failure, the airplane should be accelerated to V2, and climbed to 800 feet at that speed. No differences in the procedure were specified if an engine failure occurred after reaching V2, and in fact, the specific scenario was not addressed by the airline's procedures. Following the accident, American Airlines modified this procedure to either maintain the speed at the time of engine failure, if between V2 and V2 + 10, and to maintain not less than V2 + 10 if the failure occurred after reaching V2 + 10. The revised procedure also instructed crews to disregard the flight director pitch command (which provided guidance to achieve V2) and to maintain at least V2+10.
It was clear to the NTSB that the airplane was being decelerated from 172 knots toward V2 in an attempt to follow the engine-out climb procedure (and reacquire V2) when the left wing stalled at 159 knots. The stall resulted in the left roll and ensuing loss of control.
As the engine/pylon assembly separated from the airplane, four hydraulic lines in the vicinity of the engine pylon were severed, resulting in the loss of hydraulic pressure to the slat actuators outboard of the engine location. This loss of pressure allowed several left wing leading edge slats to retract. The hydraulic actuators did not include an alternate locking system to prevent inadvertent slat retraction in the event of hydraulic system pressure loss. The slat system was designed such that when the slats achieve the commanded position, the hydraulic system provides a "lock" that uses hydraulic pressure to maintain slat position. (view diagram of the slat actuation system PDF: 272KB) When the engine/pylon assembly separated from the wing, hydraulic fluid was pumped through the broken lines, draining the system and depleting the hydraulic locking pressure. Upon loss of this pressure, aerodynamic loads caused the retraction of the slats on the outboard portion of the left wing. With the slats retracted, the stall speed for the wing is higher than with slats extended. View Animation of the engine separation sequence during the takeoff roll, and see an illustration of the effects of slat retraction on the wing's ability to generate lift and the crew's capability to control the airplane.
Strut Failure and Associated Maintenance Procedures
Detail of aft attach spherical bearing, and location in engine pylon
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Detail of forward attach fitting
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Strut failure was determined to have been caused by unintended structural damage which occurred during engine/pylon reinstallation using a forklift. The engine/pylon removal and reinstallation were being conducted to implement two DC-10 Service Bulletins, one of which called for replacement of the aft attach spherical bearings on engines 1 and 3 (McDonnell Douglas Service Bulletin 54-48), and the other called for lubrication/replacement of the forward attach monoball bearings for engines 1 and 3 (McDonnell Douglas Service Bulletin 54-59).
Service Bulletin 54-48 superseded but did not cancel an earlier Service Bulletin (54-45), which required inspection/replacement of the aft attach bearing. Both Service Bulletins 54-48 and 54-59 required that the pylons be removed, and recommended that this be accomplished with the engines removed. The Service Bulletin instructions assumed that engines and pylons would be removed separately, and did not provide instructions to remove the engine and pylon as a unit. Additionally, removal of the engines and pylons as a unit was not an approved Maintenance Manual procedure.
Photo of CF6 engine on engine stand
Photo Copyright Julio Castillo - used with permission
The DC-10 engine strut and pylon access doors are illustrated below. This figure identifies the inspection panels and access points. If inspection per the service bulletin(s) revealed damaged parts, then removal of the engine and pylon was necessary.
For removal of the engine and pylon, the illustrated attachment points would be successively accessed and detached per maintenance manual instructions, first removing the engine and then the pylon.
The Maintenance Manual did not provide instructions for removal of the engine and pylon as an integral unit. To facilitate the time required for bearing replacement, American Airlines developed a procedure using a forklift to remove, support, and replace the engine/pylon assembly.
The airline's engine change procedure removed the engine and pylon as a unit. A forklift with an attached engine stand was used to support the engine while the forward and rear pylon/wing attachments were removed. American Airlines developed the forklift procedure for removing the engine without concurrence from McDonnell Douglas. The airline had evaluated and validated portions of the procedure, but the NTSB concluded that the procedure had not been sufficiently evaluated, and appropriate consideration had not been given to the potential for inducing damage.
Pylon access panels and inspection points
Engine and wing attachment points
The lack of precision associated with the use of the forklift, essentially an inability to perform the fine manipulations necessary to accomplish reinstallation of the engine/strut assembly, in combination with the tight clearances between the pylon flange and the wing clevis resulted in damage to the same part that had just been inspected. View animation illustrating how the damage to aft pylon attach fitting occurred.
Shown is an illustration of the pylon/wing attachments. In American Airlines' procedure, the forward and aft fittings were the only fittings that were detached in order to separate the engine and pylon from the wing as a unit. The failure, which allowed the engine and pylon to separate from the wing and cause the accident, occurred at the aft attachment point, the pylon aft bulkhead.
Diagram of Wing/pylon attachment hardware
A detailed cutaway view of the pylon attachment structure illustrates the complexity of the surrounding area and relative difficulty in inspecting and/or working in the area without removing the pylon from the wing.
Diagram of Pylon structure and wing/pylon attachments
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During reinstallation of the engine/pylon assembly, replacement of the forward pylon/wing fitting bolt (prior to replacement of the aft fitting) allowed the engine/pylon assembly to pivot around the forward pylon bulkhead fitting. This rotation of the engine/pylon assembly around the forward fitting resulted in rear mount contact with the wing clevis. The forces imparted by this contact resulted in cracking of the pylon aft bulkhead and the subsequent failure of the fitting during a takeoff.
Inspections of other DC-10 pylon mounts following the accident resulted in nine additional cracked mounts being identified. As a result, citing the possibility that the DC-10 "...may not meet the requirements of Section 603(a) of the Federal Aviation Act for a Type Certificate..." FAA Administrator Bond revoked the DC-10 type certificate for 37 days while the investigation proceeded.
The NTSB was highly critical of the FAA's lack of involvement in overseeing and approving important maintenance tasks such as engine change procedures. As a result of this accident, Type Certification Processes now formally address certain "vital" maintenance tasks. Prior to this, aircraft certification and maintenance were treated as separate, somewhat unrelated issues.
During the investigation, Aviation Week & Space Technology published a series of articles concerning various aspects of this accident and the early investigation. These articles can be viewed by clicking on the titles:
- Crash to Boost FAA Scrutiny, AW&ST, June 4, 1979, Page 12
- DC-10 type Certificate Lifted, AW&ST, June 11, 1979, Page 47
- Pylon Related Bulletins Issued Earlier, AW&ST June 11, 1979, Page 57
- FAA Probe Turns to DC-10 Slat System, AW&ST July 9, 1979, Page 30
Photo of American Airlines Flight 191 Memorial
Photo copyright Jason Rice – used with permission
In October 2011, a memorial was dedicated to the passengers and crew of Flight 191. The memorial is near the crash site, and consists of a decorative garden, bordered by a brick wall. The bricks in the wall are engraved with the names of persons who perished in the accident.