Over the holidays, I happened on a documentary that told the story of American Airlines flight #191 – to date, the single worst air crash in our country’s history. The story covered the aftermath of that accident and the resulting FAA investigation into the causes. It was the conclusions of that investigation that really raised my interest, and as you’ll see as I share what I learned, this tragic occurrence has lessons to share with all of us in our roles as professional technicians.
On May 25, 1979, 271 passengers boarded AA 191 at Chicago’s O’Hare airport. Shortly after the big jet rotated on takeoff, the left engine separated from the wing and literally fell off of the aircraft. It also ripped through vital hydraulic and electrical lines, including the “shaker” that warns the pilots of an impending stall by vibrating the primary controls.
The pilots knew they had lost an engine’s power, but didn’t realize they had lost the engine structurally. They followed emergency procedure for a loss of power as the plane continued to climb, attempting to gain enough altitude for a safe return back to the airport. When the last of the hydraulic fluid leaked out, the front wing slats on the left wing closed, reducing the amount of lift on that side of the DC-10-model aircraft, causing the plane to dip over into a hard left turn until the wings were nearly vertical to the horizon and stalled. With the wings no longer producing lift, the plane crashed shortly after, killing everyone on board.
|(Image courtesy of FAA) The aft mount of the pylon is the single “wing clevis” shown in the diagram. The mass of the assembly was enough to crack this mount during the reinstallation. This “short cut” cost over 270 people their lives.|
The aircraft had just been in for maintenance eight weeks earlier. Part of the maintenance was an inspection of the engines and their mounts. On the DC-10, the engines are attached to the wing through a highly engineered part called the pylon. This mount supports the weight of the engine and is attached at two points, fore and aft. Accessing the areas of the mount for inspection is supposed to be performed after the engine has been removed from the pylon but the procedure to remove the engine was involved and time-consuming.
And here is where the story really caught my attention.
Rather than remove the engine first, someone decided it would be so much easier if the engine and pylon were removed as an assembly. An engine stand was mounted on a forklift and the lift was used to support the weight of the two during removal. Once the work was completed, the lift was used to raise the assembly into place while the pylon was reattached to the wing. And here is where the problem occurs. During the reinstallation, proper positioning of the pylon mounts is critical to protecting them from damage. That kind of precision placement is kind of hard to do with a forklift positioned perpendicular to the mount. During the job, the pylon aft mounting plate was cracked, ever so slightly, and the damage went unnoticed by the maintenance team. That’s easy enough to understand, since they just inspected that component and found it to be serviceable.
During the next eight weeks of takeoffs and landings, additional stresses caused the crack to grow until the aft mount finally failed. It failed on the takeoff roll of AA #191, causing the full weight of the engine to pivot down on the front mount until it too sheared, allowing the engine to complete its rotation over the front edge of the wing and finally coming to rest on the runway.
There isn’t a commissioned technician I know of that hasn’t come up with ways to make a job go faster or easier. I’ll never forget an instance back in the early days of the Dodge Neon. When first introduced, they were notorious for leaking oil from the original head gasket design and we repaired dozens of them under factory warranty (I worked for the local dealer at the time) using the new MLS-style gasket. Some of the techs where I worked loved the jobs and could complete them in just a few hours. How, you ask? By cutting some major corners, that’s how.
Rather than remove the head and clean the gasket surfaces, these “techs” would remove the head bolts, leaving the cam and timing belt intact, and simply lift the head just enough to slide the old gasket out and the new one in. They weren’t concerned if the gasket failed, or some other problem arose, arguing that they would just fix it again “under warranty”.
And how many of us love to tackle A/C evaporator cores by peeling the dash back and laying it on the seat, rather than removing it completely from the car? I loved the Chrysler minivans when the book time was still something like 12 hours or so. Heck, I could have one done in four hours, and that included sucking out the refrigerant and charging it back up!
|Piercing can be an acceptable way of accessing an electrical circuit if you follow the “best practices."|