The Crash of American Airlines 587:
Accident or Terrorism?
It isn't any wonder so many fear this tragedy to be yet another act of terrorism straight from the evil mind of Osama Bin Laden. However, what we know so far doesn't seem to add up to the kind of crash that would be produced by a deliberate act of sabotage.
All airliners are design-certified to be able to continue to climb, at maximum certified gross weight and at the most critical time in the takeoff profile, in the event of a total loss of power from one engine.
Then, why did that plane crash?
Witnesses were said to have seen smoke and flames trailing from one engine, and finally observing that engine fall off, shortly before the crash. In addition, a loud "explosion" sound was also reported by ground witnesses, prior to the crash. That led to initial speculation that engine failure was the cause of this crash.
When thrust is lost from one engine, the pilot must quickly act by pushing in the rudder and rolling in aileron, towards the side of the remaining good engine. If a pilot fails to respond in that manner, the plane would roll upside down and total control would quickly be lost.
The rudder is attached to the trailing edge of the vertical stabilizer. When a vertical stabilizer breaks free from a plane, the rudder goes with it. At that point, the only way a pilot could prevent the plane from rolling over on its back, is by retarding the throttle on the remaining good engine. With the power from that good engine no longer available to the pilot, he would have no choice but to descend towards mother earth. If he tried to arrest that descent, by pushing the throttle forward again, the plane would start to roll, because the rudder was gone. Caught between a rock and a hard place, a pilot could only try to extend his glide to a certain crash, by advancing the thrust lever no more than his limited aileron roll control would permit.
One thing is very certain: That if the theory--- that an engine had a massive internal failure---had been accurate, the plane was doomed to go down once the pilot lost the stabilizer/rudder control. At that point, the power of the remaining engine would have been not only useless, it would have then became a force of liability, determined to roll the plane upside down and out of control, if that power wasn't promptly retarded. However, now that we know neither engine had such a failure, we need to examine other possible reasons for the in-flight breakup of AMR 587.
Pictures of the vertical stabilizer, as it was lifted from Jamaica Bay, revealed a very clean break at the bottom edge, where it had been bolted to the fuselage. It looked pristine, as if it was being moved from a production line to be mated to a new production fuselage. Without any visible damage to that stabilizer, one could not reasonably conclude it had been ripped away by physical contact with other parts flying off the plane. Neither would that pristine condition allow for a terrorist bomb inside of the fuselage, as a sensible explanation. The "black boxes" (CVR and FDR) were found in the main crash site area, indicating the rest of the tail section, below the departed vertical stabilizer/rudder, remained attached to the fuselage. That would tend to indicate a bomb was not the cause of the stabilizer/rudder separation from the fuselage.
The latest information:
a) There was no internal failure of either engine.
b) Both engines separated from the plane prior to impact.
c) The cockpit voice recorder (CVR) recorded a "rattling" noise and the pilots commented about it being "wake" turbulence. That means, they thought the noise (and whatever vibration and shaking they might have been experiencing at the same time) was caused by the wake left from the airplane that had taken off ahead of them. Then, a second rattling sound was heard by the pilots, one minute and 21 seconds after the start of the takeoff roll. At 1+25, the First Officer called for "max power." Control of the plane began to be lost at that point. It rolled over and dove, nose-first, into the ground shortly thereafter.
d) The vertical stabilizer (and the rudder that was attached to the trailing edge of that stabilizer) broke off the plane, probably before the engines broke free.
What could have caused that stabilizer/rudder section to separate? One possible explanation would be a severe horizontal gust load. That means a horizontal wind hit the side of that stabilizer with a force so strong that it exceeded the strength and ability of that structure to withstand such a force. That kind of force could be produced by a very rapid and severe yawing motion (the tail suddenly moves sideways). Had the theory of a sudden and catastrophic failure of one engine been accurate, then one could also reasonably suspect that the forward thrust of that failed engine might have not only suddenly been eliminated, but there could also have been a reversed flow of thrust, if the giant compressor, in the front section of that engine, had stalled. Such a compressor stall would sound like an explosion, to witnesses on the ground. However, the CVR did not record any such explosive sound.
In theory, the airplane should be able to withstand a sudden yaw, yet it is well known that severe and dangerous horizontal gust loads can be imposed on vertical stabilizers under some flight conditions. That is why they have computer monitoring of airspeed so as to reduce the limit of rudder movement, on modern airliners: because structural limits of the vertical stabilizer can be exceeded if the rudder throw is too great when accompanied by a severe side loading.
It will take a very lengthy investigation to determine why that vertical stabilizer failed.
The presence of corroded and/or cracked structure bolts, reducing the total load limit strength, is one possibility. Improper maintenance procedures and/or defective design, another. Or, something might have gone wrong with the rudder itself. Could a malfunction in the rudder computer control system have caused the rudder to start moving rapidly from side to side? I don't know if that is possible, but if that actually happened, it could have imposed heavy side loads on the vertical stabilizer to such an extent that design strength limits were exceeded. If one of the attach bolts, which attached the rudder to the trailing edge of the vertical stabilizer, broke, that might have caused the rudder to start flopping rapidly as if in a dangerous flutter mode. It is well known that flutter, if it develops on any tail control surface, can lead to in-flight breakup in a matter of seconds.
If the catastrophic failure of the vertical stabilizer/rudder section occurred first, it could have been the cause of the engines falling off. If sudden yawing movements (sideways motion) was part of the process of the failure of the stabilizer/rudder, then excessive loads likely were imposed on the 3 shear pins that attach each engine to its respective pylon. Those pins are designed to shear, in the event of a massive engine breakup, allowing the engine to fall away from the plane before it can destroy the wing.
It now seems the failure of the stabilizer/rudder is the key to analyzing the causal factors in this accident. Could it have been an act of sabotage? Nothing can be ruled out at this early stage of the investigation, but I see that to be a very remote possibility. Human failure in the design, operation, or maintenance of the airplane is much more likely.
November 14, 2001
Robert J. Boser
The Editor of this Web Page, now retired, was an airline pilot for
33 years and holds 6 specific Captain's type-ratings on Boeing Jet Airliners,
including the Boeing 747-400.