Ten Second Scenario

Submitted by Faucon X (United States), Dec 15, 2008 at 09:09

TEN-SECOND SCENARIO

This paper addresses a failure scenario of the vertical tail that the NTSB refused to consider during its investigation of the AA587 Crash. This is hard to understand considering the substantial supporting physical evidence and test data.

Simply stated, the vertical tail departed the aircraft from a sequence of events and individual structure failures that initiated with the failure of the aft right hand lug and lasted ten seconds.

The ten-second scenario differs for the NTSB findings in two ways: First, this scenario finds that the aft right hand lug failed from an overload condition caused by jet wake encounter ten seconds before the vertical tail broke off the aircraft. Secondly, the pilot did not impose an overload condition onto the vertical tail through his inputs to the rudder control system. From the moment of the jet wake encounter to the catastrophic vertical tail departure, an uncontrollable erratic motion of the aft lower portion of the vertical tail controlled the rudder and aircraft.

This scenario considers the vertical tail departure as two failure events in succession over a span of ten seconds. While it is obvious that the first failure lead directly to the departure event, the two failures can be considered as separate events for convenience since each failure is completely different and occurred ten seconds apart.

The first failure at the aft right side lug was caused by an overload condition resulting from jet wake encounter. Its mode of failure was shear tension tear-out of the lug. It is a classical failure mode considered in the design of structural joints, the lug pin loads the lug housing until the pin ripped through its housing.

The second failure occurred in skin transition area and this makes this failure unique because test and analysis identified the middle attachment to be critical at the lug. The failure occurred in the skin transition near the middle lug along a row of fasteners that attach the skin to the lower closure rib. Its mode of failure consisted of a combination of tension and out-of-plane bending. During numerous Component Lug Tests which included the skin transition area, each lug failed in shear tension tear-out and not in the skin.

Note. The NTSB claims the vertical tail departed the accident aircraft instantaneously, immediately after the aft right hand lug failed.

By simply proclaiming that a catastrophic failure of the vertical tail attachment system caused the departure of the vertical tail above ultimate load, the NTSB substantiates the structure integrity of the vertical tail and that its attachment

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system was not compromised until the departure event. The blame is put squarely on the shoulders of the pilot and solves a lot of problems for Airbus, FAA, American Airlines and aircraft composite industry.

There is no evidence to support this hypothesis; in fact, a catastrophic departure is not consistent with the Full Scale Static Test results.

The ten-second scenario finds that the aft lug failed due to jet wake encounter. The jet wake encounter load was large enough to fail the aft lug, but did not produce a load large enough to overcome the middle lug's strength. The middle support structure maintained its structural integrity, resisted the load and the vertical tail stayed on the accident aircraft.

During the next ten seconds, the lower closure rib aft of the middle spar disintegrated to the point where the rib substantially lacked structural integrity. The lost of strength and stiffness caused the aft lower portion of the vertical tail to become ineffective in supporting and stabilizing the lower skin. Since the vertical tail was no longer attached at its aft support, the vertical tail was allowed to deflect freely in a violent lateral motion.

This motion affected the rudder controls and ultimately the flight characteristics and performance of the aircraft were compromised. The rudder control mechanism consists of a set of tie-rods that link the control mechanism located in the vertical tail to a fitting mounted in the fuselage. Any deflection of the vertical tail relative to the fuselage will develop a forced displacement in the linkage independent of pilot input. Therefore, as the aft portion of the vertical tail deflected, it provided input to the rudder control system just as if receiving a pedal input from the pilot. Any attempt by the pilot to control this phenomenon at this point was fruitless. Pedal input by the pilot could have added to or reduced input to the rudder actuation system depending on the position of the vertical tail at that instant. In a sense, the vertical tail was in control of the aircraft.

At some point during the destruction of the aft lower portion of the vertical tail, the structural integrity of the skin at the line of fastener that attached the lower rib to the skin weaken to the extend that the skin failed in bending and tension. This failure was catastrophic and caused separation.

PHYSICAL EVIDENCE THAT SUPPORTS THE TEN-SECOND SCENARIO

The following provides substantiate physical evidence that the aft lower portion of the vertical tail was severely damaged at the time of the vertical tail departed the aircraft.

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Full Scale Static Test. The ten-second scenario is consistent with the Full Scale Static Test results. During this test, the aft lug failed above ultimate design load as planned, but no other lugs failed simultaneously at this load level. Testing was discontinued with the vertical tail intact, except for a failed aft support system. The structural integrity of the middle attachment was not compromised and the vertical tail could continue to carry load.

The Full Scale Static Test demonstrates that it is possible to fail the aft lug without a catastrophic departure of the vertical tail and that the aft lug failure could have occurred on the accident aircraft at any time during the sequence of vertical tail overload events. All that was needed was time and a sequence of large diverging load excursions.

Component Lug Static Test. During numerous Component Lug Tests which included the skin transition area, each lug failed in shear tension tear-out.

Significance of Failure in Skin Transition Area above Middle Lug. The fact that the middle lug did not fail in shear tension tear-out proves that the vertical tail departure and mode of failure was not consistent with the design, analysis and test results. This is substantial evidence that the tail's departure was not instantaneous.

Sometimes the fact that something does not occur can be just as significant and revealing as if something does happen.

Opportunities for Aft Lug Failure. During the ten second period prior to separation, the accident aircraft's aft lug had received a number of load events that were larger than the failing aft lug load of the Full Scale Static Test. Any one of these load events could have failed the aft lug without failing the middle lug.

Sounds from Voice Recorder. After the jet wake encounter, the vertical tail remained on the accident aircraft for ten seconds. During this time, loud noises, bumps, thumps, bangs and pops, and sounds of the pilot struggling to control the aircraft. The final sound was that of a loud bang. Anyone who has ever listened to the voice recorder tape could not help but imagine that somewhere something seriously wrong was happening to the aircraft structure.

Delamination of lug. In an incident with accident aircraft over Peru a large number of passengers was injured. Loads analysis show that Vertical Tail experienced a load equal to the ultimate load. Delamination in lug was found on other aircraft that had experience similar vertical tail overload. It can be assumed that this incident caused a delamination in the aft lug of the accident aircraft further weakening it.

Damage to Lower Closure Rib. Photos of the lower closure rib structure shows the damage being extensive and exclusively located in the area behind the middle support.

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The structural damage to the rib aft was violent and complete to the point where the rib is unrecognizable. It can be assumed that this damage occurred while the vertical tail was on the aircraft since the aft support system was no longer effective and all of its load must redistribute to the middle support through the lower closure rib.

Lateral Motion of Aft Portion of Vertical Tail. NTSB Docket No. 168606, Factual Report 02-077, page 9 of 63, Section 2.2.2. Photos of the aft lower spar surface provide witness marks that indicate grudging in the spar caused by a back and forth movement of the outboard end of the left lateral link against the spar. If the Vertical Tail separated for the aircraft instantaneously, there would be no grudging. This damage occurred during the destruction of the closure rib and indicates motion while still on the aircraft.

Bearing failure at the left spar lug bore at 1 o'clock position indicates spar movement to the right, roll-15-08-M.jpg.

NTSB Docket No. 168624, Factual Report 02-078, App. A, page 9 of 52, Figure 06. Two large areas of delamination in lower aft spar above both lug bores.

NTSB Docket No. 168606, Factual Report 02-077, page 5 of 63. Deformation of aft left yoke sleeve.

Conversely, when the vertical departed the aircraft there was no delaminaion in the forward or middle spars because as the vertical tail departed the structure was free to move aft and up in the plane of the canted spars. Hence, there was no out-of-plane resistance by the lower spars. NTSB Docket No. 168624, Factual Report 02-078, App. A, page 9 of 52, Figure 05.

Both the middle and forward left lateral links were free to rotate on the attachments and eliminate damage to the spars. NTSB Docket No. 168606, Factual Report 02-077, page 5 of 63 shows no deformation.

Impact Damage to Failed Aft Lug. Roll-13-03-M.jpg. Photo that show local impact damage on the surface of the fracture of the aft right hand lug that could only have occurred by the vertical tail coming back onto itself and the lug impacting the fairing structure mounted to the fuselage or a portion of the lower closure rib. This damage could only have occurred if the vertical tail remained on the aircraft sometime after its failure.

No Fail-safe Capability. The vertical tail attachment system has no fail-safe capability. The aft lower spar has no out-of-plane capacity. The weaken of the spar web is due to the fact that there is no support gussets.

SUMMARY OF DISCREPANCIES

Design Decision Review to Change from Aluminum to Composite Tail in 1982

Edge Distance on Aft Lug

Full Scale Static QualificationTest vs. Actual Loads

Delamination Repair on Center LHS Lug on Subject Tail

Incident over Miami.

Possible Hidden Damage and Delamination from event over Miami on Subject Tail

Load Path Weakening of Center LHS Lug

Damage Tolerance Allowable of Composites

Jetwake Encounter Loads

Initial Failure of Aft Lug

Failsafe Capacity -- Missing Failsafe Gusset

Break Up of Tail in 12 seconds

Transverse Thermal Loads between Tail and Fuselage

Development of Bending Load on Lugs

Influence of Bending loads on Strength of Lug

Aft shear spar reaction system.

Aluminum lug is 3 times stronger than composite. What were magitude of original loads used to design Metallic Fin.

Unique failure characteristics of the lug.

Freezing Cause of more Delamination

What Caused Impact Damage on Lug -- Impact damage could be caused as lug exit clevis during tail sepreration.

TIMELINE

1. The original Vertical Tail Design was made of Aluminum, but redesigned to save weight.
2. Flaw in Fail Safe Design of Composite Tail in 1982, design eliminated critical failsafe fitting in lower tail structure at each lower spar attachment. Without the fitting the lower spar had no capability to react out-of-plane loads.
3. The Composite Design and very low e/D of lug forced a large reduction in load capacity compared to the metal design. Metal design is three times stronger.
4. Component Static Test showed that the tail attachment was weakest at each lug. This is very important because on the Vertical Tail the middle lug did not fail due to shear tear-out. The failure occurred above the lug through the line of fasteners attaching the lower closure rib to the skin. The failure was a combination of out-of-bending and tension. If the aft lug failed due to a load level on the Vertical Tail above Ultimate Load, the middle lug should have also failed in the same manner, shear tear-out of the lug.
5. In an incident with accident aircraft over Peru a large number of passengers was injured. Loads analysis show that Vertical Tail experienced a load equal to the ultimate load. Delamination in lug was found on other aircraft that had experience Vertical Tail Overload. It can be assumed that this incident caused a large delamination in aft lug of the accident aircraft.
6. Over the years, delamination in the lug increased from Vertical Tail loads, weathering and thermodynamic forces.
7. Shortly after take-off the in New York the accident aircraft experience a Jet Wake encounter, load approached the ultimate capacity of Vertical Tail (Lockheed Analysis). If an undamaged lug would not have been on this aircraft, the Vertical Tail would sustained the load without incident.
8. If the load from this encounter was above ultimate, the whole tail would have flew off the aircraft at the instant of encounter. The reason why it is known the vertical tail did not depart the aircraft at this time is because the middle attachment failure was not in the lug. The Jet Wake load was not large enough to break the tail completely off the aircraft. Tail departure did not occur for another ten seconds. The Jet Wake load was large enough to fail the aft lug but at a low enough level not to fail the middle lug. This is very important because the tail remained on the aircraft after the aft lug failed.
9. The Vertical Tail stayed on the aircraft for another ten seconds. During this time, loud noises, bangs and pops, and sounds of the pilot struggling and fighting the controls was a result of the aft tail destroying itself.
11. An estimate of the Jet Wake load can be established by knowing the relative strength of the aft lug to the middle lug. The load distribution on the vertical tail is 16 percent, 40 percent and 44 percent going to the fwd, middle and aft lugs, respectively. It can be shown that the failing load on the aft lug was less than 44 percent of the total tail load because if it was larger, the center lug would have failed similarly to the aft lug. 44 percent of the aft load plus 40 percent load of the middle lug would have been instantaneously redistributed to the middle. 44 plus 40 equals 84 percent, far more load than the middle lug can carry. The middle lug did not fail at this time because it would have failed identically as the aft and fwd lug. Both these lugs failed from shear tear-out of the pin. This failure mode was duplicated in lug tests and was no surprise. The pin laterally torn through the thickness of the lug. Failure at the middle lug was completely different but equally as violent. It resulted from the tail skin being ripped away from the lug housing and came about from the subsequent tearing apart destruction of the aft lower portion of the vertical tail that started at the aft left hand lug that propegated forward until the middle lug mount failed and the tail departed the aircraft.
12. What was unusual and very important was the failure mode at the middle lug. The failure was very different from the aft lug. It is know that the middle lug could not sustain a load level near 84 percent of the tail load. The middle lug strength was in the order of 66 percent.
13. Working backward and knowing the strength of the middle lug, 64 percent, the load on the aft lug would have had to be in the order of 64/84 or about 76 percent of the 44 percent. The aft lug failed when the load on the Vertical Tail reached no more than 76 percent of the Vertical Tail's capacity.
14. So the Jet Wake Load was approximately76 percent of the aft lug design load which is just what Lockheed predicted. This load should not have failed the aft lug, but it did. The combination of Jet Wake load and delamination from the Peru event caused the aft lug to fail. But it didn't bring down the aircraft. Nor did the pilot's actions while trying to gain control the aircraft, a feat which was impossible to do. From the moment the aft lug failed, the pilot was not in control of the aircraft. The aircraft's rudder position was being manipulated by the motion of the aft end of the Vertical Tail which was now in the process of being torn apart. This process would last only ten second.
15. This aircraft was designed without a failsafe feature. Failsafe means that if a major structural component fails, there is always an alternate loadpath which continues to carry the load.
16. Airbus's Failsafe Design was flawed, the failsafe fittings were not incorporated on the new composite design in 1982. Photos show that these fittings are on the metal tail but not on the composite tail. Therefore, when the aft lug failed at less than ultimate capacity, there was nothing there to stop the propagation of failure in the remaining structure.
17. Physical evident supports the theory that the tail did not depart aircraft instantaneously. Initially the Aft Lug Failed at a load less than its ultimate capacity. The Middle lug did not fail. Evident in photos show that the lower Closure Rib aft of the middle lug sustained significant damage. If the middle lug would have fail immediate after the aft lug failed, the damage to the closure structure would have not been so extensive and resemble the damage to the closure rib forward of the middle lug. The closure rib forward of the middle lug sustained limited damage the fwd lug failed immediately after the middle lug. There was no time for the closure structure to desentigate and rip itself apart as the aft closure rib did. This damage could not have occurred because the tail departure was instantaneous. Once the middle attachment structure fail the load transferred to the Fwd Lug and failure was immediate. Tail under structure damage was extensive and indicates a sequence of back and forth motion that required time to develop.
18. Additional Physical evidence in shown in a photo that shows damage inside the fracture of the RHS aft lug. This damage could only occur by the aft lug and vertical tail coming back onto itself. This damage could only occur if the vertical tail did not departure the aircraft instantaneously.
19. A Full Scale Static Test to failure should be performed to demonstrate the mode of failure of the lugs and lower Closure Rib.

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