PJ2 & BOAC; Vertical Stabiliser separation
The following is in answer to the posts of PJ2 and BOAC; I doubt that the question of the tail separation being airborne or on water entry will be resolved readily, however it is the domain of the BEA’s investigation, and hopefully in due course will be answered satisfactorily.
The current BEA position is the tail was attached at impact, and the airframe did not depressurise. This may be the case, however there is the matter of the last CMC message that needs to be explained, which otherwise would indicate a potential structural defect in flight. The BEA observes that the O2 masks were not deployed, however also notes that the covers are off some O2 units. (The RTLU state corresponding to normal conditions is surprising given the airdata failure...)
The airdata failure confronting the crew in turbulence is conducive to developing conditions of high loads, including on the tail, and may result in in flight failure. High yaw rates and angles have not only been a problem on AA587 for Airbus, but were the cause of the structural failure for Lauda 1, over Thailand, resulting from the PW4060 being in reverse in flight, resulting in failure of the VS, and of at least one of the horizontal stabilisers through excessive loads. The failure of the horizontal tail in that case caused a rapid nose down pitch change, and a subsequent failure of the wings in negative overload.
A feature of the Airbus tail is that the secondary load paths in the VS will fail from the same loads that cause a primary load path structural failure, due to the lever arm that is generated at the secondary yoke attachment. While this is interesting where the failure of the primary is due to a sub design load component failure, where the loads applied exceed the ultimate design loads, this is moot; any system will fail in that case.
With a commencement altitude of FL350, if the aircraft remained within the normal envelope until the loss of data, the data loss would occur above around FL300. If the operational envelope was exceeded, then any altitude above sea level would be possible. A possible point of structural failure is at the time where pressurization faults are noted. If a VS failure occurs around this time through one assumes oscillatory divergent torsion-bending, then the aircraft would be commencing the event around Vs-Mmo, approximately 180-280Kts roughly. The Thales pitot tube failure of a single channel would be recognised as a degradation of a PRIM & or SEC computer, and the control laws would be degraded. An identified failure of all pitot tubes would result in the control laws going to direct law. In all failure cases, rudder limiting degrades, which increases the potential for developing high loads on the VS.
If a failure of the VS occurs airborne, the aircraft yaw divergence will result in a very high deceleration rate, rough guess is in the range of 2-3g, developing to around 40-60kts sec-1 reduction. the yaw will result in roll, however induced roll lags yaw, but will be significant, and a rapid roll would develop, until IAS has washed off, at which case the aircraft will likely oscillate in all axis due to local flow effects. If the engines are thrown off the pylons, the yaw divergence will be reduced at that time. The wings and fuselage loads may be within design loads, but would be still fairly high.
The VS if separated in flight as a single component, has the potential to develop lift through auto-rotation about its center of mass, and will not just freefall. The likely offset of the VS from the remainder of the airframe impacts is able to be modeled, but is complicated by ocean drift of the VS post impact. Roughly, for a separation at 30-35000', the impacts CEP's would be in the range of ~3-5nm +/-2 (very rough estimate). The VS will drift at near the current rate, with little effect from wind. The engines if separated at high altitude and moderate to high forward speed, would have a ballistic trajectory, modified by drag variations from tumbling, but would have a CEP of around ~5-6.5nm +/-1, and a flight time of a ~46-52 secs, with a very high water entry speed. Low density components will remain close to the point of separation, with wind determining their final impact. Wind will not affect the engine CEP significantly. The fuselage and the VS time of flight is going to be ~4-8 minutes for the fuselage, and the VS around 10-15 minutes.
The hydrodynamic effects post water entries are dependent on the structural integrity of the component. The engines will sink near vertically post water entry, but the fuselage if retaining some integrity may well sled away from the entry point to its final resting place.
In respect to jet upset, in approximately 1997, as a result of USA427 and UA585, and other events, both Boeing and AI worked on a jet upset program. This program included a comprehensive video presentation, which included advice on application of rudder for roll control in high AoA conditions, due to the reduced effectiveness of ailerons at high AoA. This advice was and remains reasonable, however, the flight control characteristics of various jets means that there is a potential for excessive loads to be generated. The worst case for flight control application is a rapid control reversal, and where coupled with high angular rates, the resulting forces are extreme, where oscillatory inputs are made, the cyclical bending-torsion loads are basically divergent, as long as the control inputs are maintained. The VS and horizontal stabiliser are the most susceptible surfaces, as they are a short rigid structure. The ailerons of cycled, will result in large flexing of the wings, which will absorb some of the loads. On the B767, B747 and B777, a very small amplitude doublet pulse of the ailerons will feel very uncomfortable, but is fairly well damped. At high altitude damping is reduced, and for the B767, I know of one case where an accidental small amplitude doublet was made which caused a very poorly damped oscillation to occur, resulting in the autopilot tripping eventually.
The training programs supported by AI and Boeing were appropriate, but the application of this training is problematic. While aerobatic training may be assumed to assist in assessing what is “judicious” in relation to a control input, the majority of aerobatic aircraft do not have swept wing effects or even large dihedral effects, tending towards low lateral-directional stability. Military aerobatic aircraft which do have swept wings, have significantly different B/A inertial coupling effects to a commercial jet…
Until the submissions of AI were received by the NTSB on AA587, the potential to develop divergent responses in the tail from moderate control inputs below Va was at best poorly understood, or little known, or probably completely unknown. The generic advice to pilots to treat their aircraft kindly with control inputs doesn’t imply knowledge existed that the empirical effect on the structure is known, it just means it is wise to avoid unknowns where possible. To place that in perspective, an unlimited aerobatic aircraft with rapid cyclical control reversal of a rudder may well result in structural loads that exceed any testing or design. The timing of the control input and the aircraft response is important in determining whether divergent response occurs.
Page 197-198 of NTSB/AAR-99/01
1.18.9.2 Postaccident Activity
On August 16, 1995, the FAA disseminated Flight Standards Handbook Bulletin
for Air Transportation (HBAT) 95-10, entitled “Selected Events Training” (SET), to its principal operations inspectors (POI). The HBAT contains “…guidance and information on the approval and implementation of ‘Selected Events Training’ for operators training under 14 CFR Part 121, who use flight simulation devices as part of their flight training programs.”
The HBAT states that the SET is “voluntary flight training in hazardous inflight
situations which are not specifically identified in FAA regulations or directives.” Some of the examples of these selected events include false stall warning in rotation, excessive roll attitude (in excess of 90°), and high pitch attitude (in excess of 35°). The HBAT further states that the SET program was developed jointly by the FAA and the aviation industry in response to previously issued Safety Board recommendations addressing the need for unusual events and unusual attitude training for Part 121 and 135 air carrier pilots.
In 1996, USAir implemented SET as a required recurrent training element for all
of its pilots. The training program at USAir included simulator training in recovering from nose high, nose low, and inverted airplane attitudes. Also, many air carriers began implementing SET/Advanced Maneuvers Package programs patterned after the guidelines of the FAA’s HBAT 95-10 and United Airlines’ program, respectively.
On October 18, 1996, the Safety Board issued Safety Recommendation A-96-120. This recommendation asked the FAA to require 14 CFR Part 121 and 135 operators to provide training to flight crews in the recognition of and recovery from unusual attitudes and upset maneuvers, including upsets that occur while the aircraft is being controlled by automatic flight control systems and unusual attitudes that result from flight control malfunctions and uncommanded flight control surface movements.
In a January 16, 1997, letter to the Safety Board, the FAA stated that it was
considering an NPRM proposing to require that air carriers conduct training that will emphasize recognition, prevention, and recovery from aircraft attitudes that are normally not associated with air carrier flight operations. In its July 15, 1997, response, the Safety Board stated that it was not aware of any training programs that specifically addressed unusual attitudes that resulted from a control system failure or for which some flight controls would not be available, or would be counterproductive to, the recovery. (This recommendation is discussed more fully in section 1.18.11.5.)
In a November 2, 1998, letter to the FAA, the Safety Board listed those safety recommendations, including A-96-120, for which no recent action had been taken by the FAA. In a January 13, 1999, letter to the Safety Board’s Director of the Office of Aviation Safety, the FAA’s Associate Administrator for Regulation and Certification stated that “14 CFR part 121, subparts N and O (Training Program and Crewmember Qualifications, respectively), are being extensively rewritten. The rulemaking is expected to contain specific requirements addressing the NTSB’s concerns.” (See section 2.7 for the Safety Board’s review and evaluation of the FAA’s action in response to Safety Recommendation A-96-120 and the recommendation’s current classification.)
During 1997 and 1998, a working group composed of representatives of aircraft manufacturers, air carriers, pilot associations, training organizations, and government agencies (including the FAA) developed the Airplane Upset Recovery Training Aid. This publication and video program provided background information for air carrier pilots and managers on jet aerodynamics, stability, control, and upset recovery. The training aid also provided a model curriculum for classroom and flight simulator training in recovering from unusual flight attitudes. As of late 1998, the Airplane Upset Recovery Training Aid publication and video program were being distributed by two major air transport manufacturers (Boeing and Airbus) to their customers. This training aid, however, does not include simulator training in unusual attitudes resulting from flight control malfunctions and uncommanded flight control surface movements.
NTSB/AAR-99/01
Recommendation #6:
Require all 14 Code of Federal Regulations Part 121 air carrier operators of
the Boeing 737 to provide their flight crews with initial and recurrent flight
simulator training in the “Uncommanded Yaw or Roll” and “Jammed or
Restricted Rudder” procedures in Boeing’s 737 Operations Manual. The
training should demonstrate the inability to control the airplane at some
speeds and configurations by using the roll controls (the crossover airspeed
phenomenon) and include performance of both procedures in their entirety.
(A-99-25)
The NTSB's recommendations were sound, as was the intent of the training that followed, but there was missing information in respect to the certified design. The training (version 1) possibly didn't result in sufficient attention to the sensitivity of the rudder although this was mentioned in passing, and the potential for a structural failure was not fullt comprehended.
The figures are rough assessments only, but the Cd of the VS is going to approximate 1.0 in unsteady flight, about the same as a parachute... the engines Cd are dependent on the structure that may separate with them and the resultant stabilised attitude, (I get the engine stabilising in an oscillatory exhaust forward attitude after mid flight...) around Cd of 0.15-0.6. The airframe Cd is dependent on final attitude, but is going to be around 0.5-0.8 or thereabouts.
Whether the airframe is is one piece is moot; the control problem confronting the crew in adverse weather/IMC was sufficient to lose flight path control, and has at the least ended in impact of the aircraft at relatively low energy state with the water. If the aircraft remained intact, then a recovery without airdata is problematic from a stalled condition, and requires at the very least the use of other data such as AoA (ATT-FPV). Flight crew are not trained to recover using such data, and IIRC on the A330 you would need to change MCP modes to get the FPV displayed, whereas on the Boeing it can be selected separately. AoA also may be displayed full time on Boeing's by operator pin selection. (I gave an ex F16/Hawk IP a UA with unreliable AS recently, and the recovery was not achieved until such time as he was talked through using the ATT/FPV cues to detect he was in a stalled condition while indicating a Mmo overspeed. Once provided the cue, the state recognition and recovery was possible. The difficulty of properly identifying and rectifying such a failure in turbulent conditions cannot be overstated, IMHO)
"doubt is not a pleasant condition, but certainty is absurd"
Voltaire (1694-1778)