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Atlas Air 767 down/Texas

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Atlas Air 767 down/Texas

Old 9th Mar 2019, 18:32
  #401 (permalink)  
 
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Originally Posted by PerPurumTonantes
Based on that, looks to me like recovered part is in full nose down position. Am I missing something?
???
I believe I see ~ 2-3 inches of spindle above the Ball nut. The silver part above the hinge appears to have the same color and diameter as the lower part. With 1,7" per unit and 0,25 units = max ND, I end up with ~2,5 -3 units. Seems like pretty much spot on for that speed and configuration.
That observation does not rally point into the direction of spindle/jackscrew failure nor runaway trim.
On top of that in the footage shown it appears indeed as if there were some kind of recovery attempt going on.
The speed being somewhere around 240kts this becomes more and more strange.
So at the moment it looks like:
- Flight at correct trim (assumption based on position of Jackscrew)
- At 240kts at 6000ft. (assumption based on FR24 data). (Probably Some 60-70kts above stall speed).
- Flight in IMC (Statement early in the process)
- Coming out of the clouds at high speed (>>250kts)
- Flightpath angle in the part visible in the footage initially ~40 - 50°ND, apparently possibly reducing to ~30 - 40°ND.
- Initial altitude in the video difficult to judge but in a wild guess somewhere in the range 2000 - 3000ft.

What plausible scenarios does that leave?
=> Stall as initiating event rather unlikely (too much speed margin, flight attitude pretty stable and straight, albeit hefty ND with rather lowish AoA in the footage)
=> Out of Trim situation somewhat unlikely if the position of the jackscrew represents the state it was in during the terminal phase of the flight.
=> failed jackscrew/ball nut can't be completely ruled out but is not really support by the picture of the jackscrew nor the apparent started recovery => rather unlikely.

At the moment I'm somewhat at a loss what could have reasonably caused that loss of Control.

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Old 9th Mar 2019, 18:42
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If there was an apparent recovery with a jackscrew failure, might the latter step have been from commanding max thrust vs spool up time?
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Old 9th Mar 2019, 19:01
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Originally Posted by henra
???
What plausible scenarios does that leave?
=> Stall as initiating event rather unlikely (too much speed margin, flight attitude pretty stable and straight, albeit hefty ND with rather lowish AoA in the footage)
=> Out of Trim situation somewhat unlikely if the position of the jackscrew represents the state it was in during the terminal phase of the flight.
=> failed jackscrew/ball nut can't be completely ruled out but is not really support by the picture of the jackscrew nor the apparent started recovery => rather unlikely.

At the moment I'm somewhat at a loss what could have reasonably caused that loss of Control.
Some instrument failure causing false and highly unexpected aircraft attitude data beeing displayed for the pilots making them push hard on the yoke, aka CRJ200 crash in Sweden....
Failure of the elevator control/servo system putting the aircraft in a steep dive, aka 737 rudder hardover......
Mechanical breakage/failure of the HS screw jack system including its attachment structure making the HS freefloating, somewhat similar to Alaska MD-80......
........
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Old 9th Mar 2019, 19:40
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Remember that the jackscrew moves the stab, not the elevator.
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Old 9th Mar 2019, 19:57
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Originally Posted by SteinarN
Some instrument failure causing false and highly unexpected aircraft attitude data beeing displayed for the pilots making them push hard on the yoke, aka CRJ200 crash in Sweden....
........
Crossing the waypoints at 7000' or 6000' depending on the transition they were in VMC conditions.

KIAH 232053Z 31010KT 10SM FEW045 BKN210 OVC250 22/08 A2989 RMK AO2 SLP121 T02220083 57010=
KIAH 231953Z 31011KT 10SM SCT039 BKN090 BKN250 23/13 A2990 RMK AO2 SLP123 T02330128=
KIAH 231853Z 32011G19KT 10SM SCT035 BKN080 BKN250 21/12 A2991 RMK AO2 SLP129 T02110117=
KIAH 231802Z 32015G24KT 10SM FEW035 SCT060 BKN080 BKN250 22/12 A2992 RMK AO2 T02220117=
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Old 9th Mar 2019, 19:57
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Concur with your observations, henra.

B2N2, they were still ~35nm away from HOU, so just keeping in mind, drawing no conclusions, Tim Vasquez's work near the start of the thread regarding cloud & turbulence in the aircraft's path and altitude approaching the Trinity Bay.

HOU ATC was apprising them of weather and possible detours. I'm sure the NTSB is examining what route all other aircraft in that area took to find out what their experience was. Same thing was done during the AF447 investigation.

Last edited by PJ2; 9th Mar 2019 at 21:20.
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Old 9th Mar 2019, 20:20
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Originally Posted by B2N2
Crossing the waypoints at 7000' or 6000' depending on the transition they were in VMC conditions.
Thanks.
In that case, less possible causes for pilot error at least.
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Old 9th Mar 2019, 20:25
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Although not a factor in this crash, always remember ceiling heights reported from surface based weather observations, i.e. METARs, are AGL, (Above Ground Level), NOT above MSL (mean sea level). Forecast cloud heights in Area Weather Forecasts are ASL, (above sea level), unless noted.

Also remember wind direction in METARs are "True", NOT magnetic. Winds on the ATIS and winds reported by Tower are, or course , magnetic. There are exceptions to the reporting standard, usually reserved at sites located in extreme latitudes.

Curiously, the prevailing visibility as reported in most North American METARs are in STATUTE MILES. (a unit of linear measure equal to 5,280 feet, or 1,760 yards (approximately 1.609 kilometers, or 0 .86 of a nautical mile).

Last edited by evansb; 9th Mar 2019 at 21:03.
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Old 9th Mar 2019, 21:02
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Originally Posted by SteinarN
Failure of the elevator control/servo system putting the aircraft in a steep dive, aka 737 rudder hardover......
Mechanical breakage/failure of the HS screw jack system including its attachment structure making the HS freefloating, somewhat similar to Alaska MD-80......
........
These would be my 2 thoughts regarding the cause of this accident.
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Old 9th Mar 2019, 21:07
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Apologies if this has already been answered, was the aircraft normal or inverted? I can't tell from the videos I've viewed.

Loss of control (for whatever reason)... followed by inversion, outside loop etc., it might give a different dimension/explanation as to why control surfaces were in unlikely positions.
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Old 9th Mar 2019, 21:12
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Do you further speculate the failure of elevator control was due to turbulence?
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Old 9th Mar 2019, 21:25
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Originally Posted by evansb
Do you further speculate the failure of elevator control was due to turbulence?
Don't know if the question was for me, but I would not rule out the turbulence possibility. Early posts showed a possibility of some severe turbulence.

However, I would lean more to a basic structural failure by metal fatigue or corrosion.

No scientific reasons, just my thoughts.
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Old 9th Mar 2019, 22:52
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Originally Posted by TheHardWay
The elevator is hinged at the leading edge, so the servo tab mounted at the trailing edge has the mechanical advantage or leverage to force the elevator to move up or down.
The stabilizer is hinged at the trailing edge with the actuator at the leading edge, so the position of the elevator mounted at the trailing edge of the stabilizer is going to have little or no effect on the position of a free floating stabilizer.
The stabilizer hinge line is slightly forward of the point where its trailing edge meets the fuselage. Due to stabilizer sweep angle, its hinge line is close to the center of lift such that horizontal tail angle-of-attack does not have a significant impact on the load applied to the jack screw. Due to the stabilizer sweep angle and the location of the elevators, deflection of the elevators generates a rotational torque on the stabilizer in the opposite direction. Note that all for the elevators are located aft of the horizontal stabilizer hinge line. The outboard end of the elevator is significantly aft of the stabilizer hinge line.
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Old 9th Mar 2019, 23:31
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Originally Posted by Raffles S.A.
Flaps would be 0 and the trim for 240KIAS about 1 or 2 units nose up
You sound very convinced and I never flew 76, but 1 or 2 units nose up sounds not much to me and more like nose down trim.
While low speed and flaps up, in any aircraft, would require quite some nose up trim.
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Old 9th Mar 2019, 23:43
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Originally Posted by FCeng84
The stabilizer hinge line is slightly forward of the point where its trailing edge meets the fuselage. Due to stabilizer sweep angle, its hinge line is close to the center of lift such that horizontal tail angle-of-attack does not have a significant impact on the load applied to the jack screw. Due to the stabilizer sweep angle and the location of the elevators, deflection of the elevators generates a rotational torque on the stabilizer in the opposite direction. Note that all for the elevators are located aft of the horizontal stabilizer hinge line. The outboard end of the elevator is significantly aft of the stabilizer hinge line.
I used to fly skydivers in an old Cessna 182 that used stab trim. The only "locking" device for the jackscrew was the ball and spring detents on the trim wheel. The pivot point was at the aft spat well behind the center of lift. When I would youthfully speed up my decent by applying down elevator without trimming to achieve yellow line, the rotational force exerted by the elevators was enough to overcome the ball detent and I would get a sudden roll back of the trim wheel resulting in an unexpected 2.5-3 g pull up. With the 767 stab hinge closer to the center of lift, I would also expect control reversal caused by the elevators acting as servo tabs if the stab trim was mechanically disconnected.
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Old 10th Mar 2019, 00:58
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This ‘thread’ officially derailed pages ago. You all will be unceremoniously stunned when the findings are far, far simpler than a stripped stabilizer jackscrew.

Last edited by FIRESYSOK; 10th Mar 2019 at 01:19.
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Old 10th Mar 2019, 01:39
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Originally Posted by FCeng84
The stabilizer hinge line is slightly forward of the point where its trailing edge meets the fuselage. Due to stabilizer sweep angle, its hinge line is close to the center of lift such that horizontal tail angle-of-attack does not have a significant impact on the load applied to the jack screw. Due to the stabilizer sweep angle and the location of the elevators, deflection of the elevators generates a rotational torque on the stabilizer in the opposite direction. Note that all for the elevators are located aft of the horizontal stabilizer hinge line. The outboard end of the elevator is significantly aft of the stabilizer hinge line.
Thanks for taking the time to explain, FCeng84. What you say does make sense.
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Old 10th Mar 2019, 03:16
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Forecast cloud heights in Area Weather Forecasts are ASL
The altitude of Houston is like 100± feet ASL.
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Old 10th Mar 2019, 03:25
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Originally Posted by Ivor_Bigunn
If the Statement:

"At full nose down (0.25 units), the top of the moving part (ballnut) is within an inch of the upper stop."

is correct, then the photographed screwjack is 80 to 90% Nose Down.

(look at the length of screwjack extending below the ballnut to the lower stop).

Whether this was how it was recovered, or if all the threads were stripped, would reduce the significance of the photo.
Let me add a little more information from the manual here:

0.00 units: ballnut is 0.5 inches from the top = stab leading edge up +2 degrees (full nose down trim with Alternate Trim?)
0.25 units: ballnut is 0.92 inches from the top = stab leading edge up +1.75 degrees. This is the nose down electric trim limit for the column switches with flaps not retracted
0.50 units: ballnut is 3 inches (?) from the top. This is the nose down electric trim limit for the column switches with the flaps retracted.
2.00 units (neutral): ballnut is 3.84 inches from the top.
12.8 units: ballnut is 22.43 inches from the top. This is the electric nose up limit for column switches. i..e 12.8 units = stab leading edge down minus 11 degrees
14.2 units: ballnut 24.96 inches from the top = stab leading edge down minus 12.5 degrees (full nose up trim for Alternate Trim)

I can't explain the irregularity in the manual with regards to inches versus units. Using up/down column switch limit values, 0.25 units is 0.92 inches and for every unit thereafter, there is an additional (approx) 1.79 inches. However, the 0.5 unit/3 inches value seems to be an aberration. It should be more like 1.3 inches. This 3 inch value is repeated throughout the manual.

For info only.
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Old 10th Mar 2019, 05:26
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Great information, thanks NSEU
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