A320 pitch control
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A320 pitch control
In Direct Law in the A320: Do the elevators have suffiicient authority to over come a full up stab & a full power pitch up couple ?
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Tubby:
Based on BEA's Preliminary Report of the Perpignan Air NZ accident it would appear that the elevator authority was not adequate to overcome the fully displaced (nose up) stab. At least not for the power settings in question.
Based on BEA's Preliminary Report of the Perpignan Air NZ accident it would appear that the elevator authority was not adequate to overcome the fully displaced (nose up) stab. At least not for the power settings in question.
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I very much doubt any aircraft would have the elevator power to overcome a stabiliser trimmed to its full extent of travel up or down. Take a punt that its the main reason for the precautions taken in design to prevent runaway trim.
Editted to add certification standard for out of trim condition of transport category aircraft;
§ 25.255 Out-of-trim characteristics.
(a) From an initial condition with the airplane trimmed at cruise speeds up to VMO/MMO,the airplane must have satisfactory maneuvering stability and controllability with the degree of out-of-trim in both the airplane nose-up and nose-down directions, which results from the greater of—
(1) A three-second movement of the longitudinal trim system at its normal rate for the particular flight condition with no aerodynamic load (or an equivalent degree of trim for airplanes that do not have a power-operated trim system), except as limited by stops in the trim system, including those required by §25.655(b) for adjustable stabilizers; or
(2) The maximum mistrim that can be sustained by the autopilot while maintaining level flight in the high speed cruising condition.
(b) In the out-of-trim condition specified in paragraph (a) of this section, when the normal acceleration is varied from +1 g to the positive and negative values specified in paragraph (c) of this section—
(1) The stick force vs. g curve must have a positive slope at any speed up to and including VFC/MFC; and
(2) At speeds between VFC/MFCand VDF/MDFthe direction of the primary longitudinal control force may not reverse.
(c) Except as provided in paragraphs (d) and (e) of this section, compliance with the provisions of paragraph (a) of this section must be demonstrated in flight over the acceleration range—
(1) −1 g to +2.5 g; or
(2) 0 g to 2.0 g, and extrapolating by an acceptable method to −1 g and +2.5 g.
(d) If the procedure set forth in paragraph (c)(2) of this section is used to demonstrate compliance and marginal conditions exist during flight test with regard to reversal of primary longitudinal control force, flight tests must be accomplished from the normal acceleration at which a marginal condition is found to exist to the applicable limit specified in paragraph (b)(1) of this section.
(e) During flight tests required by paragraph (a) of this section, the limit maneuvering load factors prescribed in §§25.333(b) and 25.337, and the maneuvering load factors associated with probable inadvertent excursions beyond the boundaries of the buffet onset envelopes determined under §25.251(e), need not be exceeded. In addition, the entry speeds for flight test demonstrations at normal acceleration values less than 1 g must be limited to the extent necessary to accomplish a recovery without exceeding VDF/MDF.
(f) In the out-of-trim condition specified in paragraph (a) of this section, it must be possible from an overspeed condition at VDF/MDFto produce at least 1.5 g for recovery by applying not more than 125 pounds of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system. If the longitudinal trim is used to assist in producing the required load factor, it must be shown at VDF/MDFthat the longitudinal trim can be actuated in the airplane nose-up direction with the primary surface loaded to correspond to the least of the following airplane nose-up control forces:
(1) The maximum control forces expected in service as specified in §§25.301 and 25.397.
(2) The control force required to produce 1.5 g.
(3) The control force corresponding to buffeting or other phenomena of such intensity that it is a strong deterrent to further application of primary longitudinal control force.
Editted to add certification standard for out of trim condition of transport category aircraft;
§ 25.255 Out-of-trim characteristics.
(a) From an initial condition with the airplane trimmed at cruise speeds up to VMO/MMO,the airplane must have satisfactory maneuvering stability and controllability with the degree of out-of-trim in both the airplane nose-up and nose-down directions, which results from the greater of—
(1) A three-second movement of the longitudinal trim system at its normal rate for the particular flight condition with no aerodynamic load (or an equivalent degree of trim for airplanes that do not have a power-operated trim system), except as limited by stops in the trim system, including those required by §25.655(b) for adjustable stabilizers; or
(2) The maximum mistrim that can be sustained by the autopilot while maintaining level flight in the high speed cruising condition.
(b) In the out-of-trim condition specified in paragraph (a) of this section, when the normal acceleration is varied from +1 g to the positive and negative values specified in paragraph (c) of this section—
(1) The stick force vs. g curve must have a positive slope at any speed up to and including VFC/MFC; and
(2) At speeds between VFC/MFCand VDF/MDFthe direction of the primary longitudinal control force may not reverse.
(c) Except as provided in paragraphs (d) and (e) of this section, compliance with the provisions of paragraph (a) of this section must be demonstrated in flight over the acceleration range—
(1) −1 g to +2.5 g; or
(2) 0 g to 2.0 g, and extrapolating by an acceptable method to −1 g and +2.5 g.
(d) If the procedure set forth in paragraph (c)(2) of this section is used to demonstrate compliance and marginal conditions exist during flight test with regard to reversal of primary longitudinal control force, flight tests must be accomplished from the normal acceleration at which a marginal condition is found to exist to the applicable limit specified in paragraph (b)(1) of this section.
(e) During flight tests required by paragraph (a) of this section, the limit maneuvering load factors prescribed in §§25.333(b) and 25.337, and the maneuvering load factors associated with probable inadvertent excursions beyond the boundaries of the buffet onset envelopes determined under §25.251(e), need not be exceeded. In addition, the entry speeds for flight test demonstrations at normal acceleration values less than 1 g must be limited to the extent necessary to accomplish a recovery without exceeding VDF/MDF.
(f) In the out-of-trim condition specified in paragraph (a) of this section, it must be possible from an overspeed condition at VDF/MDFto produce at least 1.5 g for recovery by applying not more than 125 pounds of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system. If the longitudinal trim is used to assist in producing the required load factor, it must be shown at VDF/MDFthat the longitudinal trim can be actuated in the airplane nose-up direction with the primary surface loaded to correspond to the least of the following airplane nose-up control forces:
(1) The maximum control forces expected in service as specified in §§25.301 and 25.397.
(2) The control force required to produce 1.5 g.
(3) The control force corresponding to buffeting or other phenomena of such intensity that it is a strong deterrent to further application of primary longitudinal control force.
In my old steamplane, the B707, full elevator travel was equivalent to around four units of stab trim so the stab could easily overpower the elevators. There were about 12 units of stab trim available.
On the earlier models full elevator travel could load up the stab trim drive so much it stalled so you could end up in a situation whereby if the stab ran away you would run out of elevator to counteract it.
Do the FBW aircraft have a separate stab trim mechanism or is it all handled by the autoflight system?
Regards,
BH.
On the earlier models full elevator travel could load up the stab trim drive so much it stalled so you could end up in a situation whereby if the stab ran away you would run out of elevator to counteract it.
Do the FBW aircraft have a separate stab trim mechanism or is it all handled by the autoflight system?
Regards,
BH.
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And a supplementary question:
In the event of a jammed stab, a "less than full" flap setting is used for landing in airliners I have flown, to avoid degrading pitch authority. Is this due to a downwash effect, and what might be the result of using full flap?
In the event of a jammed stab, a "less than full" flap setting is used for landing in airliners I have flown, to avoid degrading pitch authority. Is this due to a downwash effect, and what might be the result of using full flap?
Do a Hover - it avoids G
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TyroPicard
I think you have answered your own question. The result of selecting more flap would be reduced pitch authority which is why you did not do it on the types you have flown.
As to why there would be reduced pitch authority it is again as you say caused by the change of downwash from the bigger flap angle.
If there was any change of trim with the final stage of flap then part of your elevator capability would be absorbed in countering that trim change (which ever direction was involved) leaving less for you to use to manoeuvre the aircraft. If there was no trim change with the final stage of flap then I would suggest there would be no loss of pitch authority.
But I suspect you had sussed all that and were looking for confirmation.
I think you have answered your own question. The result of selecting more flap would be reduced pitch authority which is why you did not do it on the types you have flown.
As to why there would be reduced pitch authority it is again as you say caused by the change of downwash from the bigger flap angle.
If there was any change of trim with the final stage of flap then part of your elevator capability would be absorbed in countering that trim change (which ever direction was involved) leaving less for you to use to manoeuvre the aircraft. If there was no trim change with the final stage of flap then I would suggest there would be no loss of pitch authority.
But I suspect you had sussed all that and were looking for confirmation.
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you're spot on as usual, John. I did it once in the sim (by mistake) and won't do that again!!
This was a jammed stab on an A320 for which the procedure is to land with Flap 3 (of 4). Unfortunately, we selected F Full and there was no elevator authority at all for the flare/roundout. Redfaces all round including the TRE who had not spotted it, but actually he did us a favour by letting us do it!! Not to be repeated!!
Cheers
mcdhu
This was a jammed stab on an A320 for which the procedure is to land with Flap 3 (of 4). Unfortunately, we selected F Full and there was no elevator authority at all for the flare/roundout. Redfaces all round including the TRE who had not spotted it, but actually he did us a favour by letting us do it!! Not to be repeated!!
Cheers
mcdhu
