Abstract:
A method for ensuring the safety of an aircraft flying horizontally at low speed includes an operation in which, when the flaps of the aircraft are disposed in a maximum extended position and are blown on by airscrews, the flaps are at least partially retracted, automatically, based on whether the thrust of the engines is at least equal to a predetermined lift value.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for ensuring the safety of an aircraft flying horizontally at low speed, for example no more than slightly greater than the angle of attack protection speed, said aircraft comprising:
         a fixed wing supporting trailing-edge high-lift flaps and engines provided with airscrews, the latter blowing on said wings and said flaps; and   a stabilizing horizontal tail group, tilt-adjustable.       

     BACKGROUND OF THE INVENTION 
     It is known that, in such a phase of horizontal flight at low speed, the lift imparted on the aircraft by its wings and said flaps, then in the extended position, needs to be high, such that this high lift, reinforced by the blowing on the wings and the extended flaps by the airscrews of the engines and aided by the thrust of said engines, generates a high pitch-down moment relative to the center of gravity of the aircraft. 
     To balance the aircraft, the pilot deflects said adjustable horizontal tail group to nose up, so that it generates, relative to the center of gravity of the aircraft, a nose-up moment to counteract said high nose-down moment. This balancing nose-up moment must therefore be high, such that the local impact on said adjustable horizontal tail group is strongly negative. 
     The result is that if, during such a phase of flying horizontally at low speed, the pilot orders a dive, for example to abruptly avoid another aircraft by flying under it to avoid a collision or to rapidly regain speed, the local impact on said adjustable horizontal tail group risks exceeding the stalling effect of the latter, such that, at the moment when the pilot wants to stop the dive maneuver, the adjustable horizontal tail group may have lost its effectiveness: the aircraft will therefore be incapable of priming a flare and this could result in the loss of the aircraft. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to remedy this drawback. 
     To this end, according to the invention, the method for ensuring the safety of an aircraft flying horizontally at low speed, said aircraft comprising:
         a fixed wing supporting engines provided with airscrews, and trailing-edge high-lift flaps, that can be extended and retracted; and   a stabilizing horizontal tail group, tilt-adjustable, provided with elevators,
 
said flaps then being in the maximum extended position and being blown on by said airscrews,
 
is noteworthy in that said flaps are at least partially retracted when the thrust of said engines is at least equal to a predetermined high value.
       

     Thus, thanks to such a retraction, the blowing effect on said flaps by the airscrews is reduced and said nose-down moment is therefore reduced accordingly. Because of this, the adjustable horizontal tail group must supply a nose-up moment of lower intensity, meaning that the local impact on said adjustable horizontal tail group is less negative and that the latter will be effective at the moment when the flare will be invoked. 
     According to a first embodiment, which can be qualified as “preventive”, the method according to the present invention is such that said predetermined lift value corresponds to the thrust of the engines needed for take-off. Such a value is generally known by the name TOGA (Take Off-Go Around). Thus, during a possible nose-down according to the phase of flying horizontally at low speed, the harmful situation in which the maximum-extended flaps are blown on by the airscrews of the engines running at the highest speed is avoided. 
     In a second embodiment, more dynamic than the previous one, said predetermined high value corresponds to a first threshold less than the thrust TOGA of the engines needed for the aircraft to take off, but, on the other hand, the at least partial retraction of the flaps is subject to the additional condition that a nose-down deflection command greater than a second threshold signifying nose-down is addressed to said elevators. Said first threshold may be at least approximately equal to 60% of the thrust TOGA of the engines needed for take-off, while said second threshold corresponds at least approximately to 60% of the total displacement, in the nose-down direction, of the control column available to the pilot to control said elevators. 
     In order to avoid unwanted triggerings near to the ground, the method according to the present invention is applied only when the altitude of the aircraft is greater than a third threshold which, for example, is at least approximately equal to 30 meters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing clearly show how the invention can be implemented. In these figures, identical references denote similar items. 
         FIG. 1  is a plan view of an airplane to which the present invention can be applied, the wing flaps being shown in the retracted position. 
         FIG. 2  is a side view, in horizontal flight and at low speed, of the airplane of  FIG. 1 , said flaps being shown diagrammatically in the extended position. 
         FIGS. 3 and 4  illustrate two variants of embodiment of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The transport airplane  1 , shown diagrammatically in  FIGS. 1 and 2 , has a longitudinal axis X-X and comprises two symmetrical wings  2 , each supporting two engines  3  with airscrews  4 . The trailing edges of the wings  2  are provided with controllable moving high-lift flaps  5 , that can assume a retracted position (see  FIG. 1 ) and at least one extended position (see  FIG. 2 ). The transition from the retracted position to an extended position is illustrated in  FIG. 2  by the double arrow  6 . 
     In its rear part, the airplane  1  is provided with a vertical tail group  7  supporting, at its top end, a horizontal tail group  8 , tilt-adjustable as is illustrated by the double arrow  9  of  FIG. 2 . The trailing edge of the adjustable horizontal tail group  8  comprises elevators  10 , hinged on the latter. 
     As is diagrammatically illustrated by  FIGS. 3 and 4 , the flaps  5  are controlled, for extension and retraction, by at least one flap control computer HLCC, which receives commands from the flap control lever  11 , available to the pilot of the airplane  1 . 
     In horizontal flight at low speed (see  FIG. 2 ), the flaps  5  are extended to enable the wings  2  to impart on the airplane  1  a high lift L. This high lift L, augmented by blowing on the wings  2  and the extended flaps  5  by the wind W generated by the airscrews  4  and aided by the thrust T of the engines, exerts, on the airplane  1 , a nose-down moment relative to the center of gravity CG of the latter. To balance this nose-down moment, it is necessary to deflect the adjustable horizontal tail group  8  to nose-up so that it generates a negative lift D generating an opposing nose-up moment relative to said center of gravity. 
     In this case, as is illustrated in  FIG. 2 , said adjustable horizontal tail group  8  is tilted to nose-up by an angle iH relative to the axis X-X and the elevators  10  are advantageously in aerodynamic extension of said adjustable horizontal tail group  8 . The result is that, on said adjustable horizontal tail group  8 , the local impact is strongly negative. 
     Therefore, if the pilot orders an abrupt nose-down, by imposing on the elevators  10  a nose-down deflection δqp via the control column  12  available to him (see  FIG. 4 ), the local impact on said adjustable horizontal tail group can exceed the stalling effect. Subsequently, at the moment when the pilot wants to level the airplane  1  by imposing on the elevators  10  a nose-up deflection δqc via the control column  12 , the airplane will be incapable of performing the necessary flare. 
     The two variants of embodiment of the method according to the present invention, illustrated diagrammatically and respectively in  FIGS. 3 and 4 , make it possible to avoid this situation. In these  FIGS. 3 and 4 , an HLCC computer is shown which is capable of controlling the extension and the retraction of the flaps  5 , the lever  11  for deliberately controlling the flaps  5  via the HLCC computer, a sensor  13  for delivering to the latter a signal FP representative of the fact that the flaps  5  are in the maximum extended position and a radio-altimetric probe  14  sending to the HLCC computer the altitude ZRA of the aircraft  1 . 
     Furthermore, in the variant of  FIG. 3 , the HLCC computer receives, from the HLCC gas levers  15  available to the pilot and controlling the speed of the engines  3 , a TOGA signal, indicating that this speed is the maximum speed. In the variant of  FIG. 4 , instead of being linked to the gas levers  15 , the HLCC computer is linked, on the one hand, to an on-board computer  16 , for example an FADEC (Full Authority Digital Engine Control) computer capable of sending it a measurement of the current thrust T and, on the other hand, to the control column  12  transmitting to said HLCC computer at least the nose-down commands δqp that it addresses to the elevators  10 . 
     The logic systems of the two variants of embodiment of the method of  FIGS. 3 and 4  are implemented in the respective HLCC computer and, to this end:
         the HLCC computer of  FIG. 3  contains an altitude threshold HS, for example at least approximately equal to 30 meters, below which the automatic retractions of the flaps  5  are disabled, in order to avoid movements of the latter not controlled by the pilot close to the ground; and   the HLCC computer of  FIG. 4  incorporates, in addition to the altitude threshold HS, the threshold FNS for the thrust T exerted by the engines  3  and a nose-down command threshold δqps for the elevators  10 .       

     The thrust threshold FNS can correspond at least approximately to 60% of the maximum thrust of said engines  3 , while the threshold δqps can correspond at least approximately to 60% of the maximum nose-down travel of the control column  12 . 
     In the variant of embodiment of  FIG. 3 , the HLCC computer controls an at least partial retraction of the flaps  5 , when the following three conditions are satisfied:
         the measured altitude ZRA of the airplane  1  is greater than said threshold HS,   the flaps  5  are in the maximum extended position, which is indicated by the signal FP, and   the gas levers are in the TOGA position.       

     Thus, in this variant of embodiment, the airplane  1  cannot be in a critical position for which, at the same time, the flaps  5  would be in the maximum extended position and the engines  3  would be exerting their maximum thrust. In effect, thanks to the present invention there then occurs an at least partial retraction of the flaps, such that the value of the nose-up angle iH of the adjustable horizontal tail group  8  can be smaller, which improves the stalling margin of the latter at the time of a flare following a nose-down. 
     In the variant of embodiment of  FIG. 4 , the HLCC computer controls an at least partial retraction of the flaps  5 , when the following four conditions are satisfied:
         the measured altitude ZRA of the airplane  1  is greater than said threshold HS,   the flaps  5  are in the maximum extended position,   the measured thrust T, exerted by the engines  3 , is greater than the threshold FNS, and   the nose-down command δpq generated by the control column  12  is greater than the threshold δpqs.       

     Here, too, the stalling margin of the adjustable horizontal tail group  8  is improved at the time of a flare following a nose-down, starting from a phase of horizontal flight at low speed.