Abstract:
A structural element of an aircraft includes a hollow part. An indicator provides a visual indication, for example by moving, if ice forms in a given region of the hollow part.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a structural element of an aircraft having a hollow part and an indicator of formation of ice in such hollow part. 
     The outer surfaces of an aircraft (whether, for example, on the fuselage or the wing group) generally are formed by a relatively thin skin, which therefore is stiffened in standard manner by structural elements, such as frames and stringers, for example. 
     In order to play their mechanical part correctly without excessive contribution of weight, these structural elements frequently have shapes that define a hollow part within the structural element, such as the commonly used omega stringers. 
     Such hollow parts then may form an inopportune container for liquids, typically condensation and runoff water, in particular for structural elements located at a low point of the part (for example of the fuselage) concerned. 
     This presence of water proves to be problematic when the temperatures encountered lead to a changeover to the solid state (in practice the formation of ice) which, because of the changes in volume associated with the change of state inside the fixed volume of the structural element, may be accompanied by mechanical stresses detrimental to the mechanical strength of the structural element, by the way, however, with no damage being visible from the outside. 
     This risk is particularly serious in the case of structural elements made of composite material in which the frost may lead to a separation of the element or to a delamination. 
     A visual signal of the presence of frost already has been proposed in document FR 2 680 871. This document, however, applies to the general detection of frost on the airplane and therefore in no way considers the possible consequences of frost on the structural elements. 
     SUMMARY OF THE INVENTION 
     In this context, the invention proposes a structural element of an aircraft comprising a hollow part, characterized by means able to provide a visual indication in case of formation of ice in a given region of the hollow part. 
     In this way it is possible, for example during a routine inspection, to easily detect the parts having sustained a flood and in which there is a risk of damage to the mechanical qualities of the structural element. 
     These means able to provide a visual indication comprise, for example, an indicator installed in the structural element in a first position and able to assume a second position under the effect of forces exerted by ice in formation in the said given region. The visual indication thus is provided in particular by the displacement of the indicator because of the increase in volume associated with the change in state, which is an especially simple manner of detecting the formation of ice. 
     The indicator further may comprise anti-return means able to limit a movement of the indicator from the second position to the first position, so that the visual indication may relate, if need be, to a presence of water at a time prior to that of the inspection. 
     In practice the indicator is installed, for example, sliding in an aperture of the structural element, along with possibly at least one elastic tab bearing at least one boss such that the diameter of the indicator at the boss is greater than the diameter of the aperture. 
     In this context, the boss is located, for example, inside the structural element (that is to say generally in the hollow part) in the first position and outside the structural element in the second position. 
     In this way the functionalities contemplated above are achieved in a manner easy to implement. 
     The invention also proposes an indicator of the presence of water such as set forth above as well as an aircraft with a structural element such as mentioned above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will become more apparent in the light of the description that follows, presented with reference to the attached drawings in which: 
         FIG. 1  shows a structural element (here a stringer) able to be equipped with an indicator in accordance with the teachings of this invention; 
         FIG. 2  shows a section according to line II-II of  FIG. 1 ; 
         FIG. 3  shows the stringer of  FIG. 1  in which the indicator in accordance with the teachings of the invention is installed; 
         FIG. 4  is a sectional view of the indicator of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There has been shown on  FIGS. 1 and 2  an omega stringer  10  fastened (for example by bonding) onto a skin  20  with a view to reinforcement of the latter. 
     The stringer extends according to a general direction perpendicular to the section plane of  FIG. 2 , on which the omega shape is clearly visible. 
     Stringer  10  thus has tabs  11 ,  12  essentially flat, parallel and bonded to skin  20 . These tabs  11 ,  12  are located on both sides of a central region of stringer  10  made up of a base-plate  13  essentially parallel to skin  20  but at a distance therefrom and which continues on both sides respectively in a side-plate  14 ,  15  that extends obliquely in relation to skin  20  as far as the corresponding tab  11 ,  12 . 
     Stringer  10  is produced, for example, from a part made of composite material. 
     Side-plates  14 ,  15  of stringer  10  have, for example at regular intervals, drainage holes  16 ,  17  (one of which for each side-plate is visible on  FIGS. 1 and 2 ) with diameter D 1  here equal to 10 mm, in order in particular to drain off liquids (mainly water) likely to end up (for example through runoff or through condensation) inside hollow space  30  formed between stringer  10  and skin  20  (precisely between skin  20  and side-plates  14 ,  15  and base-plate  13  of stringer  10 ). 
     Despite the presence of drainage holes  16 ,  17 , it is possible, however, that liquids will accumulate in hollow space  30  defined between stringer  10  and skin  20 , for example in the case in which drainage holes  16 ,  17  are clogged, or also in regions of the hollow space in which holes  16 ,  17  do not allow a proper drainage of the liquids (such as, for example, at a stringer end located below drainage holes  16 ,  17 ). 
     For this reason there is provided (particularly in the sensitive zones that have just been mentioned) an ice-formation indicator  40  able to signal by means of a visual indication the formation of ice in a given region of hollow space  30  defined by skin  20  and stringer  10 . 
       FIG. 3  shows such an indicator  40  installed sliding in an aperture (here with diameter D 2  equal to 6 mm) of base-plate  13  of stringer  10  and provided with anti-return elements (here anti-return pawls  45 ) so as to remain in an “extended” position (visible in dotted lines on  FIG. 3 ) subsequent to a movement of indicator  40  toward the outside because of the formation of ice in the hollow part. 
     As visible on  FIG. 4 , indicator  40  is made up, for example, of a general cylindrical part  41  with outside diameter slightly smaller than diameter D 2  of apertures  18  formed in the base-plate so as to be able to slide as already indicated in these apertures  18 . 
     The indicator also comprises a head  42 , for example set onto cylindrical part  41 . Head  42  has a diameter greater than that of aforementioned aperture  18  so as to stop the sliding of indicator  40  (downward on the Figures). 
     Indicator  40  also comprises elastic tabs  43  located in the continuation of cylindrical part  41  at rest and each ending with a boss  44 , so as to form at bosses  44  located on elastic tabs  43  a diameter greater than diameter D 2  of aforementioned aperture  18 . 
     Indicators  40  (several indicators being able to be disposed each in an aperture  18  of a sensitive zone mentioned above) are inserted into their aperture  18  by retracting bosses  44  toward the inside of cylindrical part  41  by action on elastic tabs  43  (for example by means of a suitable tool) until head  42  of the indicator comes into contact with base-plate  13  of stringer  10 , where indicator  40  has a position referred to as “retracted,” in which bosses  44  stop the sliding of indicator  40  (upward on the Figures). 
     As long as the space defined between skin  20  and stringer  10  is not filled with ice (and the presence of water or ice therefore is not detrimental to the mechanical qualities of the structural element), the indicator remains in this retracted position by virtue of the relative stiffness of elastic tabs  43  and the presence of bosses  44  that immobilize indicator  40  in relation to base-plate  13 . 
     On the other hand, in case of formation and of significant presence of ice in the space defined by stringer  10  and skin  20 , here if the ice level is greater than level h visible on  FIG. 3 , the indicator is pushed toward the outside of space  30 , in particular during the formation of ice because of the increase in volume associated with this change of state, so that elastic tabs  43  bend inward until the bosses no longer prevent the rising of indicator  40  toward the outside of the space (upward on  FIG. 3 ) and indicator  40  thus reaches the position shown in a dotted line on  FIG. 3 . Bosses  44  preferably have a shape adapted for facilitating this rising of indicator  40 . 
     Once indicator  40  is moved into extended position, bosses  44  prevent return into retracted position even if the ice melts and the water is drained off (for example by evaporation) from space  30 , which makes it possible to detect the prior presence of ice (and the risk of weakening of the associated structural element) even during a later inspection when the ice no longer is present. 
     In this way, during a routine inspection it is easy to detect the zones of the structure for which icing has taken place and to conduct in these zones a more precise determination of the actual presence of a separation or a delamination, for example with the aid of non-destructive monitoring means. 
     The example described above is only one possible embodiment of the invention. It is conceivable, for example, to detect the formation of ice in the hollow part by electric or electronic means (for example by tripping a switch as a result of movement of the indicator described above or by means of sensors of the forces exerted by the ice in formation) and to generate a visual indication of this detection for example by means of a warning light connected to these electric or electronic detection means.