Issue with function that stopped my program - c

Question

I have to do create a tree to register some passengers in it (from a plane flight) and then i will search them by their first letter.
My issue is that insert and print function work very well, but the search function do well for first time but when I want to do some other thing after in the main, it doesn't run the other function or things
(I tried to ask the user a letter, then use the function on research (works well here) but then for a second time to ask an other letter to user, and it prints "first letter of the person you are looking for" but i can't write the letter and it does not launch the function after this. so it stop the program there
int i;
char c;
typedef struct Passager
{
char nom[20];
char prenom[20];
int age;
int num_siege;
} Passager;
Passager liste_passagers[30]; //30 = nombre de passagers
typedef struct Arbre
{
Passager value;
struct Arbre *fils_gauche;
struct Arbre *fils_droit;
struct Arbre *racine;
} Arbre;
Arbre *const empty = NULL;
Arbre *passengers = empty; //passengers = arbre des passagers
/*--------------------------------------------*/
void liste_lettre_nom(Arbre *tree, char a)
{
Arbre *temp;
temp = tree;
if (!temp)
{
return;
}
else
{
if (a < temp->value.nom[0])
{
liste_lettre_nom(temp->fils_gauche, a);
}
if (a > temp->value.nom[0])
{
liste_lettre_nom(temp->fils_droit, a);
}
if (a == temp->value.nom[0])
{
printf("passager : \n");
print_passager(temp->value);
liste_lettre_nom(temp->fils_gauche, a);
liste_lettre_nom(temp->fils_droit, a);
}
}
}
/*--------------------------------------------*/
int main()
{
FILE* fichier = NULL;
fichier = fopen("/Users/Patoch/Desktop/Patoch /UNI/Informatique/info sem 2/Structure de données/Labo/TP3/Passager2.txt", "r");
if (fichier == NULL)
{ //test de la bonne ouverture du fichiers
printf("Impossible d'ouvrir le fichier Passagers.docx");
exit(EXIT_FAILURE);
}
for (i=0; i<(sizeof(liste_passagers)/sizeof(liste_passagers[0])); i++)
{
fscanf(fichier, "%s %s %d %d", liste_passagers[i].nom, liste_passagers[i].prenom, &liste_passagers[i].age, &liste_passagers[i].num_siege);
}
passengers = insertion(passengers, liste_passagers[1]);
passengers = insertion(passengers, liste_passagers[2]);
passengers = insertion(passengers, liste_passagers[0]);
passengers = insertion(passengers, liste_passagers[5]);
passengers = insertion(passengers, liste_passagers[26]);
print_arbre(passengers);
printf("-----------------------------\n");
printf("1ere lettre du nom de la personne recherchée: \n");
scanf("%c", &c);
liste_lettre_nom(passengers, c);
printf("-----------------------------\n");
printf("1ere lettre du nom de la personne recherchée: \n");
scanf(" %c", &c);
liste_lettre_nom(passengers, c);
destroy(passengers);
fclose(fichier);
return 0;
}