new bool:pilot[500];

public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid)
{
    if( (GetPlayerVehicleID(issuerid) == 476) && (pilot[playerid] == true) )
    {
        new Float:health, Float:armour;
        GetPlayerHealth(playerid, health);
        GetPlayerArmour(playerid, armour);
        SetPlayerArmour(playerid, 0);
        SetPlayerHealth( playerid, (health + armour - 100) );
        // This all means that if you are a pilot and you get hit by another
        //  player flying a Rustler, you will be greatly damaged: if you have
        //  no armour you will die instantly, and if you do have armour you will
        //  have it removed, and your health may be reduced depending on your
        //  previous armour value.
    }
    return 1;
}

new bool:pilot[MAX_PLAYERS]; // How a bool is setup, automatically false.
public OnPlayerEnterVehicle(playerid,vehicleid,ispassenger)
{
    if(ispassenger == 0) // If they are not a passenger.
    {
        if(GetVehicleModel(playerid) == 476) // If they enter a Rustler
        {
            pilot[playerid] = true; // Then they are a pilot
            return 1;
        }
    }
    return 1;
}
public OnPlayerExitVehicle(playerid,vehicleid)
{
    // If they exit any vehicle
    pilot[playerid] = false; // Then they are no longer a pilot
    return 1;
}
public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid)
{
    // You added a lot of uneccesary paranthesis here. I fixed it.
    if(GetPlayerVehicleModel(issuerid) == 476 && pilot[playerid] == true) // If they are in a Rustler
    {
        // Floats for health and armour are not needed since you are just killing the player.
        SetPlayerArmour(playerid, 0);
        SetPlayerHealth(playerid, 0);
    }
    return 1;
}