function read_apt_dat()

  local ii = 0
  local jj = 0
  local kk = 0
  local line_trim = ""
  local line_char = ""
  local line_lenght = 0
  local apt_line = ""
  local apt_word = {}
  local mm = 0

  local apt_first = 0

  apt_data_num = 0
  apt_data = {}

  local file_navdata2 = io.open(current_dir .. "B738X_apt.dat", "r")
  if file_navdata2 ~= nil then
    apt_line = file_navdata2:read()
    if string.sub(apt_line, 1, 7) == "log.txt" then
      apt_line = file_navdata2:read()
    end
    while apt_line do
      if apt_first ~= 0 then
        ii = 0
        jj = 0
        line_trim = ""
        line_char = ""
        line_lenght = string.len(apt_line)
        if line_lenght > 0 then
          for kk = 1, line_lenght do
            line_char = string.sub(apt_line, kk, kk)
            if line_char == " " then
              if ii == 1 then
                jj = jj + 1
                apt_word[jj] = line_trim
                ii = 0
                line_trim = ""
              end
            else
              line_trim = line_trim .. line_char
              ii = 1
            end
          end
          if string.len(line_trim) > 0 then
            jj = jj + 1
            apt_word[jj] = line_trim
          end

          if jj > 0 then
            if string.len(apt_word[jj]) > 1 and string.byte(apt_word[jj], -1) == 13 then  -- CR
              apt_word[jj] = string.sub(apt_word[jj], 1, -2)
            end
          end


          if jj == 6 then
            apt_data_num = apt_data_num + 1
            apt_data[apt_data_num] = {}
            apt_data[apt_data_num][1] = apt_word[1]   -- ICAO
            apt_data[apt_data_num][2] = tonumber(apt_word[2])   -- lat
            apt_data[apt_data_num][3] = tonumber(apt_word[3])   -- lon
            mm = tonumber(apt_word[4])    -- trans alt
            if mm == nil then
              apt_data[apt_data_num][4] = 0   -- trans alt
            else
              apt_data[apt_data_num][4] = mm    -- trans alt
            end
            mm = tonumber(apt_word[5])    -- trans lvl
            if mm == nil then
              mm = tonumber(string.sub(apt_word[5], 3, -1))
              if mm == nil then
                apt_data[apt_data_num][5] = 0   -- trans lvl
              else
                apt_data[apt_data_num][5] = mm * 100    -- trans lvl
              end
            else
              if mm < 1000 then
                apt_data[apt_data_num][5] = mm * 100    -- trans lvl
              else
                apt_data[apt_data_num][5] = mm    -- trans lvl
              end
            end
            apt_data[apt_data_num][6] = tonumber(apt_word[6])   -- longest rnw
          end
        end
      end
      apt_first = 1
      apt_line = file_navdata2:read()
    end
    file_navdata2:close()
  end

end

function create_idx_table()

  local ii = 0
  local tmp_byte = 0
  local tmp_idx = 0

  -- clear idx tables
  idx_earth_nav = {}
  idx_apt = {}
  idx_ils = {}
  idx_ils_icao = {}
  idx_rnw = {}
  idx_awy = {}

  for ii = 48, 57 do
    idx_earth_nav[ii] = {}
    idx_earth_nav[ii][99999] = 0
    idx_apt[ii] = {}
    idx_apt[ii][99999] = 0
    idx_ils[ii] = {}
    idx_ils[ii][99999] = 0
    idx_ils_icao[ii] = {}
    idx_ils_icao[ii][99999] = 0
    idx_rnw[ii] = {}
    idx_rnw[ii][99999] = 0
    idx_awy[ii] = {}
    idx_awy[ii][99999] = 0
  end
  for ii = 65, 90 do
    idx_earth_nav[ii] = {}
    idx_earth_nav[ii][99999] = 0
    idx_apt[ii] = {}
    idx_apt[ii][99999] = 0
    idx_ils[ii] = {}
    idx_ils[ii][99999] = 0
    idx_ils_icao[ii] = {}
    idx_ils_icao[ii][99999] = 0
    idx_rnw[ii] = {}
    idx_rnw[ii][99999] = 0
    idx_awy[ii] = {}
    idx_awy[ii][99999] = 0
  end

  if earth_nav_num > 0 then
    for ii = 1, earth_nav_num do
      tmp_byte = string.byte(string.sub(earth_nav[ii][4], 1, 1))
      if (tmp_byte >= 48 and tmp_byte <= 57) or (tmp_byte >= 65 and tmp_byte <= 90) then
        tmp_idx = idx_earth_nav[tmp_byte][99999]
        if tmp_idx < 99998 then
          tmp_idx = tmp_idx + 1
          idx_earth_nav[tmp_byte][99999] = tmp_idx
          idx_earth_nav[tmp_byte][tmp_idx] = ii
        end
      end
    end
  end

  if apt_data_num > 0 then
    for ii = 1, apt_data_num do
      tmp_byte = string.byte(string.sub(apt_data[ii][1], 1, 1))
      if (tmp_byte >= 48 and tmp_byte <= 57) or (tmp_byte >= 65 and tmp_byte <= 90) then
        tmp_idx = idx_apt[tmp_byte][99999]
        if tmp_idx < 99998 then
          tmp_idx = tmp_idx + 1
          idx_apt[tmp_byte][99999] = tmp_idx
          idx_apt[tmp_byte][tmp_idx] = ii
        end
      end
    end
  end

  if ils_nav_num > 0 then
    for ii = 1, ils_nav_num do
      tmp_byte = string.byte(string.sub(ils_nav[ii][1], 1, 1))
      if (tmp_byte >= 48 and tmp_byte <= 57) or (tmp_byte >= 65 and tmp_byte <= 90) then
        tmp_idx = idx_ils[tmp_byte][99999]
        if tmp_idx < 99998 then
          tmp_idx = tmp_idx + 1
          idx_ils[tmp_byte][99999] = tmp_idx
          idx_ils[tmp_byte][tmp_idx] = ii
        end
      end
      tmp_byte = string.byte(string.sub(ils_nav[ii][7], 1, 1))
      if (tmp_byte >= 48 and tmp_byte <= 57) or (tmp_byte >= 65 and tmp_byte <= 90) then
        tmp_idx = idx_ils_icao[tmp_byte][99999]
        if tmp_idx < 99998 then
          tmp_idx = tmp_idx + 1
          idx_ils_icao[tmp_byte][99999] = tmp_idx
          idx_ils_icao[tmp_byte][tmp_idx] = ii
        end
      end
    end
  end

  if awy_data_num > 0 then
    for ii = 1, awy_data_num do
      tmp_byte = string.byte(string.sub(awy_data[ii][1], 1, 1))
      if (tmp_byte >= 48 and tmp_byte <= 57) or (tmp_byte >= 65 and tmp_byte <= 90) then
        tmp_idx = idx_awy[tmp_byte][99999]
        if tmp_idx < 99998 then
          tmp_idx = tmp_idx + 1
          idx_awy[tmp_byte][99999] = tmp_idx
          idx_awy[tmp_byte][tmp_idx] = ii
        end
      end
    end
  end
  --dump_idx_table()
end

function apt_exist(eee)

  local ii = 0
  local res = false
  local nd_x = 0
  local idx_rec = 0

  icao_latitude = 0
  icao_longitude = 0

  if apt_data_num > 0 then
    nd_x = string.byte(string.sub(eee, 1, 1))
    if (nd_x >= 48 and nd_x <= 57) or (nd_x >= 65 and nd_x <= 90) then
      if idx_apt[nd_x][99999] > 0 then
        for ii = 1, idx_apt[nd_x][99999] do
          idx_rec = idx_apt[nd_x][ii]
          if eee == apt_data[idx_rec][1] then
            icao_latitude = apt_data[idx_rec][2]
            icao_longitude = apt_data[idx_rec][3]
            icao_tns_alt = apt_data[idx_rec][4]
            icao_tns_lvl = apt_data[idx_rec][5]
            res = true
            break
          end
        end
      end
    end
  end
  return res

end