Esto no es exactamente un error
Notice: Undefined index: FirstClass in C:\xampp\htdocs\new_aircraft_process.php on line 13
Es simplemente porque ese campo lo tenés como disabled y no se envía.
Te recomendaría que revises el MySql y los campos de la tabla
hacé esta prueba, cambiá tu php por esto
Código PHP:
Ver original<?php
//include('./db.inc.php');
$ICAO = $_POST['ICAO'];
$Name = $_POST['Name'];
$WeightEmpty = $_POST['WeightEmpty'];
$WeightFull = $_POST['WeightFull'];
$CargoFull = $_POST['CargoFull'];
$CruiseSpeed = $_POST['CruiseSpeed'];
$Range = $_POST['Range'];
$Price = $_POST['Price'];
$FirstClass = $_POST['FirstClass'];
$BusinessClass = $_POST['BusinessClass'];
$EconomyClass = $_POST['EconomyClass'];
// mysql_query("INSERT INTO aircrafts (ICAO, Name, WeightEmpty, WeightFull, CargoFull, CruiseSpeed, Range, Price, FirstClass, BusinessClass, EconomyClass) VALUES ('$ICAO', '$Name', '$WeightEmpty', '$WeightFull', '$CargoFull', '$CruiseSpeed', '$Range', '$Price', '$FirstClass', '$BusinessClass', '$EconomyClass')") or die(mysql_error());
echo "INSERT INTO aircrafts (ICAO, Name, WeightEmpty, WeightFull, CargoFull, CruiseSpeed, Range, Price, FirstClass, BusinessClass, EconomyClass) VALUES ('$ICAO', '$Name', '$WeightEmpty', '$WeightFull', '$CargoFull', '$CruiseSpeed', '$Range', '$Price', '$FirstClass', '$BusinessClass', '$EconomyClass')";
?>
y procesá el form, con y sin disabled para FirstClass, vas a ver que recoge las variables correctamente
Saludos