Factual error: In circling over Washington Dulles, a plane would fly over several airports that they could land at with perfect communication, including Richmond, Baltimore, Andrews AFB, etc. Also, there is no communication from the airport to the plane but the plane would be in range of no fewer than 15 transmitting stations that could have relayed messages.
Factual error: It is impossible for a stream of burning jet fuel to follow a plane through snow and catch up. Not only is jet fuel extremely hard to ignite, almost as soon as the plane was off the ground the fuel stream would be too dispersed for the flame to climb up into the tank, and even if not it wouldn't burn fast enough to catch the plane.
Factual error: When McClane is trapped in the cockpit of Esperanza's plane, and the terrorists throw grenades in, it sure takes them a long time to explode, almost 30 seconds before he ejects from the plane and the grenades go off. Normal grenades have about a 5 second fuse. With a delay that long he could even have thrown them all back out at the bad guys.
Factual error: The payphone McClane uses in Dulles airport has the "Pacific Bell" logo. Pacific Bell is a West Coast phone company. (00:04:20)
Factual error: There is no airport in the world with a manhole cover in the middle of a runway. (01:14:00)
Factual error: Firing a full magazine of blank cartridges from an automatic weapon in the police station office as McClane does would be painfully loud. Nobody shows the slightest effect - nobody even winces. Some of the men wince slightly but their reaction is grossly underplayed. Obviously the sound was looped in later. (01:37:40)
Factual error: When Holly's plane finally lands, the cockpit shot shows the pilot with his hands on four engine throttles. But Holly's plane is an L-1011 TriStar, which only has three engines. (01:52:00)
Factual error: After recovering a pistol from a slain baddie, John McClane explains to airport security that it's a "Glock 7" (in actuality a Glock 17) and says that it's made of porcelain that makes it "undetectable by X-ray." Excusing the fact that not only Glock, but no major firearms company has ever mass-produced such a weapon due to functionality issues, it doesn't change that the ammunition would still be visible on X-ray. The armorer for the production team pointed these facts out on set and tried to convince them to change this detail, but they filmed it with these mistakes anyway.
Factual error: At the end when John lights the fuel with a lighter, this isn't possible with Jet A-1 which is what the Boeing 747 is fuelled with. This type of fuel is extremely hard to light. Even if you drop a match into a bowl of jet A-1, it will not ignite. Jet A-1 has to be compressed or have its temperature raised to a minimum of 38c/100f in order to ignite (which the compressing also does) so fuel in snow would never ignite in the way shown.
Factual error: Near the beginning of the film when John McClane goes to the pay phone and calls his wife Holly on the airplane, that was impossible at the time. Those phones you see on the seats on the plane can only be used from the air to the ground not the other way around. Holly does answer on the plane phone and not on any cell phone she had in her purse. Not to mention even if it was possible to call the planes from the ground, they could do that to warn them of what's going on below and prevent more crashes. (00:04:20)
Factual error: Several runway numbers are mentioned in the movie: runway 10, 15, 25 right and 25 left. Runway 25 means course 250. The opposite end would then be Runway 7 (course 70). It would be unusual for an airport to have runways running course 70, 100 and 150, as that is way too close. And Dulles certainly does not. In the church occupied by the terrorists, the radar display (the upright one with the circular LED pattern, not a scope) shows the runway configuration of Las Vegas McCarran airport, not that of Washington Dulles airport. McCarran does have runways 25R and 25L, which are mentioned in the film.
Factual error: When McClane (Bruce Willis) is brought into Capt. Lorenzo's (Dennis Franz) office at Dulles Airport for the first time, Lorenzo mentions that McClane was in violation of several District of Columbia laws. Dulles Airport is in Virginia, not DC, therefore any DC laws would not apply to Dulles. (Although VA may very well have similar laws about airport misconduct.) (00:19:55)
Factual error: You cannot fire 'blanks' and 'live' ammunition from the same weapon without fitting a BFA (blank firing attachment). When a live round is fired, the bullet moving up the barrel seals the barrel. The exploding gas pushes in two directions. One - forcing the bullet out the end and two - pushing the working parts back. The working parts then move forward and pick up a new round. The BFA seals the top of the barrel, as a live round would do. If a live round is fired through a BFA you would be lucky not to blow the barrel.
Factual error: The terrorists are using Kenwood 46AT Handheld Radios that operate on 440Mhz. The Army moves in with a Kenwood TS-850 Radio that operates on 30Mhz-3Mhz. Nowhere near the frequency that the terrorists are using. (01:11:30 - 01:25:40)
Factual error: At the end of the movie, all of the planes come in to land by the fire. However, the planes are far, FAR closer than any large aircraft would land to each other. FAA regulations would have the aircraft about 5 miles away from each other. If they're landing this close due to "emergency", there would be wicked wake turbulence at such close distances.
Factual error: The plane from which Bruce Willis escapes by using an ejector seat is a non-existing plane. The fuselage is from an old transport plane from the US Air Force made by Chase. But there has never been a Chase with 4 engines buried in the wing. America uses underslung engines. In this case, it is the wing of an Arado Blitz prototype from the second world war and the fuselage of a Chase cargo plane. (01:15:10)