McClain over hears on the CB the landing of the cargo plane, he's talking to the janitor guy with the maps and wonders how quickly he can get to the landing runway.
We next see him running through the series of tunnels (!) under the runways of the airport, he finds the place his looking for and climbs up a ladder. At the top of the ladder is a large drainage grate -probably three feet on each side, at least- that is locked to the ground with a pad-lock. McClain shoots at the lock with his gun, breaking it (and likely giving him a good ring in his ears) and allowing him to try and force the heavy gate open with his back and shoulders.
The gate proves to heavy for him as he ends up wedged between it and the runway surface as the C-130 (?) lands at the runway threshold. I believe he uses his gun to try and support the grate as he tries to wedge through as the slowing plane closes in on him. He manages to dislodge himself from the grate just as the plane gets there and rolls out of the way, the plane's nose wheel strikes the propped-up grate with a crash, dislodging the gun it getting lost to the bottom of the drain shaft, the gate slams "shut."
Yes it should be noted that there's no likely to be any large drainage grates on a runway's surface. Such a thing would simply be too dangerous with too much opportunity to go wrong resulting in "at best" a heavy bump in the middle of the runway as age, weight and rust cause it to cave-in and at worst a very large hole should the grate ever fail or somehow become missing.
Also the plane would have struck the propped up grate going around 40 or 50 miles an hour if not much more and airplane landing gear hardly have the heartiest of suspension systems. The pilot (the captured leader the mercenaries/terrorists were trying to retrieve) almost certainly
would have felt the jolt of the plane striking the grate if not plain lost control of the plane on the slick runway (the ground crews were not allowed out to clear the runway of the heavy snow) with his own admission to low visibility.
Personally, I wouldn't let your knowledge of how things work in the real world preclude you from being able to enjoy a movie based entirely in fiction. You may know that's not how things work, I know that's not how things work, but it's one of the cases where you check your knowledge at the door and roll with it.