Left 4 Dead: Dead Air Trick

Well, apparently there's a new way to hide yourself on top of the aircraft. Ever since Valve removed the mounted machine gun glitch that would launch someone sky high and land on the carrier turbo-prop, people have started finding other ways to avoid combatting against the Tanks and hopefully get the Untouchables achievement on their Steam accounts. Note that this trick is only confirmed on the PC and is technically not a glitch but how the Source Engine's physics works.

So, this trick works by having either throwing a gas or propane tank toward a a baggage container. Be sure to NOT shoot the gas or propane tank. While someone is throwing the tank, be sure to have another person pushing the baggage container forward between the truck and the nose cone of the C-130 Hercules. Have 2 baggage containers in between the C-130 and the truck, then jump on the truck and then platform your character over to the plane's nose cone. This will allow you to get on board of the aircraft. In addition to throwing the tanks, you can use the mounted machine gun to shoot the containers to aid the people pushing. However, try not to shoot your teammates because it results in an instant incapitation which no one would want.

Once aboard the roof of the turbo-prop, be sure to stick together at the complete centre of the aircraft to give you room in case you get dragged by a smoker. Note that since the infected can't get you, they will stand completely still, so just peg them off with a long range weapon or your pistol. Be sure to keep whacking each other to disable the Smoker's grabbing abilities and therefore not having anyone pulled by the smoker. The Tank should not be able to touch you at all and will simply vanish.

The only "hard part" other than getting this trick working is to jump down on the aircraft and get into the cargo bay. Be sure to have pipebombs and/or molotovs handy and throw them to buy your team time to get into the fuselage without getting killed and minimize getting yourself damaged. You may be lucky enough to even get the "Untouchables" achievement.

Goodluck!