AH-64 Apache

Good

The AH-64 Apache is a US military attack helicopter used by the HECU and the Black Ops during the Black Mesa Incident. It is armed with rockets and a 360 degree swiveling turret, making attempts to engage it without cover futile. Several Apaches were stationed at Black Mesa before the Resonance Cascade occurred, and more were brought in from nearby Santego Military Base in Arizona after the event.

Half-Life
The Apache is first seen during the train ride in Half-Life's first chapter, Black Mesa Inbound, idling on a helipad. The player first encounters it in a combat situation in the chapter Surface Tension, in which one is providing close air support for some dug-in HECU troops guarding the hydroelectric dam the player must cross.

Half-Life: Blue Shift
The Apache can be seen flying overhead, possibly the same one that Gordon saw, taking off. This seems likely considering the fact that the Resonance Cascade occurs shortly after the first tram ride.

Half-Life: Opposing Force
In Opposing Force, a Black Ops Apache can be seen in the chapter "The Package".

Half-Life: Decay
In Decay, an Apache can be briefly seen passing by in the fourth chapter, Resonance.

Tactics
The Apache is a tough and effective foe. It is armored against small arms fire, meaning that, although it is possible to take one down with bullets, destroying the Apache with anything less than explosives or experimental weaponry is difficult. The RPG is by far the most effective weapon against the Apache, but it (or sufficient ammunition for it) may not always be available. In these cases the Tau Cannon can destroy it with only a couple of hits with a charged shot (only one in Easy mode). If both are unavailable, shooting the main rotor works, though it takes a long time.

Trivia

 * The Apache is primarily used by the US Army. The United States Marine Corps uses the AH-1 SuperCobra as their attack helicopter.
 * In Half-Life: Source, the Apache is vulnerable anywhere on its body because the engine did not include the ability to have both bulletproof and vulnerable surfaces.

List of appearances

 * Half-Life: Day One
 * Half-Life
 * Half-Life: Opposing Force
 * Half-Life: Decay
 * Half-Life 2: Raising the Bar