Anti-aircraft warfare
Another example of using RPGs against helicopters is Operation Anaconda in March 2002 in Afghanistan. Taliban insurgents defending Shah-i-Kot Valley used RPGs in a direct fire role against landing helicopters. Four rangers were killed when their helicopter was shot down by an RPG, and SEAL team member Neil C. Roberts fell out of his helicopter when it was hit by two RPGs. In other instances helicopters have been shot down in Afghanistan during a mission in Wardak province. One feature that makes RPGs useful in air defence is that they are fused to automatically detonate at 920 m. If aimed into the air this causes the warhead to airburst which can release a limited but potentially damaging amount of shrapnel hitting a helicopter landing or taking off.
For insurgents the most effective method of countering aircraft is to attempt to destroy them on the ground, either by penetrating an airbase perimeter and destroying aircraft individually, e.g. the September 2012 Camp Bastion raid, or finding a position where aircraft can be engaged with indirect fire, such as mortars.  A recent trend emerging during the Syrian Civil War is the use of ATGM against landing helicopters.

See also: Air defence forces
Air supremacy
Gun laying
List of anti-aircraft weapons
Self-propelled anti-aircraft weapon
The bomber will always get through

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"Flak (1943)" on YouTube
1914 1918 war in Alsace – The Battle of Linge 1915 – The 63rd Anti Aircraft Regiment in 14 18 – The 96th poste semi-fixed in the Vosges
Archie to SAM: A Short Operational History of Ground-Based Air Defense by Kenneth P. Werrell (book available for download)
Japanese Anti-aircraft land/vessel doctrines in 1943–44
2nd/3rd Australian Light Anti-Aircraft Regiment