Hardened Aircraft Shelters (HAS), or Protective Aircraft Shelter (PAS), are a reinforced structure to house and protect military aircraft from enemy attack. Cost considerations and building practicalities limit their use to fighter size aircraft.
As with many military items, whether structures, tanks or aircraft, its most prolific use was during the Cold War. NATO and Warsaw Pact countries built hundreds of HAS's across Europe. In this context Hardened Aircraft Shelters were built to protect aircraft from conventional attacks as well as nuclear, chemical and biological strikes. NATO shelters, built to standard designs across the continent, were designed to withstand a direct hit by a 500lb (226kg) bomb, or a near miss by a larger one (i.e., 1,000 lb+). In theory HAS's were also built to protect aircraft in a nuclear strike; however, the effect of such an attack on airfield taxiways, runways, support facilities and personnel would have made any retaliatory mission extremely difficult and subsequent return and rearming almost impossible.
In the post-cold war era the value of the HAS concept was further eroded by the introduction of precision-guided munitions. Iraqs HAS hangers were built to a standard somewhat higher than NATO or Warsaw Pact shelters, but nevertheless proved almost useless during the Gulf War. Early attempts to defeat them typically used a "one-two punch" using a TV guided missile to blast open the doors, followed by bombs tossed in the front. US efforts soon turned to simply dropping a 2,000 lb laser guided bomb on the top, which would easily penetrate the roof and explode within. Although NATO hangars would remain useful against any conceivable attack on Europe in the short term (which would generally lack precision guidance systems), the value of HAS hangers against western air forces is effectively zero.
Wider dispersal (distance between aircraft) at airfields would decrease the vulnerability of aircraft. This would also force an enemy to increase the number of attacking aircraft greatly, or spend more time over the target. Either way the effect of airfield defences would take a heavy toll on the aggressor. However like HAS, dispersal can be expensive, requiring massive construction of hardstanding.
Dispersing aircraft between many bases greatly increases the cost of attacking a given number of aircraft, as measured by the number of attack aircraft required. However, this option similarly increases the defenders' cost of operation and degrades their efficiency.