A hardened aircraft shelter (HAS) or protective aircraft shelter (PAS) is a reinforced hangar to house and protect military aircraft from enemy attack. Cost considerations and building practicalities limit their use to fighter size aircraft.
HASs are a passive defence measure (i.e., they limit the effect of an attack, as opposed to active defences; e.g., surface-to-air missiles) which aim to prevent or at least degrade enemy attacks. The widespread adoption of hardened aircraft shelters can be traced back to lessons learned from Operation Focus; in the 1967 Arab-Israeli Six-Day War when the Israeli Air Force destroyed the unprotected Egyptian Air Force, at the time the largest and most advanced air force in the Arab world, at its airbases.
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 HASs 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 500 lb (226 kg) bomb, or a near miss by a larger one (i.e., 1,000 lb+). In theory HASs 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. Iraq's HAS hangars 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. With that being said, however, NATO hangars would still remain useful against the forces of any enemy as might conceivably engage Europe in an armed conflict in the short term (whose capabilities generally lack precision guidance systems needed to defeat the defensive shield such hangars offer).
Sweden developed its own alternative during the cold war. In the late 1950s the Swedish Air Force introduced the Bas 60 air base system, which revolved around both the principle of dispersal between bases and dispersal at bases simultaneously. Road runways (highway strips) were also introduced as alternative bases. Bas 60 was primarily a response to the nuclear threat, so in the aftermath of the Six-Day War and the introduction of long range attack aircraft, such as the Su-24, the system was further developed into Bas 90. Improvements in the Bas 90 system included construction of short backup runways in the direct vicinity of the air bases and further dispersion of aircraft positioning and ground operations. Flygbasjägare (Air Force Rangers) were also raised specifically to counter the threat of infiltrating enemy special forces.