Hardened aircraft shelter

A hardened aircraft shelters (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.

Background
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 in the 1967 Arab-Israeli Six-Day War when the Israeli Air Force decimated 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. 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 hangars against well-equipped first world air forces is effectively zero.

Advantages

 * Reduces vulnerability of aircraft to all but the most accurate precision weaponry
 * Combined with active airfield defences increases survivability of defender's aircraft and cost to enemy's forces.
 * An alternative option, dispersal of aircraft to many different bases, reduces the efficiency of aircraft at both squadron and air force level.
 * Nuclear weapons can be stored in the HAS near the aircraft, in a vault; e.g., the United States Air Force Weapons Storage and Security System (WS3).

Disadvantages

 * Hardened shelters are expensive.
 * Hardened shelters are usually too small to easily accommodate large aircraft such as strategic transport aircraft and large surveillance aircraft.
 * Time taken for construction requires forward planning regarding most likely combat zones. If a conflict flares up quickly aircraft may be afforded no protection; e.g., in both the Gulf War and 2003 Iraq War many coalition aircraft had only sun shelters, not hardened facilities.
 * When first developed, the likelihood of a direct hit was minimal. Today, with precision-guided munitions (PGMs) and adequate training, delivering a direct hit on a HAS is trivial.  Coalition aircraft destroyed over half of Iraq's HASs during the Gulf War.
 * Germany's Luftwaffe has conducted tests to establish the effect of humidity inside a shelter on its aircraft's avionics. Results suggested the higher relative humidity has a higher corrosive effect than outside its shelters.  Supply of dry air to avionics compartments has decreased corrosion and increased serviceability.

Alternatives

 * Deployable shelters: Kevlar lined deployable shelters could protect aircraft from bomblets (a common anti-airfield weapon). However this would provide no protection from PGMs.
 * Dispersal at bases: 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.  Defense against enemy special forces is also more difficult.
 * Dispersal between bases: 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.
 * Underground hangars: Several air forces have used tunnels dug into a mountainside as underground hangars.