40th ARRS
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The 40th Helicopter Squadron is a missile support unit of the
United States Air Force The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
. As the 40th Aerospace Rescue and Recovery Squadron it was a helicopter rescue squadron of the USAF during the
Vietnam War The Vietnam War (1 November 1955 – 30 April 1975) was an armed conflict in Vietnam, Laos, and Cambodia fought between North Vietnam (Democratic Republic of Vietnam) and South Vietnam (Republic of Vietnam) and their allies. North Vietnam w ...
.


History

The squadron was activated as the 40th Aerospace Rescue and Recovery Squadron at
Udorn Royal Thai Air Force Base Udorn Royal Thai Air Force Base (Udorn RTAFB) is a Royal Thai Air Force (RTAF) base, the home of 23rd Wing Air Command. It is in the city of Udon Thani in northeastern Thailand and is the main airport serving the city and province. The RTAF 23 ...
in March 1968 with
Sikorsky HH-3 The Sikorsky SH-3 Sea King (company designation S-61) is an American twin-engine anti-submarine warfare (ASW) helicopter designed and built by Sikorsky Aircraft. A landmark design, it was one of the first ASW rotorcraft to use turboshaft engine ...
s which were nicknamed "Nitnoy." Later Sikorsky HH-53s arrived and these were nicknamed "BUFF." In March 1968, Detachment 2 of the
37th Aerospace Rescue and Recovery Squadron The 37th Helicopter Squadron is a United States Air Force unit assigned to the 582d Helicopter Group in support of the 90th Missile Wing located at Francis E. Warren Air Force Base, Wyoming. The unit is tasked with flight operations in support ...
at Udorn RTAFB operating HH-3s and HH-53Bs, was transferred to the 40th ARRS. The 40th moved to
Nakhon Phanom Royal Thai Air Force Base The Nakhon Phanom Royal Thai Navy Base (NKP), formerly Nakhon Phanom Royal Thai Air Force Base, is a Royal Thai Navy facility used for riverine patrols along the Mekong River. It is approximately northeast of Bangkok, west of Nakhon Phanom ...
on 21 July 1971. 20 August 1972, the local base rescue detachments of the 3d Aerospace Rescue and Recovery Group each operating 2 HH-43s were transferred to the 40th ARRS, comprising: *Detachment 2
Takhli Royal Thai Air Force Base Takhli Royal Thai Air Force Base is a Royal Thai Air Force (RTAF) facility in central Thailand, approximately 144 miles (240 km) northwest of Bangkok in Takhli District, Nakhon Sawan Province. Units Takhli is the home of the Royal Thai ...
*Detachment 3
Ubon Royal Thai Air Force Base Ubon Royal Thai Air Force Base is a Royal Thai Air Force (RTAF) facility located near the city of Ubon Ratchathani, in Ubon Ratchathani Province. It is approximately 488 km (303 miles) northeast of Bangkok. The Laos border is about dire ...
*Detachment 4
Korat Royal Thai Air Force Base Korat Royal Thai Air Force Base or Nakhon Ratchasima Royal Thai Air Force Base is a base of the Royal Thai Air Force (RTAF) in northeast Thailand, approximately 200 km (125 mi) northeast of Bangkok and about 4 km (2.5 mi) sou ...
*Detachment 5 Udorn RTAFB *Detachment 12 U-Tapao Royal Thai Naval Airfield *Detachment 14
Tan Son Nhut Air Base Tan Son Nhut Air Base () (1955–1975) was a Republic of Vietnam Air Force (RVNAF) facility. It was located near the city of Saigon in southern Vietnam. The United States used it as a major base during the Vietnam War (1959–1975), stationing ...
30 November 1972, with the inactivation of the 37th ARRS at
Danang Air Base Da Nang Air Base () (1930s–1975) (also known as Da Nang Airfield, Tourane Airfield or Tourane Air Base) was a French Air Force and later Republic of Vietnam Air Force (RVNAF) facility located in the city of Da Nang, Vietnam. During the Vietnam ...
, 5 of its HH-53s were transferred to the 40th ARRS, while its two HH-43s remained at Danang as Detachment 7 of the 40th ARRS to provide base rescue during
Operation Linebacker II Operation Linebacker II, sometimes referred to as the Christmas bombings and, in Vietnam, Dien Bien Phu in the air, was a strategic bombing campaign conducted by the United States against targets in North Vietnam from 18 December to 29 December ...
. Following the
Paris Peace Accords The Paris Peace Accords (), officially the Agreement on Ending the War and Restoring Peace in Viet Nam (), was a peace agreement signed on January 27, 1973, to establish peace in Vietnam and end the Vietnam War. It took effect at 8:00 the follo ...
all remaining US Forces were withdrawn from South Vietnam by 27 March 1973. Detachment 7 at Danang Air Base and Detachment 14 at Tan Son Nhut Air Base were inactivated during this period. Following the withdrawal from South Vietnam the 40th's force level was 11 HH-53s and 14 HH-43s. The USAF continued combat operations over Cambodia until 15 August 1973 and the 40th provided CSAR support during this period. Following the end of combat operations the 40th kept 2 HH-53s at Nakhon Phanom on 15-minute alert during daylight and 45-minute alert at night. In July 1974 Detachment 10 at Takhli was disbanded, followed in August by Detachment 3 at Ubon. On 20 February 1975, Detachment 1 at Nakhon Phanom was disbanded. At this time the 40th's force level had dropped to 8 HH-53Cs and 4 HH-43Fs. The 40th moved to
Korat Royal Thai Air Force Base Korat Royal Thai Air Force Base or Nakhon Ratchasima Royal Thai Air Force Base is a base of the Royal Thai Air Force (RTAF) in northeast Thailand, approximately 200 km (125 mi) northeast of Bangkok and about 4 km (2.5 mi) sou ...
on 1 October 1975. On 15 October 1975 with the inactivation of the 56th ARRS its 4
HC-130 The Lockheed HC-130 is an extended-range, search and rescue (SAR)/combat search and rescue (CSAR) version of the C-130 Hercules military transport aircraft, with two different versions operated by two separate services in the U.S. armed force ...
Ps joined the 40th. On 31 January 1976 the 40th ARRS was inactivated at Korat RTAFB.


Operations and losses

*30 May 1968, Jolly Green, an HH-3E was attempting to rescue
Colonel Colonel ( ; abbreviated as Col., Col, or COL) is a senior military Officer (armed forces), officer rank used in many countries. It is also used in some police forces and paramilitary organizations. In the 17th, 18th, and 19th centuries, a colon ...
Norman Phillips the pilot of a downed
F-105 The Republic F-105 Thunderchief is an American fighter-bomber that served with the United States Air Force from 1958 to 1984. Capable of Mach 2, it conducted the majority of strike bombing missions during the early years of the Vietnam War. It ...
, near
Savannakhet Savannakhet (, ; ), officially named Kaysone Phomvihane (); since 2005 and previously known as ''Khanthaboury'' (; ), is a city in western Laos. It is the capital of Savannakhet Province. With a population of 125,760 (2018), it is the second-l ...
, Laos. Pararescueman
Sergeant Sergeant (Sgt) is a Military rank, rank in use by the armed forces of many countries. It is also a police rank in some police services. The alternative spelling, ''serjeant'', is used in The Rifles and in other units that draw their heritage f ...
Thomas A Newman descended into a hostile jungle environment to rescue the pilot. Hampered by darkness and concentrated automatic weapons fire, Newman requested the rescue helicopter to enter a nearby orbit, both for the safety of the crewmembers and to prevent the hovering aircraft from establishing their location for the unfriendly ground forces. When the HH-3 returned, he secured the injured Philips to the forest penetrator and protected him with his own body as they ascended to the helicopter. Newman was awarded the Air Force Cross. *25 December 1968, ''Jolly Green 17'', an HH-3E was attempting to rescue
Major Major most commonly refers to: * Major (rank), a military rank * Academic major, an academic discipline to which an undergraduate student formally commits * People named Major, including given names, surnames, nicknames * Major and minor in musi ...
Charles R Brownlee, the pilot of ''Panda 01'' an F-105 shot down near Ban Lathama, Mahaxia District, Khammouan Province, Laos. Pararescueman Airman First Class Charles Douglas King descended by rescue hoist to rescue the injured pilot. With the pilot attached to the hoist, the HH-3E and King were hit by enemy fire, seriously injured, King instructed the helicopter to depart. King was posthumously awarded the Air Force Cross,. The bodies of King and Brownlee were not recovered and both were listed as killed in action - body not recovered. *18 January 1969, ''Jolly Green 67'', HH-53B, ''66-14430'', was hit by ground fire while on a CSAR mission for ''Sandy 02'', an A-1H lost the previous day. Helicopter made an emergency landing 15 km southeast of Tchepone, Laos. The crew and the A-1 pilot were rescued by ''Jolly Green 70''. An airstrike was ordered to destroy ''JG-67'' *28 January 1970, ''Jolly Green 71'', HH-53B, ''66-14434'', on a CSAR mission for the pilot of ''Seabird 02'', an F-105G, was shot down by a missile fired from a
Mig-21 The Mikoyan-Gurevich MiG-21 (; NATO reporting name: Fishbed) is a supersonic jet aircraft, jet fighter aircraft, fighter and interceptor aircraft, designed by the Mikoyan, Mikoyan-Gurevich OKB, Design Bureau in the Soviet Union. Its nicknames in ...
piloted by Vu Ngoc Dinh of the 921st Fighter Regiment. The 6-man crew were all killed. *30 June 1970, ''Jolly Green 54'', HH-53C, ''66-8283'', was on a CSAR mission for the crew of ''Nail 44'' an
OV-10 The North American Rockwell OV-10 Bronco is an American twin-turboprop light attack and observation aircraft. It was developed in the 1960s as a special aircraft for counter-insurgency (COIN) combat, and one of its primary missions was as a forw ...
A, over Savannakhet, Laos. Abandoning the first rescue attempt due to heavy fire, the pilot
Captain Captain is a title, an appellative for the commanding officer of a military unit; the supreme leader or highest rank officer of a navy ship, merchant ship, aeroplane, spacecraft, or other vessel; or the commander of a port, fire or police depa ...
Leroy C Schaneberg decided to make a second rescue attempt. ''JG54'' was hit by ground fire and crashed. The 5-man crew were all killed. The crash site was excavated in December 1993 and remains were identified as a group on 7 March 1995. Schaneberg was posthumously awarded the Air Force Cross. *21 November 1970, 5 of the squadron's HH-53s took part in
Operation Ivory Coast Operation Ivory Coast was a mission conducted by United States Special Operations Forces and other American military elements to rescue U.S. prisoners of war during the Vietnam War. It was also the first joint military operation in United State ...
, the raid on the
Son Tay A son is a male offspring; a boy or a man in relation to his parents. The female counterpart is a daughter. From a biological perspective, a son constitutes a first degree relative. Social issues In pre-industrial societies and some current ...
POW camp. *21 June 1971, ''Jolly Green 54'', HH-53, ''66–8285'', was shot down over Laos while attempting to recover an AQM-34 ''Buffalo Hunter'' drone. Crew were all rescued. *27 March 1972, ''Jolly Green 61'', HH-53C, ''66-10359'', crashed over Stoeng Treng Province, Cambodia, about southeast of
Siem Pang District Siem Pang District is a district located in Stung Treng Province, in north-east Cambodia. According to the 1998 census of Cambodia, it had a population of 13,517. See page 246. The Khmer Khe language is spoken in Siem Pang District. References< ...
due to unknown causes. A pararescueman was lowered to the ground at the site of the crash to check for survivors, but due to the intense heat from the burning helicopter, he could not approach near enough to determine if there were crew members inside the aircraft. Some three hours later a second rescue specialist was deployed in the immediate area, who reported the wreckage was still burning, precluding close inspection. The 5-man crew were all killed in action - body not recovered. *13 April 1972, Captain Bennie D Orrell pilot of a Jolly Green, rescued a downed pilot near Tchepone, Laos. Orrell was awarded the Air Force Cross. *2 June 1972, Captain Dale E. Stovall pilot of a Jolly Green, flew through intense hostile ground fire to complete the rescue of Captain
Roger Locher Roger Clinton Locher (born September 13, 1946) is a retired Colonel in the U.S. Air Force and a former McDonnell Douglas F-4D Phantom II Navigator/Weapon Systems Officer (WSO) and subsequent Pilot who, during Operation Linebacker in the Vietnam ...
, an F-4 copilot who had evaded capture for 23 days after being shot down in North Vietnam. This rescue was the deepest CSAR mission into North Vietnam and took place 8 km northeast of
Yên Bái Air Base Yên Bái Air Base is a Vietnam People's Air Force (VPAF) ''(Không quân Nhân dân Việt Nam)'' military airfield located north-northwest of Yên Bái. History Vietnam War On 15 November 1967 F-105s of the 357th Tactical Fighter Squadron bo ...
, one of the most active
Vietnamese People's Air Force The Vietnam People's Air Force (VPAF; ), officially the Air Defence - Air Force Service (ADAF Service; ) or the Vietnam Air Force (), is the aerial, air and space defence service branch of Vietnam. It is the modern incarnation of the so-cal ...
MiG airfields. Stovall was awarded a Bronze
Oak leaf cluster An oak leaf cluster is a ribbon device to denote preceding decorations and awards consisting of a miniature bronze or silver twig of four oak leaves with three acorns on the stem. It is authorized by the United States Armed Forces for a spec ...
in lieu of a second award of
Silver Star The Silver Star Medal (SSM) is the United States Armed Forces' third-highest military decoration for valor in combat. The Silver Star Medal is awarded primarily to members of the United States Armed Forces for gallantry in action against a ...
. Stovall was also awarded the 1973
Jabara Award The Jabara Award for Airmanship, named in memory of Colonel James Jabara (1923-1966), is awarded each year to the United States Air Force Academy graduate whose accomplishments demonstrate superior performance in fields directly involved with aero ...
for Airmanship. *27 June 1972, Pararescueman Sergeant Charles D. McGrath was on a CSAR mission on ''Jolly Green 77'' over North Vietnam. While penetrating dense jungle to rescue F-4 crewman, Captain Lynn A. Aikman, who had a broken leg, knee, elbow and jaw, McGrath exposed himself to intense ground fire in order to drag the incapacitated Aikman to a suitable recovery area. Seeing ''JG73'' crippled by the hostile fire, he directed air strikes against surrounding hostile ground forces until he was able to secure CAikman and himself to the penetrator of ''Jolly Green 57'', the backup helicopter. Rising through constant accurate ground fire, he shielded Aikman with his own body until they were successfully recovered. McGrath and the pilot Captain Dale Stovall were both awarded the Air Force Cross. *27 December 1972, ''Jolly Green 73'', HH-53C, ''69-5788'', on a CSAR mission for crew of ''Jackel 33'', an
F-111 The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, multirole combat aircraft. Production models of the F-111 had roles that included attack (e.g. interdiction), strategic bombing (including nuclear weapons capabilit ...
A downed on 22 December 1972 over North Vietnam, lost its fuel probe due to enemy ground fire and crash-landed due to fuel starvation. ''JG-73'' was then destroyed by an A-7D Corsair. *14 June 1973, ''Jolly Green 64'', HH-53C, ''66-10362'', lost its tail rotor and crashed into the Tonle Sap, Cambodia. 3 of the crew were killed and 2 were rescued. *12 April 1975 the squadron supported
Operation Eagle Pull Operation Eagle Pull was the United States military evacuation by air of Phnom Penh, Cambodia, on 12 April 1975. At the beginning of April 1975, Phnom Penh, one of the last remaining strongholds of the Khmer Republic, was surrounded by the Khmer ...
, the evacuation of
Phnom Penh Phnom Penh is the Capital city, capital and List of cities and towns in Cambodia, most populous city of Cambodia. It has been the national capital since 1865 and has grown to become the nation's primate city and its political, economic, industr ...
, Cambodia. *29–30 April 1975, 2 of the squadron's HH-53s operating from took part in
Operation Frequent Wind Operation Frequent Wind was the final phase in the evacuation of American civilians and "at-risk" Vietnamese from Saigon, South Vietnam, before the takeover of the city by the North Vietnamese People's Army of Vietnam (PAVN) in the Fall of Sai ...
, the evacuation of
Saigon Ho Chi Minh City (HCMC) ('','' TP.HCM; ), commonly known as Saigon (; ), is the most populous city in Vietnam with a population of around 14 million in 2025. The city's geography is defined by rivers and canals, of which the largest is Saigo ...
. *15 May 1975, 7 of the squadron's HH-53s participated in the recapture of the
SS Mayaguez SS ''Mayaguez '' was a U.S.- flagged container ship that is best known for its 12 May 1975 seizure by Khmer Rouge forces of Cambodia, which resulted in a confrontation with the United States at the close of the Vietnam War. Service history ...
.


Lineage

* Constituted as the 40th Aerospace Rescue & Recovery Squadron, activated and organized on 21 March 1968 : Inactivated on 31 January 1976 : Activated on 1 July 1978 : Inactivated on 31 December 1987 * Redesignated 40th Rescue Flight on 1 April 1993 : Activated on 1 May 1993 * Redesignated 40th Helicopter Flight on 1 May 1998 * Redesignated 40th Helicopter Squadron on 11 October 2005


Assignments

* 3d Aerospace Rescue and Recovery Group: 21 March 1968 * 41st Aerospace Rescue and Recovery Wing (later 41st Rescue and Weather Reconnaissance Wing): 20 August 1972 – 31 January 1976 * 39th Aerospace Rescue and Recovery Wing: 1 July 1978 – 31 December 1987 *
341st Operations Group The United States Air Force's 341st Missile Wing is an intercontinental ballistic missile unit headquartered at Malmstrom Air Force Base, Montana. Up until 1 July 2008, it was designated as the 341st Space Wing. Established as a World War II T ...
: 1 May 1993 *
582d Helicopter Group The 582nd Helicopter Group was activated in January 2015 at F. E. Warren Air Force Base, Wyoming to provide a unified headquarters for the helicopter squadrons located on the intercontinental ballistic missile bases of Air Force Global Strike C ...
: 5 January 2015


Detachments

* Detachment 1, Nakhon Phanom RTAFB, Thailand: 18 March 1968 – 1 July 1971 : Detachment 1,
Homestead Air Force Base Homestead Air Reserve Base (Homestead ARB), previously known as Homestead Air Force Base (Homestead AFB) is located in Miami–Dade County, Florida, to the northeast of the city of Homestead. It is home to the 482nd Fighter Wing (482 FW) o ...
, Florida: 8 January 1981 – 1 October 1985 * Detachment 3, Ubon RTAFB, Thailand: 20 August 1972 – 20 August 1974 * Detachment 4, Hill Air Force Base, Utah: 1 July 1978 – 31 December 1987 * Detachment 5, Udorn RTAFB, Thailand: 20 August 1972 – 30 September 1975 * Detachment 5,
Edwards Air Force Base Edwards Air Force Base (AFB) is a United States Air Force installation in California. Most of the base sits in Kern County, California, Kern County, but its eastern end is in San Bernardino County, California, San Bernardino County and a souther ...
, California: 1 July 1978 – 31 December 1987 * Detachment 6
Holloman Air Force Base Holloman Air Force Base is a United States Air Force base established in 1942 located six miles (10 km) southwest of the central business district of Alamogordo, which is the county seat of Otero County, New Mexico, United States. The b ...
, New Mexico: 1 July 1978 – 31 December 1987 * Detachment 7 Da Nang Air Base, South Vietnam: 11 December 1972 – 10 February 1973 * Detachment 8 Takhli RTAFB, Thailand: c. Dec 1972 – 31 January 1973 * Detachment 10 Takhli RTAFB, Thailand: 31 January 1973 – 30 July 1974 * Detachment 10
MacDill Air Force Base MacDill Air Force Base (MacDill AFB) is an active United States Air Force installation located 4 miles (6.4 km) south-southwest of downtown Tampa, Florida. The "host wing" for MacDill AFB is the 6th Air Refueling Wing (6 ARW), assig ...
, Florida: 8 January 1981 – 31 March 1981 * Detachment 12 U-Tapao RTAFB, Thailand: 20 August 1972 – 31 January 1976 * Detachment 14
Tan Son Nhut Air Base Tan Son Nhut Air Base () (1955–1975) was a Republic of Vietnam Air Force (RVNAF) facility. It was located near the city of Saigon in southern Vietnam. The United States used it as a major base during the Vietnam War (1959–1975), stationing ...
, South Vietnam: 15 September 1972 – 10 February 1973 * Detachment 18
Plattsburgh Air Force Base Plattsburgh Air Force Base is a former United States Air Force (USAF) Strategic Air Command (SAC) base covering 3,447 acres (13.7 km) in the extreme northeast corner of New York, located on the western shore of Lake Champlain opposite Burl ...
, New York: 8 January 1981 – 30 November 1987 * Detachment 22
Mountain Home Air Force Base Mountain Home Air Force Base is a United States Air Force (USAF) installation in the Western United States, western United States. Located in Southwestern Idaho, southwestern Idaho in Elmore County, Idaho, Elmore County, the base is southwest ...
, Idaho: 1 July 1978 – 30 November 1987 * Detachment 24
Fairchild Air Force Base Fairchild Air Force Base (AFB) is a United States Air Force base, located in the northwest United States in eastern Washington, approximately southwest of Spokane. The host unit at Fairchild is the 92nd Air Refueling Wing (92 ARW) assigned ...
, Washington: 1 July 1978 – 30 November 1987


Stations

*
Udorn Royal Thai Air Force Base Udorn Royal Thai Air Force Base (Udorn RTAFB) is a Royal Thai Air Force (RTAF) base, the home of 23rd Wing Air Command. It is in the city of Udon Thani in northeastern Thailand and is the main airport serving the city and province. The RTAF 23 ...
, Thailand, 21 March 1968 *
Nakhon Phanom Royal Thai Air Force Base The Nakhon Phanom Royal Thai Navy Base (NKP), formerly Nakhon Phanom Royal Thai Air Force Base, is a Royal Thai Navy facility used for riverine patrols along the Mekong River. It is approximately northeast of Bangkok, west of Nakhon Phanom ...
, Thailand, 28 July 1971 *
Korat Royal Thai Air Force Base Korat Royal Thai Air Force Base or Nakhon Ratchasima Royal Thai Air Force Base is a base of the Royal Thai Air Force (RTAF) in northeast Thailand, approximately 200 km (125 mi) northeast of Bangkok and about 4 km (2.5 mi) sou ...
, Thailand, 15 September 1975 – 31 January 1976 *
Hill Air Force Base Hill Air Force Base is a major U.S. Air Force (USAF) base located in Davis County, Utah, just south of the city of Ogden, and bordering the Cities of Layton, Clearfield, Riverdale, Roy, and Sunset with its largest border immediately adja ...
, Utah, 1 July 1978 – 31 December 1987 *
Malmstrom Air Force Base Malmstrom Air Force Base is a United States Air Force base in Cascade County, Montana, United States, adjacent to the city of Great Falls. It was named in honor of World War II POW Colonel Einar Axel Malmstrom. It is the home of the 341st Mis ...
, Montana, 1 May 1993 – present


Aircraft

*
Sikorsky HH-3 The Sikorsky SH-3 Sea King (company designation S-61) is an American twin-engine anti-submarine warfare (ASW) helicopter designed and built by Sikorsky Aircraft. A landmark design, it was one of the first ASW rotorcraft to use turboshaft engine ...
(1967–1969) *
Kaman HH-43 Huskie The Kaman HH-43 Huskie is a helicopter developed and produced by the American rotorcraft manufacturer Kaman Aircraft. It is perhaps most distinctive for its use of twin intermeshing rotors, having been largely designed by the German aeronautical ...
(1972–1976) * Sikorsky HH-53B & C (1968–1976) *
Lockheed HC-130 The Lockheed HC-130 is an extended-range, search and rescue (SAR)/combat search and rescue (CSAR) version of the Lockheed C-130 Hercules, C-130 Hercules military transport aircraft, with two different versions operated by two separate service ...
P Hercules(1975–1976) *
Bell UH-1N Iroquois The Bell UH-1 Iroquois (nicknamed "Huey") is a utility military helicopter designed and produced by the American aerospace company Bell Helicopter. It is the first member of the prolific Huey family, as well as the first turbine-powered helic ...
(1973–present) *
Boeing MH-139A Grey Wolf The Boeing MH-139 Grey Wolf is a twin-engine helicopter operated by the United States Air Force (USAF) for security and support missions. Developed by Boeing, the Grey Wolf is a variant of the Leonardo AW139, an Italian-built multi-role helicopt ...
(2024-present)


References

* *


Notes

; Explanatory notes ; Citations {{USAF South Vietnam Military units and formations of the United States Air Force in the Vietnam War Helicopter squadrons of the United States Air Force