Question: In what month was the crash that Dennis Nielsen is most widely known for having participated in the rescue effort for?

Context articles: Dennis Nielsen
Maersk Alabama hijacking
Umberto Nobile
Rescue of Roger Mallinson and Roger Chapman
Walter Abel
Cross Mountain Mine disaster
Clinton Bush Haiti Fund
Rouge Bouquet
United Airlines Flight 232
Iowa Hog Lift

Dennis Nielsen is a retired United States Air Force Colonel who is most widely known for having participated in rescue effort of the United Airlines Flight 232 crash in Sioux City, Iowa in 1989.
 Born in Shelby, Iowa, he has lived in the small town of Nashville, North Carolina since he retired from the Air National Guard in 2002.
The "Maersk Alabama" hijacking was a series of maritime events that began on 8 April 2009 with four pirates in the Indian Ocean seizing the cargo ship  240 nmi southeast of Eyl, Somalia.
 The siege ended after a rescue effort by the U.S. Navy on 12 April 2009.
 It was the first successful pirate seizure of a ship registered under the American flag since the early 19th century.
 Many news reports referenced the last pirate seizure as being during the Second Barbary War in 1815, although other incidents had occurred as late as 1821.
 It was the sixth vessel in a week to be attacked by pirates who had previously extorted ransoms in the tens of millions of dollars.
Umberto Nobile (] ; 21 January 1885 – 30 July 1978) was an Italian aviator, aeronautical engineer and Arctic explorer.
 Nobile was a developer and promoter of semi-rigid airships during the period between the two World Wars.
 He is primarily remembered for designing and piloting the airship "Norge", which may have been the first aircraft to reach the North Pole, and which was indisputably the first to fly across the polar ice cap from Europe to America.
 Nobile also designed and flew the "Italia," a second polar airship; this second expedition ended in a deadly crash and provoked an international rescue effort.
The rescue of Roger Mallinson and Roger Chapman occurred between 29 August and 1 September, 1973 after their Vickers Oceanics' small submersible "Pisces III" was trapped on the seabed at a depth of 1575 ft , 150 miles off Ireland in the Irish Sea.
 The 76-hour multinational rescue effort resulted in the deepest sub rescue in history.
Walter Abel (June 6, 1898 – March 26, 1987) was an American stage and film character actor.
 Known as a prolific and very dependable character actor, Abel appeared in over 200 films, beginning in the silent film era.
 Often portraying characters of "responsibility," (the district attorney prosecuting the lynch mob in "Fury", the minister keeping morale up in a war zone in "So Proudly We Hail!
" and the colonel leading a rescue effort in "Island in the Sky"), Abel was a regular in films of the 1940s and 1950s, in particular.
 A distinctive bearing and direct gaze were two of his trademarks.
 His eyes were brown and his height was 5'10" (178 cm).
The Cross Mountain Mine disaster was a coal mine explosion that occurred on December 9, 1911 near the community of Briceville, Tennessee, in the southeastern United States.
 In spite of a well-organized rescue effort led by the newly created Bureau of Mines, 84 miners died as a result of the explosion.
 The likely cause of the explosion was the ignition of dust and gas released by a roof fall.
The Clinton Bush Haiti Fund was a 501(c)(3) nonprofit organization founded on January 16, 2010, by former US Presidents Bill Clinton and George W. Bush, to aid the victims of the 2010 Haiti earthquake.
 The organization was formed in response to the request of President Barack Obama for the former Presidents to assist in the Haitian rescue effort.
 The fund concluded formal operations on December 31, 2012.
Rouge Bouquet is a part of the Forêt de Parroy near the French village of Baccarat that was the site of a German artillery bombardment of American trench positions on 7 March 1918 at 15:20 on the Chausailles sector of the Western Front during World War I.
 The bombardment resulted in the burial of 21 men of the 165th Infantry Regiment, 42nd Rainbow Division (originally the 69th Regiment of the New York National Guard) of which only a few survived.
 The 22 men, including their platoon commander 1st Lieutenant John Norman, were assembled in a dugout when a German artillery shell landed on the roof of the dugout.
 A rescue effort by Major William J. "Wild Bill" Donovan was begun in haste attempting to dig the men but their efforts were hampered by mud-slides and continued enemy shelling.
 Two men were rescued and five dead were recovered before efforts had to be halted.
 The voices other men could be heard for a while, but the remaining fifteen men died before rescue efforts could resume.
 Donovan was awarded the French Croix de Guerre for his actions during the attempted rescue.
United Airlines Flight 232 was a DC-10, registered as N1819U, that crash-landed at Sioux City, Iowa in July 19, 1989 after suffering catastrophic failure of its tail-mounted engine, which led to the loss of all flight controls.
 The flight was en route from Stapleton International Airport in Denver, Colorado to O'Hare International Airport in Chicago.
 Of the 296 passengers and crew on board, 111 died in the accident and 185 survived in total.
 Despite the deaths, the accident is considered a prime example of successful crew resource management due to the large number of survivors and the manner in which the flight crew handled the emergency and landed the airplane without conventional control.
The Iowa Hog Lift was a 1960 rescue effort by the agriculture sector in the U.S. State of Iowa following significant damage resulting from typhoons in Yamanashi Prefecture, Japan.
 It is widely accepted that the hog lift was largely responsible for the development of the sister-state relationship between Iowa and Yamanashi and marked the beginning of currently strong relations in agriculture between Japan and the United States.