He transferred to the Luftwaffe (Air Force) in 1935 and following flight training, Hackl was posted to Jagdgeschwader 77 (JG 77–77th Fighter Wing) in April 1938.

Hackl claimed four victories during the Norwegian Campaign and then flew missions on the Channel Front in aftermath of the Battle of Britain.

Hackl then fought in the aerial battles of Operation Barbarossa, the German invasion of the Soviet Union.

On 19 September 1942, Hackl claimed his 118th and last victory on the Eastern Front, and was then transferred to the North Africa, fighting in the Tunisia Campaign.

Hackl was born on 25 March 1915 in Regensburg, Upper Palatinate of the Kingdom of Bavaria, as part of the German Empire.

In 1937, holding the rank Obergefreiter (senior lance-corporal), Hackl was sent to Halberstadt where he received flight training.

He was promoted to Unteroffizier (staff sergeant) in 1937, received fighter pilot training, and in April 1938 was posted to the II.

Gruppe (2nd group) of Jagdgeschwader 77 (JG 77–77th Fighter Wing),[Note 2] at the time under the command of Oberstleutnant (Lieutenant Colonel) Carl-Alfred Schumacher.

[5] World War II in Europe began on Friday, 1 September 1939, when German forces invaded Poland.

On the invasion day, Hackl was promoted to Feldwebel (staff sergeant) and did not participate in the Polish campaign.

Gruppe, flying combat air patrol missions along Germany's western border during the period dubbed the Phoney War.

Staffel flew from Stavanger-Sola and encountered a flight of Lockheed Hudson light bombers from the Royal Air Force (RAF) No.

Following preliminary repairs at Trondheim, Scharnhorst began its return voyage to Germany on 20 June.

[10] His four aerial victories in Norway earned Hackl the Iron Cross First Class (Eisernes Kreuz erster Klasse) on 2 July 1940.

[6][Note 4] In July 1941 he was posted with JG 77 to the Eastern Front, supporting Generalfeldmarschall (Field Marshal) Gerd von Rundstedt's Heeresgruppe Süd (Army Group South), with the objective of capturing Ukraine and its capital Kiev.

[13] Hackl became an "ace-in-a-day" for the first time on 19 April 1942, claiming two Mikoyan-Gurevich MiG-3, two Polikarpov R-Z and one I-18 shot down over the Isthmus of Perekop.

Hackl and together with Oberfeldwebel Franz-Josef Beerenbrock were presented the Oak Leaves by Adolf Hitler at the Führerhauptquartier at Rastenburg.

Gruppe arrived in North Africa on 5 December where it was based at Zazur airfield, approximately 15 km (9.3 mi) west of Tripoli.

[1][19] In combat with P-38 Lightnings escorting a flight of 24 Boeing B-17 Flying Fortress' on 4 February 1943, he was badly wounded resulting in a forced landing in his Bf 109 G-2 trop (Werknummer 10787—factory number) near Matmata.

Gruppe, including three Consolidated B-24 Liberator bombers shot down on 18 March 1944 taking his total to 139 aerial victories.

[25] In April 1944, he briefly acted as commander of JG 11, replacing Oberstleutnant Hermann Graf who had been wounded in combat on 29 March.

[26] On 15 April 1944, Hackl was shot down in his Focke-Wulf Fw 190 A and wounded in combat with United States Army Air Forces (USAAF) P-47 Thunderbolts, grounding him for a period of convalescence.

According to an Ultra deciphered message sent by Hackl to the General der Jagdflieger on 20 May, he had made recommendations on how to best utilize the young and inexperienced new fighter pilots in combat.

[29] The Geschwaderstab (headquarters unit) of JG 76 was formed on 21 July at Rotenburg an der Wümme and equipped with factory new Bf 109 G-6 aircraft.

On 31 August, the Geschwaderstab was ordered to Wiesbaden-Erbenheim Airfield and to Freiburg im Breisgau on 5 September where it again came under control of the 5.

Gruppe was withdrawn from combat operations and moved to Reinsehlen Airfield for conversion training to the new inline engine powered Fw 190 D-9, the second unit to receive this aircraft.

[34] In late February, he was appointed Geschwaderkommodore of JG 11, succeeding Major Jürgen Harder who was killed in a flying accident.

On 3 May 1945, two JG 11 pilots, Leutnant Hermann Gern and Feldwebel Franz Keller, attempted to steal a Messerschmitt Bf 108 Taifun aircraft to desert and fly to southern Germany.

A number of non-commissioned officers of JG 11 spoke to Hackl on behalf of Keller, no attempt was made to save Gern.

[38] Bergström and Mikhailov state that Hackl flew about 1,000 combat missions and also list him with shooting down 192 enemy aircraft plus another 24 unconfirmed aerial victories.