Viljoen served in 1 SAAF Squadron in May – June 1940 before being posted on attachment to 112 Squadron on 19 June 1940.

He left 112 Squadron in July 1940 and returned to 1 SAAF Squadron. This unit operated over Eritrea from July 1940 and was initially equipped with Gloster Gladiators.

In the beginning of October Captain Brian Boyle took a detachment of six Gladiators to Azaza, near Gedaref. Only six mechanics accompanied the detachment to this rudimentary base, where the pilots even had to prepare their own belts of ammunition for their guns. Early warning was almost non-existent, consisting of a native in the bush with a field telephone, whose only words in English were ”Aircraft over Gallabat”.

After an early warning on 4 October, Captain Brian Boyle led Lieutenants Viljoen and Andrew Duncan in a scramble from Azzoza. They were climbing over Metema when they saw three CR.42s in line astern approaching them 500 feet higher. Climbing to meet them head-on, the Gladiators attacked and the six aircraft broke up into a dogfight. Lieutenant Viljoen shot down one Fiat in flames, the pilot bailing out. Captain Boyle’s aircraft was hit and damaged but despite this he continued to attack one of the Fiats, seeing it fall away apparently out of control, trailing smoke.
Subsequently the ground forces sent in a report that this second aircraft had crashed, but the Italian recorded the loss of only one aircraft (that destroyed by Viljoen), claiming in return one of the Gladiators shot down. It is possible that Boyle’s victim force-landed in a repairable condition, or managed to struggle back to base. Almost certainly their opponents were from the 412^a Squadriglia.

On 11 November, a reconnoitring Hardy of 237 (Rhodesian) Squadron bombed Italian lorries on the road to Gondar. Around 12:30 some Fiat fighters scramble to intercept it, but instead had a clash with three Gladiators which were patrolling the area.
Lieutenants John Coetzer, Andrew Duncan and Viljoen of 1 SAAF Squadron had set off for Gallabat to intercept bombers. West of the town they ran into a reported eight enemy fighters instead. These were stepped up in echelon in three groups, 1524 meters above the three Gladiators. Despite tactical and numerical disadvantage, the South Africans attacked, chasing two of the Fiats down to ground level before they escaped, while the rest made one pass on the Gladiators and then fled. One Gladiator returned damaged by a single explosive bullet.
The Italians from 412^a Squadriglia returned claiming a Gladiator destroyed (probably by Tenente Mario Visintini) and two more probables, the latter one each to Sottotenente Giovanni Levi and Sergente Maggiore Lugi Baron.

In December 1940 1 SAAF Squadron started to receive Hurricanes, which the unit operated together with the Gladiators until all of these aircraft was replaced in the end of February 1941.

At 13:00 on 16 December Lieutenants Leonard le Clues Theron and Viljoen and Second Lieutenant Jarvis scrambled from Azaza to intercept bombers. One CR.42 was seen, but it saw them early enough, outdistancing their pursuit in a dive.

On the morning on 27 January 1941, six Hurricanes and six Gladiators of 1 SAAF Squadron, using Sabderat as an advanced landing ground for refuelling, attacked Gura airfield during the day, many aircraft being seen there. While Major Wilmot, Lieutenant Theron and a third pilot remained above as top cover, the other nine strafed, claiming hits on nine S.79s, seven S.81s and three Ca.133s.
The Gladiators taking part in the strafe were led by Captain Brian Boyle (Gladiator N5815) and the other five pilots were Lieutenants Viljoen, Robin Pare, Andrew Duncan, John Coetzer and Taylor. Hurricane pilots included Major Wilmot, Captain K. W. Driver, Lieutenant John Hewitson and Lieutenant Leonard le Clues Theron.
It seems that the attack was contested since Tenente Luciano Cacciavillani (attached to 412^a Squadriglia) reported in his logbook that he scrambled at 12:35 and fought against a couple of Hurricanes, with no results.
After the strafe at Gura, Captain Driver and Lieutenant Hewitson continued to Adi Ugri where Driver strafed four more S.81s, claiming damaged to two of them. Hewitson meanwhile strafed other ground target.
The Italians reported that three S.79s and four S.81 were substantially damaged.

On 3 February six Gladiators from 1 SAAF Squadron flew forward to a new landing strip called ‘Pretoria’ early in the morning where they refuelled. Five of them (Captain Brian Boyle (N5824), Lieutenant Andrew Duncan, Lieutenant Robin Pare, Lieutenant Viljoen and Lieutenant H. P. Smith) took off again at 11:45 to strafe airfields in the Gondar area, to the south. A landing site was spotted to the south of Azozo on which five Ca.133s were bombed up. The Gladiators attacked and claimed all five in flames. The Italians reported only one as actually destroyed in this attack.
Seeing CR.42s scrambling from Azozo, they flew over to investigate and Boyle, Pare, Viljoen and Smith claimed a shared damaged S.81 on the ground there when they reported that they had shot the wing off it while Lieutenant Duncan chased after a CR.42. The Gladiators were then attacked by Fiats and a big dogfight began. In the combat Captain Boyle claimed a Fiat shot down. Boyle reported that his claim was made towards the end of the dogfight and he was so close to the Fiat that he saw the enemy pilot clearly as he tried to bale out before crashing into the mountains.
Sergente Maggiore Enzo Omiccioli was shot down and killed in this combat. Reportedly he had scrambled alone.
The Gladiators then carried out a reconnaissance in the area, looking for reported Italian movements. On return, Lieutenant Smith crashed on landing at Azaza when starboard tyre burst and the aircraft slewed onto its side while Boyle suffered a flat tyre from the fight.

On 11 February, 1 SAAF Squadron had 11 aircraft on patrols over Keren during the day. During the morning, two of them encountered three CR.42s flown by Tenente Mario Visintini, Tenente Ubaldo Buzzi and Sergente Maggiore Lugi Baron engaged in strafing British troops in front of Keren. After the first brief clash the Fiats hid in a thick cloud. Lieutenant Viljoen followed and failed to return. Running low on fuel, he had to land near a village, fortunately within territory in British hands. He obtained petrol, and attempted to take off next morning, but crashed. He returned to Akordat on foot. His aircraft was later recovered and repaired.
Tenente Buzzi and Sergente Maggiore Baron landed at ‘Sabarguma’ airstrip (Gahtelay, in the Wekiro river valley between Asmara and Massawa) due to bad weather, while Tenente Visintini had reached Asmara.
In the afternoon, Visintini took off to fly back to guide these pilots home. It seems that during the flight he was perhaps blown off course by high winds and while descending through clouds he crashed into the sides of Mount Bizen, near the hamlet of Nefasit, and was instantly killed.

Air reconnaissance had reported that new CR.42s being assembled at Massawa after delivery by transport aircraft, and at 08:00 on 21 February, Major Lawrence Wilmot from 1 SAAF Squadron took off from Kassala with Captain Brian Boyle and Lieutenants Robin Pare, John Coetzer, Leonard le Clues Theron, de K. Viljoen and E. A. Jarvis. The Hurricanes refuelled en route at Agordat and took off at 11:00, attacking Massawa an hour later. A small number of aircraft was seen outside the hangars, but the number inside was not known, so all six hangars were attacked, the pilots approaching at zero feet and firing directly into them; all were left in flames, the roof of one being blown right off by Major Wilmot. Six Ca.133s and a CR.42 were strafed in the open, and claimed in flames, while Lieutenant Robin Pare wiped out an anti-aircraft position and crew, which were firing on Major Wilmot. His guns then jammed but he continued to make dummy runs on the AA posts, being the last to leave the area. Lieutenant Coetzer was however shot down and killed by AA fire, his Hurricane (V7658) was seen crashing in flames.
In fact, three Ca.133s and two S.81s, all of which had been damaged the previous day, were destroyed, and one more of each was damaged.

On 23 February 1941, seven Hurricanes of 1 SAAF Squadron flew forward to Tole where they refueled. At noon, Major Lawrence Wilmot (V7733?) led Captain Brian Boyle and Lieutenant Andrew Duncan to strafe Makele airfield while Captain K. W. Driver, Lieutenant de K. Viljoen, Lieutenant E. A. Jarvis and Lieutenant Leonard le Clues Theron provided top cover to the Vickers Wellesley acting as the navigation leader and which was intended to distract the Italians from the three low-flying Hurricanes as they went in. However, the RAF bomber lost its way soon after take-off, being dutifully followed by the top cover formation. In the target area, Major Wilmot realised that something was wrong when he saw a rising dust cloud to his left. He led his two companions toward this.
Finding Makele, the trio went in to strafe. Major Wilmot set fire to a CR.32, which was on the ground with its pilot Sottotenente Vincenzo Forcheri of the 410^a Squadriglia in the cockpit, about to take off. He leapt out under fire and managed to reach a trench.
Without their top cover, the Hurricanes were vulnerable to surprise attack, and at that moment reportedly three CR.32s arrived (the dust had been the sign of their scramble). Two fled but Sottotenente Alberto Veronese made good use of their advantageous position and dived on Major Wilmot’s aircraft, shooting it down. Lieutenant Duncan was on him like a flash and shot him down in flames. Veronese managed to parachute to safety, slightly wounded.
Wilmot had in the meantime carried out a crash-landing after that his Hurricane had been hit in the radiator, and became a POW.
Captain Driver, leading the four top-cover Hurricanes, now saw smoke rising, and dived down to join the strafing, the formation claiming five S.79s and three CR.32s set on fire and three S.79s as damaged.

He left the Squadron in 1941 but returned again on 18 August 1942. The unit was at this time operating in the Western Desert and equipped with cannon-armed Hurricane Mk.IIcs.

At 10:30 on 14 October, twelve Hurricanes of 1 SAAF Squadron provided top cover to 33 Squadron, escorting two tactical reconnaissance aircraft. Over Daba, five Bf 109s and MC.202s attacked, joined by three more. One Bf 109 was seen to crash, and it had to be decided which of three pilots, Captain Viljoen (Hurricane IIc …798/AX-T), Lieutenant H. F. Smith (HL892/AX-H) and Lieutenant D. T. Gilson (…666/AX-K), was responsible. All protested that it should be credited to the others, and eventually it was decided that it should be shared between them.

At 15:45 on 2 November, the Hurricane IIcs of 1 SAAF Squadron took off with 213 Squadron (15:45-16:25) as top cover. The 24 aircraft met 25 Ju 87s with twelve Bf 109s as close cover, about ten more being seen above these. The Ju 87s were just going down to bomb when attacked, many of them being seen to have Italian markings.
1 SAAF Squadron reported combat 24km west of El Alamein at 16:05 and Captain J. M. Faure (Hurricane IIc HL892/AX-H) claimed one and a probable, Lieutenant I. R. R. Pryde (HL797/AX-C) one and a damaged Bf 109, Captain Viljoen (HL845/AX-N) and 2nd Lieutenant McC. E. S. “Robbie” Robinson (HV314/AX-R) shared one, and Lieutenant H. F. Smith (HL833/AX-I) damaged two.
For 213 Squadron, Squadron Leader Peter Olver (BP401) claimed one, one probable and one damaged, Pilot Officer G. Carrick (BP592/F) and Sergeant D. C. Usher (BP345) one each destroyed, Flight Lieutenant S. P. V. Bird (HV309), Pilot Officer C. Luxton (HL613) and Sergeant R. W. Jones (BP599) one each as a probable, and Pilot Officer H. A. Aitken (BP583) one damaged.
Eleven Bf 109s of I/JG 27 and ten from III/JG 77 had taken off at 15:35 to escort the Stukas, attacking ten fighters before bombing and 25-40 afterwards.

From late 1942, he served as flight commander.

The Squadron re-equipped with Spitfire Mk.Vs from the beginning of November.

At 17:50 on 27 March 1943, twelve Spitfire Vs from 1 SAAF Squadron scrambled and were vectored 48 kilometres west-north-west of Gabes onto an Axis formation identified as eight Me 210s in two groups of four, flying at 3,000 meters. The Spitfires were at 6,000 meters and one section went down after the first four, two more sections following to attack the second Schwarm. The bombers jettisoned their bombs and broke up in disorder; two Bf 109s were seen, but these disappeared. One Me 210 was claimed by Lieutenant Douglas Rogan (Spitfire V AB320/AX-L). Three more were claimed as damaged by Lieutenant W. M. Langerman (AX-M), Lieutenant Bruce Rose-Christie (ER941/AX-E) and Captain Viljoen (ES150/AX-S) respectively. Lieutenant D. L. Quin’s aircraft (EP827) was hit by return fire or Flak and he was led by Lieutenant Gilson to territory occupied by the New Zealanders, where he crash-landed inverted and broke his back. He was rescued by own troops and subsequently recovered.
There is a possibility that the formation attacked by the South Africans was actually - or partially - comprised of Henschel Hs 129s of 8.(Pz)/SG 2 which lost three of these twin-engined anti-tank aircraft operating in the El Hamma area on this date, although reportedly all were the victims of Allied AA fire. Lost aircraft were WNr. 0259 reportedly shot down by AA 20km SW El Hamma (Unteroffizier Hans-Joachim Hesse KiA), WNr. 0352 65% damaged in combat over El Hamma (pilot safe) and WNr. 0388 hit by AA and crash-landed north-west of El Hamma being destroyed (Unteroffizier Karl Krauss WiA).

He was posted away from the unit on 11 April 1943.

During 1943, he served as air fighting instructor at 73 OTU.

Viljoen ended the war with 1 biplane victory.

Claims:

Kill no.	Date	Time	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1940
1	04/10/40		1	CR.42 (a)	Destroyed	Gladiator II		Metema	1 SAAF Squadron
	1941
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/5	Ca.133 (b)	Shared destroyed on the ground	Gladiator		S Azozo	1 SAAF Squadron
	03/02/41	11:45-	1/4	S.81	Shared damaged on the ground	Gladiator		Azozo	1 SAAF Squadron
	21/02/41	12:00-	1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41	12:00-	1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41	12:00-	1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41	12:00-	1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41	12:00-	1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41	12:00-	1/7	Ca.133 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	21/02/41	12:00-	1/7	CR.42 (c)	Shared destroyed on the ground	Hurricane		Massawa	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	CR.32	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	CR.32	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	CR.32	Shared destroyed on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared damaged on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared damaged on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	23/02/41	p.m.	1/4	S.79	Shared damaged on the ground	Hurricane		Makele airfield	1 SAAF Squadron
	1942
	14/10/42	11:00	1/3	Bf 109 F (d)	Shared destroyed	Hurricane IIc	798/AX-T	El Daba	1 SAAF Squadron
	02/11/42	16:05	1/2	Ju 87 (e)	Shared destroyed	Hurricane IIc	HL845/AX-N	15m W El Alamein	1 SAAF Squadron
	1943
	27/03/43	18:15	1	Me 210 (f)	Damaged	Spitfire V	ES150/AX-S	48km WNW Gabes	1 SAAF Squadron

Biplane victories: 1 destroyed, 5 shared destroyed on the ground, 1 shared damaged on the ground.
TOTAL: 1 and 2 shared destroyed, 1 damaged, 20 shared destroyed on the ground, 4 shared damaged on the ground.
(a) Almost certainly claimed in combat with CR.42s from the 412^a Squadriglia. 1 SAAF Squadron claimed two CR.42s for no losses while 412^a Squadriglia claimed one Gladiator for the loss of one of their own.
(b) The Italians reported only one as actually destroyed in this attack.
(c) Three Ca.133s and two S.81s, all of which had been damaged the previous day, were destroyed, and one more of each was damaged.
(d) This claim can’t be verified with Luftwaffe records.
(e) Hurricane IIcs from 1 SAAF and 238 Squadrons claimed 6 destroyed Ju 87s, 4 probably destroyed and 5 damaged, which none can be confirmed with Luftwaffe records.
(f) Possibly claimed in combat with Hs 129s from 8.(Pz)/SG 2, which lost 3 Hs 129s to reportedly AA (1 pilot KiA and 1 pilot WiA). 1 SAAF Squadron claimed 1 and 3 damaged Me 210 while losing 1 Spitfire V (pilot WiA).

Sources:
Luciano Cacciavillani's personal logbook courtesy of Cacciavillani family (Luciano jr and Alberto)
A History of the Mediterranean Air War 1940-1945: Volume Two – Christopher Shores and Giovanni Massimello with Russell Guest, Frank Olynyk & Winfried Bock, 2012 Grub Street, London, ISBN-13: 9781909166127
Dust Clouds in the Middle East - Christopher Shores, 1996 Grub Street, London, ISBN 1-898697-37-X
Fighters over the Desert - Christopher Shores and Hans Ring, 1969 Neville Spearman Limited, London
Springbok Fighter Victory: East Africa Volume 1 1940 – 1941 – Michael Shoeman, 2002 African Aviation Series No. 11, Freeworld Publications CC, ISBN 0-958-4388-5-4
Those Other Eagles – Christopher Shores, 2004 Grub Street, London, ISBN 1-904010-88-1
Additional info kindly provided by Stefano Lazzaro, Michele Palermo and Ludovico Slongo.