It does indeed seem unlikely that an object with clearly visible windows, capable of executing a sharp turn close to the ground and carrying bright lights, was an AWACS , an Ultra Light Motorized vehicle, or a hologram (sic). But why not a helicopter?
Helicopters are perfectly capable of performing 360 degree turns at low altitudes. Many army helicopters have portholes which look like "illuminated panels" when the lights inside the craft are on. Furhtermore, the majority of helicopters carry white lights underneath the fuselage, often in combination with a red rotating light. And finally, all army and police helicopters have one or two bright searchlights in front.
Actually, only the absence of sound seems to argue against the helicopter explanation.
We checked the weather conditions for the evening of December 11, 1989 to see if the wind or the presence of an inversion layer may help explain why no sound was heard. The official weather data  tell us that their was a light to moderate breeze in the evening, and near gale toward midnight. The direction of the wind was south to south-southeast. When Lt. Col. AMOND first spotted the panel-shaped lights in the distance, the object was in the west. At that moment, the wind, coming from a generally southern direction, may have carried the sound away from the witnesses. When the object executed the 360 degree turn, it was closer by and in the south-southwest. Under these circumstances one would expect that at least some of the noise had reached the couple. However, from his letter of December 19, we know that Lt. Col. AMOND restarted the engine of the car immediately after the big light appeared. It is therefore conceivable that, during this phase, the sound of the car engine drowned the sound of the unidentified object.
Balloon soundings from the French weather station in Trappes (275 km/171 miles southwest of Ernage), the Dutch station in De Bilt (175 km/109 miles north of Ernage) and the German station in Idar-Oberstein (220 km/137 miles east-southeast of Ernage) further indicate that the weather conditions were ideal for an inversion layer to form over large parts of Western Europe . Inversions occur when cold air near the ground gets trapped under a layer of warmer air on top. The sound of a helicopter flying above an inversion layer may bounce back upward off the layer, so that it doesn't reach an observer on the ground. It is not known how strong this effect can be, but it may be an additional factor in helping to understand why no sound was heard.
Whatever the cause, the fact is that, in rare cases, helicopters manoeuvring in close proximity of observers can remain unheard. CAELESTIA has several cases in its files which involve sightings of helicopters that directed bright spotlights at cars, people and houses, and which, despite their being close by, appeared completely silent. Actually, the helicopters in these cases could only be recognized for what they were because, at a given moment during the sighting, the craft moved off to a different part of the sky where there was more background illumination, making it possible for the observers to discern the typical silhouette of a helicopter. As is explained in our introductory note for the helicopter picture gallery, not only weather conditions, but also a state of dissociation caused by an emotional shock, and the use of special mufflers to reduce rotor noise, can help explain the absence of sound in helicopter encounters.
The article by BOUGARD & EYCKMANS from which we borrowed this case, describes 23 more UFO incidents that occurred in the French speaking part of Belgium that same night. Included in their list is the spectacular encounter that took place in the early hours of December 12 at Jupille-sur-Meuse, 67 km (42 miles) east of Ernage. Here the object, which was shaped like an egg with a dark "windscreen" and three spotlights underneath, did make a sound. The principle witness to the event described this sound as "a kind of vibration, not continuous but intermittent; a dull buzzing sound, comparable to an axle having play in a roller bearing". In other words, much like the sound of a helicopter. If the object sighted by this witness was indeed less than 80 m (262 feet) away and only 10 m (33 feet) off the ground, as was claimed, it may well have been hovering below the inversion layer. Under these conditions, sounds will be audible to an observer on the ground because they are trapped between the ground and the base of the inversion. Still, the question remains whether the object sighted at Jupille-sur-Meuse was the same, or just a similar object, as the one observed more than seven hours earlier at Ernage.
To illustrate the similarities between the object sighted by Lt. Col. AMOND and an army helicopter executing a sharp turn, we made a composite illustration in which stills from a computer-animated rendering of the sighting were placed next to images of a model helicopter. In creating the latter images, we opted for the Aérospatiale PUMA SA 330 because it is one of the few helicopters we know of that carries three white lights underneath in a similar configuration as described by the witness . Moreover, three PUMA SA 330 helicopters were operated by the Belgian Army in 1989 (all three craft were stationed at Brasschaat Air Base, 73 km or 45 miles north of Ernage).
Left: five computer generated images representing the phenomenon observed at Ernage. Right: five comparative pictures depicting a scale model of a PUMA SA 330 helicopter with computer-added lights.
[The images on the left originate from a computer animation made by the Centre Laique de l'Audiovisuel (CLAV) in the company of Lt. Col. AMOND; they were scanned from Vague d'OVNI sur la Belgique / Images on the right: © CAELESTIA]
A helicopter flying in a straight line from the military air base at Brasschaat to the military air base at Florennes, located 111 km (69 miles) south of Brasschaat, will pass over Ernage. It is not outrageous to imagine that a pilot, who spots a parked car on a lonely country-road, decides to take a closer look, makes a U-turn, and switches on his searchlight. Noticing that the car starts to roll as he approaches, the pilot will assume that everything is all right and will continue his turning manoeuvre to regain his initial course, one that will take him towards the air base of Florennes, which is located less than 40 km (25 miles) south of Ernage.
In evaluating this case, one should not forget that the UFO fever which broke out in Belgium at the end of November 1989, had reached its climax by mid December. Despite the cold, many people were spending the evening outside hoping to catch a glimpse of one of the mysterious triangles. On an almost daily basis, stars, planets and airliners were mistaken for UFOs, so why not the much rarer sight of an army helicopter with its bright lights skimming the landscape at night .
Unfortunately, letters to our country's military information services and to pilots who flew the Belgian Puma helicopters in 1989, yielded no response.