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According to your web site, when the craft turned and moved in the direction of the witnesses, it was at a distance of 1 km (0.62 mile or 3,280 feet), a distance that can be measured on the scaled map....  read more 

In this case, an Army officer, Lt. Col. AMOND [now retired], and his wife, stopped their car on what Wim [VAN UTRECHT] describes as a "lonely road" and wound down the window with "ears pricked" to try and detect any sound from an object of evidently large angular size showing details of lit "panels" or windows as well as various other separate lights which circled apparently nearby for up to 8 minutes.....  read more 

It is correct that an inversion will refract sound towards the surface, and the recommendation to avoid flying in an inversion makes sense therefore. But the inference that flying above the top of an inversion will have the inverse effect of abnormally refracting sound upward is unwarranted I think. The manual [see "Our reply" to Martin's previous mail] is recommending that you try to avoid unduly amplifying your helicopter noise for people on the ground, and that if you can avoid flying in inversions you can keep your ground noise footprint merely normal.

Normal sound propagation in a standard atmosphere has a small upward tendency because of the temperature/density lapse rate, which is what is referred to here, but that doesn't mean that flying machines are normally inaudible unless there is an abnormal atmosphere (inversion), just that sound waves launched horizontal will normally tend to rise a little - but of course not much of the sound energy is launched horizontally by (say) a helicopter: the net air momentum is shed very definitely in the downward direction and carries rotor and engine noise with it. Helicopters flying by are normally very audible (as I can tell you from frequent first hand experience with a variety of choppers on military exercises, mountain rescue operations/exercises and hydroelectric power-line surveys that make a pest of themselves in this glen, not to mention occasional air-ambulance helicopters that have been known to use a nearby flat pasture as the only practical landing spot).

In any case, if a helicopter was involved at Ernage, as I pointed out, there are several interlocking lines of evidence - latent and explicit - that suggest it must have been quite close and quite low, especially during the close approach upwind of the witnesses where it was reported against the backdrop of the moonlit trees at an altitude of only a few tens of metres. Such a helicopter could not have been above the top of the surface inversion recorded, which was at a few hundred metres (higher than 220 m AGL - we don't know where the actual top is, only that it is somewhere in the big gap between about 220 m and the next reading at ~1160 m AGL), and not very strong (compared with gradients that cause optical mirage for example, which tend to be many times as severe).

It's possible that the witnesses' guess of 2-300 m places the object above it during the start of the sighting. But this is a secondary matter. There's no evidence that flying above an inversion does more than help avoid intensifying the normal noise footprint. In fact even flying above an inversion would be expected to intensify that footprint above the normal for the reasons I discussed, though of course not by as much as flying within it.

Apart from the fact that this not the acoustically interesting phase of the sighting, I'd only note that specific quantities are probably the least reliable of things in witness descriptions of objects seen at night in the absence of valid distance cues. Much more reliable IMO is information latent in basic topological relationships, such as that an object was in front of A or behind B etc, where angles and distances of A or B can be physically determined afterwards. In this case that procedure applied to the most relevant portion of the sighting - when the object appeared nearest, upwind of the alert witnesses with motor off and window open - indicates an altitude of only a few tens of metres, i.e. order of 1/10 the height of the Uccle surface inversion. That isn't unarguable, but it is interesting.

I think it's worth emphasising that the witnesses noted, whilst driving, that they could not hear anything and took steps specifically designed to test the possibility that sound from the object was being drowned out: They stopped the vehicle, turned off the engine, opened the window and listened with "ears pricked". Their actions show that they thought about this possibility. They did have some expectation of being able to hear the object (i.e., had it been a familiar thing such as an Army helicopter) if conditions were right. And were the conditions in which they tested that expectation actually rather poor, by bad luck? No. We can show on the contrary that their own actions coincided with the external conditions for sound propagation in their direction becoming most favourable.

I also pointed out that all factors appear to conspire to enhance the conditions for audibility at this time. When the witnesses were listening, with the car engine off, the window open and their "ears pricked", the object was:

- upwind of the witnesses
- due South within about 20 deg of the SSE surface wind (Uccle wind at 280 m AGL was virtually due S, 00:00Z 12 Dec)
- at its lowest (according both to report details and to the "investigating a stopped car" helicopter scenario)
- at its nearest point to the witnesses
- approaching the witnesses (a typical helicopter noise footprint ellipse extends ahead of it)

Its noise carrying downwind would be enhanced by earthward refraction due to a temperature gradient inversion and further enhanced by the downward refraction of sound waves due to the wind velocity gradient

According to Lt. Col. AMOND he stopped the car so that the object, which had lagged behind on their right, now came abreast of them on their right. This was when they opened the window after 2-4 min. The object then continued past them on the right, heading SE. So this fly-past, and the entire 360° circling manoeuvre that followed, with the object between SSW and due S, took place with the car silent and the window open. These events account for at least half of the total estimated sighting duration of 5-8 min.

The story of your personal encounter with a near-silent helicopter (see "Our reply" to Martin's previous response) is very interesting. Evidently dissociative states such as you describe can occur, and in combination with special conditions unfavourable for sound propagation towards the witness, it makes sense that sometimes people can be surprised and misled by unnaturally quiet helicopters.

But considering all the factors I personally think it unlikely in this case, where conditions (so far as we can ascertain) appear to have been rather favourable for sound propagation, and where the witnesses' actions (more telling than merely subjective self-reports re state of mind) seem to me to indicate some presence of mind and alertness to the need to test initial impressions.

Martin SHOUGH
Strathconon, Scotland

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Re the Ernage case in particular and silent helicopters in general, I looked a bit further into the sound propagation question and helicopter noise sources. Perhaps this will help us decide the merits of the theory. Anyway I've collected some information for future reference - and just because it's interesting.....  read more 

I'd like to point out that Mr. PAQUAY treats Lt. Col. AMOND's "2x moon diameter" light as relating to the angular size of a light source, when clearly this should be treated as an impression of the circle of glare (overloaded retina) caused by this "huge" and "brilliant" light shining into the witness's face......  read more 

I would like to correct the approach of Mr. PAQUAY who claims that the observation of the "spotlight" was made while the craft was 1,000 m from my observation point. This is COMPLETELY WRONG.......  read more 

In his response, dated June 5, 2008, Col. AMOND claims that my distance estimation of 1,000 m for the phase during which the craft turned towards him is completely false and that the real distance at that moment was 100 to 200 m. He also disputes my calculations and qualifies them as unscientific.......  read more 

The Belgian airspace is surveyed by four powerful radars, two military and two civilian, which are all interlinked, i.e. any duty controller can select the image of any of these radars at any one time. All radar registrations are recorded and these recordings are kept during a well determined period......  read more 

A little comment about the strawman argument at the end of Wilfried DE BROUWER's email......  read more 

In my opinion, the important information in DE BROUWER's e-mail is not in his last paragraph about the reliability of Col. AMOND. Of course human testimony is not always reliable, but Jean-Michel ABRASSART tends to say that it's NEVER reliable. I think he pushes his argument too far here, turning it into some kind of indisputable doctrine.....  read more 

Now I am beginning to wonder : how could it be that the four Belgian radars did not detect these objects in the sky which were neither aircraft nor helicopters...  read more 

Gen. DE BROUWER's first-hand testimony confirms not only that no helicopter should have been flying (as previously reported by Col. AMOND) but that, in point of fact, no illicit helicopter or other aircraft was detected by radars covering the area....  read more 

I will try to contribute to this debate by sharing with you that I have the following documents concerning the Ernage case....  read more 

Re your reply to Prof. MEESSEN, you're right, the statement about flight plans in the Defence Minister's reply to the Parliamentary question is different from what we understood Gen. DE BROUWER to say....  read more 

Thanks for Mr. PAQUAY's follow-up. I was not (as Mr. PAQUAY believes) suggesting that Col. AMOND was "dazzled" to explain a large image. As he says, an "overloaded retina" is not a very good explanation....  read more 

Allow me to clarify a few points. 1. Your statement: "A helicopter is perfectly capable of executing such a manoeuvre". The report of André AMOND suggests that the object was making a tight turn with a considerable angle of bank (45 degrees?) at very slow speed (20-30 km/h?)....  read more 

I don't claim to have the detailed aeronautical experience of General DE BROUWER, but during my own military service, I did have some exposure to helicopters as a passenger and as an observer from the ground.....  read more 

Thanks for your input Joe. Yes, clearly it can happen that when the conditions for sound propagation and other witness circumstances are unfavourable people can fail to hear helicopters......  read more