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. They heard nothing but "silence", even though the wind was from the S and the object approached them directly and at low level in the SSW. Wim concedes this but says:
"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 A.A. 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".
I'm not sure that I follow the structure of this argument. Lt Col. AMOND said that they stopped the car, wound down the window and listened whilst the object was in the SSW performing its apparent 360 turn and continued listening when it seemed to bear down on them with its brilliant headlight. Only after this had happened did his wife urge him to restart the car, precisely because the object seemed to have approached. He then did so, and this action coincided with the end of this phase of the event:
"As I start the car again, the big light disappears and three white spotlights, less important than the preceding one, become visible... The object then actually performs a turn of 180 degrees to the left".
Was this an Army helicopter, perhaps a Puma or a Sea King, as Wim suggests? Lt. Col. AMOND does not think so and explicitly dismissed that suggestion in an early interview. Not only was there no sound but even with the light of a favourably-placed full moon illuminating the landscape and the object from behind the witnesses, no reflected shape or structure was visible at any time. Coming from a couple no doubt familiar with Army helicopters one feels inclined to give that opinion some weight; but it's true, as Wim says, that the main obstacle from our point of view is the silence. Wim also points out that there is evidence of a probable temperature inversion that night which could have reflected sound upward and away from the witnesses.
I looked at the radiosonde profiles. The De Bilt ascent [De Bilt is the most important weather station in The Netherlands, located 174 km N of Ernage] does show a very slight inversion above ~100 m developing through the afternoon, Idar-Oberstein inland [a German weather station 215 km ESE of Ernage] shows a more severe one in place somewhat higher by midnight. But is it really the case that an intervening inversion will deflect the sound propagation upwards away from the listeners? I don't think so. On the contrary, an anomalous lapse rate would tend to bend sound upwards and away from the witnesses. The sound waves will actually refract in the direction of cooler air, so an inversion will bend sound towards the ground not away from it.
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I can imagine that an inversion boundary having a very sharp gradient might tend to partially reflect some sound frequencies where the waves impinge on the layer at a shallow angle. The frequencies that might tend to reflect would be those whose wavelength is of the same order as the depth of the gradient layer. But analogously with partial reflection of EM waves, the bulk propagation would probably continue through the layer by refraction in the usual way, and the overall intensity near the ground may well still be enhanced by the inversion.
This needs to be taken up with experts on acoustics in the free atmosphere, but I'm suspicious of the theory as it stands, especially given the extreme efficiency required of this process in the case in question where two alert witnesses consciously optimised the conditions for the express purpose of detecting sound and heard none despite a favourable wind.
Does someone have a reference for a scientific discussion of this supposed sound-reflecting property of inversions?
Another effect that might be considered is noise cancelling by interference between direct and ground-reflected crests and nulls of the sound waves. Maybe such a thing can occur briefly by a rare chance. For it to persist for a long time I imagine we'd need a helicopter to fly at constant altitude over a perfectly flat and acoustically uniform reflecting surface in a curve of constant radius with the witness's ear at the exact focus - we could even say that this condition is approximated during the first 2 - 4 mins of the Ernage sighting. But of course this is the phase of the least sonic interest, since the car engine was running then anyway. It is later, when the car was stopped and the object was manoeuvring in the SSW, that the absence of sound becomes problematical. That such a cancellation effect should happen to occur then, and persist, whilst the helicopter is turning 360 deg and manoeuvring through changes of altitude and attitude, seems to me to be unlikely in the extreme.
Note also that the effect of the wind gradient should also have been favourable. This is the velocity gradient between the surface friction wind and the wind aloft which has the effect of refracting sound waves towards the ground in the downwind direction and away from the ground in the upwind direction. In this case at the very time when the car was stopped with the window open on the silent road and with the occupants' ears "pricked" the downward refraction due to an hypothetical temperature inversion would have been enhanced by the direction of the wind and by the downward refraction of sound waves due to the velocity gradient.
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A further factor is the shape of a helicopter's sound "footprint" which is sensitive to direction and flight aspect. From various web sources I find that in general the footprint tends to form an ellipse whose major axis extends ahead of the flight path. In other words because they were immobile and with the window wound down when the object approached them, this factor also favours the propagation of noise towards our witnesses just at the time when they are listening, and when the helicopter is closest to them, when it is upwind of them and when they are also getting the benefit of the wind gradient refraction. (The footprint seems to be extremely sensitive to the helicopter aspect because of main rotor pitch and tail rotor noise etc; but in any case, the circling manoeuvre of the helicopter at this point would expose the witnesses to 360 deg of the helicopter's noise emission.)
Of course these considerations can be minimised if the "helicopter" is far away. It's interesting to work out just how far away it could realistically have been based on the cardinal directions of the terminal sighting points and the rough estimated duration, which can?give us an estimated average angular rate and thereby places an upper bound on distance for a given average true ground speed. According to Lt. Col. AMOND the initial phase of motion from the NW to SSW (about 130 deg on Wim's map) took 2 - 4 min (the total sighting duration including the circling manoeuvre and recession being 5-8 min). This corresponds to an average angular rate of about 0.5 - 1.0 deg/sec. By translating a realistic helicopter ground speed to this angular rate we can then find a realistic limit for the distance of an hypothetical helicopter from the witnesses.
A fast attack helicopter like the Apache has a max speed of 365 km/h going balls-out, but a realistic speed in this situation is going to be the speed of a more typical general utility helicopter - and not max speed, but typical cruise speed, because after all we know the thing has to be turning at low level (supposedly slowing and circling to inspect a possible car in trouble according to Wim's scenario, so not in a panic to get somewhere) and we don't want the engine screaming and rotors roaring any more than they have to. The typical max rated speeds of such helicopters tend to to be in the region of 250 km/hr or about 135 kt, so let's assume a reasonable round figure of 100 kt in cruise then. We find that at this speed, for the bracketed duration estimated, the maximum bracketed distance away would be 1.5 - 3 nmi (2.75 - 5.5 km).
True distance would probably be less than this upper limit. Proximity is implied by Wim's hypothesis - a chopper crew reacts with curiosity/concern to the presence of the witnesses' stopped car, thus explaining the circling and approach with bright headlight followed by retreat when the car is restarted. There is other internal evidence that would rather strongly support this conclusion. The witnesses described seeing a series of four illuminated "panels" which in the helicopter hypothesis become windows on the side of the helicopter and several separate lights which become running lights and a red strobe on the fuselage. In other words a good deal of visual structure was reported implying significant angular size. The circular glare of the white "headlight" was alone estimated at some two moon diameters during the approach (of course this is not going to be an accurate measure of the size of a lamp housing but a subjective impression of the apparent size of the light halo, nevertheless it follows the trend of significant angular size).
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Could these discerned details be consistent with a helicopter at the maximum bracketed range found above, 2.75 - 5.5 km? Highly unlikely. Let's allow generously that the "windows" on the side of a big helicopter fuselage span 20ft - then they would subtend only 6 - 12 arcmin at this range, with divisions between the windows being only about 1.5 - 3 arcmin apart. Detecting this structure at all would be close to the limit of normal visual acuity for a 20/20 human eye, and already probably flatters the hypothesis considerably. If the windows in the rear of the chopper fuselage span only a more realistic 10 ft (we don't want a huge and thunderous Chinook remember, just a small and quiet-ish helicopter) then the figures are halved and become very implausible indeed - only a speck of light would be discernable.
These lines of argument are strengthened with Lt. Col. AMOND's statement that at closest approach the object dropped to an estimated 40 m altitude and was visible in front of the treeline to the SW (he tells us the landscape was well lit by the full moon), which Wim's page tells us was found to be 1500 m from the car. This would place the object in the order of 1 km distant, which is consistent with the reasonings above and is indeed very close to the distance shown in the map based on Lt. Col. AMOND's own drawings (and incidentally would also place a helicopter well below the altitude of the recorded temperature inversions that night).
So how loud ought a helicopter to have been? I found some measurements of helicopter noise made for different machines in different flight phases at different distances:
A SuperPuma approaching the measuring equipment somewhat off the line of sight (drifting over 70 deg of bearing) when at a slant range of 280 m it had a noise level of just over 83 dBA, and at 220 m it was over 85 dBA.
Using the inverse square law we can calculate that all else being equal this Super Puma at a distance of 2.75 km (plausible based on angular rate) would generate about 1/10^2 or 0.01 of this noise, or in the order of -20 dB down on the measured level, i.e about 64 dBA, and at the favoured shorter distance of 1 km would be 13 dB down or about 72 dBA...
Examples of some types of noise in this region of 60-70 dBA are average factory noise, a vacuum cleaner running a couple of metres from your ear, average-to-busy road traffic, conversation at one metre, a large transformer, a busy restaurant, or TV shows. At 80 dB are heavy city traffic within 10 m, loud-radio music or a garbage disposal machine.
This is a very crude procedure I admit, but these noises are maybe indicative of the sort of sound level that we require to be completely mitigated by our hypothetical inversion "partial reflection" effect or other possible noise-cancelling effect - despite the several other factors tending to favour audibility in the circumstances.
Martin SHOUGH is the author of several articles and papers on UFOs and (alleged) radar detection thereof. He has compiled and evaluated an extensive catalogue of radar cases which forms the core of RADCAT, an exhaustive on-line resource presently under construction by Jan Aldrich and associates at Project 1947. SHOUGH has worked with Dr. David CLARKE on analyses of a number of UK military radar reports. He is a UK Research Associate for the National Aviation Reporting Center for Anomalous Phenomena - NARCAP, whose science director is Dr. Richard F. HAINES (see also our discussion of the 1990 Montreal case). Martin SHOUGH has worked on two major historical cases.
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Our reply :
The above reaction was posted on the EuroUFO list, an Internet forum created for long-time European UFO/UAP researchers (the web site of the European UFO Network can be found at www.euroufo.net). Martin SHOUGH's comments provoked a lenghty debate on the forum in which several noted investigators were to participate. Reactions were also received from non-members of the EuroUfo list, including Lieutenant Colonel André AMOND himself and Major General Wilfried DE BROUWER who was Chief of the Operations Division of the Belgian Air Staff in 1989. An extensive digest of this debate is presented here.
In our reply to Martin SHOUGH we pointed out that our source for the sound-refracting properties of inversion layers was a brief instructions manual in which it was explained how helicopter pilots can reduce the noise of their craft. We found it on http:/avstop.com/Helicopters/91. Unfortunately, this manual is no longer available at that URL. We do have a print though. What the manual tells us about sound and inversions is that "pilots should avoid flying under or in an inversion" because "an inversion layer has a tendency to 'bounce' the noise to the surface" (it is not sure whether the author used the term 'bounce' intentionally, or meant to say that sound waves refract (curve) when going through layers of different temperatures/densities). Anyway, we concluded from this that pilots who want to reduce the noise of their helicopters, should fly above an inversion layer so that part of the noise will be "bounced" upward. The question then was to find out if there was an inversion close enough to the ground for such a situation to occur.
For our web site article we relied on soundings from meteorological balloons released at Trappes (France), De Bilt (The Netherlands) and Idar-Oberstein (Germany). In addition, we consulted the synoptic data retrieved from nine Belgian weather stations surrounding the sighting location. Soundings from the station at Uccle (34 km NW of the sighting location), were not available when we visited the Brussels Royal Meteorological Institute, but thanks to Dutch investigator Frits WESTRA we now know that these data can be consulted at weather.uwyo.edu (the data from Uccle can be accessed by entering the station number, which is 06447). The data thus retrieved tell us that, at 01:00 am local time on December 12, 1989, i.e. 6 hrs and 15 min. after the sighting (the object was seen at approximately 6:45 pm), there were three inversions: one on the ground, one roughly between 1300 and 1900 m and one between 12,000 and 14,000 m (too high to be of any interest to us). These are the temperatures recorded at altitudes between 104 m (= station elevation) and 5,344 m:
|104 m||-3.1 deg|
|105 m|| -3.1 deg|
| 382 m||2.2 deg|
|1301 m||-3.5 deg|
|1403 m||-2.9 deg|
|1886 m||0.0 deg|
|2941 m||-6.5 deg|
|5344 m||-21.7 deg|
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Lt. Col. AMOND estimated that,at the beginning of the sighting, the unknown "craft" was at an altitude between 200 and 300 m. With the sighting location being approximately 160 m above sea level, this suggests that the object was flying just above the ground-based inversion layer. As such, part of the noise may have been refracted upward (possibly to be refracted downward again by the second layer and becoming "trapped" in between these two inversions). The bad thing is that there's a gap of more than six hours between the sighting and the soundings. In consequence, much of this remains guesswork. But perhaps the presence of a temperature inversion close to the ground is not so crucially important to explain the absence of sound. Even with no inversion present, it can be argued that the witnesses were located in what is referred to as the "shadow zone", a region that surrounds the upward curving waves and is free of noise (see illustration below gleaned from www.kettering.edu).
The synoptic data from the weather station at Gosselies,approximately 20 km WSW of Ernage, further tell us that, at 7:00 pm local time, there was a moderate to fresh breeze coming from the SSE. The object was in the west, so the wind may indeed have carried the sound away from the witnesses. The distance between the witnesses and the object (more than 1 km), and the fact that Mr. AMOND and his wife were inside a moving car, may also help explain why no sound was heard during the first minutes of the sighting.
So far so good, but the biggest problem of course is the phase during which the object made the turn and headed in the direction of the witnesses with the car now parked on the lonely road and one of the windows rolled down. In our web site article we rather quickly "solved" this problem by pointing out that it was at this moment that Lt. Col. AMOND restarted the car, meaning that, during that phase, the noise from the engine could have drowned a hypothetical sound coming from the unidentified object.
However, in his message of April 20, Martin rightfully points out that, according to the lieutenant-colonel, the car was started when the object was at its closest point, meaning that the noise of the car engine couldn't have drowned the sound of the object during the approach itself. So it seems like we have a problem here. Still, let's not forget that all this happened in a matter of seconds with the witnesses being overwhelmed with a feeling of anxiousness, and this from the moment the craft started to move in their direction.
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Many years ago the author of these lines had a rather terrifying encounter with a helicopter himself. The experience was powerful enough to convince him that the majority of close encounter cases mentioning "silent" objects with bright "searchlights" can be attributed to helicopters. The incident in question dates back to the early 1980s. It was described as follows in my reply to Martin SHOUGH on the EuroUFO forum :
"It was a calm evening with little or no wind. The sun had already set and I was walking on the narrow asphalt road that borders the Albert Channel (Albertkanaal) in the small village of Grobbendonk, Belgium. All at once, out of nowhere, a beam of light came from the sky and I found myself "trapped" in a cone of light. On the road and on the grass beside it was a very sharply delineated circular patch, about three or four metres across. I was standing in the middle of the circle and was terrified. I couldn't move and didn't hear a sound. It was only when the light went out and I looked around to see where it had gone, that I spotted a rather large helicopter in the distance, silhouetted against a part of the western sky that was illuminated by the streetlights of the nearby highway. Only at that moment did I hear a faint rotor sound. At no time had I felt any downdraft or heard any noise when the helicopter was straight above me. I never understood how that was possible, and the only explanation I could come up with is that I was in a sort of trance or dissociation by which the visual focus and emotional impact were so strong that they overrode the workings of the other senses (perhaps even affect my memory because I couldn't even remember having heard anything prior to the searchlight shining down on me). British UFO investigator Jenny RANDLES has coined the expression "the Oz factor" to describe this unusual state during a UFO close encounter. Others have invoked true UFOs mimicking helicopters (but forgetting to make noise). The same eerie impression of soundlessness also occurs during accidents : seconds before the car hits you (I'm drawing from personal experience again), all sounds seem to disappear, to only come back several seconds after you regain your senses. The effect is often used in movies, at times accompanied with slow motion images of the accident itself".
The biggest problem with this "emotional shock theory" is that Lt. Col. AMOND mentions explicitly that he "pricked his ears", something you would not normally do when in a state of dissociation. Could this have been just his way of emphasizing that absolutely nothing was heard?
Perusing the Internet for UFO reports mentioning silent UFOs that turned out to be a helicopter, we found only one such example (and not even a well-documented one). It can be found at www.ovni.ch (you need to scroll down to the sixth entry to find it). The report is in French and unfortunately no date is given. Below is an English (slightly edited) translation of the text.
"What can we say? We were on a trip by car in the area of Ypres, Belgium, when a blinding white light shone down on the car from above!! When we got out of the car, the light went out. No noise, except a dull sound. Then I saw a shape above us, I decided to follow this shape. I was thinking about a UFO and wanted to go to the bottom of this!! When I arrived at the army base at Ypres, I was speechless!!! An electric helicopter!! You can believe it or not but the helicopter was TOTALLY silent!! With its 3 lights, one in front and two at the back. All sorts of flying machines had crossed our minds, but we were dumbfounded because this helicopter didn't make any noise. It's true that our thoughts sometimes wander to unidentified flying objects, but now I had proof that this was not a UFO but this craft that hovered above us in silence!! Technology can be beautiful, but this scared the pants off of me!? "