TRIGGER POINT OF WIND DETECTION

Abstract

During flight, without means of measurement or data transmitted by the ground, while exploiting the possible hints (smoke, dust, ripples on water surfaces, etc...), it is extremely difficult to detect and assert the strength and the direction of the wind at ground level, and more precisely the effective wind on track of the selected final leg to an outlanding. It is not always possible to make the few spiral tums that would allow a more precise apprehension of the wind before one has to commit oneself to the beginning of the "downwind leg". If the wind is in the flow of the selected track, only the appreciation of the ground speed allows some kind of pertinent estimation of the true wind. Having these considerations in mind, it is interesting to determine the precise minimum speed from which the true wind may be detected, and consequently, if it is sufficient to have to be taken into consideration for the safety of the final approach. Our experience, double checking, and the bibliography currently available, have shown that the minimum detected true wind is not less than 10 or 12 kt and may reach 15 kt and even more in the case of some tailwinds when there has been reports of a belated detection of a 20 kt wind.When reference to the altimeter readings is out of the question, particularly while on the base and final legs, the appreciation of the height, produced by and in disassociable from the appreciation of the distances, is completed by the understanding of the relative motion of all the reference points which are on both sides of the trajectory. This phenomenon appears right from the downwind leg, is intensified with time, and makes it much easier to appreciate "too high, too near" or "too low, too far" while on the base leg. During the short final, this phenomenon totally replaces the appreciation of the slope of descent. It is what happens when a pilot, in his flare out, with an excessive speed, 1 m off the ground at one end of the runway, wishes to stop at the other end,at a distance of 1000 or 1200 m right abeam his hangar. At this moment, the stopping point is seen under an angle of about 1/1000, i.e. totally unusable in a vertical plane. However, the displacement speed and the closing-up speed of all visible elements feed the pilot at every moment with data concerning the relation between the present and the remaining distances, and allow him to set the air brakes precisely and to stop within a few meters of the desired spot.