Eilmer, like William of Malmesbury who recorded his feats, was a monk at Malmesbury Abbey, which was at that time a very rich establishment both materially and culturally. Until shortly before Eilmer's birth, Malmesbury had been the (first) capital of all England under King Athelstan. Domesday Book records that it owned 50,000 acres of land. Eilmer was, for his time, a knowledgeable astronomer and capable mechanic.
William, a well-respected and reliable historian, recorded Eilmer's achievements in his "Chronicle of the Kings of England". Although William did not know Eilmer personally, he would have been told of the events by eye-witnesses.
Thanks to an error by Ralph Higden in his "Polychronicon" of 1352, Eilmer's name is frequently mis-spelled as Oliver, an error to which Prof. Lynn White (professor of history at University of California, Los Angeles) has drawn attention. William's book, written of course in Latin, spells it Eilmerus.
Prof. White was the first modern worker to research Eilmer's efforts in depth, considering the cultural situation that could have led him to consider flight, the mechanical and scientific knowledge he would have had and other competing claims to have achieved the first flight using a heavier than air device.
The technical aspects, such as the materials then available, the necessary clear height needed to achieve the appropriate glide angle and the options for that in Malmesbury, and the impact of the wind on the venture have all been analysed in some depth by Maxwell Woosnam in his booklet "Eilmer; 11th Century Monk of Malmesbury: The Flight and the Comet" published in 1986.
According to Messrs. White and Woosnam, Malmesbury Abbey at that time would have had a typical Saxon high tower, probably about 80 feet tall. The abbey stands on a hill top with a sharp 50-60 feet drop to the river about 600 feet away to the north and west, less sharply to the river 1,800 feet away to the south and east. The High Street runs down the south slope. The prevailing wind is from the south-west, and leads to considerable uplift as it meets the hill and the abbey walls, which local jackdaws delight in using to soar to considerable heights. Eilmer would have made use of this, and was probably blown along the ridge by the wind. He is reported to have landed near the High Street and a little to the west of it, precisely where modern experiments predict would be the case.
Calculations show that Eilmer must have been quite a small man, and would have used wings with a total area of about 100 square feet. His main difficulty would have been to hold the wings steady on top of the tower in a wind strong enough for his purpose. This is what puts the upper limit on the wing size, and hence indicates he must have been quite light for that size of wing to be adequate to support him. The structure would have been such that it could not fold upwards, and was probably made of ash or willow (both readily available locally) with a covering of fine cloth or parchment. It is known that his wings were attached to his hands and feet, which is consistent both with the wing area needed and with the required delta shape necessary to achieve stability without a tail. That he lacked a tail is known from his own (erroneous) belief that he would not have been injured on landing if he had had one.
The glide ratio he would have needed to achieve his distance of 200-220 yards on the route used would have been about 4.3:1, which is quite feasible in that location with a suitable wind.
Eilmer was unfortunate in being thrown about by gusts, causing the precipitate end of his flight and resulting in his breaking both legs on landing. His injuries resembled those from a skiing accident, almost certainly because he landed at speed on a downward sloping hillside. He lived to a ripe old age despite being partially lame, and gained more fame at the time and for some time after his death for having seen Halley's Comet twice in his lifetime (and somehow realising it was the same one) than for his flying!