An amateur owner and her aged bay jumper were anything but amateur yesterday afternoon as they flew over the Capital Centre jump-off course to win the Insilco Jumper Classic by three seconds over For maggio at the Washington International Horse Show.

Owner-rider Brendan Damon of Honolulu and Atticus negotiated the twisting course without a fault in 30.599 seconds. Susan Dickey, on the Rhiannon Farm's Formaggio, gave it a good try with a clean round, but in a clocking of 33.741 seconds.

The third fastest time of the six who managed to survive the first round and make it to a jumpoff was posted by Lt. Alice Rowan, U.S. Navy, on her Ryewacket, but the aged brown gelding took out a rail along the way for four faults. Ryewacket's time was 31.358 seconds.

Atticus' victory in the Insilco clinched the amateur-owner jumper championship and the Foxbrook Trophy that goes with it. The campion had 16 points, while the reserve tricolor went to Pyewacket.

County Wicklow's victory in the junior jumper stake class was not enough for the Gary Young-ridden jumper to overcome Donald Cheska's Close Encounter in the battle for the Greenberg Challenge Trophy as Junior jumper champion. Although Close Encounter failed to score in the stake close, the 11 points bunld up over the competition was enough for the tri-color.

The Ri-Arm Farm of Franklin Lakes, N.J., scored an impressive double in the junior working hunter divisions with its small (under 16 hands) hunter, Rain Forest, winning the Junior Equitation School Challenge Trophy and its large (16 hands and over) hunter, Bridgegroom, taking the Dilwyne Challenge Trophy.

Bridgegroom, handled by Mark Leone, won the Fairfax Junior Hunt Challenge Trophy in the final stake class and closed out the battle with 12 points. Reserve went to Bonnie Blake on Gozzi with eight points.

John Carnes rode his Short Stirrup to the Small Pony Hunter tricolor with eight points, gaining the Stom-bock Saddlery Challenge Trophy as champion.

The Pegasus Stable Perpetual, one of the oldest trophies in the show was captured by Sarah Deal on her Had N' Hardy as large pony titlist after a backoff with Shelly Stigler's Hi Fi.