DCA86AA001

Drg an ILS to rwy 35, the apch was normal until the acft was aprx 100' abv the decision height, except there was condensation on the windshield. Touchdown was hard. A witness said the acft lndd aprx 1/3 of the way down the 5200' rwy. After lndg, with the props in 'ground fine,' braking action was ineffective on the wet rwy. The acft was not equipped with an anti-skid braking sys. Subsequently, the acft contd off the rwy & went down an embankment into a field. Scrub marks on the rwy & reverted rubber on the tires indicated the acft was hydroplaning. According to the flt manual, 4500' (plus a 10% factor for no anti-skid) would have been required to stop on a dry rwy in the prevailing wx conds, which included a slight tailwind. Two pax, acting as flt attendants (f/a's), assisted the other pax in evacuating. An investigation revealed the f/a's were not properly trained; the pax were not completely briefed before the flt; & the pax briefing cards were inadequate. The acft was on a business flt to transport an entertainment troupe.