Endo-1,5-alpha-L-arabinanase involved in degradation of pectin. Its preferred substrate is linear 1,5-alpha-L-arabinan (By similarity). Endohydrolysis of (1->5)-alpha-arabinofuranosidic linkages in (1->5)-arabinans. Glycan metabolism; L-arabinan degradation. Belongs to the glycosyl hydrolase 43 family.