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Gravitational collapse of molecular cloud or cloud core/clump may lead to theformation of geometrically flattened, rotating accretion flow surrounding thenew born star or star cluster. Gravitational instability may occur in suchaccretion flow when the gas to stellar mass ratio is high (e.g. over$\sim$10\%). This paper takes the OB cluster-forming region G10.6-0.4 as anexample. We introduce the enclosed gas mass around its central ultra compact(UC) H\textsc{ii} region, addresses the gravitational stability of theaccreting gas, and outline the observed potential signatures of gravitationalinstability. The position-velocity (PV) diagrams of various molecular gastracers on G10.6-0.4 consistently show asymmetry in the spatial and thevelocity domain. We deduce the morphology of the dense gas accretion flow bymodeling velocity distribution of the azimuthally asymmetric gas structures,and by directly de-projecting the PV diagrams. We found that within the 0.3 pcradius, an infall velocity of 1-2 km\,s$^{-1}$ may be required to explain theobserved PV diagrams. In addition, the velocity distribution traced in the PVdiagrams can be interpreted by spiral arm-like structures, which may beconnected with exterior infalling gas filaments. The morphology of dense gasstructures we propose appears very similar to the spatially resolved gasstructures around the OB cluster-forming region G33.92+0.11 with similar gasmass and size, which however is likely to be approximately in a face-onprojection. The dense gas accretion flow around G10.6-0.4 appears to be Toomreunstable, which is consistent with the existence of large-scale spiral arm-likestructures, and the formation of localize gas condensations.

Interpreting observations of edge-on gravitationally unstable accretion flows