Context : Gravitational collapse of molecular cloud or cloud core/clump may lead to the formation of geometrically flattened , rotating accretion flow surrounding the new born star or star cluster .
Gravitational instability may occur in such accretion flow when the gas to stellar mass ratio is high ( e.g .
over \sim 10 % ) .

Aims : This paper takes the OB cluster-forming region G10.6-0.4 as an example .
We introduce the enclosed gas mass around its central ultra compact ( UC ) H ii region , addresses the gravitational stability of the accreting gas , and outline the observed potential signatures of gravitational instability .

Methods : The dense gas accretion flow around the central UC H ii region in G10.6-0.4 is geometrically flattened , and is approximately in an edge-on projection .
The position-velocity ( PV ) diagrams of various molecular gas tracers on G10.6-0.4 consistently show asymmetry in the spatial and the velocity domain .
We deduce the morphology of the dense gas accretion flow by modeling velocity distribution of the azimuthally asymmetric gas structures , and by directly de-projecting the PV diagrams .

Results : We found that within the 0.3 pc radius , an infall velocity of 1-2 km s ^ { -1 } may be required to explain the observed PV diagrams .
In addition , the velocity distribution traced in the PV diagrams can be interpreted by spiral arm-like structures , which may be connected with exterior infalling gas filaments .
The morphology of dense gas structures we propose appears very similar to the spatially resolved gas structures around the OB cluster-forming region G33.92+0.11 with similar gas mass and size , which however is likely to be approximately in a face-on projection .

Conclusions : The dense gas accretion flow around G10.6-0.4 appears to be Toomre unstable , which is consistent with the existence of large-scale spiral arm-like structures , and the formation of localize gas condensations .
The proposed approaches for data analyses may be applied to the observations of Class 0/I low-mass protostars , to diagnose disk gravitational instability .