When a Charter flight crew attempted to exit the CYUL FBO ramp adjacent the Runway 24L approach end; they taxied onto a grass infield area which they could not see on the snow covered ground. The Commercial Airport Chart in use did not depict the grass infield areas.

Narrative

We landed on Runway 24R at CYUL. The aircraft taxied to the FBO ramp adjacent to the departure end of Runway 24L. While on the FBO Ramp; the aircraft was catered for the next flight. Several hours later we started engines to reposition from the FBO ramp to the gate. We exited the ramp at the same point of entry earlier; between previously plowed snowbanks adjacent to Taxiway R. The aircraft taxied onto Taxiway R and turned right towards the Hold Short position for Runway 24L as instructed by ATC. Prior to reaching the Hold Short Point; the aircraft felt as though the nose wheel had turned sideways on the hard packed snow on Taxiway R and power was added to assist the turn. The aircraft did not continue to move forward or turn and it was determined that the right main wheel assembly had either become stuck in a small snowbank; had displaced a grate on the tarmac under the snow; or had left the paved surface under the snow; contrary to what is depicted on the Commercial Airfield diagram as a fully paved area. The engines were immediately secured (the APU was still online and providing electrical power and air conditioning for the repositioning of the aircraft). FBO personnel brought out stairs and the right main assembly was inspected by the flight crew and flight mechanic. The flight mechanic decided that there was no damage to the right main wheel assembly or tires and suggested the aircraft be towed forward by a tug. Multiple attempts to free the aircraft by tug were unsuccessful and maintenance control directed that no further attempts would be made. The event occurred because the ramp & taxiway (Taxiway R) adjacent to the Northeast FBO ramp were covered/obscured by snow and the Commercial Airfield diagram for CYUL did not accurately depict the unpaved areas which existed between the ramp and Taxiway R and unpaved area northeast of the FBO ramp and the Hold Short area for Runway 24L. Aircrew should realize that; despite their best efforts to safely taxi aircraft in snow covered or otherwise adverse conditions; that errors in publications upon which all pilots rely may; in fact; contain errors which may lead to incidents such as this one. These errors must be brought to the attention of those responsible for the publication of such information to ensure as much accuracy as possible; and aircrew must be aware that these errors can and do exist in the publications we are to utilize and to exercise extreme caution when adverse weather conditions exist which increase aircrew reliance on these publications.