The current paper contains construction and method of calculating of heat and hydraulic characteristics of appliance for air cooling of heat-generating equipment, mounted outside aircraft. Oncoming air flow is used as cooling medium. It passes through flow cavity of air thermolplates on surface of which heat-generating equipment is mounted with provision of guaranteed thermal contact. Method of calculating allows determining the ranges of high and speed of aircraft flight for given heat load and temperature of oncoming air flow, at which temperature of equipment is maintained no higher than the preset level. Also method allows to justify the need to split air path of appliance on independent parts to ensure uniform distribution of air flow through flow cavity of air thermolplates.