Flammability of aerosols of hydrocarbon liquids was studied using a spinning disk atomizer, which simulates accidental aerosolization of a fuel from an aircraft. We determined the critical droplet sizes and fuel flow rates (or mass flux) needed to create an aerosol mixture that forms a stable flame upon ignition. At lower droplet sizes or higher flow rates than the critical values the aerosol mixtures were flammable. The critical flow rate increases with the droplet size for JP5 and are lower by a factor of 3 to 5 for heptane. The flammability diagram enables a quantitative comparison of different liquids in aerosol form. This is significantly different from the thermodynamic behavior of a liquid pool, which is dictated by the flash point or vapor pressure.