Environmental control system supply precooler bypass

A precooler (7) for cooling compressor bleed air (38) for an environmental control system (6) includes a heat exchanger (56) in fluid communication with a source of cooling air (57) and operable for cooling the bleed air (38). A variable bypass valve (74) between a bleed air source (116) and environmental control system (6) is operable for bypassing at least a portion of the compressor bleed air (38) around the heat exchanger (56). The cooling air may be a portion of fan air (33) modulated by a variable fan air valve (76). The bleed air source may be selectable between the low pressure bleed air source (116) and a high pressure bleed air source (118). One method includes flowing the compressor bleed air (38) from a single low pressure source only and increasing thrust sufficiently to meet a minimum level of pressure of the bleed air (38) during one engine out aircraft operating condition during approach or loitering.