There are sometimes extraordinary circumstances which require alternative methods of refueling a jet. Let me tell you about a situation at a Central American aeropuerto that does not have adequate ground air start equipment to start a large high bypass ratio turbofan, which would require a minimum of two air start units, preferably three. And suppose that your APU is out of service, what to do? Well, the answer is self explanetory. One engine is left running during the offloading/loading and refueling process.
