Proof of the required reliability, basically. Both showing that you've done it at a lower certification level, and actually doing demonstration flights where yo

Proof of the required reliability, basically. Both showing that you've done it at a lower certification level, and actually doing demonstration flights where you run around with only one engine. Certification authorities check and make sure the required reliability is available (because having your second engine fail 5 1/2 hours from the nearest diversion airport sucks). ICAO conveniently has some info: https://www.icao.int/SAM/Documents/2014-EDTO/EDTO%20Module%20%204%20%E2%80%93%20Aircraft%20certification%20considerations.pdf https://www.icao.int/SAM/Documents/2014-EDTO/EDTO%20Module%2...