Even if you keep an extensive spares inventory at outstation, great chance it will not save the day for ULH. Troubleshooting the problem, getting the spare (which could be 30 min drive airside one way in large airports), replacement, testing, paperwork - and the crew is out of duty. The way to go almost bulletproof is to keep a hot standby aircraft for departure but if I am not mistaken only BA (and possibly AF) did it in JFK for Concord operation. It is simply way too expensive for the normal business.