During the 747-8 flight testing, we had a lot of issues with in-flight starting of the GEnx-2B engine. The GEnx engine simply didn't want to light off a lot of the time (although once it lit, it would start fine - it was getting the fire going that was a problem). Lots of tweaks to the FADEC light-off software eventually resolved the issues satisfactorily, but my buddy that was responsible for certifying the in-flight start envelope remained nervous that we'd get an in-flight non start for some reason and it would re-open that can of worms.
I was on a flight test shortly before we got type cert - they were doing some engine-out autothrottle testing and we'd been cruising around at 37,000 for about an hour with an engine shutdown. After they'd completed the test condition, the pilot asked me if he could go ahead and try a windmill restart of that engine. Not only were we several thousand feet above the restart envelope (30k IIRC), the engine was cold soaked which is pretty much a worst case for getting a light-off. I responded 'sure - worst that can happen is it won't start'. Well it started right up, first try - no problem.
Next day I was in the office and stopped by my buddies desk and told him about it - he was aghast that we'd even tried - what if it had failed?
'Who cares, we were well outside the start envelope - I figured it didn't hurt to try'. But he was paranoid that if the FAA heard we'd had
any in-flight start failure the FAA would want a complete investigation and want to re-do some of the in-flight start testing...
