I read somewhere else that high energy SAM fragmentation particles have such a high velocity/energy that an aircraft will start burning when being hit. Looks like MH17 parts started to burn on the ground and not in the air. Any experts
I've never seen an airliner hit by a SAM but when I was making my living flying over North Vietnam i saw plenty of my colleagues flying U.S. Navy fighter and light attack aircraft (and in one case a SH-3A helicopter) hit by SA-2 missiles. The results varied widely. In some instances there was warhead detonation resulting in a huge fireball of burning aircraft fuel with no large pieces of the aircraft exiting the short-lived fire. What came out of the fireball looked like confetti. Yet, in some cases, the pilot survived.
In other cases the aircraft seemingly survived the warhead detonation (which by itself produced a pretty good sized fireball) and remained essentially intact, or at least in one large piece without any fire other than the result of the missile warhead. Some pilots subsequently ejected and some did not. Sometimes the aircraft impacted the ground and burned. Others either didn't burn or perhaps I didn't notice. It's usually a busy time when the air is filled with SAMs, some of which seem to have your name on them.
In one case, I saw the aircraft main fuel cell rupture producing a big ball of white fuel vapor which did not ignite.
I would say that a large and very noticeable in-flight fuel fire might be expected when any aircraft is shot down by a large SAM, but that outcome is certainly not guaranteed. The fact that MH 17 apparently burst into flames only after impact with the ground is not all that surprising, at least to me.