Whenever Hellkite Charger attacks, you may pay. If you do, untap all attacking creatures and after this phase, there is an additional combat phase.
10/1/2009 You decide whether to pay as Hellkite Charger’s ability resolves.
10/1/2009 If you pay , the new combat phase immediately follows the current combat phase. There is no main phase in between.
10/1/2009 Hellkite Charger’s ability may trigger multiple times in the same turn, since its own ability gives it multiple chances to attack. Each time it resolves, you may create an additional combat phase.
10/1/2009 If two Hellkite Chargers attack at the same time, both of their abilities trigger. If you pay for each, two new combat phases will be created. However, all attacking creatures untap as those abilities resolve, not as the combat phases start. Any creature that attacks in the second combat phase will remain tapped during the third combat phase.