CMpilot1& gaMGle3
As the aircraft climbs the pressure is dropping and the air mixed in the fuel keeps bubbling out much the same way as beer going flat. The time it takes to climb to 30000ft plus 30mts all the air should have gone out but if the aircraft hasn't climbed to 30000ft or hasn't spent 30mts above 30000ft then there can be significant amount of air in the fuel. The only option available is to keep it under higher pressure to prevent the bubbling. That is achieved by descending to 15000ft or 30000ft in the later case. Anything below is even better.