Over flight clearances
under ICAO Article 5, a private, non commercial, non scheduled flight does not require a clearance for over flight or landing for a fuel stop.
FLIGHT OVER TERRITORY OF
CONTRACTING STATES
Article 5
Right of non-scheduled flight
Each contracting State agrees that all aircraft of the other
contracting States, being aircraft not engaged in scheduled international air
services shall have the right, subject to the observance of the terms of this
Convention, to make flights into or in transit non-stop across its territory
and to make stops for non-traffic purposes without the necessity of obtaining
prior permission, and subject to the right of the State flown over to require
landing. Each contracting State nevertheless reserves the right, for reasons
of safety of flight, to require aircraft desiring to proceed over regions which
are inaccessible or without adequate air navigation facilities to follow
prescribed routes, or to obtain special permission for such flights.
All that is required is a flight notification or flight plan.
How ever a lot of third world countries are not used to private flights and ignore ICAO Article 5, insisting on clearances.
PNG and SOLOMON ISLANDS are ICAO registered states that are very confused regarding ICAO obligations.
When planning into or over 3rd world countries a lot of grief can be avoided by obtaining a clearance prior to the intended flight, even though it is NOT REQIRED !