The decision comes after the Welsh government said it would not step in to rule on the application at Llanbedr, near Harlech.
Snowdonia National Park Authority says the plans will be approved, subject to conditions, which are yet to be decided.
Park charity Snowdonia Society said the site was inappropriate.
It wanted the Welsh government to call in the plans and to hold a planning inquiry.
But it has now referred the decision back to the park authority.
The airfield employed 130 people before it closed in 2004.
Llanbedr Airfield Estates was previously granted a certificate to use the site to test unmanned aerial vehicles at the former military air base.
Earlier this month it applied to the park, as planning authority, to re-use hangars and associated buildings for aircraft maintenance including decommissioning / disassembly, parts recovery and refitting together with engineering training.
It says it would take up to 28 days to dismantle narrow bodied commercial airliners such as a Boeing 737 with bigger Boeing 747 taking up to 42 days.
After the airfield closed, Llanbedr Airfield Estates wanted to use the land for private flights and let empty buildings for business use.
Plans to turn a former airfield in Snowdonia into a yard to dismantle airliners will be given permission.