Suffix -ING is a vowel suffix. It is added to the end of a base word to change its meaning to action now or materials, as in “land” to “landing.”

Suffix -ING can name an action. Landing can mean the action of coming down, as in:
The plane is landing now.

Suffix -ING can also name materials or a thing.

“Landing” can name a place or object, as in:
The landing strip was ready for the pilot.

Suffix -ING does not reduce to schwa in speech.

Suffix /ēng/, Suffix /ēng/,Suffix /ēng/, landing

When I-N-G is part of a base word, like “swing”, it is acting as a glued sound. If you take the glued sound I-N-G away, you are left with only part of a word, which cannot stand alone as a base word.

When I-N-G is a suffix, as in “landing”, it is added to the end of a base word. If you take the Suffix -ING away, the entire base word “land” remains.