Final answer:
The purchase of airplanes by the United States Air Force is classified as Government spending in GDP expenditure approach, as it is government expenditure on goods and services.
Step-by-step explanation:
The purchase of airplanes by the United States Air Force from a United States corporation falls into the Government spending category of the GDP (Gross Domestic Product) expenditure approach. Government spending accounts for expenditures on goods and services made by the government. Since the Air Force is a branch of the government, its purchases are classified as government spending. This category covers all levels of government and includes spending on defense, education, public safety, and more.