The rule allows the city's air regulators to impound planes for non-payment of charges and that aircraft can be sold after 60 days if the account is not settled.
Hong Kong Airlines, which has a fleet of 39 planes, said the impounded aircraft were not currently operating passenger routes.
"Our operation remains normal," the carrier said in a statement. The seizures will deepen concerns over the health of Hong Kong Airlines. The firm is owned by struggling Chinese conglomerate HNA Group, which has been looking to lower its debt burden.
The airline has been hammered by a sharp drop-off in tourists to Hong Kong as the city convulses from six months of violent pro-democracy protests.
Earlier this month, regulators threatened to suspend its licence unless it found new cash revenues after the company announced it was delaying some salary payments to staff.
The carrier found a last-minute injection of funds, earning a reprieve. Visitor arrivals have tanked during the protests with passengers from the Chinese mainland plummeting, hammering retail sales and helping to tip the city into recession.