Kalitta Air will begin operating the world's first Boeing 777-300 converted freighters in October, following certification delays of more than two years. The aircraft were modified by Israel Aerospace Industries, reports New York's FreightWaves.

The Michigan-based cargo airline received two retrofitted aircraft at its Oscoda maintenance facility last month. The planes are part of a seven-unit order leased from AerCap and will replace ageing Boeing 747s.

IAI launched the 777-300 conversion programme over five years ago. Certification from US and Israeli authorities was granted in August. Modifications include reinforced floors, a wide cargo door, cockpit barrier and container handling systems.

The redesigned jets, dubbed "Big Twin," offer 25 per cent more volume than the 777-200 and 14 per cent more than a 747-400 freighter. They are also 21 per cent more fuel-efficient, making them ideal for lightweight e-commerce cargo.

Kalitta Air operates 21 Boeing 747-400s and 10 factory-built 777 freighters. Its clients include DHL Express and the US military. Chief executive Connie Kalitta said the new aircraft will enhance range and payload capacity.

IAI and AerCap have dozens of orders for converted 777s. Customers include Emirates SkyCargo, Challenge Airlines, EVA Air and Fly Meta. The latter will lease aircraft to Air Atlanta Europe.

Mr Kalitta was originally scheduled to receive its first "Big Twin" in 2023. Delays were caused by supply chain issues, FAA scrutiny of Boeing, and disruptions from Israel's conflict with Hamas.

Mammoth Freighters, a US competitor, has 35 orders and began test flights of its 777-200 in July. It is producing aircraft in Fort Worth and Manchester. Qatar Airways is its launch customer.

A third firm, Kansas Modification Center, is also developing a 777-300 conversion but trails behind IAI and Mammoth.

Industry leaders warn of a looming freighter shortage as older aircraft retire and new models face production delays. About 20 per cent of the 650 widebody freighters in service are over 30 years old.

IBA estimates the cost of a mid-life 777-300 converted freighter at $75 million to $80 million, rising to $100 million with engine overhaul. IAI has partnered with Etihad Engineering, Sharp Technics K and Ascent Aviation to accelerate deliveries.