In a tweet posted this morning to his personal account, President-Elect of the United States Donald Trump expressed his disdain for the procurement program which will see the US Air Force replace its aging fleet of VC-25 VIP aircraft, used entirely as official presidential transports, with newer Boeing 747-8i airliners.

Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion. Cancel order! — Donald J. Trump (@realDonaldTrump) December 6, 2016

Labeled with the world-famous callsign “Air Force One” when the President is aboard, the VC-25, based on the Boeing 747-200B, has been in service with the US Air Force’s 89th Airlift Wing since 1990, having been bought and completed by 1986, flying for the first time the following year, and being delivered to the Air Force after an extensive refit three years later. Ever since, the two VC-25s have accumulated hundreds of thousands of miles traveling across the globe, ferrying the President, journalists, aircrew and ever-present Secret Service protective details from country to country on business.

The US Air Force’s Air Mobility Command, the parent unit of the 89th Airlift Wing, which oversees all Air Force-related Presidential travel, began looking into replacing their VC-25s in 2009, when it was found that due to their age (Boeing 747-200s first flew some forty five years ago), the aircraft have become considerably less budget-friendly to fly as frequently as they do. Parts are hard to come by, especially as Boeing 747-200s have been entirely retired from airline service and maintenance costs, as a result, are through the roof. A number of the companies that were originally contracted to build spares and parts for the 747-200 don’t even exist anymore. Currently, the average operating cost of a VC-25 is over $200,000 USD per hour. Hence the need for a new aircraft.

The base requirements in the request for proposals outlined that the new aircraft would be a “new-build, commercial derivative, wide-body aircraft”. It would be fully modified with a communications suite, a defense countermeasures system, air-to-air refueling capabilities and built to be as comfortable as possible for whichever President may fly on it. Ultimately, the Air Force announced Boeing as the winner of the contract in January of last year. Boeing had put forth concepts on all three of its widebody airliners – the 787 Dreamliner, the 777-300ER, and the 747-8i. The 747-8i was chosen, due to its four-engine build and probably also because it just looks downright presidential. This means that Boeing has been the Air Force’s go-to for VIP transport since the company first delivered modified early-generation 707 jet airliners for John F. Kennedy’s use in 1962.

Trump pushing for a cancellation of the VC-25 replacement project, known as PAR – Presidential Aircraft Recapitalization – would result in one of four things:

The Air Force being forced to investigate other low-cost alternatives to the 747-8i, which is currently the largest aircraft in production by Boeing. As the Airbus A380 is the only other four-engined commercial airliner in production, it would likely be the only other option to the 747-8i; four engines is a hard requirement for safety purposes. However, Airbus did not take part in the PAR program as it was commonly understood that it would only be an American aircraft considered for the job of flying… well, the American President. The Air Force staying with picking up two 747-8is, though on a delay, and refitted with off-the-shelf hardware it currently uses to fulfill the mission requirements. The Air Force sticking with the VC-25s and shifting more of the Presidential airlift burden to C-32 VIP aircraft (essentially Boeing 757 narrow-body airliners), which are already used from time to time as Air Force One and Air Force Two (for when the Vice President is the primary passenger). This would really only just delay the inevitable, which is buying the 747-8i as the replacement for the VC-25. VC-25s are virtually the best option for Presidential travel (due to their space, defense measures and communications gear), and thus need a dedicated follow-on that capitalizes on the strengths of the older airframe and mitigates weaknesses in the new one. Trump using his own personal Boeing 757 narrowbody private jet for Presidential travel in his official capacity. This would prove to be the least-likely option, as Trump’s aircraft is certainly not modified to the military standards required for Presidential airlift missions, and it isn’t crewed by US Air Force Air Mobility Command pilots and flight crew. The cost of modifying the aircraft to serve in this function would be unjustifiable as after Trump eventually leaves office, the aircraft will either have to be retired or put through a very costly refurbishment program that would see all of the necessary modifications removed.

At the moment, the Air Force has already invested $93 million USD into the PAR program to replace the VC-25, and the program’s overall costs are expected to reach around $2.9 to $3 billion USD for two highly-modified aircraft.