You may have heard that United has decided not to fly Washington/Dulles – Dakar and Newark – Stockholm next year. Many have assumed that this is due to weak demand, but that doesn’t appear to be the primary driver. After speaking with several sources, it seems United has an engine shortage, and it has no choice but to cut capacity. Presumably these routes are the weakest performers, but it’s unclear that they would be canceled if not for this engine problem.

To be clear, this is completely unrelated to the Pratt & Whitney GTF engine manufacturing issues that have plagued airlines flying the A320neo and A220 family aircraft. This is also not a technical fault. The problem is with availability of PW4000 engines that power the pre-merger United 777 fleet. (Continental opted for GE.) These are just not easy to find anymore.

The PW4000 was a popular engine that powered all sorts of different widebody aircraft when it was rolled out in the late 1980s, including the A300/A310, A330, B767, B747, and even the MD-11. The biggest version of that engine was the PW4000-112 which was developed specifically for the B777 with more thrust and a bigger 112-inch fan diameter. Because these engines are substantially different in size and thrust from the 94- and 100-inch thrust versions, I’m guessing that versions not built for the B777 can’t really help United in its quest to find more engines. If that’s wrong, well, it could slightly broaden the pool of possibilities, but this would still be an issue. (Then again, there are a lot of MD-11s not using engines right now….)

The -112 was an early option for the B777, and it’s one that most airlines did not choose. Of the 629 B777-200/200ER/300s flying today, only 81 of them are Pratt-powered. United has 46 of these airplanes flying along with another six that were parked at some point this year, including N775UA which was just sent to Victorville at the end of last month. That means all the other airlines in the world have a total of 32 aircraft in service.

United’s breakdown is that it has 19 of the old B777-200 “A” models which have the PW4077, meaning they had only 77,000 pounds of thrust. At last check, three of those were parked. It has another 33 B777-200ERs with the PW4090 engine (you guessed it, 90,000 pounds of thrust). Twenty-nine of those are configured in the airline’s international configuration and are flying regularly (or in regular maintenance). There are four aircraft that are ERs with the higher-thrust engines, but they are configured in the domestic configuration. Of those, it appears only one is flying right now. I imagine these are the airplanes that United will be robbing first to fund the international aircraft.

So, if an engine shortage is the problem, then what can United do? The first thing is can do is get new airplanes that don’t have this problem. Too obvious? Of course, United has already done that. It has scores of B787s on order, but, well, those airplanes have taken significant delays thanks to Boeing’s issues. That has meant the B777s have to keep on keepin’ on for longer.

The other option is to find PW4000s on the open market. According to Cirium data, here is a look at the airplanes that are in service today with PW4000-112 engines.

Those PW4077s powering the A models for United? There appears to be only one other airframe flying, and that’s owned and operated by Max Air in Nigeria. I don’t imagine that’s going to be a good opportunity for United. I don’t know how easy it is to use engines with different thrust and get more life out of them. So it’s possible that the seven ANA airframes with the slightly less powerful PW4074 could be made to work. Anybody know? I’m not sure.

The situation is a little better for finding PW4090s that power United’s ER fleet. There’s United’s joint venture partner ANA once again with three -200ERs and five -300s. Maybe ANA would like to help its partner-in-crime by retiring some airplanes. If not, well, it looks like Korean Air is the only other option.

Korean has four B777-300s of its own that are PW-powered while its low-cost subsidiary Jin Air has four of Korean’s old B777-200ERs. And then there’s new Korean Air subsidary Asiana with eight of its own. I’m sure Delta’s best friend in the Pacific would love to help United out, right? RIGHT?! Ok, maybe not.

This is unfortunately what happens when aircraft age, and it’s even worse when there’s an unpopular engine. To break it down further — excluding the B777-300ER which could only be GE-powered — 70 percent of those 629 B777s in service have GE on the wing with another 18 percent having Rolls. The Pratts were the least popular, and United flies more than half of them already. The numbers do not work in United’s favor here.

I don’t know if the Dakar and Stockholm flights are just the tip of the iceberg or how many aircraft will need to be grounded. But it seems pretty clear that these airplanes are running out of time, and those B787s can’t arrive soon enough.