Last month, airline giant United Continental (NYSE:UAL) unveiled a notable shift in its capital spending plans. Under newly installed CFO Andrew Levy, United is reining in capex, starting with 61 Boeing (NYSE:BA) 737s that were deferred indefinitely.

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United Airlines deferred an order for Boeing 737s last month. Image source: The Motley Fool.

Levy has signaled that more changes to United's order book are on the horizon. This could be bad news for Boeing, which is United's No. 1 aircraft supplier. However, it all depends on how aggressive United is regarding reducing capital spending in the long term.

A historic bond

Nearly a century ago, Boeing and United Airlines were part of the same company. Even though they parted ways in the 1930s, United has remained one of Boeing's biggest customers. Today, United has roughly 200 outstanding orders for Boeing airplanes.

The bulk of those orders (about 160) are for the 737 MAX, following last month's 737 order deferral. But 737 MAX orders are a dime a dozen; Boeing currently has more than 3,300 on the books.

United's widebody orders are (and have been) even more important for Boeing. United ordered 65 Dreamliners, making it one of the largest customers for the new aircraft type. In the past two years, it has converted 14 of those orders to the 777-300ER, throwing Boeing a lifeline for its slow-selling 777 product line. These widebody orders are far more valuable for Boeing than 737 orders.

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United is one of Boeing's biggest customers for the 787 Dreamliner. Image source: The Motley Fool.

United is tightening its purse strings

In and of itself, United Continental's recent decision to defer the delivery of 61 Boeing 737-700s was not as problematic for Boeing as some pundits suggested. Boeing never stood to make much profit on the deal. It had sold the planes to United at a bargain price in order to block its rivals and keep United loyal to the Boeing 737 for its domestic fleet. Additionally, the 737 assembly line is overbooked for the next few years.

The bigger concern for Boeing (and Airbus) is that United CFO Levy indicated at the investor day conference that everything is on the table going forward. It's too late to significantly modify orders for 2017, but United Continental is reviewing all of its orders for 2018 and beyond, with an eye toward reducing capex.

Levy specifically stated a few weeks ago that United is thinking about converting its 35 orders for A350-1000s either to the smaller A350-900 model or to the cheaper A330 (or A330neo). The company is also considering buying more used aircraft going forward.

Why it matters for Boeing

Boeing has a lot to lose from a potential crackdown on capital spending at United Continental, because it has seemed likely in recent years that United would eventually order additional 787s.

United continues to fly 51 Boeing 767s, with an average age of nearly 20 years. Even the youngest 767s in its fleet were built in 2002. Meanwhile, United also operates 70 777s that were built in 2002 or earlier. Thus far, United has planned for the replacement of, at most, a third of these two widebody fleet types.

That could potentially mean dozens of additional 787 Dreamliner orders. And Boeing really needs more orders for the Dreamliner right now, as 2016 will be the third consecutive year that 787 deliveries outpace orders by a wide margin. Boeing has walked back its plan to increase 787 production from 12 per month to 14 per month, and some pundits think Boeing might have to slash output to 10 per month within a few years.

Dreamliner

Dreamliner orders have fallen short of deliveries for three consecutive years. Image source: Boeing.

If United Continental starts to focus more on minimizing capex rather than on improving its fuel efficiency, it might consider buying cheap 10- to 15-year-old A330s or 777s to replace its oldest 767s and 777s. It could also turn to Airbus' new A330neo, which isn't quite as fuel-efficient or capable as the Dreamliner, but much cheaper to acquire.

Of course, the Boeing 787 still has a lot of things going for it. It's critical to United's growth in Asia -- particularly its ability to expand into secondary cities in China. The 787 is also extremely fuel-efficient relative to the other small widebodies on the market. Lastly, it's very expensive to refurbish a mid-aged widebody, even if the purchase price is low.

Thus, United Continental could decide to order more 787s in the coming years in spite of its desire to hold down capital spending. But Boeing can't be nearly as confident about winning those orders as it may have been six months ago.

Adam Levine-Weinberg owns shares of Boeing. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.