Strong aircraft demand and the improving profitability of its 787 Dreamliner family have driven incredible earnings and cash flow growth at Boeing (NYSE:BA) in recent years.

This upward trajectory continued in 2018, despite production delays at an engine supplier that slowed Boeing 737 MAX output for much of the year. Boeing capped off its most successful year yet earlier this week with a strong Q4 earnings report. Furthermore, the aerospace giant forecast that earnings and cash flow will reach new heights in 2019. As a result, Boeing stock jumped 6% on Wednesday, approaching the all-time high it reached last year.

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Boeing smashes its forecasts, as usual

Boeing's growth rate accelerated last quarter, as the company did its best to catch up on aircraft production following delays earlier in the year. Boeing still missed its forecast of delivering 810 to 815 commercial jets in 2018 -- it delivered 806 -- but Boeing's fourth-quarter delivery total rose 14% year over year, driving a 12% revenue increase in its commercial airplanes segment.

The defense and services segments also grew at double-digit rates last quarter. As a result, revenue rose 14% year over year to $28.3 billion. For the full year, Boeing generated $101.1 billion of revenue. That was up nearly 8% year over year and comfortably beat the company's most recent guidance range of $98 billion to $100 billion.

The acceleration in revenue growth enabled Boeing to blow past its earnings forecast. Core earnings per share reached $5.48 last quarter, bringing full-year core EPS to $16.01: up 30% year over year. Boeing's most recent guidance had called for core EPS between $14.90 and $15.10, and its initial outlook for 2018 had projected that core EPS would come in between $13.80 and $14.00.

A rendering of a Boeing 737 MAX 9 flying over clouds.

Image source: Boeing.

Not surprisingly, Boeing also surpassed its forecast for free cash flow. At the beginning of 2018, management had expected full-year free cash flow of approximately $12.8 billion. Free cash flow ultimately reached $13.6 billion.

Check out the latest Boeing earnings call transcript.

The 2019 outlook is spectacular

Boeing is carrying great momentum into 2019, and management doesn't expect that to stop. For the upcoming year, Boeing is projecting revenue of $109.5 billion to $111.5 billion, which would represent 8% to 10% growth. The commercial airplanes division will drive most of that increase.

Meanwhile, Boeing expects core EPS to surge roughly 25% to a range of $19.90 to $20.10: well ahead of the analyst consensus of $18.31. (And if history is a guide, Boeing's guidance could prove to be conservative.) Finally, the company's cash flow forecast implies that free cash flow will rise about 10% to a range of $14.7 billion to $15.2 billion.

Strong order activity last year supports Boeing's optimistic forecast for 2019. The company ended 2018 with a contractual backlog worth $462 billion, up from $457 billion a year earlier.

A tougher climb after 2019

While Boeing may seem invincible right now, the company has benefited over the past several years from periodic production rate increases for its 737 and 787 programs. Furthermore, while the 787 Dreamliner was extremely expensive to build at first, production costs have declined steadily over the past five years, boosting profitability.

However, the last scheduled production increases for the 737 and 787 aircraft families will be implemented later this year. And while Boeing would like to increase 737 output again -- suppliers' production capacity is the main obstacle right now -- the next production adjustment for the Dreamliner is likely to be downward.

Boeing ended last year with 622 net firm orders for the 787 family. That's less than four years of production at the 14-per-month rate planned for later this year. (For comparison, the 737 has a seven-year production backlog.) Additionally, 787 orders have routinely lagged production in recent years.

In the short term, declining 787 production costs should drive further profit growth for Boeing. But it is virtually inevitable that Boeing will have to reduce its 787 production rate a few years down the road, possibly by a substantial amount. Boeing stock's rally may not be over yet, but investors should remember that the production increases that have powered its recent earnings growth may not be sustainable -- particularly in the event of a global recession.