New York CNN Business  — 

United Technologies, the 84-year-old maker of elevators and jet engines, is breaking itself apart.

The company announced plans late Monday to spin off its Otis elevator division as well as its Carrier building systems business. The remaining company, squarely focused on aviation, will be led by jet engine maker Pratt & Whitney and newly acquired parts maker Rockwell Collins.

Splitting United Technologies (UTX) into three separate public companies marks the end of one of America’s last great industrial conglomerates. UTC, a member of the Dow Jones Industrial Average, was founded in July 1934 under the name United Aircraft Corporation.

“Our decision to separate United Technologies is a pivotal moment in our history,” UTC CEO Gregory Hayes said in a statement on Monday.

Investors have soured on the conglomerate model typified by General Electric (GE), which at one point provided everything from movies and microwaves to mortgages, MRI machines and locomotives. Today, GE is racing to break itself apart to pay down a mountain of debt caused by poorly timed acquisitions.

Conglomerates are out of style

Wall Street was once enamored of the benefits created by joining dissimilar businesses under one roof: shared sales forces, more attractive financing and diversification to protect against downturns in one industry. Investors have since grown frustrated with the difficulty of valuing these vast empires.

UTC’s booming aerospace business, for instance, is much more attractive than Carrier, which makes heating and air conditioning systems. That’s why activist investors have pressured UTC to break itself up.

“The company has been more hampered by its model than it’s been helped,” said Jim Corridore, an analyst at CFRA Research. “It makes a lot of sense for United Technologies to break itself up.”

By splitting up, UTC said the three companies will have the resources, flexibility and visibility to be successful.

“I’m confident that each company will continue our proud history of performance, excellence and innovation while building an even brighter future,” Hayes said.

The breakup will take place as a tax-free spinoff of the Otis and Carrier businesses to existing shareholders. The move was announced just as UTC completed its acquisition of Rockwell Collins, a $23 billion deal unveiled in September 2017. That deal received final regulatory approval from China last week.

UTC shares retreated nearly 6% on Tuesday following the breakup news. Corridore said investors may be frustrated that UTC said the split won’t be completed until 2020.

Pratt & Whitney makes engines for commercial planes and military jets like the F-35. The breakup of United Technologies will allow the remaining company to focus squarely on aviation.

GE, Honeywell are shrinking too

UTC is just latest conglomerate to split up. Last year, Honeywell announced plans to split up its thermostat and security division from its transportation systems business. Honeywell (HON) is keeping its aerospace division.

Industrial manufacturer Danaher (DHR) announced plans in July to spin off its dental business. ITT, which was founded nearly a century ago, has spun off its defense and water technology businesses in recent years.

And GE is in the process of saying goodbye to its railroad and healthcare divisions as well as its oil-and-gas business. Former GE CEO Jeff Immelt recently said he wished he moved faster to separate the industrial business from GE Capital, the money-losing financial arm.

Of course, some storied conglomerates are staying put, at least for now. For instance, 3M (MMM), which makes Post-it notes, cleaning supplies and backup-camera monitors for cars, remains intact.

But the remaining conglomerates are under pressure to sell off pieces or at least review their portfolios to make sure they still make sense.

“The conglomerate space is not likely to be the same over the next five years as it was over the past 50,” said Corridore.