What happened

Electric plane start-up Vertical Aerospace (EVTL 4.19%) is reportedly nearing an important development milestone, the first flight of its VX4 aircraft. The stock is airborne as a result, rising as much as 17% in Wednesday trading.

So what

Vertical Aerospace is one of a number of so-called "flying taxi" start-ups that have hit the scenes in recent years, promising to revolutionize local air travel with small, battery-powered planes capable of vertical takeoffs and landings. Vertical's flagship VX4 is designed to transport a pilot and up to four passengers more than 100 miles at speeds exceeding 200 miles per hour.

It is an intriguing concept, and Vertical has lined up a number of potential airline customers, including U.S. heavyweight American Airlines Group. But those sales, and revenue, will only come if the VX4 proves itself to be as capable in real-world use as it is on the drawing board.

Investors might have an idea whether the VX4 has the right stuff before too long. Vertical Aerospace is preparing a VX4 prototype for a first flight scheduled for later this summer, according to Flight Global, after completing a series of ground tests.

Now what

Obviously, a first flight is just one smart part of a complex testing and certification process. But getting off the ground is a huge step in the right direction.

In theory, these small aircraft make a lot of sense as both a helicopter replacement to avoid gridlock on the ground and as a green option to replace small regional aircraft on routes between minor markets and airline hubs. But there are at least a dozen companies worldwide working on developing the technology, and some are likely to never make it past the design phase.

Vertical Aerospace has already lined up a strong list of customers interested in buying its plane and seems to be making steady progress bringing that plane to market. There is still a lot of risk here, but the company appears to be traveling in the right direction.