Question: In Star Trek Beyond, they fix up the old Federation ship, USS Franklin. It wasn't made to fly in atmosphere, so in order to take off they need to propel it off a cliff and reach terminal velocity before the impulse engines can fire. (From what I can recall from a single viewing - if any of those assumptions are wrong please correct.) 

What I don't understand is *how* does terminal velocity allow impulse engines to just "start working"?

Answer: 
The quote from the film is 

> 
> **Kirk:** *Scotty, can you get this thing started?*
> 
> 
> **Scott:** *Started yes, Sir. Flying, that's a different thing. These old vessels, they were built in space. They were never supposed to
>  take off from atmosphere.*
> 
> 
> 

then

> 
> **Chekov:** *We have to achieve terminal velocity **in order for the stabilizers to provide lift.***
> 
> 
> 

Quite why this is the case isn't explained other than it being [treknobabble](https://memory-alpha.fandom.com/wiki/Technobabble) to explain why they need to push the ship off of the cliff.

---

Stabilizers are, at least in the Abramsverse, an integral part of starship atmospheric flight

> 
> “Mr. Sulu,” Spock exclaimed, “divert all remaining power to stabilizers!”
>  “Doing what I can, sir,” the helmsman replied as he desperately fought to comply. “Doing what I can.”  
> 
>  Spock tried his best to see that the Enterprise’s vanishing energy resources were parceled out meticulously among the ship’s most critical active systems. While life support drew the most attention, he and Sulu attempted to steady the starship’s wildly skewing and rapidly failing artificial gravity. If he couldn’t stabilize it any better, there was a good chance a large percentage of the ship’s crew would never be able to make it to their assigned evacuation stations. Yet if he shunted power from life support to the precessers, there was a chance atmospheric pressure would fall too low and kill everyone on board.
> 
> 
> [Star Trek Into Darkness - Official Novelisation](http://kn)
> 
> 
>
Spaceships in atmosphere are usually depicted as flying due to downward force from an engine, like a hovering harrier. Since the Franklin wasn't designed to fly in atmosphere, it may not be able to direct sufficient force in a downward direction to effect a vertical takeoff.

So, if it can't just take off straight up, it must need to generate lift a different way. Assuming that the ship is producing lift via [Bernoulli's Principle](https://en.wikipedia.org/wiki/Bernoulli's_principle) and an [airfoil](https://en.wikipedia.org/wiki/Airfoil), needing a jump start makes perfect sense.

The lift generated by an airfoil is dependent upon the speed at which the craft is moving through the air. This means that the craft needs to get going fast before it can actually fly. This is why commercial aircraft require long runways, so they can get up to speed. As you may have noticed, there was no runway in front of the Franklin.

Therefore, before the ship could fly under its own power, it needed to get going fast enough that the stabilizers could keep it flying, despite providing minimal lift on their own. This required falling off a cliff.

In other words, it's just what Chekov said:

> 
> We have to achieve terminal velocity in order for the stabilizers to provide lift.
> 
> 
>