How do airline crews make sure they have enough runway for a safe takeoff? Check out our latest video to find out:

When you're flying an aircraft under Part 91, the FAA doesn't have much to say about how much runway length you need for takeoff.

But, if you're flying under Part 121, the rules are more limiting. Your runway length, and associated max takeoff weight, is governed by the FAR Takeoff Field Length.

It computes distances to make a normal, all engine takeoff. And it considers the distance to reject a takeoff, as well as continue after an engine failure.

And then, you limit your weight to make sure all three can safely happen on the runway.

It's not the only weight limiting calculation done in Part 121 preflight planning, but it's a key ingredient. So, let's see how field length limits your takeoff weight, starting with an all engine takeoff.

The All Engine Takeoff

The All Engine Takeoff Distance is the total length to accelerate from a full stop on all engines, rotate at Vr, and climb out to reach 35 feet at an airspeed that's at least V2, which is your Takeoff Safety Speed. The FARs require 115% of that value, and that's your All Engine Takeoff Field Length.

What Happens If An Engine Fails?

FAR Field Length also plans for an engine failure: one where you stop, and one where you continue the takeoff.

That brings us to Accelerate-Stop and Accelerate-Go distances. It's also where V1 comes into play: the point where you switch from stopping, to going. You can think of it as a "Takeoff Action Speed."

V1 is the fastest speed where the pilot must take the first action to reject a takeoff so that the aircraft will come to a stop within the calculated Accelerate-Stop distance. That action might mean applying brakes, reducing thrust or deploying speed brakes.

V1 is also the minimum speed during the takeoff where, following the failure of the critical engine, the pilot can continue the takeoff and reach 35 feet above the takeoff surface at V2 airspeed, within the computed Accelerate-Go distance.

Accelerate-Stop and Accelerate-Go Distances

FAR Part 121 requires a runway long enough to meet the Accelerate-Stop Distance for your weight at V1. You can include a stopway, which is a hard-surfaced portion after the runway intended for rollout during a rejected takeoff that won't structurally damage the aircraft. But, airlines often won't add in stopway distances when computing performance. That gives them an extra margin of safety.

Accelerate-Go distance considers the opposite situation. You lose an engine at Vef (V Engine Failure), just prior to V1, and continue your takeoff.

This is the total distance to accelerate from a standing start, through the engine failure and V1, and continue the takeoff to 35 feet at your takeoff safety speed, V2, using only aerodynamic controls and average piloting skills.

Balanced Field Length

V1 is usually positioned so that Accelerate-Stop and Accelerate-Go and distances are the same.

This is called a "balanced field," and it allows you to take off with the most weight possible for a specific runway length. As V1 gets faster, it takes less energy and distance to continue takeoff, reach V2 and 35 feet after an engine failure.

So, as you increase V1, you can meet Accelerate-Go requirements for a runway at a higher gross weight. But, as you increase V1, it takes more distance and energy to stop following a failure. So as you increase V1, you must be lighter to meet Accelerate-Stop requirements for the runway.

The point where these lines meet is the Balance Field Limit V1 Speed. And this weight is called the Balanced Field Limit Weight. That V1 speed allows you to take off with the most weight on a specific runway.

Making Every Takeoff Safe

With airline training, normal takeoffs, single-engine takeoffs, and rejected takeoffs are rehearsed, routine procedures. And, while rejecting a takeoff isn't common, the procedures centered around V1 make the process simple: you've made your decision before anything happens.