Each takeoff is different. The weight is different, the winds, temperature, pressure, terrain ahead, etc..
We have to calculate the thrust & speeds needed to get off the ground before we run out of runway, and do so in a fashion that will allow us to clear terrain…assuming we lose an engine.
So, we plug the details into our computer. It sends that off to another computer at the airline, which does the calculations & sends back a solution.
This solution tells us how much thrust we need from the engines, flap settings, trim setting for the elevator & at what speeds we will perform certain actions. These speeds are referred to as “V speeds”, for “Velocity”.
V1 is arguably the most important. As part of the computers solution, it is using the “balanced field” concept. There is no need to use maximum thrust on a 12,000’ runway…you can take it easy on the engines, reducing wear & tear. On the other hand, a 6,000’ runway is going to require a lot of flaps & maybe maximum thrust. Depends on the weight, temperature, etc.
So the computer does this work, with the goal of creating a situation where at some point in the takeoff, the distance required to safely continue the takeoff is equal to the distance required to reject the takeoff & stop. There is a point along the runway where these two are equal.
That point isn’t going to be clearly marked on the runway. As I said, the numbers change for every takeoff depending on the conditions. So instead of looking for a distance, we use a speed.
You can think of this as V1. I’m simplifying here, but I think that will help you better understand.
At V1, we have hit the balanced field point. We now have enough runway in front of us to either safely continue the takeoff (even if an engine fails) or safely stop in the runway remaining.
Hopefully by now you can see what is happening after V1…you can safely continue the takeoff, but you have passed the point where you can safely attempt to stop. If you attempt to stop, you are going off the end of the runway & into the dirt at an unknown speed. It’s going to be a bad day.
Procedurally, 5 knots before reaching V1, the pilot not flying calls out “V1”. This triggers the pilot flying to take their hand off the thrust levers (throttles) & put it on the yoke (or wherever, just get it off the thrust levers. Keep in mind procedures vary slightly between airlines).
It’s a commitment that almost no matter what happens now, we are going flying. We will not attempt to stop for almost anything.
(you might attempt to stop for some extreme event, like a wing falls off or something, but emergencies like engine fires, failures, hydraulics…even a fire in the cabin will not stop us from taking off past V1. It’s safer to get in the air & work the problem)
So, V1 is a decision point. You don’t actually do anything to the airplane, you just change your mindset.
Now, our calculations & procedures are all based on losing an engine at precisely V1 speed or after, where you can no longer stop. Don’t worry, we’ve done the math. The airplane will fly & it will clear the terrain, though it’s going to be very busy for the pilots for a while.
Hope that helps. As usual, I’m simplifying some for a general audience & there is more to it, but hopefully you now have an understanding of the term V1.
Another ill-fated take-off over V1 (in this case, V2!)
June 13 1996: Garuda Indonesia Flight 865, a DC-10, crashes in Fukuoka (Japan) 3 of 275 aboard die. On take-off, the jet had just left the ground when one of the engines failed. Crew opted to reject the maneuver; the aircraft was unable to stop safely and left the runway: landing gear collapsed, and the airplane caught fire.
Inquiry noted pilot actions by aborting takeoff above V1, where SOP dictated pilots should have continued takeoff and deal with the engine later. More info below from Aviation Safety Network – Aftermath video is from Aircrashdaily (go give them a follow on YouTube)
“The DC-10 accelerated for takeoff. The nose was raised, and at a speed of 158 kts, the first officer called "Rotate". It was 12:07:40. Three seconds later, at a radio altitude of 9 feet, a fan blade of the 1st stage HP turbine from the no. 3 engine separated. The N1 dropped to 23,7% within a few seconds. At 12:07:45, the flight engineer called "Engine failure number one." Takeoff was aborted at about the V2 speed, and the airplane contacted the runway one second later at a vertical acceleration force of 2.1 Gs.
The thrust reversers were deployed and ground spoilers were extended. The DC-10 skidded off the runway through a ditch, fence and a road, before coming to a halt 620 m past the runway threshold.
Investigation revealed that the turbine blade that failed, had operated for 30913 hours and 6182 cycles. General Electric had advised customers to discard blades after about 6000 cycles.
Accident cause
Although the Aircraft was well in excess of V1 and the aircraft had already lifted off from the runway, the takeoff was aborted. Consequently, the aircraft departed the end of the runway, came to rest and caught fire. It is estimated that contributing to the rejection of the takeoff under this circumstance was the fact that the CAP's judgement in the event of the engine failure was inadequate."