Thrust Set...V1, Rotate - What are Takeoff V Speeds?
What are Takeoff V Speeds? Why and how do pilots use them?
Have you ever heard what happens inside a flight deck during the takeoff roll? If you haven’t then you’d be interested to know that there’s a lot that happens before the aircraft gets airborne.
Most of the call-outs are related to the aircrafts V Speeds, and they are really important.
In this post, I’m going to answer the questions, “What are these V Speeds and why are they so important? Let’s get to it!
What is the Takeoff phase?
So the takeoff phase — what is it? It begins from the moment takeoff power is applied, until the aircraft is airborne and the first power reduction is actioned (thrust reduction altitude). If the pilot is planning on staying in a visual pattern, then the takeoff phase ends when the aircraft reaches the visual pattern of 1000 feet above the runway end elevation. Finally, the takeoff phase can also end if the takeoff is aborted on the runway.
As I’m sure you’ve seen, during the takeoff phase, an aircraft needs to travel pretty fast down the runway. The reason for this is because there needs to be enough airflow over the wings to generate the lift required to actually get airborne. On a typical day, a jet aircraft can easily reach speeds of over 180mph (nearly 300kph). So yeah… quite fast!
Now imagine going that fast with an aircraft that weighs anything up to a few hundred tonnes. Now imagine you need to STOP all of a sudden. The amount of energy that the brakes have to absorb during the aborted takeoff is just enormous. This is why we need specific speeds to tell us when we can and cannot abort a takeoff, when we should get the aircraft off the ground, and what speed we should maintain once airborne. This is where the V Speeds come in.
V1 - Make a Decision
Now if there’s any V Speeds people know about it’s going to either be V1, Vr or V2 (we’ll talk about Vr and V2 soon). These three speeds are the most commonly used during the takeoff roll. I don’t know of any airline that doesn’t require the pilots to announce them during the takeoff, but if you know of any, please leave a comment below, I’d be really interested to know!
So let’s talk about V1. What exactly is this infamous speed?
V1 - This is the speed that, when reached, a takeoff should not be aborted.
The way that I like to think of V1 is simply the “GO” speed. Once you’ve passed this speed, when hurtling down the runway at nearly 300kph, there may not be much runway left. Also, the aircraft may have too much energy for the stopping devices to enable it to come to a complete halt before the end of the runway. If this is the case, and a takeoff is aborted after the V1 speed, there is a serious risk of overrunning the runway, and no pilot wants this to happen.
The V1 speed is calculated based on many different things. Some of the factors that determine V1 are, but are not limited to:
- Aircraft weight
- Runway condition (dry, wet, slippery or contaminated)
- Weather (wind direction, temperature, pressure etc)
- Takeoff thrust being used
- Type of brakes fitted to the aircraft (steel or carbon)
When are the speeds calculated?
What happens below V1?
Below V1, if a failure occurs, the takeoff can be aborted. But it’s not that simple. There are two different “speed regimes” during the takeoff roll, and these are
- The “Low Speed” regime and…
- The “High Speed” regime
During the low speed regime, the pilot can decide to stop for more or less any reason. This is because below 80-100 knots, the brakes are easily able to cope with an aborted takeoff as the aircraft isn’t carrying too much kinetic energy. Above 80-100 knots though, things change as the aircraft moves into the high speed regime.
Above 80-100 knots, there are only a few things the pilot should stop for (these are serious malfunctions). Some of these things are:
- Engine failure
- Fire or fire warning
- Uninhibited warnings (aircraft specific)
- If the aircraft is unsafe or unable to fly
- (Some airlines may have more criteria than this)
As you can see, if one of these malfunctions were to happen above 80-100 knots and before V1, it would definitely be safer to stay on the ground. But as you get faster and faster down the runway, the brakes will need to absorb more and more energy to bring the aircraft to a stop, so as you can expect, the risk of aborting becomes greater the closer you get to V1.
So the gist of it is, in the low speed regime, the pilot can stop for anything. In the high speed regime, the pilot should only stop for the major failures as mentioned above.
Now… what about the other two important speeds, Vr and V2?
Vr - Rotation Please
The next speed that comes along in the takeoff regime is known as Vr. This speed is often called “rotate” and verbalised as such by the pilot monitoring.
The definition of Vr is:
The speed where the lift forces over the wings are greater than the weight of the aircraft and the pilot uses the controls to rotate the nose-wheel away from the ground. At this speed, wing tip vortices are created and move rearwards.
Quite simply, Vr is the speed that the pilot gently pulls back on the flight controls, bringing the aircraft nose gently into the air. During the rotation, usually at just over 10 degrees pitch in many jet aircraft, the main wheels get off the ground and the aircraft becomes fully airborne. This is known as Vlof (lift off speed), but we won’t go into detail about that now, as if calculations have been done correctly, Vlof should always occur very soon after Vr.
Vr is also calculated before push back, just like V1.
Now, have you ever see an aircraft gently pull back and takeoff on a day where there’s a lot of moisture in the air? Well if you have, you’ll have probably noticed the “mist” that forms on the upper side of the wing and the contrails that form over the wing tips and mover rearwards of the aircraft.
The mist over the wing is formed because the air on the upper side of the wing has a lower pressure than the surrounding air, and this lower pressure causes a sudden decrease in temperature. The air condenses and the water vapour is the mist or fog you see.
Something interesting about Vr, is that very often (usually on dry runways) it is the same as V2. Another fun fact is that the pilot does not necessarily need to rotate at Vr. In strong, gusty winds, the device that measures air pressure (which is converted into airspeed) also picks up gusts of wind. This gust could cause the airspeed to show that Vr has been reached. But once the gust is gone, the airspeed then falls below Vr. This is why, during gusty winds, the pilot can rotate a bit later than Vr, to ensure there is definitely enough forward speed to safely get airborne.
So that’s Vr. Now let’s have a look at V2.
V2 - Screen Heights and Rate of Climb
The final speed we’re going to talk about in this post is V2. This is often called the “Takeoff Safety Speed”. Let’s look at a definition.
V2 is the speed that the aircraft must maintain in case of an engine failure after takeoff to reach a height of 35 feet by the end of the runway. It must also enable the aircraft to maintaina climb rate of at least 200 feet per minute until acceleration altitude.
There are two very important factors when it comes to V2. These are the screen height and the climb rate.
The screen height is the height at the end of the runway that the aircraft must reach in case of an engine failure. For us, this screen height is 35 feet, and will ensure that the aircraft will pass over any obstacles beyond the runway end.
The climb rate after takeoff, in case of an engine failure, has to be at least 200 feet per minute. This is because when the authorities plan departure routings, they take into account the fact that the aircraft will be able to maintain this rate of climb or greater (as well as achieve a screen height of 35 feet), in order to clear all obstacles beyond the runway.
The main thing to take away from this is that the Takeoff is a very busy phase of flight and needs to be managed properly. The V Speeds, which are a generic set of speeds used for all aircraft, give the pilots the characteristic speeds they need to use to safely get the aircraft off the ground, whether that’s with all engines running, or in an engine failure situation.
Below I have added a video of me rolling an Airbus A320 down Runway 08R at Gatwick Airport, and I’ve shown where and when (roughly) the V Speeds happen. Hope you enjoy it!
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The Humble Pilot