Acceleration Of A Jet Plane When Taking Off

The acceleration of a jet plane is a crucial part of the flight, which requires a significant amount of concentration from both of the pilots.

Imagine sitting in a cockpit and just lining up on the runway at a busy airport like JFK. Things can get hectic on the ground and require a lot of situational awareness, especially with minimal traffic separation, wet runways, bad visibility, and strong winds. Nevertheless, the engines accelerate, and once you take off and break through the clouds, you start to relax a little and enjoy the beauty of flying. To enable you to master the takeoff phase confidentially, we have prepared this article containing all you need to know.

There are three important call-outs you need to memorize during the acceleration and takeoff phase of your flight. First, at 80 knots, you will check your instruments. V1 is when you are too fast to abort the takeoff and rotate when you lift the nose of your aircraft upwards.

This was just a concise explanation. We will cover all essential phases during the acceleration of a jet plane to allow you to prepare and know what to expect mentally.

And don’t worry if you’re struggling to get your head around all of this - I know I certainly did when I was doing my own flight training, so you’re in good company!

‍How Can You Calculate Your Takeoff Speed?

To start, we have to discuss how jet pilots calculate their takeoff speeds. First, the plane's weight is one of the most important factors. For example, a fully loaded 747 jumbo jet can weigh up to 675 tons.

To calculate the takeoff weight, the pilot needs to know the plane's empty weight and the fuel and cargo weight. The empty weight is how much the plane weighs when there's nothing in it. The fuel and cargo weights will be listed in the flight manual.

Another factor that affects the acceleration and the takeoff speed is the density altitude. This is dependent on how high above sea level the plane is taking off from. The higher the altitude, the less dense the air is. And that means the plane won't generate as much lift as it needs to take off.

So, a pilot taking off from Denver, Colorado, which has an altitude of 5,280 feet (1,609 meters), will have to use a higher takeoff speed than a pilot taking off from sea level.

Finally, the pilot has to consider the wind speed and direction. A headwind will help the plane take off, while a tailwind will make it more difficult.

Now, after we have learned the factors that affect takeoff speed, let's look at how pilots calculate it. They use a formula that considers the weight of the plane, the density altitude, and the headwind.

The formula is:

Takeoff speed = (weight / 10) + (density altitude / 1,000) + (headwind / 2)

For example, a plane is taking off from Denver in a 50-mile-per-hour (80-kilometer-per-hour) headwind. The aircraft weighs 600,000 pounds (272,155 kilograms), and the density altitude is 5,280 feet (1,609 meters).

Using the formula, we get:

Takeoff speed = (600,000 / 10) + (5,280 / 1,000) + (50 / 2)

= 60,000 + 5.28 + 25

= 60,035 feet per minute

= 100 miles per hour (160 kilometers per hour)

So, the plane will need to go 100 miles per hour (160 kilometers per hour) when it leaves the ground. I have to add that this is an example from a small aircraft. Modern jets usually have takeoff speeds between 135-165 knots.

How to Calculate The Takeoff Distance?

The other crucial aspect of knowing the takeoff speed is how long the runways need to be before you take off.

So, let's say the plane is taking off from Denver, Colorado, on a hot day with strong winds. In this scenario, the temperature is 90 degrees Fahrenheit (32 degrees Celsius). The wind is blowing at 30 knots from the west. The density altitude would be about 10,000 feet (3,048 meters).

The ground roll would be 8,000 feet (2,438 meters) in this case. The flap extension speed would be 175 knots (201.16 mph or 324.17 km/h). And the headwind component would be 30 knots (34.52 mph or 55.56 km/h). So, the total distance the plane will travel before taking off is 8,000 feet (2,438 meters) + 175 knots (201.16 mph or 324.17 km/h) + 30 knots (34.52 mph or 55.56 km/h).

Next, the pilot must add the runway slope to the ground roll. The runway slope is the amount of downhill or uphill grade on the runway. In this case, it's 0.5%. So, the total distance the plane will travel before taking off is 8,000 feet (2,438 meters) + 175 knots (201.16 mph or 324.17 km/h) + 30 knots (34.52 mph or 55.56 km/h) + 0.5%.

The last step is to add the acceleration distance to the ground roll. The acceleration distance is the amount of runway that the plane needs to accelerate from its takeoff speed to its lift-off speed. For this plane, it's 1,500 feet (457 meters). So, the total distance the plane will travel before taking off is 8,000 feet (2,438 meters) + 175 knots (201.16 mph or 324.17 km/h) + 30 knots (34.52 mph or 55.56 km/h) + 0.5% + 1,500 feet (457 meters).

This means the plane will need 9,905 feet (3,023 meters) of runway to take off.

Why Should You Prefer To Take off With Headwinds?

You might wonder why airports are designed with runways with a headwind component. After all, wouldn't it be easier to take off into a tailwind?

There are a few reasons for this. First, headwinds help the plane take off. That's because wind speeds can add to the plane's lift.

Also, headwinds help to reduce noise. But, again, that's because the engine noise is directed away from the airport.

But the main reason is safety. For example, a headwind will help keep the plane on the runway if there's a problem with the engine during takeoff. On the other hand, a tailwind will push the aircraft off the runway.

What are Essential Speedmarks During Take-off?

To mention all the essential speed marks during the acceleration and takeoff of a jet airplane, we go through them in a synchronous order.

80 Knots

The first call-out pilots will do on acceleration during takeoff is "80 knots". The pilot flying calls out "80 knots," and the pilot monitoring will respond with "checked" While still at relatively low speed, the main reason to do it is to check for any discrepancies on the flight instruments on either the captains or the first officer's side.

In case any problems occur during this phase of takeoff, it is crucial to identify them early, and the chances of a takeoff abortion are high. In most airlines, the pilot in command will actually be the one to make this decision, so his hands are placed on the thrust levers, whether he is the pilot flying or not.

V1

V1 is the speed at which a decision must be made to continue or abort the takeoff. If an engine fails after this point or any other problem occurs, the takeoff should be continued as it is impossible to stop the aircraft on the remaining runway. At this point, it is safer to take the airplane airborne and deal with the problem after than risk overshooting the runway.

After V1 was called out in many airlines, the pilot will remove his hands from the thrust lever to not abort a takeoff in a state of shock when something unforeseen happens.

Rotate

VR - the speed at which rotation should be initiated. This is usually between 135-165 knots for modern jets. The pilots flying at this point will call out "rotate," and the nose of the aircraft will be rotated upwards to a takeoff attitude. The pitch to rotate should be around 15 degrees.

V2

The speed at which the aircraft should be climbing away from the runway. This is usually around 155-185 knots for modern jets. If an engine fails at this point, the takeoff can continue as the aircraft will have enough power to climb away on one engine.

Once the aircraft is airborne, it will accelerate to its initial climb speed. This is usually around 250 knots for modern jets. The pilot flying will call out a "positive rate" at this point, meaning that the aircraft is climbing at a rate greater than 1,000 feet per minute. The pilot monitoring will then retract the landing gear and flap settings in the correct order.

What Are Potential Risks During The Acceleration Phase?

The first risk to mention might be slightly unexpected: Fatigue. In a 2017 survey of over 800 commercial airline pilots by the pilot's union, 55% of respondents cited fatigue and a loss of situational awareness as the biggest threat to flight safety, ahead of terrorism.

During takeoff, the most common potential risks for commercial planes are bird strikes, engine fire or failure, and tire blowouts. A bird strike can cause severe damage to an engine, while a tire blowout can create debris on the runway that could puncture another tire or cause damage to the landing gear.

Pilots conduct a thorough pre-flight check of the plane and its systems to minimize these risks. It is also essential to use a checklist to ensure that all procedures are followed during takeoff.

Finally, as the last safety barrier, you will receive training on how to deal with potential problems during takeoff. For example, in the event of an engine fire or failure, you will know the proper procedures in this case.

Generally speaking, you will differ between the low-speed phase (below 80 knots) when you almost ever abort the takeoff if any problems occur. It is safer to stay on the ground and deal with the situation. However, you should have a more go-oriented mindset in the high-speed phase (more than 80 knots). Therefore, the risks of abortion may be higher than the risk of takeoff with that problem.