Four Reasons Why A Plane Might Not Be Able To Take Off

When we talk about takeoff, it is easier to define it in terms of distance.

Takeoff may seem like a simple maneuver, but it involves a lot. When we talk about takeoff, it is easier to define it in terms of distance. The Takeoff Distance (TOD) required for a transport category aircraft is the distance from the brake release point to a screen height of 35 ft.

The takeoff consists of a ground segment known as the takeoff roll or the ground run, and an airborne segment which is the distance required to reach the screen height. Many factors can affect the takeoff distance. In this article, we will look at four of them.

Atmospheric conditions

Aircraft performance is very much connected to the atmosphere and among this, the ambient temperature plays a major role. High temperatures reduce the aircraft's performance as it increases the density altitude. The increase in density altitude decreases engine performance and the aerodynamics of the aircraft are also affected. You can read this article regarding the effects of density altitude on aircraft.

Aircraft weight

The weight of the aircraft increases the takeoff distance due to several reasons. Foremost, a heavier aircraft has more inertia. This requires more acceleration. To accelerate, more runway is required, and this increases the takeoff distance. A heavier aircraft also puts in more load on the ground, and this increases the wheel drag which in turn increases friction. To overcome the increased friction, the aircraft needs a longer distance to reach the speed for takeoff.

As aircraft can only lift off at a certain speed, a heavier aircraft must roll on the runway a lot more than a less heavy aircraft. This again increases the takeoff distance. The weight also affects the angle of climb, which requires more horizontal distance to reach the screen height. This is because an increase in weight in a climb increases the horizontal component of weight. This component acts against the thrust and works more like a drag component. This increases the takeoff distance as well.

Prevailing winds

Airplanes generally take off into the wind. This is because a headwind reduces the takeoff distance. The opposite is true in tailwind conditions. So, why does that happen?

It is important to understand at this point that pilots fly with Indicated Air Speed (IAS), which is measured by the sensors fixed to the airframe. The density of the air affects the actual air speed of the aircraft. This is known as True Air Speed (TAS). On the ground, the TAS is affected by, yet another speed called ground speed. Headwinds reduce the ground speed and tailwinds increase ground speed.

During a takeoff, for instance, the IAS for lift-off might be 130 knots. With the density correction say the TAS is 135 knots. So, the aircraft needs to accelerate to a speed of 135 knots to get airborne. If there is a headwind of 10 knots, only a ground speed of 125 is required to lift off. As this is a lower speed, to reach this speed a lower acceleration is required, and this reduces the takeoff distance. If there is a tailwind of 10 knots, the ground is increased to 145 knots, and this requires more acceleration which results in more runway distance.

Runway conditions and slope

A perfectly smooth runway does not exist. And due to this, there is friction developed between the aircraft tyres and the surface. When the runway is contaminated by water, snow, or slush the friction increases a lot more due to fluid resistance. There is also a drag known as the precipitation drag or impingement drag. This type of drag occurs as water sprays and hits the airframe as the aircraft accelerates on the runway.

The runway slope is another factor. In an upslope runway, the horizontal component of weight acts against the acceleration of the aircraft, which increases the takeoff roll. In a downslope, the horizontal component of the weight adds to the thrust which increases acceleration and reduces the takeoff distance. It is very much like trying to push a box down a down-angled plane.

What can pilots do to reduce the takeoff distance and improve takeoff performance?

One way to reduce takeoff distance and improve takeoff performance is to modify aircraft configuration. This can be done by lowering flaps. When flaps are lowered, it increases the coefficient of lift which allows the aircraft to take off at a lower speed. This reduces takeoff distance. However, flaps increase the drag. Due to this reason, full flaps cannot be lowered for takeoff. Each aircraft has an optimum takeoff flap configuration. This is determined during the testing phase of the aircraft and is specified in the aircraft flight manual. An increase in flap setting beyond this setting increases drag and the benefit it gives by increasing the coefficient of lift no longer exists.

A higher flap setting also reduces the climb performance. So, while a higher flap setting can give a reduced takeoff distance, it can negatively affect the obstacle clearance. Thus, when a higher climb gradient is required the lowest takeoff flap setting gives the greatest benefit.

What happens when pilots fail to realise the performance decrement?

The above-mentioned conditions can reduce the takeoff performance to the point where safety can be affected. So, a pilot needs to realise the impact of those factors before takeoff. Aircraft manufacturers provide charts in the aircraft manual for pilots to determine this. These charts called WAT (Weight, Altitude, and Temperature) charts can be helpful to quickly determine if the aircraft can safely take off from a runway in given conditions.

In the airline world, these days this is done through manufacturer-made performance software which saves time and gives very accurate results. As things are much more standardised and pilots in the airlines undergo rigorous training, takeoff related mishaps are low.

Unfortunately, the same cannot be said about general aviation. Many pilots in general aviation are less experienced and because of the sometimes very basic training they receive some of them have a tendency not to look at the manuals. This has led to many takeoff-related accidents, incidents, and close calls. As a pilot, it is important to know your airplane well and to respect its limitation at all times.