If you have ever been on an airplane, there is a fair chance you would have experienced turbulence. For nervous flyers, turbulence can be frightening, especially when it’s severe. Some might even think that these strong forces would be enough to break the aircraft.

In reality, airplanes, especially large airliners, are built with enough strength to withstand almost all naturally occurring turbulence.

The load factor and v-n diagram

There are three main loads when it comes to the structural safety and integrity of the aircraft. They are:

  • Limit load: The maximum load the aircraft might see in service
  • Ultimate load: The load at which structural failure can occur
  • Safety factor: The ratio of ultimate load to limit load.

When an aircraft is pushed to its limit load, its structure should be able to handle it with no issues. However, when taken to the ultimate load, structural failure can occur. The safety factor occurs between these two limits. When within the safety factor limit, permanent deformations of aircraft structures are very unlikely.

For airplanes, the safety factor is 1.5 of the limit load. This is a compromise between the safety and weight of the aircraft. The higher the safety factor, the stronger the airframe and aircraft structures have to be. This adds to the weight of the aircraft. Due to this reason, the safety factor cannot be made infinitely high.

What this shows is that as the lift increases, the load factor increases. For instance, if the lift is four times the weight, the load factor is equal to 4.0 or 4 g. When designing airplanes, it must be within something called a maneuver envelope, which is also known as the V-n diagram.

The V-n diagram, as shown above, is plotted with the load factor against the aircraft speed. There are three speeds of importance labeled in the graph: Vs (stall speed), Va (maneuvering speed), Vc (design cruise speed), and Vd (design dive speed). The envelope in the diagram has its limit at the limit load of the aircraft.

The most important thing to note here is the Va speed. This is the maximum maneuvering speed. When the aircraft is flown at or below this speed, the pilot can positively load his or her aircraft freely without risk of airframe damage. This is because if the pilot were to recklessly pull back on the controls, the aircraft would enter a stall before it sustains damage. Va is thus one of the most important speeds in an aircraft, and due to this reason, pilots should take caution when maneuvering above this speed.

The gust envelope

The gust envelope is built based on the maneuver envelope. It was first developed in the 1940s.

According to the gust envelope, an aircraft by design must be able to withstand a vertical gust of 66 ft/sec when flown at speed Vb (design speed for maximum gust intensity). At or below this speed, the aircraft stalls before it reaches a load factor that could cause structural damage.

It can also be seen that when at speed Vc, the aircraft can handle a gust of 55 ft/sec. This strength requirement came about because of the huge difference between Vb and Vc. You can imagine being at Vc when you hit turbulence, and it may take some time to decelerate to Vb. Hence, to account for this delay, the aircraft should be able sustain a considerable amount of gust even at Vc.

This speed, however, is not a very practical one, as stalling an aircraft to protect it from exceeding its limit load factor in turbulence is unacceptable. So, an operational called rough air speed, Vra, is used. This speed is low enough to prevent damage and, at the same time, high enough to give protection from an inadvertent stall condition. To calculate the Vra, the designers first calculate the speed Vb and then build enough strength into the airframe to come up with an acceptable Vra speed which gives the aircraft the capability to withstand a 66ft/sec gust.

The speed for Vra has to thus lie between the line CE and MN. An example of this speed is represented by the line PO. When flown at PO, the aircraft has enough strength to sustain a sudden gust without both stalling and accidentally exceeding its limit load factor.

Wing design

The wing design also plays a role in how an aircraft behaves when encountered with a gust. A vertical gust causes a change in the angle of attack, which ultimately leads to an increase in load factor.

For example, an aircraft is flying with a Cl (coefficient of lift) of 0.50. If a 1-degree change in angle of attack increases Cl by 0.3 what will be the load factor when a gust increases the angle of attack by 5 degrees?

Load Factor = Lift/Weight

1 = 0.50/0.50

A 5-degree angle of attack will increase Cl by 5 x 0.3, which is 1.5. So, 1.5 + 0.50 = 2

Load Factor = Lift/Weight

= 2/0.5

= 4

A gust that increases the angle of attack by 5 leads to a load factor of 4 g.

The swept wings used on high-speed airliners are less susceptible to vertical gusts when compared to those aircraft with straight wings. This is because, for a given angle of attack, a swept wing generates less amount of lift compared to a straight wing.

This is one of the reasons why jet aircraft tend to be smoother in turbulence when compared to smaller turboprops.

What actions do pilots take when flying through turbulence?

When pilots encounter turbulence, the speed is reduced to turbulence penetration speed Vra. This speed varies from aircraft to aircraft and is provided in the aircraft documentation. The speed for Vra changes with altitude until it changes to a constant Mach number.

Normally, pilots try to avoid areas of heavy turbulence using the weather radar system, which scans the area ahead of the aircraft. The pilots use the radar to fly deviation maneuvers many times, reducing severe turbulence.