Aviation

How airplanes survive extreme turbulence

If you have ever been on the plane, there’s a good chance you’ve encountered turbulence. Turbulence can scare nervous pilots, especially when it’s strong. Some might even think that these strong forces would be enough to break the plane.


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


Loading factor and vn diagram

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

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  • Limit load: The maximum load that the aircraft can expect during operation
  • End load: The load at which the collapse of the structure can occur
  • Safety factor: The ratio of ultimate to ultimate load.

When an aircraft is pushed to its load limit, its structure must be able to handle it without a problem. However, when moving to the maximum load, structural failure may occur. The safety factor lies between these two limits. Within the limits of the safety factor, permanent deformations of aircraft structures are very unlikely.

For airplanes, the safety factor is 1.5 of the ultimate load. It is a compromise between security and the weight of the aircraft. The higher the safety factor, the stronger the airframe and aircraft structures must be. This increases the weight of the aircraft. For this reason, the safety factor cannot be infinitely high.

Loads on an aircraft are introduced through overload forces, which in technical terms is known as the load factor (n). It is represented by the equation:

Load factor (n) = lift / weight

This shows that as the lift increases, the load factor increases. For example, if the weight is four times greater, the load factor is 4.0 or 4 g. In aircraft design, this must be within what is called the maneuvering envelope, also known as the Vn diagram.

Vn diagram. Photo: Oxford ATPL

The Vn diagram as shown above is constructed with the loading factor as a function of the aircraft speed. The graph shows three important speeds: Vs (stall speed), Va (maneuvering speed), Vc (estimated cruise speed) and Vd (estimated dive speed). The envelope in the diagram has its limit at the ultimate load of the aircraft.

What can be done from the schedule:

  • As the load factor increases from O to S, at speed Vs, the aircraft enters the 1g dump.
  • When flying at Va and retracting the controls, the aircraft stalls at about 2g.
  • As you accelerate from Vc to Vd (C to D and F to E), the maximum load factor the aircraft can handle decreases. This is because at high speeds, aircraft structures experience high dynamic pressure, which requires a reduction in the maximum load factor.

The most important thing to note here is the Va speed. This is the maximum maneuvering speed. If the aircraft is flying at or below this speed, the pilot is free to load his aircraft without risking damage to the airframe. This is because if the pilot were to carelessly pull back on the controls, the aircraft would enter the dump before sustaining any damage. Therefore, Va is one of the most important airspeeds in an airplane, and for this reason pilots should exercise caution when maneuvering above this airspeed.

During high-g maneuvering, care must be taken to keep the maneuvering speed below maximum. photo:
LGLiao by Wikimedia

A burst envelope

Burst fire is built on the basis of maneuver fire. It was first developed in the 1940s.

A burst envelope. Photo: Oxford ATPL

Based on the aircraft’s gust range, the aircraft must be designed to withstand a vertical gust of 66 ft/s when flying at Vb (the design speed for maximum gust intensity). At or below this speed, the aircraft stalls before it reaches a load factor that can cause structural damage.

It is also seen that at Vc the airplane can withstand a wind gust of 55 ft/sec. This strength requirement came about because of the huge difference between Vb and Vc. You can imagine that you are at Vc when you hit turbulence and it may take some time to get down to Vb. Therefore, to account for this delay, the aircraft must be able to withstand significant wind even at Vc.

This speed, however, is not very practical, since stopping the aircraft to protect it from exceeding the limiting load factor in turbulence is not acceptable. Therefore, the operational speed called Vra is used. This speed is low enough to prevent damage, and at the same time high enough to provide protection against inadvertent disruption. To calculate Vra, designers first calculate the speed Vb and then stiffen the airframe to obtain an acceptable speed Vra that allows the airplane to withstand a 66 ft/s wind. This is shown in the graph below:

Photo: Oxford ATPL

The graph above shows the three main speeds:

  • CE line: Stall speed when the aircraft experiences a gust of 66 ft/s.
  • GHI line: maximum operating speeds of the aircraft.
  • MN: Maximum Burst Velocity 66 ft/s.

Therefore, the velocity Vra must lie between the lines CE and MN. An example of such a speed is the PO line. When flying at PO, the aircraft has sufficient power to withstand a sudden gust without stalling or exceeding the load limit.

Wing design

Wing design also plays a role in how the aircraft behaves when encountering gusts. The vertical gust causes a change in the angle of attack, which ultimately results in an increase in the load factor.

For example, an airplane flies 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 if the gust increases the angle of attack by 5 degrees?

Load factor = lift / weight

1 = 0.50/0.50

An angle of attack of 5 degrees will increase Cl by 5 x 0.3, which equals 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 results in a load factor of 4g.

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

The swept wings of jetliners respond much better to turbulence than straight wings. Photo: Air Canada

This is one of the reasons why jets are smoother in turbulence compared to smaller turboprops.

Turboprop engines, with their predominantly straight wings, jerk violently in turbulence. Photo: ATR.

What actions do pilots take when flying through turbulence?

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

Airbus A320 Turbulence Penetration Rate Graph. Photo: Airbus A320 FCOM.

Pilots usually try to avoid areas of severe turbulence by using meteorological radar system, which scans the area in front of the aircraft. Pilots many times use radar to perform deflection maneuvers, reducing severe turbulence.

https://simpleflying.com/how-airplanes-survive-in-extreme-turbulence/ How airplanes survive extreme turbulence

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