In brief
- Turbulence penetration speed is a recommend speed the aircraft can be flown at to stop the aircraft’s structure from becoming overloaded
- Turbulence penetration won’t help to reduce or avoid turbulence, but protect the aircraft as you ride through it
- Pilots may slow to turbulence penetration speed when they expect to encounter moderate to severe turbulence, mountain wave activity, convective chop, or strong jet stream shear
When travelling through turbulence, slower = safer
Turbulence penetration speed is the manufacturer’s recommended target speed for travelling through turbulent air. Also known as rough air speed, it’s calculated by the plane manufacturer to give the plane the best balance between structural protection and aerodynamic stability.
We’re going to explore the science behind turbulence speed, including a basic explainer on how it’s calculated and when it’s used. We’ve also linked to some of our favourite resources to help you understand the concept.
In the end, the science may be complex but the principle is that turbulence penetration speed is slightly slower to protect the structure of the plane – and the passengers inside it.
The science behind the number
This may be a little complicated, but turbulence penetration speed is calculated according to the V–n diagram. Often called the flight envelope, this relates speed to load factor.
There are two crucial elements to this. The first is the aircraft’s stall boundary. This is a measurement of how much load the wing can generate before it reaches its maximum lift coefficient.
The second is the gust line. This measures how much additional load a certification-strength vertical gust produces at a given speed.
Turbulence penetration speed is where these two meet. Specifically, it’s the point where a strong gust of wind is more likely to make the wing stall aerodynamically before structural load limits are exceeded.
Turbulence penetration speed isn’t a fixed number, but is calculated according to several factors, including aircraft weight, altitude, airspeed, and mach number in cruise. This is because the aircraft’s stall boundary changes according to its weight.
As you’d expect, a heavier aircraft can use a higher penetration speed than a smaller and lighter plane.
Thankfully, pilots aren’t having to calculate the turbulence penetration speed in the air, it’s provided by the aircraft manufacturer.
We know this is quite complicated stuff, but thankfully, there are resources that can help.
This video is a great place to start:
Why a safe speed matters more than comfort
A lot of explanations online describe turbulence penetration speed as providing a smoother ride. While that may be true, it’s not the primary reason why pilots use it.
Turbulence penetration speed is actually about structural load management. While it may be more comfortable, that’s a secondary benefit. The real purpose is to protect the aircraft’s wings, control surfaces, empennage and fuselage.
If you want to know more about how planes fly, check out our Fear of Flying section.
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FAQs
Turbulence penetration speed is the manufacturer’s recommended target speed that’s used to protect the structural load of the aircraft. While it may result in a smoother ride, it’s about protecting the plane – and the passengers inside.
Turbulence penetration speed isn’t a fixed number. Instead, it’s calculated based on several factors, including the aircraft’s weight, altitude, airspeed, and Mach number in cruise. Thankfully, pilots don’t have to calculate turbulence penetration speed, it’s provided by the plane manufacturer.
No. It’s unlikely a pilot would announce the fact that they’re slowing to turbulence penetration speed. However, you may notice a feeling of the plane slowing and a different tone from the engines. If the aircraft is in turbulence penetration speed, the seatbelt sign will be illuminated. It’s best to sit down, strap in and stay safe.