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Is It OK To Fly In The Yellow Arc?

Jim Raeder

Has someone ever told you to avoid flying in the yellow arc? In many airplanes, that's harder said than done.

What Is Vno?

The FAA defines Vno in the Pilot's Handbook of Aeronautical Knowledge as "the maximum speed for normal operation or the maximum structural cruising speed. This is the speed at which exceeding the limit load factor may cause permanent deformation of the aircraft structure." The definition sounds a little clunky, right? As you'll read about below, "the maximum structural cruising speed" gets confused with Vne all the time, but that couldn't be farther from the truth.

When taking a look at the Vg diagram, you'll notice a line that designates maximum structural cruising speed. As you fly faster and faster, any change in angle of attack will result in a greater G load imposed upon the airplane. This could cause the aircraft to reach the upper or lower area of structural damage shown in the diagram below.

Manufacturers publish Vno to provide a safe buffer zone between normal airspeeds, and airspeeds that could cause a structural failures during high speed flight. During slow speeds, moderate turbulence will result in relatively small G-loading. As speed increases, the G forces associated with turbulence have a much stronger, adverse affect on aircraft load limitations. This is one reason you'll start to feel uneasy sooner, as you fly into turbulence at faster airspeeds. Your body is experiencing more G-loading, just like your airplane.

On your airspeed indicator, Vno is represented by the point at which the green arc ends and the yellow are begins. The Vno speed for your plane is published in the POH.

Wikipedia

In Some Airplanes, Flying In The Yellow Arc Is "Normal"

In an article on mooneypilots.com, a test pilot reports that...

"The top of the green arc/beginning of the yellow arc on the M20C is an impossibly low 130 KIAS. My gosh, in the C model you are almost cruising in the yellow arc in level flight! This is a significant limitation for descent if you are to abide by the airspeed markings."

Wikimedia

If you're flying something like a Cessna 172, it's pretty hard to reach Vno, and impossible in level flight. According to the C172S POH, 129KIAS is maximum structural cruising speed. The POH instructs pilots: "Do not exceed this speed except in smooth air, and then only with caution." If you've ever flown a C172, you know that to reach Vno, you have to fly a high descent rate, combined with a high power setting. There just aren't many cases where you'll find yourself in the yellow arc.

Lane Pearman

But in planes like the Mooney, Cirrus SR22, and other high performance singles, you have to make a judgment about whether to descend using cruise power, or a reduced power setting. This is, in part, determined by how smooth the air is. Judging this "smoothness" isn't as easy as it sounds, and the FAA provides no definition for "smooth air."

Occasional small bumps are nothing to worry about. When you start noticing controllability problems or substantial airspeed changes, it's always smart to slow down below Vno. Smoother air is better for airframes when it comes to flying at fast airspeeds. Plus, you and your passengers will feel much more comfortable at slower speeds during turbulence.

Wikimedia

What Should You Do?

Always reference your aircraft POH about Vno speeds, definitions, and procedures. Don't be afraid of flying in the yellow arc, but always be cautious about the air conditions around you. If you find yourself flying too fast, you can control your speed by reducing power or slowing your descent rate. Both of these options in combination are a great way to ensure you're flying at a safe speed.

Vno doesn't need to be treated as a Vne (never exceed) speed, because your airplane is certified to fly within that range under the right conditions. As long as you're cautious, you won't damage the aircraft. That said, repeated flights in the yellow arc in anything but smooth air will impose repeated stress on your airframe. Over time, this stress could lead to a structural failure. Simply put, if the air isn't smooth, slow down below Vno.

What do you think? How often do you fly in the yellow arc?

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and commercial pilot for Mokulele Airlines. He holds multi-engine and instrument ratings, and is an aviation student at the University of North Dakota. He's the author of the articles, quizzes and lists you love to read every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures at http://www.swaynemartin.com.

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