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Jet airspeed limits

Why are there two, or even three, red lines?

By J. Mac McClellan

Piston-powered airplanes have a single red line on the airspeed indicator. It’s the indicator of VNE, which stands for never exceed airspeed. Sounds draconian, doesn’t it? Blow past that airspeed and who knows what bad things may happen?

Photography by Mike Fizer.
Photography by Mike Fizer.
Photography by Mike Fizer.

In jets we have a red line at VNO airspeed, and another at MMO indicated Mach. And with some airplanes, particularly the light jets, there may be another airspeed red line that appears and disappears at 8,000 feet. More on that later.

At some point in the history of certification rules development, jet pilots were apparently given more credit for skill and airspeed awareness than pilots of piston airplanes. Instead of trying to scare the bejabbers out of jet pilots with the ominous “never exceed” airspeed limit, the FAA limits jet pilots with “maximum operational” edicts. Yes, do not intentionally ignore a VMO or MMO red line, but if you do happen to exceed the limit because of unexpected operational conditions you can know that disaster is not imminent. It’s an operational limit, not an implied life sentence.

The reason there are two airspeed red lines in jets is because VMO is a structural limit while MMO is a performance and aerodynamic behavioral limit. At piston airplane speeds Mach is never a flying qualities consideration, so the single red line there is entirely a structural design limitation.

I’m sure you’ve seen images of airplanes with sandbags, or even people, spread across the wings to demonstrate the structural strength of the wing. That’s called static loading, and it’s interesting. But the key certification standards and testing are dynamic loading caused by what the engineers call gusts—what the rest of us call turbulence—and by maneuvering. The dynamic loading of an airframe structure is a combination of the velocity of a turbulence gust, or the abruptness of a pilot maneuver, and the airspeed. The higher the airspeed in any situation, the greater the structural load. Engineers call this “Q pressure” and it is this strain that at some elevated point will break an airplane. Flying at VNO keeps the Q pressure within the certification envelope and assures us the structural margins we count on are available during turbulence encounters.

At some altitude, typically in the high twenties, the indicated airspeed in controlled flight has decreased to the point that Q pressure in turbulence—and thus, VNO—is not the immediate concern, but Mach effects are. That’s why the maximum operating airspeed limitation transitions to MMO.

It sounds silly to be concerned about Mach effects when flying well below the speed of sound, but airflow over parts of the airframe—called local airflow—may in fact be nearing or exceeding transonic velocity at normal jet climb and cruise speed. This happens because the air must accelerate to pass over the wings, tail, fuselage, and other parts of the airframe.

When airflow becomes transonic a shock wave develops, which can disrupt normal air flow over critical surfaces. The shock wave moves aft with increased velocity and can move the center of lift with it, causing unexpected and unwelcome pitch changes. If the shock wave moves far enough aft it can impinge on control surfaces, making their behavior unpredictable. None of this is consistent with stable and easily controlled flight.

Again, that’s why we use the “maximum operating” limit instead of the “never exceed.” Mach effects do not turn on suddenly, so a few hundredths Mach over the MMO limit do not eat into control margins. But exceeding that limit does reduce the predictable flying qualities margins at least a little, so the red line must be honored under all normal conditions.

The third airspeed redline in some jets is a VMO reduction when flying below 8,000 feet. Obviously, the potential loading from turbulence and maneuvering doesn’t change at that altitude, but certification standards say large birds do. According to FAA standards, big birds don’t fly above 8,000 feet so we don’t need to account for damaging impact with one above that altitude.

Structural certification of jets requires testing the damage caused by impact with a large bird while flying a VMO. The manufacturer and FAA agree on what is the most vulnerable part of an airplane to bird impact damage, and it’s usually the outboard section of the horizontal stabilizer.

The test requires the vulnerable section to be struck by a bird—now a mass that duplicates an actual bird, but for many years an actual bird—shot at the speed of VMO. Obviously, the structure cannot break at bird impact, but the shape of the resulting damage must be tested in flight to demonstrate that aircraft control is not compromised.

The structures of larger jets are, obviously, more robust and able to withstand the bird impact when struck at full VMO speed. But lighter jets lack the massive structural reserves, so there is the option of conducting the bird test at a lower velocity. That’s the reason for the lower VMO when flying below 8,000 feet. That lower red line is the equivalent airspeed of the maximum bird strike test during certification.

Some large birds do fly above 8,000 feet, but I’m sure it’s rare. And with the 250-knot airspeed limit below 10,000 feet in the United States and some other countries our speed is limited anyway, so the 8,000-foot lower VMO may not matter to most of us. But since the 10,000-foot speed limit is not universal across the globe, the bird strike VMO must be part of the certified limitations.

In the days of mechanical airspeed indicators, showing the various red lines in a jet was complicated. Some indicators had a striped “barber pole” that moved down to display MMO. Some had a mask that would display the lower “bird strike” V below 8,000 feet and then cover it as you climbed.

Those complications have disappeared with the electronic displays nearly all of us fly with now. Whatever red line applies for your altitude and speed is prominently displayed on the PFD, and flashes warnings if you exceed the limit, along with annoying audible warnings. The good news is that the electronic brains that control our airspeed displays can also give us a cushion of at least a few seconds to slow down before the beeping and honking begin if we slip past one of those maximum operation red lines. Don’t forget to mute the cockpit speaker so passengers won’t hear the racket if you do ever nudge past a red line.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan

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