System Synopsis: The new redline

The mystery of the disappearing yellow arc

May 1, 2011

System synopsis

While an MU–2 can certainly cruise faster than 300 KTAS, its ASI redline (right) is set at 250 KIAS. This prevents overspeeds at low altitudes. Up in the flight levels, indicated airspeeds are lower, owing to less-dense atmospheric pressure.

The typical general aviation pilot sitting in the cockpit of a turbine-powered airplane for the first time is confronted with many unfamiliar controls, instruments, and systems. One might be something as mundane as the airspeed indicator.

May 2011 AOPA Pilot Turbine Edition

The airspeed indicator in piston-powered lightplanes is adorned with colorful arcs and radial markings. The same gauge in a turbine-powered airplane has fewer or none of these. Often gone are the familiar white, green, and yellow arcs. In place of a redline, there might be instead a maximum-airspeed pointer, colloquially called a barber pole because of its diagonal striping. When flying an airplane with piston power, a pilot routinely avoids flying beyond V NO, the maximum structural cruising speed. V NO is represented on an airspeed indictor by the upper limit of the green arc (or bottom limit of the yellow arc). It is permissible to exceed V NO and operate in the caution range (the yellow arc) when in smooth air. Although foolish, it is even legal to nudge the redline and fly at but not beyond V NE, the never-exceed speed.

There is little danger of inadvertently exceeding the redline in most piston airplanes. The increase in parasitic drag of these airplanes in high-speed flight is so great that it imposes somewhat of an aerodynamic barrier to flight beyond V NE. When flying turbine aircraft, however, it would be easier to inadvertently exceed V NE. These aircraft create relatively less drag at any given airspeed in a “clean” configuration. Everything else being equal, turbofan airplanes, for example, create only half as much drag as piston-powered airplanes. They can glide twice as far from any given altitude. Some are so clean and have so much power that they can exceed the redline in a moderate climb. Even turboprop airplanes are cleaner than their piston counterparts because of reduced nacelle frontal area and the elimination of cooling drag.

Reducing the drag of turboprop airplanes, however, has consequences. Because of the relative ease with which pilots could exceed the redline, such an airspeed limit is necessarily reduced by simply eliminating the yellow arc and placing the redline where V NO used to be. This makes it more difficult for pilots to inadvertently zip through what used to be V NE. The new airspeed redline is designated as V MO, the maximum operating speed for turbine-powered airplanes. This applies to both turboprop and turbofan airplanes and explains why turbine airspeed indicators do not have a yellow caution range.

When a piston-powered airplane is converted to a turboprop, the pilot might be dismayed to discover that the conversion necessitates reducing the maximum-allowable airspeed. V NE and the yellow arc are eliminated and V MO replaces V NO to conform with the certification requirements of turbine airplanes. In other words, the new redline is where the “top of the green” used to be. These aircraft typically cruise at higher altitudes where indicated airspeeds might be lower, but where true airspeeds are much higher.

At high altitude, the true airspeed when operating at what used to be V NE could be so high as to induce flutter, a potentially catastrophic phenomenon that can lead to airframe destruction. This is because the onset of flutter is directly related to the actual speed of the air passing by the airframe. In other words, a high true airspeed is the primary cause of flutter, not necessarily a high indicated airspeed. This is why in some airplanes, the maximum-allowable indicated airspeed is gradually reduced above a certain altitude to keep true airspeed in check and avoid the excitation of flutter. (This airspeed reduction is unrelated to reductions in indicated airspeed necessitated by Mach limitations.)

Because turbine airplanes can more easily accelerate to and beyond V MO, most are equipped with aural warnings to alert the pilot when airspeed reaches the allowable limit. In some cases, stick-pullers and autopilots apply nose-up back-pressure to the control wheel and make it more difficult for a pilot to exceed V MO. In other cases, autoflight and fly-by-wire systems make it impossible for a pilot to exceed V MO no matter how hard he might try.