Turbine airplanes with a "Caution" airspeed range

The Pilatus PC-6 Porter's yellow caution arc on its airspeed indicator is a direct artifact of the aircraft's certification heritage rather than an anomaly introduced by its turbine powerplant. The yellow arc — representing the caution range between Vno (maximum structural cruising speed, the top of the green arc) and Vne (never-exceed speed, the red radial line) — is a standard instrument marking requirement under FAR Part 23 for aircraft certified in the normal, utility, and acrobatic categories. The PC-6 was originally designed and certified with piston power, and its airspeed indicator markings carried forward from that baseline certification. When Pilatus transitioned to the PT6A turboprop, the airframe's certification basis did not fundamentally change the ASI marking scheme — the yellow arc remained because the aircraft remained a Part 23 design, not because the turbine demanded it.

Most turbine-powered aircraft, including virtually all turboprops and jets, use a fundamentally different airspeed limitation philosophy rooted in Vmo/Mmo (maximum operating speed and Mach number) rather than Vno/Vne. Under this framework, the "barber pole" — alternating red and white segments on the ASI — marks Vmo, and there is no yellow caution arc because the structural speed margin concept is handled differently. Aircraft like the King Air series, the PC-12, the TBM 700/900/960, the Cessna Citation family, and the Pilatus PC-24 all use this system. Pilots operating these platforms are taught to respect the barber pole as an absolute operating limit rather than a smooth-air-only caution zone, which is a meaningfully different discipline than managing a yellow arc. The King Air 350, for instance, publishes a Vmo of 226 KIAS and an Mmo of 0.58 — no yellow arc, no Vne in the traditional sense.

The PC-6's situation is not entirely unique in the turbine world, but it is genuinely rare. Some turbine engine conversions fitted to older airframes — agricultural aircraft like the Ayres Thrush in turbine configuration, or various STC'd piston-to-turbine conversions — may retain yellow-arc ASIs if the underlying airframe certification was never restructured. Similarly, certain older light turboprops certified in earlier regulatory eras under Part 23 frameworks may carry the traditional marking scheme. However, purpose-designed modern turboprops almost universally move to Vmo/Mmo conventions from the outset, making the PC-6 a recognizable outlier among production turbine aircraft. Pilots transitioning to the Porter from other turbine platforms sometimes find this marking counterintuitive precisely because the rest of the aircraft — PT6 engine management, prop control, turbine systems — behaves like a turboprop while the ASI reads like a Cessna 172.

For working pilots, the practical significance lies in the different mental models required. In a yellow-arc aircraft, operating between Vno and Vne is legal but explicitly restricted to smooth air; encountering turbulence above Vno requires immediate speed reduction. In a Vmo/Mmo aircraft, exceeding the barber pole under any conditions is an immediate structural concern with no smooth-air exception. Pilots who fly both types — not uncommon in charter or specialty operations where the PC-6 coexists with other turbine types — must be disciplined about which ruleset applies to the specific aircraft they are operating. The PC-6's continued production and global deployment, particularly in parachute operations, humanitarian airlift, and bush flying roles, means this distinction remains operationally relevant for a meaningful segment of the professional turbine pilot community.