Piper J-3 Cub: Pro 360 Digital Protractor

While the base Pro 360 model does not include a full calibration certificate, PJi offers an alternate model (KS5549-WC) that comes with a certificate of calibration.

Accessories include a magnetic base to secure the protractor to metal surfaces for hands-free stability. The unit comes with a carrying case, manual, and battery.

The Pro 360 is used to measure and verify angles on aircraft components with high precision, including propeller blade pitch, flap deflections, control surface settings, and component alignment. It provides angle readings over a full 360° range, making it versatile for level, plumb, and inclined surfaces. This tool helps ensure compliance with OEM and regulatory tolerances, improving both safety and aircraft performance.

The Pro 360 digital protractor offers a resolution of 0.1° and accuracy of ±0.1° when measuring within about ±10° of level or plumb; in other angular positions, the maximum error is ±0.2°.

The unit can be recalibrated in the field without needing special fixtures – using Kell-Strom’s Superset procedure. This procedure allows users to test accuracy and reset reference points to maintain alignment and measurement reliability.

The Pro 360 runs on a standard 9-volt battery. With a good alkaline battery, users can expect about 500 hours of operating time. It also has features to preserve battery life, such as an automatic shut-off when unused for several minutes.

For safe and reliable use, the Pro 360 operates between about 23°F and 122°F (-5°C to 50°C), and can be stored between -4°F and 149°F (-20°C to 65°C).

Built with a machined aluminum frame, the Pro 360 is rigid, lightweight, and made for durability. The construction helps it withstand the typical rigors of aircraft maintenance shops and field use.

ALT ZERO allows you to set any surface or angle as a custom zero reference instead of relying only on true level or plumb. This is useful when you need to measure relative angles or when the baseline is not perfectly level.

The HOLD button lets you freeze the displayed angle reading. This is helpful when the protractor is in a hard-to-view position or when you need to record the angle without having to hold the unit steady throughout the reading.

Yes. The display intelligently flips over when the unit is upside down or rotated so that the user can read the angle in any position. Combined with the device’s 360° capability, this makes it highly flexible for many measurement orientations.

Maintenance should include regular calibration checks (especially if the unit is dropped or used in temperature extremes), cleaning with mild soap and a damp cloth (avoiding solvents), and replacing the battery when the low-battery indicator “LO bAt” appears. Also, store it in a clean, dry environment and protect it from extreme temperatures.