Chapter 7: Synchronising and Ice Protection

Propeller synchronising

When two or more propellers turn at slightly different rpm, their sound waves interfere and produce a pulsing beat at the difference frequency — tiring for passengers and crew and a source of structural vibration. Synchronising holds all propellers at the same rpm.

• Manual synchronising — the pilot adjusts the propeller levers until the beat disappears (aided by a synchroscope on older aircraft).
• Master–slave synchroniser — one engine is the master. Pulse pickups or tachometer generators send each engine's rpm to a control box, which compares them and sends a correction signal to a trimming device (a coil or stepper actuator) on the slave governor(s), adjusting their speeder-spring setting.
• The synchroniser has limited authority — it can only move the slave rpm within a small range (typically about 1–3 %). If the master fails, the slave cannot follow it down far.
• Synchronisers are normally switched off for take-off and landing (and engaged only when the rpm are already close), so a fault cannot affect power when it matters most. Operating limits are given in the AFM.
• On FADEC/electronic propeller control systems, synchronising is a software function using shaft-speed sensors.

Synchrophasing

• Synchrophasing goes a step further: it keeps a fixed phase angle between the blades of the different propellers (for example, the left propeller's blade leading the right's by a set angle). This places the pressure pulses from the blade tips so they partly cancel in the cabin.
• A magnetic pulse pickup on each propeller produces one pulse per revolution (or per blade) as a target on the spinner bulkhead or hub passes it. The control box compares the timing of the pulses.
• Some systems let the pilot select the phase angle with a cockpit control to find the quietest setting.
• Testing (on the ground or in flight, per the AMM): check the system engages only within its capture range, holds rpm/phase, has the correct limited authority, and that the slave returns to its own governor setting when switched off. Check pickups, gaps, wiring and the trim actuator.

Active noise and vibration suppression

• Some turboprops add an active noise and vibration suppression (ANVS) system. Microphones and/or accelerometers in the cabin sense noise at the blade-passing frequency. A controller drives active tuned vibration absorbers (force generators on the fuselage frames) or loudspeakers to generate anti-phase vibration and sound that cancel it.
• It works together with synchrophasing, using propeller speed and phase signals as references.

Propeller ice protection

Ice on propeller blades changes the aerofoil shape, cuts thrust and efficiency, and causes imbalance and vibration. Ice shed from the blades can damage the fuselage, so fuselage panels in the plane of rotation may be reinforced. Ice forms first near the hub, where blade speed and centrifugal force are lowest.

Fluid anti-icing

• Anti-icing prevents ice from forming, so it must be switched on before entering icing conditions.
• Fluid — usually isopropyl alcohol (flammable) or an alcohol/glycol mix — is pumped from a tank by an electric pump whose output is set by a rheostat in the cockpit.
• The fluid goes to a slinger ring: a U-section channel on the rear of the hub. Centrifugal force spreads the fluid into delivery tubes and out along feed shoes (grooved rubber strips) on the blade leading edges.
• Inspection: tank, filter, pump, lines, slinger-ring outlets clear and secure, feed shoe condition and bonding; flow check per the manual.

Electrical de-icing

• De-icing removes ice after it has formed. Electric heater elements are built into rubber boots (overshoes) bonded to the blade leading edges, often with separate inboard and outboard elements.
• Power from the aircraft bus goes through a switch and timer, to a brush block on the engine, then through slip rings on the spinner bulkhead, to the boots.
• The timer energises the elements in a sequence (e.g. outboard then inboard, or one propeller then another) for set periods. This reduces the electrical load. The heat melts the bond between ice and boot, and centrifugal force throws the ice off.
• An ammeter (or load meter) shows current flow, and its reading changes as the timer steps through the cycle — a check that each element is working.
• Ground operation is limited to short tests because there is no cooling airflow and the boots can overheat.

Inspection, maintenance and testing (electrical)

• Operational check: with the engine running (or per the AMM procedure), check that the ammeter indicates within limits for each timer step; feel the boots heat in sequence (only briefly).
• Brush block: check brush length (wear limits), free movement in their holders, spring pressure and correct alignment with the slip rings. Measure brush-to-slip-ring contact and alignment as the AMM requires.
• Slip rings: check for wear, pitting, burning or arcing, contamination by oil or grease, and run-out. Clean with an approved solvent.
• Timer: check the cycle sequence and step times with a test lamp or test set.
• Resistance checks: measure heater element resistance against limits, and insulation resistance between elements and the blade.
• Overshoes: inspect for cuts, tears, lifting edges, burns and erosion. Small damage may be repaired with approved compounds. To replace a boot: remove the old one with solvent (e.g. MEK or toluene) and a gentle peeling action, clean and prepare the blade surface, apply the specified adhesive, position the new boot accurately, roll out air bubbles, allow full cure, then perform resistance and functional tests and check balance.