Turbine Pilot

First, go into reverse

Pilots new to Garrett (now officially called Honeywell—formerly AlliedSignal) turboprop engines often are perplexed by some of the systems and procedures unique to these powerplants. Dealing with one of them—the propeller start locks—can be an awkward proposition the first few times a pilot fires up and shuts down a Garrett. After all, going into reverse propeller pitch seems unnatural at times like these. "Why do I have to go into reverse before I can taxi?" a neophyte might reasonably ask, or "Why the urgency to go back into reverse before engine speed spools down during a shutdown?"

There are several very good answers. First of all, remember that Garrett/Honeywell turboprop engines use a single-shaft design. The propeller and its gearbox are directly connected to the rotating engine components via one lone shaft. Turn the blades of a parked Garrett’s propeller, and you turn all the moving components in the rest of the powerplant assembly.

This design is quite unlike the engine/propeller setup in Pratt & Whitney turboprop engines. With Pratts, the propeller assembly rotates free of the powerplant—hence their classification as "free turbines." In this design, engine exhaust gases drive a free-wheeling power turbine, which in turn drives the propeller.

It’s during the start sequence that you discover one of the consequences of this difference between the single-shaft and free-turbine designs: rotational drag. When you start a Pratt & Whitney engine, its compressors and turbines turn relatively easily. With the Garrett, the starter has to drive the entire power train, gearbox and propeller included. Needless to say, this puts a lot of strain on the electrical system.

Any turbine engine start has to be watched carefully for signs of the dreaded hot start—a condition where combustion flares up uncontrollably (see "Avoiding Redline Fever," June 1999 Turbine Pilot). In seconds, a hot start can ruin a turbine engine and run up a six-digit repair bill. The best way to prevent a hot start is to make sure that there’s plenty of intake air flowing through the engine. Anything that could slow the engine’s rotation during the start could impede the flow of air to the combustion chamber.

What could cause an engine to turn too slowly during a start? A sick battery could do it, but imagine for a minute that you’re trying to start a Garrett/Honeywell turboprop and for some reason the propeller blades are feathered. The starter has to turn all that hardware, and engine acceleration will be slow indeed as a feathered propeller fights air loads in its struggle to turn. This means that less air will make it to the combustion chamber, and the chance of a hot start increases.

If the propeller blades were somehow held at a flat-pitch setting, though, the propeller—and therefore the engine components—would turn more easily and admit more air.

And here’s where the start locks come into play. Start locks, located at the propeller blade roots, hold propeller pitch at a near-zero setting. The locks themselves are spring-loaded pins that prevent the propeller blades from traveling toward reverse-thrust settings when the engine is turning at low speed.

The start locks must be engaged during a ground start. Preparing a Garrett/Honeywell turboprop for starting begins with the previous shutdown procedure, when the start locks are engaged. In 690-series Twin Commanders, you do this by turning the engine control to the Off position, then selecting full reverse thrust with the power levers as engine rpm decays below the 50-percent mark. This holds the propeller blades behind the locks until the locks have a chance to extend. At this point, the blades are heading toward the feather position, but they’ll never get there because the locks will stop them. Result: The propeller blades are left in flat pitch, ready for the next start.

After the next start, the start locks must be disengaged. To do this, you once again pull the power levers into the reverse thrust range. You do this as the propeller speeds up; reversing pitch at this time causes the start locks to retract by centrifugal force. Forget to disengage the start locks? You’ll know, because as you advance power, the airplane won’t taxi away. The propeller blades, held in flat pitch, can’t produce enough thrust.

You’ll know that the start locks have disengaged by observing a slight rise in fuel flow and torque (or horsepower) as the propeller blade angle changes and the prop disc takes on aerodynamic loads as it takes a bigger bite out of the air. You can also see the blades change pitch and feel the airplane want to move forward. Now you’re ready to taxi.

After a few training sessions the start-lock drill becomes ingrained. Just remember two cardinal rules: Don’t even think about starting a Garrett/Honeywell engine with the propeller blades unlocked (a hot start awaits), and don’t move the power levers above the Ground Idle position with the start locks still engaged (the propeller will act like a fixed-pitch prop and could go into an overspeed condition).

Links to additional information about aircraft employing start locks may be found on AOPA Online ( www.aopa.org/pilot/links/links0002.shtml). E-mail the author at [email protected].