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Turbine Pilot: Turboprop TransitionTurbine Pilot: Turboprop Transition

One of the classic ironies of aviation is that modern turboprop engines—paragons of power and reliability they may be—are far more fragile during the startup sequence than piston engines. Two things have to happen to ensure a successful turboprop start.

WHAT IT LOOKS LIKE: Brake wear indicator

By Mark R. Twombly

Floatplanes and skiplanes may not need them, but any aircraft that rolls on wheels and tires sure does. Brakes, that is.

Brakes on turbine-powered airplanes are used for more than simply slowing to taxi speed on the landing roll, and to assist in steering. Brakes are critical to achieving calculated takeoff and landing distances. If it’s a short runway and an engine fails precisely at V 1, you want to have complete confidence that when you apply maximum pressure to the brake pedals (simultaneously pulling the throttle(s) to idle and deploying spoilers and thrust reversers, if applicable), the airplane will run out of forward motion before you run out of runway.

So how does a pilot determine that the brakes will indeed step up to that challenge if called upon? On light piston-powered aircraft, it’s usually possible during the preflight inspection to get down and dirty and visually examine the brake rotors and pads to ensure there is sufficient lining on the pads and that the rotors are in good shape. That doesn’t work so well on most turbine-powered airplanes. The brakes are larger and more complex, and on most aircraft the components are not easily examined in a preflight visual inspection.

There’s an easier way to confirm during a walkaround inspection that brake disks are in good shape. Simply check the brake wear indicators. The specific configuration of a wear indicator differs among aircraft makes and models, but the idea is basically the same—a metal pin protrudes from the brake piston housing. As long as some designated length of each pin on the brake piston housing is visible, the disks should be good; no maintenance required.

Some small amount of work is required to do a proper brake wear indicator check. The preflight procedure calls for pressurizing the brakes, typically by applying the parking brake, and then examining the wear indicators. If the indicators protrude from the brake piston housing, the brake disks are within tolerances. If the wear indicators are flush with their access holes, brake maintenance is required before further use. A mechanic can do a more accurate check of disk wear by precisely measuring the length of the indicator pin.

The brake wear indicator in the accompanying photos are on a Citation 550. The brakes are not pressurized, but since so little of the pin is showing, it’s probably getting close to time for some brake work.

Mark R. Twombly flies a Citation II and a Citation VII based in southwest Florida.

One of the classic ironies of aviation is that modern turboprop engines—paragons of power and reliability they may be—are far more fragile during the startup sequence than piston engines.

Two things have to happen to ensure a successful turboprop start. The compressor must be turning fast enough to generate a minimum required level of airflow, and just the right amount of air/fuel mixture must be in the combustion chamber at the moment of ignition.

Most new gas turbine engines combine these functions, coordinating the timing with electrical circuitry and pre-empting any unhappy human intervention at a critical moment. But with older turboprops and jets, pilots can still make the thing go boom in spectacular fashion.

If the airflow is inadequate or the fuel puddle too large, the result is not merely a failed start—the resulting ignition will most likely heat the engine’s core or “hot section” to a point well beyond its maximum temperature limit.

In the wake of “hot start” events such as these, standard practice typically calls for a detailed inspection of the hot section, and these can often come with a price tag in the $25,000 range. Extreme cases have been known to prompt complete engine overhauls; these can cost upward of $300,000.

Keeping things cool

To keep transitioning turboprop pilots from falling into the overtemp trap, thorough instruction from an experienced hand is invaluable. The first phase will most likely include the elimination of habits learned during piston engine operations.

One of these habits is pushing the mixture knob forward prior to engaging the starter. With a turbine, the fuel, or condition lever is typically handled with care, and kerosene is introduced only when the pilot is absolutely certain that the compressor (controlled with the starter button) has reached the correct speed.

Battery voltage is perhaps the best indicator that everything is proceeding as it should. If the battery charge is weak for whatever reason, the starter is typically unable to rotate the compressor fast enough to prevent a hot start. Checking and confirming proper voltage prior to engaging the starter is, therefore, important.

Most turbine aircraft require operators to alternate the source of startup voltage between onboard and auxiliary (ground power) systems. This ensures the aircraft batteries are fully charged if and when ground power is unavailable.

Also, if the airplane isn’t faced directly into the wind, insufficient airflow through the engine’s pointy end can have the same net effect as a too-slow compressor rotation speed. Most turbine aircraft/engine installations come with a downwind start limit, and frontal passage has been known to trigger frantic towing activity on FBO ramps as business jet crews scramble to get their aircrafts’ noses back into the wind.

Monitoring engine exhaust gas temperature (EGT) must be done throughout the start, as this is often the first sign of trouble. While the various temperature limits and “normal” indications are good to know, what you’re really looking for is a rapid rise—needle movement that appears faster than the norm. Unfortunately, cockpit gauge indications tend to lag behind actual temperatures, and if the pilot delays cutting off the fuel supply until the gauge hot start limit is reached, even the fastest remedial action will probably come too late.

If a start is abandoned, it’s reasonable to assume that unburned fuel remains in the combustion chamber. If this liquid is not ventilated or purged somehow, the next start is almost guaranteed to be a hot one. Typically, the approved “blow out” procedure is to rotate the compressor with the starter, with the condition lever and ignition switches off.

Re-lighting in flight

Overtemping is less likely during a windmilling air-start, since the compressor core is probably rotating quickly enough to support safe ignition. Just remember that engaging the starter on an already-running engine can damage the starter mechanism.

There are exceptions. Some air restart procedures call for the starter; one rule of thumb says to engage it if the core rotation speed falls below five percent. Check your specific aircraft flight manual before reaching for the switch, however. Most turbine aircraft come with a “relight envelope” diagram, which shows you the permissible airspeed and altitude ranges for a relight.

A few points about ignition switches are probably worth mentioning. Typically, continuous ignition is used any time when engine power must be absolutely guaranteed, such as during takeoff, landing, in turbulence, or in icing conditions.

As for the precise moment to throw the ignition switch during startup, this can vary between aircraft and engine models. With some, ignition comes on automatically with the start lever; on others it can be selected separately. If there is a choice, it’s probably good to remember that ignition should be activated before, or at least with, the introduction of fuel—just like a World War II flamethrower.

David Dewhirst is a 10,000-hour ATP with 6,000 hours as an instructor. He is an FAA FAASTeam counselor.

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