Technique: Off to a good start
Preventing damage means watching engine temps and rotation
By Neil Singer
With some light jet engine overhauls costing more than a half million dollars, it’s understandable why pilots monitor their starts so carefully. No other event in a modern engine with full authority digital engine control (FADEC) is as likely to rapidly cause extreme damage. During normal operations, the FADEC does a masterful job of keeping the engine within its operating parameters. During the transitional period of a start, however, the FADEC has limited control; and while on many new jets the FADEC will attempt to abort the start should it go south, a cautious pilot is still on the proverbial edge of his seat until both engines have passed into a stable idle condition.
Illustration by Charles Floyd
Why are starts so potentially catastrophic? In one word, air—specifically, the lack of it. When a jet engine is running, massive quantities of compressed air flow through the combustion section—air that hasn’t been mixed with fuel and ignited. In fact, for every pound of compressed air mixed with fuel and burned, four unmixed pounds flow through and around the metal of the combustion chamber, forming in essence a wall of dense, relatively cool air and protecting the material of the hot section from the immense heat of combustion.
Given that the temperature of combustion can be nearly double the temperature at which the metal in the combustion section will fail, it’s essential that this wall of air keep the flame and metal apart.
Because this air is generated by the compressor sections of the engine, the quantity that can be produced is a function of the engine’s rpm. Once a stable idle is achieved, the engine can produce enough air to insulate the hot section; before the engine is started, of course, there’s no heat. It’s in the transition period that things can go wrong quickly. Light-off has occurred and heat is being generated, while at the same time the engine isn’t yet rotating at a rpm high enough to compress the same quantity of air it will at idle. The challenge of an engine start, then, can be thought of as the effort to pass through the “danger zone” below a stable idle as quickly as possible.
All light jets utilize starter-generators for engine start. The same component that turns the rotation of the engine into electricity during flight can be used to turn electricity—from the aircraft battery or a ground power unit (GPU), for example—into rotation for starting. Getting this starter-generator to turn as quickly as possible, then, is key to minimizing the chance of danger during the start, and only one parameter determines how quickly the starter turns: the voltage that is applied to it.
A well-charged light jet battery puts out a little more than 24 volts without any load on it, but will drop several volts during the demand of an engine start. An operating generator, in contrast, puts out 28 volts, as does a properly set GPU. Furthermore, the amount of power being asked from a GPU, in particular, is typically a smaller fraction of its total power capability, so the voltage will drop less when the load of start is applied to it. For these reasons, a GPU-powered start will tend to be much faster and cooler than a battery start.
Voltage is only one of three parameters that can assist or stress a start. The other two are environmental: the density of the ambient air where the start is being attempted, and the wind conditions through the engine. The lower the density altitude present at start, the more air there is in any volume the engine pulls in and compresses, so the cooler the hot section will be. So a start on a summer afternoon in Aspen, Colorado (elevation 7,820 feet), will be more challenging to the engine than one at sea level at zero Celsius. Similarly, a headwind during start will assist airflow through the engine, while a tailwind creates wrong-way flow and, if strong enough, will rotate the low pressure section of the engine backwards—creating more work for the starter to overcome.
I tell pilots to think of these three variables as points on a triangle—you can start an engine as long as you have two of three points in your favor. No GPU available and need to do a battery start with a questionable battery? Only consider this if you are in a low density altitude condition and have the airplane pointed directly into the wind. Starting at high altitude on a hot day? Make sure you use a GPU and don’t try a tailwind start. Tailwind right at the start limit on a crowded ramp and the FBO is saying it can’t turn you into the wind? Grab a GPU, but don’t try this if the air is thin.
Once the pilot has optimized the initial conditions for the engine start, the actual process in FADEC-controlled airplanes is trivial. In some light jets there is only one step to take—turn a knob and the FADEC does the rest. In others two steps are required: a button-push to engage the starter, and a movement of the thrust lever out of the cutoff position to allow for fuel introduction. The pilot’s real work is in monitoring the start.
Confirming that the high pressure (N2) section begins rotating immediately with starter activation is step one. Concurrent with rotation should be activation of the ignition. It’s essential for the spark to be there before fuel is introduced, so unburned fuel doesn’t pool and ignite in a spectacular manner rather than in a controlled burn.
With proper rotation and ignition the conditions are set for the introduction of fuel. In some airplanes the pilot triggers this step with the thrust lever at a specified rotational speed, while in others it’s completely automatic. In either case, the pilot needs to confirm that fuel is actually flowing, as seen by activation of the electric boost pump and by confirming that the fuel flow value is as expected, typically between 100 and 200 pounds per hour.
Right around the time fuel is introduced, the core section of the engine should be rotating fast enough that the mechanically linked oil pump begins producing measurable pressure. This is another key step to verify. Even at idle the engine is spinning at thousands of rpm, and inadequate lubrication is a quick way to destroy the engine. As the core section spins faster, it pushes enough air through the turbines to spin the fan (N1) section. With no mechanical linkage between the two sections, seeing N1 accelerate is an indication of proper movement of gas through the engine, as well as an indication that the fan section hasn’t seized because of damage or any icing between the fan and the inlet.
Finally, the pilot should see the fruits of the starting labor—a rising inter-turbine temperature (ITT) readout that shows combustion is occurring. This should bootstrap the rest of the start. The gas produced by combustion further accelerates the core, which compresses more air, which can be burned with more fuel to allow for even more acceleration, and so on until a stable idle rpm is achieved.
As acceleration is occurring the pilot must carefully monitor the ITT gauge for the two most common start problems: a hot start, or ITT rapidly rising in a way that could exceed the ITT limits—or a hung start, where the core engine speed stops climbing and plateaus before reaching the proper idle rpm. Most light jets’ FADECs will automatically shut off fuel to the engine (almost immediately causing a decrease in temperature) if either is sensed. But the pilot should still be carefully watching ITT and N2 rpm, in case the automatic protections fail. AOPA
Neil Singer is a Master CFI with more than 9,500 hours in 15 years of flying.
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