Art of the start

Spooling you up on spool-up

December 1, 2012

art of the start

Around the time I started flying my first jet, I remember a King Air pilot telling me, “The best thing about turbine engines is that they’ll run forever as long as you don’t blow ‘em up during start.” Piston engines have hundreds of moving parts that reverse direction several times a second and are subject to immense stress; something as insignificant as a rapid change in temperature (i.e., shock cooling) can cause critical parts to fail. Turbine engines have relatively few moving parts (and they spin in the same direction all the time), and these components are made from advanced metals developed to withstand intense temperature and pressure. During a start sequence, the combustor and turbine of the engine (the “hot section”) are exposed to the greatest, most rapid changes in temperature and airflow these components will see during an entire flight—that’s when there is the most potential for things to go wrong.

A thorough preflight of the aircraft should be made before attempting to bring the engines to life. For each engine, ensure that the oil dipstick and filler cap are secure, all access panels on the cowling are latched and secure, and that all inlets are free of contamination. If you fly a jet, be aware of one wintertime gotcha: ice and snow collecting at the bottom of the inlet may prevent the fan from turning freely. Starting the engine in this condition could be a very expensive mistake.

Every engine start begins with a starter. Smaller, lighter engines make use of an electric starter-generator. Pneumatic starter motors are used in larger engines because an electric starter pulls too much amperage, creates too much heat, and is heavier than a pneumatic starter. Pneumatic starters use high-pressure bleed air from an auxiliary power unit (APU), a compressed air cart, or a running engine. The starter motor or starter-generator is attached to the accessory section of the engine and applies torque to the N2 (inner) spool of a jet engine—or the gas generator section of a turboprop or turboshaft engine—through the accessory gearbox.

Are you ready? Press the Start button. This action engages the starter and arms the igniters in the combustion chamber. In airplanes with automatic electric fuel pumps, the starter circuit will also turn on the appropriate boost pump. The inner section of the engine begins to spin up, causing air to flow through the engine. This airflow will overcome the inertia of the fan section (jet engine) or power section (turboprop) and the rest of the engine will rotate. The main oil pump and scavenge pumps (also components on the accessory section) will begin circulating oil, so oil pressure should rise. It’s safe to introduce fuel and proceed with the start sequence.

Modern turbine engines will incorporate one of three types of control units: a hydromechanical unit (HMU), an electronic engine control unit (EEC or ECU), or a full-authority digital engine control (FADEC) unit. These devices serve the same function: meter fuel to the fuel nozzles in response to pilot input in a way that results in smooth, predictable, and reliable operation of the engine. A HMU is a very complex piece of machinery that operates with precision. It takes engine speed, outside temperature, and ram air pressure into account and adjusts fuel flow appropriately. An ECU is fundamentally a HMU, but with the addition of a computer and an electronic servo. The servo overrides mechanical functions of the HMU as long as the electronics are operating normally. The ECU assists the pilot by monitoring various parameters and preventing limitations from being exceeded unintentionally. FADEC is the final iteration of engine controls; a FADEC unit incorporates a basic mechanical fuel controller which is manipulated entirely by the FADEC computer. FADEC offers many possible benefits, including no need for a mechanical connection between the power levers and the engine; better efficiency; and automatic recovery from flameouts, bird strikes, compressor stalls, and icing encounters.

With only a few exceptions, HMU- or ECU-equipped aircraft require the pilot to manually initiate fuel flow to the engine during start. Because of certification requirements, or due to design choice, many FADEC-equipped aircraft also require the pilot to perform an action (i.e., moving the power lever through a cutoff gate) to initiate fuel flow. The FADEC unit’s software can monitor the engine start from beginning to end and instantly take action if an abnormal condition is detected, thereby preventing further damage to the engine (not all FADECs have this capability).

Once the recommended motoring N2 or NG rpm is reached by the starter and air is flowing freely through the engine, you can add fuel. It is essential to wait for the proper rpm, because the hot section is about to get hot, and we want as much airflow as possible. Some pilots prefer to allow the starter to motor the engine up to the highest rpm it is able to attain before adding fuel, but this practice could cause no-light-off scenarios because the initial fuel mixture will be very lean above the recommended motoring rpm (this is also the reason many turbine aircraft have a finite airspeed/altitude envelope for in-flight restarts). You can confirm that fuel is actually flowing by noting a positive indication on the fuel-flow gauge.

When the fuel ignites, there will be a rise in exhaust gas temperature (EGT) or interstage turbine temperature (ITT). ITT is the temperature of the hot gas stream between the first and second stages of the turbine, and EGT is the temperature of the exhaust stream aft of all the turbines (an engine manufacturer will choose whether to use ITT or EGT based on where it makes more sense to locate the thermocouple probes in that engine; there are some other terms, too, like TGT and TOT). The temperature limit for start usually is well above the continuous temperature limit because it’s expected that the highest temperature attained during start will be transient (abort the start otherwise).

As hot gas begins to flow through the turbine section, the engine will accelerate off the starter. Fuel flow starts at a minimum value, and increases with increasing engine speed according to the acceleration schedule of the fuel controller. Typically, only half the fuel nozzles are used initially, with the second half brought online at a specific point on the acceleration schedule. This helps reduce the initial temperature of the start while ensuring the engine accelerates smoothly up to idle speed. Halfway through the start sequence, there will be a slight second rise in temperature, indicating that the secondary fuel nozzles have become active. On hot days or at high elevations, engine starts will be noticeably warmer. This relates to density altitude and the mass of air flowing through the engine. Some ECUs and FADEC units have the capability to “trim fuel” off the acceleration schedule to keep ITT/EGT low when starting at high density altitudes.

Once the engine is stable at idle, the fuel controller will fall off the acceleration schedule (reduce fuel flow to sustain steady idle speed), and ITT/EGT should roll back. Continuously scan the engine instruments throughout the start sequence. Timely action is the key to preventing costly damage to the engine if an abnormal situation occurs. Make a thorough check of all instruments after the start sequence to catch an insidious problem.

The following procedures are meant to be general procedures only. Should you experience any of the following conditions, consult your aircraft flight manual and maintenance manual for specific actions.

  • A hot start is when temperature limitations are exceeded during start, possibly damaging the engine’s hot section. If the condition continues unchecked, a fire may result. Hot starts may be caused by improper pilot action (introducing fuel before reaching the proper motoring rpm), a failure of the fuel controller or fuel nozzles, a tailwind component (the wind inhibits exhaust flow at the back of the engine), or a weak battery causing the electric starter to peter out after light-off. If you see ITT/EGT increasing abnormally quickly, abort the start with prompt action before an actual temperature exceedance occurs, resulting in a more mellow “warm start.” The correct pilot action for a hot start typically is to cut fuel off immediately and leave the starter engaged. Motor the engine long enough to purge all the fuel and hot gas from the combustor and allow airflow to cool the hot section.
  • A hung start occurs when the engine starts but doesn’t accelerate to idle speed. This indicates a problem wherein the HMU/ECU falls off the acceleration schedule or fails to bring the secondary fuel nozzles online. Cut the fuel off and motor the engine.
  • The two items required for combustion are, obviously, fuel and ignition. In the absence of one of these, the engine will not start. You can usually identify the cause of this problem immediately: no indication of ignition or no fuel-flow indication.
  • A typical jet engine will sustain its own acceleration from 35 percent N2 onward without the help of the starter. The pilot manually disengages the starter or a speed switch automatically “kicks” it off. If the starter continues to operate, it will generate heat until it fails. To force a pneumatic starter to stop, close the main bleed valve to the engine. To manually disengage an electric starter-generator, pull the starter relay circuit breaker.

Cyrus Sigari is president and founder of JetAVIVA in Santa Monica, California.