Learning turbine-talk
Pilots new to turbine aircraft may find that engine instrumentation is at once both strange and familiar. Oil pressure and temperature mean the same thing in either type, for instance. Fuel flow is familiar too, but in turbine aircraft it is calibrated in pounds per hour (or kilograms per hour in some countries) rather than gallons per hour. However, a number of gauges are unique to turbine engines. Knowing how to interpret their normal and abnormal indications is basic to safe operation.
In a jet engine, each major rotating section usually has a separate gauge devoted to monitoring its speed of rotation. Depending on the model, for instance, a jet engine will have a gauge labeled N 1 that monitors the low-pressure compressor section and another labeled N 2 that monitors the high-pressure compressor section. So-called triple spool engines will have an N 3 gauge as well. These sections rotate at many thousands of rpm, and their gauges are calibrated in percent of rpm, rather than actual rpm, for ease of display and interpretation.
Some jet aircraft have so-called engine pressure ratio, or EPR, gauges. EPR is a measurement of thrust produced by an engine. The gauge actually displays the ratio of turbine discharge pressure to engine inlet pressure as sensed by probes in the nacelle and exhaust sections of the engine. The EPR or N 1 gauge is the primary reference used to establish power settings in jets, although it is a good practice for pilots to observe all of the engine gauges when setting takeoff thrust.
The temperature of turbine gases must be closely monitored to prevent heat damage to turbine blades and other components. Gas temperature can be measured at a variety of different locations within an engine, and the associated engine gauges have different names according to the chosen location. For instance, these are variously referred to as exhaust gas temperature (EGT), turbine outlet temperature (TOT), interturbine temperature (ITT), or turbine inlet temperature (TIT) gauges.
Another engine instrument found on some turbine aircraft is a vibration detector. Turbine engines run far more smoothly than reciprocating engines. The combustion cycle occurs as one continuous event, much like the burner on a gas stove operates without interruption. There are no pounding pistons, furiously responding to thousands of closely timed cycles of intake, compression, ignition, combustion, and exhaust. A serious problem in a turbine engine may not manifest itself to the pilot with obvious engine roughness or unusual sounds, as might a sick piston engine. Vibration detectors are therefore useful in picking up subtle engine vibrations that might signal an impending problem.
All these gauges work in concert to help pilots monitor an engine's well-being. Turbine engines are known for their incredible reliability, but pilots still need to be aware of some seldom-seen indications that could spell trouble.
Take the normal start process. In a typical jet engine, it goes something like this: Placing the start switch to On starts the compressor section turning. Air is drawn through the compressor and into the combustion chamber. Fuel and ignition are introduced there when the mass of air is sufficient to support combustion. Light-off occurs, and the engine begins to accelerate as combusted gases drive the turbine section. As the engine accelerates, fuel flow, EGT, N 1, and N 2 all increase until the engine stabilizes at its normal idle speed.
But suppose all is not normal, and a hot start is about to occur. The first hint of an impending hot start may well be higher-than-normal initial fuel flow. The fuel control unit usually meters just enough fuel to match the available airflow from the compressor section, thus keeping start temperatures within limits. As the compressor accelerates during start, more air becomes available for combustion, and the FCU increases fuel flow accordingly. But too much fuel early in the process causes combustion gas temperatures to rise too rapidly. Airflow through the engine may be insufficient to maintain start temperatures within limits.
Higher-than-normal start temperatures can therefore be expected to quickly follow on the heels of higher-than-normal fuel flows. Knowing this can mean the difference between a pilot's preventing a fully developed hot start or ending up with an expensive collection of ruined engine parts.
Pilots who reference EPR values for setting takeoff power should be especially cautious when operating in icing conditions. If the engine probes that measure engine inlet pressure should become iced over, the resulting EPR gauge reading, and therefore the desired power setting, will be incorrect. This is believed to have been a contributing factor in the 1982 crash of an Air Florida Boeing 737 departing Washington National Airport. What appeared to the crew to be a normal takeoff power setting was considerably less. The airplane crashed into a bridge after takeoff because its climb performance was compromised by erroneous EPR indications that resulted from probe icing. To prevent this kind of occurrence, engine heat systems need to be activated during icing conditions to ensure reliable EPR readings. It is recommended that whenever setting takeoff power based on EPR, pilots should also reference a minimum N 1 setting in order to be certain that engines are producing desired takeoff thrust.
In flight, erratic or fluctuating EPR indications could mean a number of problems. The first thing pilots should think of is possible icing of the EPR probes. Turning on engine heat usually reveals if this is the case, since the problem normally clears up immediately once heat is applied. But in conjunction with other erratic indications, such as fluctuating fuel flow, EGT, N 1, or N 2, it could signal a more serious problem. Fuel contamination should be considered as a potential cause at this point, especially if more than one engine seems to be affected. If these fluctuations are seen in combination with loud banging noises from the engine audible in the cockpit, compressor stalls are probably occurring. In any case, the engine should be run at a reduced power setting. If this doesn't result in stable engine operation, the crew would be well-advised to consider an in-flight engine shutdown.
More than one crew has had the misfortune to inadvertently fly through a volcanic ash cloud. Nighttime encounters are most likely, since weather radar is not especially good at detecting these clouds. Volcanic dust will rapidly erode fan blades and vanes, and block air passages within the engine that are needed to keep temperatures within normal ranges. What indications might show up on the gauges in this scenario? Loss of thrust, higher-than-normal EGT readings, and surging power settings have all been reported, as have compressor stalls and flameouts. Visible dust in the cockpit and St. Elmo's fire on windscreens have been experienced too. In case a crew facing this situation has any lingering doubts about the cause, volcanic dust can be expected to affect all engines, and it may be accompanied by an acrid, electrical smoke-like smell. With these indications, the only thing to do is reduce power in order to minimize engine damage, and get out of the cloud as quickly as possible.
If an engine flameout occurs for unexplained reasons, the gauges will usually reveal whether it is wise for the crew to attempt a relight or not. A seized or seriously damaged jet engine will be evident by an indication of no rotation of the N 1 or N 2 sections. A relight attempt is not advised. A complete loss of oil quantity is another good reason not to try a restart, as is a fire indication. But if the engine is windmilling freely and oil quantity is normal, a restart is probably worth considering.
A high vibration detector reading could be evidence of an internal engine problem. It could also be a sign of fan blade icing. Just like an erratic EPR indication, a pilot's first response should be to turn on engine heat to see if that clears up the problem. If it doesn't, a reduced power setting ought to be tried, followed by engine shutdown if the vibration is serious enough.
Some turbine aircraft have warning lights that monitor fuel filter and oil filter status. A clogged main oil filter could be an indication of metal contaminants and may be cause for a precautionary engine shutdown. A clogged fuel filter likely means that ice crystals have formed in the fuel. If the aircraft is equipped with a fuel heat system, turning fuel heat on should melt the crystals and clear the light. However, if it remains illuminated, it may instead be a sign of solid contaminates in the fuel. This is a potentially serious problem, especially if affecting more than one engine, and could lead to an engine flameout.
Learning the nuances of engine instrumentation is an important part of mastering turbine aircraft.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 757/767 captain for a major U.S. airline.
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