This page was taken from an original document at http://fromtheflightdeck.com/Stories/turbofan/
The following article was developed to assist flight crews in understanding the operation and typical malfunctions of turbofan engines. As this information was designed to apply to a generic turbofan engine, it can be used to develop new instructional material when upgrading from turboprop to jet operations, or to enhance current flight crew training and operational understanding.
*** Updated 16 Sep 2016 ***
To provide effective understanding of and preparation for the correct responses to engine in-flight malfunctions, this article will describe turbofan engine malfunctions and their consequences in a manner that is applicable to almost all modern turbofan-powered airplanes. These descriptions, however, do not supersede or replace the specific instructions that are provided in the Airplane Flight Manual and appropriate checklists.
It is most important to provide an understanding of compressor surge. In modern turbofan engines, compressor surge is a rare event. If a compressor surge (sometimes called a compressor stall) occurs during high power at takeoff, the flight crew will hear a very loud bang, which will be accompanied by yaw and vibration. The bang will likely be far beyond any engine noise, or other sound, the crew may have previously experienced in service.
Compressor surge has been mistaken for blown tires or a bomb in the airplane. The flight crew may be quite startled by the bang, and, in many cases, this has led to a rejected takeoff above V1. These high-speed rejected takeoffs have sometimes resulted in injuries, loss of the airplane, and even passenger fatalities.
The actual cause of the loud bang should make no difference to the flight crew's first response, which should be to maintain control of the airplane and, in particular, continue the takeoff if the event occurs after V1. Continuing the takeoff is the proper response to a tire failure occurring after V1, and history has shown that bombs are not a threat during the takeoff roll – they are generally set to detonate at altitude.
A surge from a turbofan engine is the result of instability of the engine's operating cycle. Compressor surge may be caused by engine deterioration, it may be the result of ingestion of birds or ice, or it may be the final sound from a "severe engine damage" type of failure. The operating cycle of the turbine engine consists of intake, compression, ignition, and exhaust, which occur simultaneously in different places in the engine. The part of the cycle susceptible to instability is the compression phase.
In a turbine engine, compression is accomplished aerodynamically as the air passes through the stages of the compressor, rather than by confinement, as is the case in a piston engine. The air flowing over the compressor airfoils can stall just as the air over the wing of an airplane can. When this airfoil stall occurs, the passage of air through the compressor becomes unstable and the compressor can no longer compress the incoming air. The high-pressure air behind the stall further back in the engine escapes forward through the compressor and out the inlet.
This escape is sudden, rapid and often quite audible as a loud bang similar to an explosion. Engine surge can be accompanied by visible flames forward out the inlet and rearward out the tailpipe. Instruments may show high EGT and EPR or rotor speed changes, but, in many stalls, the event is over so quickly that the instruments do not have time to respond.
Once the air from within the engine escapes, the reason (reasons) for the instability may self-correct and the compression process may re-establish itself. A single surge and recovery will occur quite rapidly, usually within fractions of a second. Depending on the reason for the cause of the compressor instability, an engine might experience:
1) A single self-recovering surge
2) Multiple surges prior to self-recovery
3) Multiple surges requiring pilot action in order to recover
4) A non-recoverable surge.
For complete, detailed procedures, flight crews must follow the appropriate checklists and emergency procedures detailed in their specific Airplane Flight Manual. In general, however, during a single self-recovering surge, the cockpit engine indications may fluctuate slightly and briefly. The flight crew may not notice the fluctuation. (Some of the more recent engines may even have fuel-flow logic that helps the engine self-recover from a surge without crew intervention. The stall may go completely unnoticed, or it may be annunciated to the crew – for information only – via EICAS messages.)
Alternatively, the engine may surge two or three times before full self-recovery. When this happens, there is likely to be cockpit engine instrumentation shifts of sufficient magnitude and duration to be noticed by the flight crew. If the engine does not recover automatically from the surge, it may surge continually until the pilot takes action to stop the process. The desired pilot action is to retard the thrust lever until the engine recovers.
The flight crew should then SLOWLY re-advance the thrust lever. Occasionally, an engine may surge only once but still not self-recover.
The actual cause for the compressor surge is often complex and may or may not result from severe engine damage. Rarely does a single compressor surge CAUSE severe engine damage, but sustained surging will eventually over-heat the turbine, as too much fuel is being provided for the volume of air that is reaching the combustor. Compressor blades may also be damaged and fail as a result of repeated violent surges; this will rapidly result in an engine which cannot run at any power setting.
Additional information is provided below regarding single recoverable surge, self-recoverable after multiple surges, surge requiring flight crew action, and non-recoverable surge. In severe cases, the noise, vibration and aerodynamic forces can be very distracting. It may be difficult for the flight crew to remember that their most important task is to fly the airplane.
Single self-recoverable surge
The flight crew hears a very loud bang or double bang. The instruments will fluctuate quickly, but, unless someone was looking at the engine gage at the time of the surge, the fluctuation might not be noticed.
For example: During the surge event, Engine Pressure Ratio (EPR) can drop from takeoff (T/O) to 1.05 in 0.2 seconds. EPR can then vary from 1.1 to 1.05 at 0.2-second intervals two or three times. The low rotor speed (N1) can drop 16% in the first 0.2 seconds, then another 15% in the next 0.3 seconds. After recovery, EPR and N1 should return to pre-surge values along the normal acceleration schedule for the engine.
Multiple surge followed by self-recovery
Depending on the cause and conditions, the engine may surge multiple times, with each bang being separated by a couple of seconds. Since each bang usually represents a surge event as described above, the flight crew may detect the "single surge" described above for two seconds, then the engine will return to 98% of the pre-surge power for a few seconds. This cycle may repeat two or three times. During the surge and recovery process, there will likely be some rise in EGT.
For example: EPR may fluctuate between 1.6 and 1.3, Exhaust Gas Temperature (EGT) may rise 5 degrees C/second, N1 may fluctuate between 103% and 95%, and fuel flow may drop 2% with no change in thrust lever position. After 10 seconds, the engine gages should return to pre-surge values.
Surge recoverable after flight crew action
When surges occur as described in the previous paragraph, but do not stop, flight crew action is required to stabilize the engine. The flight crew will notice the fluctuations described in "recoverable after two or three bangs," but the fluctuations and bangs will continue until the flight crew retards the thrust lever to idle. After the flight crew retards the thrust lever to idle, the engine parameters should decay to match thrust lever position. After the engine reaches idle, it may be re-accelerated back to power. If, upon re-advancing to high power, the engine surges again, the engine may be left at idle, or left at some intermediate power, or shutdown, according to the checklists applicable for the airplane. If the flight crew takes no action to stabilize the engine under these circumstances, the engine will continue to surge and may experience progressive secondary damage to the point where it fails completely.
When a compressor surge is not recoverable, there will be a single bang and the engine will decelerate to zero power as if the fuel had been chopped. This type of compressor surge can accompany a severe engine damage malfunction. It can also occur without any engine damage at all.
EPR can drop at a rate of .34/sec and EGT rise at a rate of 15 degrees C/sec, continuing for 8 seconds (peaking) after the thrust lever is pulled back to idle. N1 and N2 should decay at a rate consistent with shutting off the fuel, with fuel flow dropping to 25% of its pre-surge value in 2 seconds, tapering to 10% over the next 6 seconds.
A flameout is a condition where the combustion process within the burner has stopped. A flameout will be accompanied by a drop in EGT, in engine core speed and in engine pressure ratio. Once the engine speed drops below idle, there may be other symptoms, such as low oil pressure warnings and electrical generators dropping off line – in fact, many flameouts from low initial power settings are first noticed when the generators drop off line and may be initially mistaken for electrical problems. The flameout may result from the engine running out of fuel, severe inclement weather, a volcanic ash encounter, a control system malfunction, or unstable engine operation (such as a compressor stall). Multiple engine flameouts may result in a wide variety of flight deck symptoms as engine inputs are lost from electrical, pneumatic and hydraulic systems. These situations have resulted in pilots troubleshooting the airplane systems without recognizing and fixing the root cause – no engine power. Some airplanes have dedicated EICAS / ECAM messages to alert the flight crew to an engine rolling back below idle speed in flight; generally, an ENG FAIL or ENG THRUST message.
A flameout at take-off power is unusual – only about 10% of flameouts are at takeoff power. Flameouts occur most frequently from intermediate or low power settings, such as cruise and descent. During these flight regimes, it is likely that the autopilot is in use. The autopilot will compensate for the asymmetrical thrust up to its limits and may then disconnect. Autopilot disconnect must then be accompanied by prompt, appropriate control inputs from the flight crew if the airplane is to maintain a normal attitude. If no external visual references are available, such as when flying over the ocean at night or in IMC, the likelihood of an upset increases. This condition of low-power engine loss with the autopilot on has caused several aircraft upsets, some of which were not recoverable. Flight control displacement may be the only obvious indication. Vigilance is required to detect these stealthy engine failures and to maintain a safe flight attitude while the situation is still recoverable.
Once the fuel supply has been restored to the engine, the engine may be restarted in the manner prescribed by the applicable Airplane Flight or Operating Manual. Satisfactory engine restart should be confirmed by reference to all primary parameters – using only N1, for instance, has led to confusion during some in-flight restarts. At some flight conditions, N1 may be very similar for a windmilling engine and an engine running at flight idle.
Engine fire almost always refers to a fire outside the engine but within the nacelle. A fire in the vicinity of the engine should be annunciated to the flight crew by a fire warning in the flight deck. It is unlikely that the flight crew will see, hear, or immediately smell an engine fire. Sometimes, flight crews are advised of a fire by communication with the control tower.
It is important to know that, given a fire in the nacelle, there is adequate time to make the first priority "fly the airplane" before attending to the fire. It has been shown that, even in incidents of fire indication immediately after takeoff, there is adequate time to continue climb to a safe altitude before attending to the engine. There may be economic damage to the nacelle, but the first priority of the flight crew should be to ensure the airplane continues in safe flight.
Flight crews should regard any fire warning as a fire, even if the indication goes away when the thrust lever is retarded to idle. The indication might be the result of pneumatic leaks of hot air into the nacelle. The fire indication could also be from a fire that is small or sheltered from the detector so that the fire is not apparent at low power. Fire indications may also result from faulty detection systems. Some fire detectors allow identification of a false indication (testing the fire loops), which may avoid the need for an IFSD. There have been times when the control tower has mistakenly reported the flames associated with a compressor surge as an engine "fire."
In the event of a fire warning annunciation, the flight crew must refer to the checklists and procedures specific to the airplane being flown. In general, once the decision is made that a fire exists and the aircraft is stabilized, engine shutdown should be immediately accomplished by shutting off fuel to the engine, both at the engine fuel control shutoff and the wing/pylon spar valve. All bleed air, electrical, and hydraulics from the affected engine will be disconnected or isolated from the airplane systems to prevent any fire from spreading to or contaminating associated airplane systems. This is accomplished by one common engine "fire handle." This controls the fire by greatly reducing the fuel available for combustion, by reducing the availability of pressurized air to any sump fire, by temporarily denying air to the fire through the discharge of fire extinguishant, and by removing sources of re-ignition, such as live electrical wiring and hot casings. It should be noted that some of these control measures may be less effective if the fire is the result of severe damage – the fire may take slightly longer to be extinguished in these circumstances. In the event of a shut down after an in-flight engine fire, there should be no attempt to restart the engine unless it is critical for continued safe flight, as the fire is likely to re-ignite once the engine is restarted.
One of the most alarming events for passengers, flight attendants, ground personnel and even air traffic control (ATC) to witness is a tailpipe fire. Fuel may puddle in the turbine casings and exhaust during start-up or shutdown, and then ignite. This can result in a highly-visible jet of flame out of the back of the engine, which may be tens of feet long. Passengers have initiated emergency evacuations in these instances, leading to serious injuries.
There may be no indication of an anomaly to the flight crew until the cabin crew or control tower draws attention to the problem. They are likely to describe it as an "Engine Fire," but a tailpipe fire will NOT result in a fire warning on the flight deck.
If notified of an engine fire without any indications in the cockpit, the flight crew should accomplish the tailpipe fire procedure. It will include motoring the engine to help extinguish the flames, while most other engine abnormal procedures will not.
Since the fire is burning within the turbine casing and exhaust nozzle, pulling the fire handle to discharge extinguishant to the space between casings and cowls will be ineffective. Pulling the fire handle may also make it impossible to dry motor the engine, which is the quickest way of extinguishing most tailpipe fires.
During engine start, the compressor is very inefficient, as already discussed. If the engine experiences more than the usual difficulty accelerating (due to such problems as early starter cut-out, fuel mis-scheduling, or strong tailwinds), the engine may spend a considerable time at very low RPM (sub-idle). Normal engine cooling flows will not be effective during sub-idle operation, and turbine temperatures may appear relatively high. This is known as a hot start (or, if the engine completely stops accelerating toward idle, a hung start). The AFM indicates acceptable time/temperature limits for EGT during a hot start. More recent, FADEC-controlled engines may incorporate auto-start logic to detect and manage a hot start.
Airplane engines ingest birds most often in the vicinity of airports, either during takeoff or during landing. Encounters with birds occur during both daytime and nighttime flights.
By far, most bird encounters do not affect the safe outcome of a flight. In more than half of the bird ingestions into engines, the flight crew is not even aware that the ingestion took place.
When an ingestion involves a large bird, the flight crew may notice a thud, bang or vibration. If the bird enters the engine core, there may be a smell of burnt flesh in the flight deck or passenger cabin from the bleed air.
Bird strikes can damage an engine. The photo on the next page shows fan blades bent due to the ingestion of a bird. The engine continued to produce thrust with this level of damage. Foreign Object Damage (FOD) from other sources, such as tire fragments, runway debris or animals, may also be encountered, with similar results.
Bird ingestion can also result in an engine surge. The surge may have any of the characteristics listed in the surge section. The engine may surge once and recover; it may surge continuously until the flight crew takes action; or it may surge once and not recover, resulting in the loss of power from that engine. Bird ingestion can result in the fracture of one or more fan blades, in which case, the engine will likely surge once and not recover.
Regardless of the fact that a bird ingestion has resulted in an engine surge, the first priority task of the flight crew is to "fly the airplane." Once the airplane is in stable flight at a safe altitude, the appropriate procedures in the applicable Airplane Flight Manual can be accomplished.
In rare cases, multiple engines can ingest medium or large birds. In the event of suspected multiple-engine damage, taking action to stabilize the engines becomes a much higher priority than if only one engine is involved – but it is still essential to control the airplane first.
Severe engine damage
Severe engine damage may be difficult to define. From the viewpoint of the flight crew, severe engine damage is mechanical damage to the engine that looks "bad and ugly." To the manufacturers of the engine and the airplane, severe engine damage may involve symptoms as obvious as large holes in the engine cases and nacelle or as subtle as the non-response of the engine to thrust lever movement.
It is important for flight crews to know that severe engine damage may be accompanied by symptoms such as fire warning (from leaked hot air) or engine surge because the compressor stages that hold back the pressure may not be intact or in working order due to the engine damage.
In this case, the symptoms of severe engine damage will be the same as a surge without recovery. There will be a loud noise. EPR will drop quickly; N1, N2 and fuel flow will drop. EGT may rise momentarily. There will be a loss of power to the airplane as a result of the severe engine damage. It is not important to initially distinguish between a non-recoverable surge with or without severe engine damage, or between a fire and a fire warning with severe engine damage. The priority of the flight crew still remains "fly the airplane." Once the airplane is stabilized, the flight crew can diagnose the situation.
Engine seizure describes a situation where the engine rotors stop turning in flight, perhaps very suddenly. The static and rotating parts lock up against each other, bringing the rotor to a halt. In practice, this is only likely to occur at low rotor RPM after an engine shutdown, and virtually never occurs for the fan of a large engine – the fan has too much inertia, and the rotor is being pushed around by ram air too forcefully to be stopped by the static structure. The HP rotor is more likely to seize after an in-flight shutdown if the nature of the engine malfunction is mechanical damage within the HP system. Should the LP rotor seize, there will be some perceptible drag for which the flight crew must compensate; however, if the HP rotor seizes, there will be negligible effect upon airplane handling.
Seizure cannot occur without being caused by very severe engine damage, to the point where the vanes and blades of the compressor and turbine are mostly destroyed. This is not an instantaneous process – there is a great deal of inertia in the turning rotor compared to the energy needed to break interlocking rotating and static components.
Once the airplane has landed, and the rotor is no longer being driven by ram air, seizure is frequently observed after severe damage.
Symptoms of engine seizure in flight may include vibration, zero rotor speed, mild airplane yaw, and possibly unusual noises (in the event of fan seizure). There may be an increased fuel flow in the remaining engines due to aircraft automatic compensations; no special action is needed other than that which is appropriate to the severe engine damage type failure.
Engine separation is an extremely rare event. It will be accompanied by loss of all primary and secondary parameters for the affected engine, noises, and airplane yaw (especially at high power settings). Separation is most likely to occur during take-off/climb-out or the landing roll. Airplane handling may be affected. It is important to use the fire handle to close the spar valve and prevent a massive overboard fuel leak; refer to the airplane flight or operations manual for specific procedures.
Fuel System Problems
Major leaks in the fuel system are a concern to the flight crew because they may result in engine fire, or, eventually, in fuel exhaustion. A very large leak can produce engine flameout.
Engine instruments will only indicate a leak if it is downstream of the fuel flowmeter. A leak between the tanks and the fuel flowmeter can only be recognized by comparing fuel usage between engines, by comparing actual usage to planned usage, or by visual inspection for fuel flowing out of the pylon or cowlings. Eventually, the leak may result in tank imbalance.
In the event of a major leak, the crew should consider whether the leak needs to be isolated to prevent fuel exhaustion.
It should be noted that the likelihood of fire resulting from such a leak is greater at low altitude or when the airplane is stationary; even if no fire is observed in flight, it is advisable for emergency services to be available upon landing.
Inability to shutdown Engine
If the engine fuel shut-off valve malfunctions, it may not be possible to shut the engine down by the normal procedure, since the engine continues to run after the fuel switch is moved to the cutoff position. Closing the spar valve by pulling the fire handle will ensure that the engine shuts down as soon as it has used up the fuel in the line from the spar valve to the fuel pump inlet. This may take a couple of minutes.
Fuel filter Clogging
Fuel filter clogging can result from the failure of one of the fuel tank boost pumps (the pump generates debris which is swept downstream to the fuel filter), from severe contamination of the fuel tanks during maintenance (scraps of rag, sealant, etc., that are swept downstream to the fuel filter), or, more seriously, from gross contamination of the fuel. Fuel filter clogging will usually be seen at high power settings, when the fuel flow through the filter (and the sensed pressure drop across the filter) is greatest. If multiple fuel filter bypass indications are seen, the fuel may be heavily contaminated with water, rust, algae, etc. Once the filters bypass and the contaminant goes straight into the engine fuel system, the engine fuel control may no longer operate as intended. There is potential for multiple-engine flameout. The Airplane Flight or Operating Manual provides the necessary guidance.
Oil System Problems
The engine oil system has a relatively large number of indicated parameters required by the regulations (pressure, temperature, quantity, filter clogging). Many of the sensors used are subject to giving false indications, especially on earlier engine models. Multiple abnormal system indications confirm a genuine failure; a single abnormal indication may or may not be a valid indication of failure.
There is considerable variation between failure progressions in the oil system, so the symptoms given below may vary from case to case.
Oil system problems may appear at any flight phase, and generally progress gradually. They may eventually lead to severe engine damage if the engine is not shut down.
Leaks will produce a sustained reduction in oil quantity, down to zero (though there will still be some usable oil in the system at this point). Once the oil is completely exhausted, oil pressure will drop to zero, followed by the low oil pressure light. There have been cases where maintenance error caused leaks on multiple engines; it is therefore advisable to monitor oil quantity carefully on the good engines as well. Rapid change in the oil quantity after thrust lever movement may not indicate a leak – it may be due to oil "gulping" or "hiding" as more oil flows into the sumps.
Bearing failures will be accompanied by an increase in oil temperature and indicated vibration. Audible noises and filter clog messages may follow; if the failure progresses to severe engine damage, it may be accompanied by low oil quantity and pressure indications.
Oil pump failures
Oil pump failure will be accompanied by low indicated oil pressure and a low oil pressure light, or by an oil filter clog message.
Contamination of the oil system – by carbon deposits, cotton waste, improper fluids, etc. – will generally lead to an oil filter clog indication or an impending bypass indication. This indication may disappear if thrust is reduced, since the oil flow and pressure drop across the filter will also drop.
No Thrust Lever Response
A "No Thrust Lever Response" type of malfunction is more subtle than the other malfunctions previously discussed, so subtle that it can be completely overlooked, with potentially serious consequences to the airplane.
If an engine slowly loses power – or if, when the thrust lever is moved, the engine does not respond – the airplane will experience asymmetric thrust. This may be partly concealed by the autopilot's efforts to maintain the required flight condition.
As is the case with flameout, if no external visual references are available, such as when flying over the ocean at night or in IMC, asymmetric thrust may persist for some time without the flight crew recognizing or correcting it. In several cases, this has led to airplane upset, which was not always recoverable. As stated, this condition is subtle and not easy to detect.
Symptoms may include:
If asymmetric thrust is suspected, the first response must be to make the appropriate trim or rudder input. Disconnecting the autopilot without first performing the appropriate control input or trim may result in a rapid roll maneuver.
Generally, thrust reverser malfunctions are limited to failure conditions where the reverser system fails to deploy when commanded and fails to stow when commanded. Failure to deploy or to stow during the landing roll will result in significant asymmetric thrust, and may require a rapid response to maintain directional control of the airplane.
Uncommanded deployments of modern thrust reverser systems have occurred and have led to Airworthiness Directives to add additional locking systems to the reverser. As a consequence of this action, the probability of inadvertent deployment is extremely low. The airplane flight or operations manual provides the necessary system information and type of annunciations provided by the airplane type.
No Starter Cutout
Generally, this condition exists when the start selector remains in the start position or the engine start valve is open when commanded closed. Since the starter is intended only to operate at low speeds for a few minutes at a time, the starter may fail completely (burst) and cause further engine damage if the starter does not cut out.
Vibration is a symptom of a wide variety of engine conditions, ranging from very benign to serious. The following are some causes of tactile or indicated vibration:
It is not easy to identify the cause of the vibration in the absence of other unusual indications. Although the vibration from some failures may feel very severe on the flight deck, it will not damage the airplane. There is no need to take action based on vibration indication alone, but it can be very valuable in confirming a problem identified by other means.
Engine vibration may be caused by fan unbalance (ice buildup, fan blade material loss due to ingested material, or fan blade distortion due to foreign object damage) or by an internal engine failure. Reference to other engine parameters will help to establish whether a failure exists.
Vibration felt on the flight deck may not be indicated on instruments. For some engine failures, severe vibration may be experienced on the flight deck either during an engine failure or possibly after the engine has been shut down, making instruments difficult to read. This large amplitude vibration is caused by the unbalanced fan windmilling close to the airframe natural frequency, which may amplify the vibration. Changing airspeed and/or altitude will change the fan windmill speed, and an airplane speed may be found where there will be much less vibration. Meanwhile, there is no risk of airplane structural failure due to vibratory engine loads.
The tabulation of engine conditions and their symptoms below shows that many failures have similar symptoms and that it may not be practicable to diagnose the nature of the engine problem from flight deck instrumentation. However, it is not necessary to understand exactly what is wrong with the engine – selecting the "wrong" checklist may cause some further economic damage to the engine, but, provided action is taken with the correct engine, and airplane control is kept as the first priority, the airplane will still be safe.
X = Symptom very likely.
O = Symptom possible.
Note: blank fields mean that the symptom is unlikely.
This page was taken from an original document at http://fromtheflightdeck.com/Stories/turbofan/