Table of Contents
4.2 Operating data
During the early stages of a flight accident investigation, the most immediate interest is in determining the sequence of and operating data from the crash. The pilots plays an important role in this. Because they can track and interpret the flight occurrences through the instruments, their perceptions are very important (Fig. "External signs"). Important data can be recovered from recording systems such as flight and crash recorders. If the flight and engine data are being closely monitored, as in military test aircraft, then a wealth of usable data will be available.
Experts can draw important conclusions from the damage symptoms of the engine parts and components ( ). These are invaluable, especially with regard to the reconstruction of possible damage sequences.
Traces on mechanical components of the regulator system (Fig. "Postcrash markings") can indicate data at the moment of impact and can be compared with data from the recording systems in order to determine plausibility.
The damage symptoms of rotor blading can give important clues as to the RPM and therefore also the operating conditions at the moment of impact ( ).
Figure "External signs": This diagram is made up of impressions and observations from pilots from comprehensive research of documents. An especially interesting source was NTSB reports about flight accidents in the years between 1983 and 1995 (Refs. 4.2-1,-2,-4,-5,-6,-8 ,-9,-12).
Naturally, the impressions are subjective and often occurred under high stress and great distraction due to necessary actions. However, experience has shown that observations by the pilot can be very applicable and helpful for determining the damage cause in the course of an investigation, even if the pilot`s statement seems implausible at first. The statements of pilots are often treated skeptically, especially with fighter aircraft due to the extreme pilot work load. It sometimes even happens, that the pilot is suspected of having acted in a way that promoted the damage. Therefore, proving that this was not the case can be very important for the pilots themselves.
For example, in one case it was suspected that the engine stall of a single-engine fighter aircraft and the repeated attempts at restarting the engine were due to improper actions by the pilot that resulted in a crash. At first, causal engine damage due to compressor blade failure was dismissed, since the pilot did not notice any vibrations. Comprehensive investigations then finally confirmed the pilot`s claim. There had undoubtedly been a blade failure in the forward compressor region. Research-based experience then showed that in the affected engine type, rotor blade damage can occur without any unusual vibrations in the cockpit.
With commercial aircraft, there is naturally the possibility that passengers may make important observations. However, the researched documents reveal that these observations were often horribly misinterpreted and resulted in improper action being taken during the damage process (Example "Tailcone fire" and Example "Ingestion of tire rubber"). For example, flames or smoke escaping from the exhaust pipe of an engine or APU may be mistaken for a dangerous fire and provoke panicked reactions. The evaluation of different observations and combinations of these can be important when reconstructing damage causes and sequences. Here, the emphasis is on plausibility and freedom from contradictions. Depending on the characteristic damage symptoms, a combination of factors such as sound (volume, time) and vibrations (frequency) can enable differentiation between and identification of unusual damage processes. The following text discusses the value of typical observations and impressions from pilots during accident sequences.
It must first be emphasized that, in order for evaluation of sounds to be somewhat accurate, experience with the engine type/aircraft in question is absolutely essential.
Sounds can be caused by very different physical processes, including contact between hard bodies, but also by gas vibrations (e.g. in combustion chambers). Sounds are also created/emitted by vibrating surfaces (e.g. blades or housings).
A single bang can indicate a stall. If it is followed by high-frequency vibrations, this indicates rotor damage.
Several bangs can also be caused by a stall. In this case, they are referred to as surge events, which are caused by repeated pressure buildup in the compressor and result in few, relatively low-frequency surges.
There have been some reports of grinding sounds in helicopters. Evidently, the close proximity of the pilot to the engine makes it possible to perceive these sounds, something that is not possible in large commercial aircraft.
Grinding sounds are indicative of rubbing and friction in the engine.
High-frequency sounds (“singing”) are a sign of a high-frequency process such as is typical for gearing problems (tooth frequencies), rotor unbalance (especially in low-power engines with extremely high RPM), or roller bearing damage (Example "Change in noise level").
Sounds and vibrations with low frequencies (growling) can indicate, for example, main rotor problems
in helicopters or damage to the low-pressure region of the fan in large turbofan engines. Another characteristic that can indicate a specific damage process is whether the sound is consistent or if it increases and decreases in waves.
For example, resonance can occur that leads to stiffness changes in the affected engine parts during the damage sequence and changes their dynamic behavior.
Wear processes during rubbing of components change the mass and stiffness of the parts, which influences their frequencies.
Rubbing of rotor spacer rings, for example, can result in heat creation and considerable changes in rotor elasticity. This can bring the system out of resonance, causing it to stop rubbing and cool down. When the part cools, its stiffness increases again (increase in the modulus of elasticity), and it may again reach resonance.
If the running sounds change during steady-state operation, it indicates a change in the operating behavior of the engine. These changes may not be serious, however.
If the running sounds change during transient operation, e.g. during start-up or rotation, they should first be attributed to this operating condition before thinking about problems. The temperature changes in the gas flow alone can change the sound of the engine in many different ways.
If unusual vibrations from the propulsion system are noticed by the pilot (Example "Change in noise level"), then this should indicate a malfunction or damage to or in a component (Example "Ingestion of tire rubber"). The frequency is an important indicator as to the affected parts. It can enable conclusions concerning the affected shaft system (LP, HP). The volume of the sounds does not necessarily indicate the seriousness of a problem
In order to estimate the seriousness of a fire, its location must be determined. If the fire is escaping around the engine, then it indicates a fuel or oil fire and the failure of auxiliary systems. This must be treated seriously. The same holds true if flames are shooting out of the front or back of the bypass duct.
Only with large fan engines is it possible to tell from a large distance, whether flames are coming out of the bypass duct or the exhaust pipe (core engine). In this case, the flames are usually caused by wear products (in extreme cases, dust explosions). Causes include:
- Blade failures
- Heavy rubbing
Flames from the bypass duct:
Flames from the bypass duct must be taken very seriously, since this duct does not contain hot gases. Every longer-duration fire in this duct must therefore be due to a failed seal in the hot area of the engine (hot gas escaping) or an oil or fuel leak. In these cases, one can assume that an engine fragment has escaped and destroyed auxiliary components and pipe lines, or punctured the housing and allowed hot gas to escape (e.g. combustion chamber gases).
However, even damage to the aggregate components such as pumps and gear boxes can result in flammable liquids escaping and igniting.
Explosive fires may be due to a dust explosion after heavy fan damage.
Flames that shoot out of the exhaust pipe (core engine):
These flames do not necessarily a serious malfunction. There are far less alarming causes for this phenomenon.
Causes of “non-dangerous” flames:
- Burning off fuel remnants after an aborted engine start (Example "Tailcone fire")
- In aircraft with afterburners, misfiring of the afterburner by a flame from the torch igniter or a darting flame from the combustion chamber
- A pronounced stall. The flames are caused by a temporary lack of air. This causes unburned fuel to collect and then ignite behind the turbine. This can be a spectacular process, where the flames occur as a low-speed detonation several meters behind the back of the running engine. The escaping flames are not classified as dangerous, but evaluating a correspondingly pronounced stall in the engine compressor is a different issue.
- Flammable remnants of cleaning materials or de-icing liquid in the exhaust pipe igniting when the engine is started (Example "Glycol ignition").
Dangerous causes of flames:
- Oil fires due to poor seals or damage to the bearings (Example "Smoke in the cabin") or oil supply system
- Heavy rubbing (rotor damage, blade failure)
- Ignition of the blading (titanium fire)
- Damage to the fuel system, fuel nozzles, or the combustion chamber.
Changes to the hardware (outer appearance):
With many engines, especially nacelle engines, it is possible to recognize heavy damage to the of compressor intake blading or turbine outlet blading from outside of the engine.
Holes in the nacelle that are obviously not designed openings (e.g. for maintenance purposes) are a clear indication of serious engine damage with uncontained fragments. This conclusion is supported by observed flames and/or smoke coming from the engine.
Missing parts of the nacelle cover indicate external factors (e.g. bird strike, lightning) or a failure of the cover itself (e.g. dynamic fatigue). In general, this damage is not as serious as that resulting from uncontained fragments.
Damage to the thrust reversers must be treated very seriously, as there may be a possibility of the actuation or blocking system being affected, which would present an immediate danger to the aircraft.
Damage to adjustable thrust jets, as are common in fighter aircraft with afterburners, must not prevent them from providing sufficient thrust (e.g. locking in open position).
Deformations of the nacelle are a clear indicator of force applied from outside, e.g. a collision with vehicles during loading and unloading.
Unusual odors in the cockpit or cabin (Example "Smoke in the cabin") can indicate impending damage. Characteristic smells, such as burning oil or some synthetic materials, can give important clues as to the damage sequence and affected components. “It smells like amperes” is a common description given for the intense smell of burning synthetic insulation. However, in only a few cases will this smell originate in the engine.
This refers to acceleration due to speed changes of the airplane. The pilot will notice the resulting steering forces. Examples include engine failure, the opening of thrust reversers during flight (Ref. 4.2-3), or altitude and direction changes independent of pilot action.
Excerpt:“ The no.3 eng. torched during start attempt and resulted in an internal eng. tailcone fire. Several paxs panicked upon seeing the torch flame/tailcone fire and between 30 to 40 paxs made an unauthorized evacuation…The flight crew was at first neither aware of the eng. torch and subsequent tailcone fire nor the evacuation. When they learned of the tailcone fire they motored the eng, in order to extinguish the fire which they believed was under control and therefore requested no emergency assistance…The nos. 1 6 2 engs. remained running during the incident and created a hazardous condition for CFR/evacuees.”
Comments: A “tailcone fire” is the burning off of drained fuel after a start attempt. This type of fire is relatively common and usually occurs without causing any damage.
Excerpt: “…the no. 1 engine had been started. during engine start of the no. 2 engine, a high fuel flow was observed by the flight crew and a passenger in the right side of the airplane observed a flame aft of the exhaust. The passenger shouted “fire”, opened the no. 2L door and deployed the emergency escape slide.”
Comments: The unnecessary emergency measure was taken by an unauthorized person who misinterpreted what he saw.
Excerpt: “… a rotorcraft flown by a commercial pilot, was substantially damaged upon impact with terrain during an auto-rotative emergency landing following a total loss of power.”
The pilot reported: “ ..that the rotorcraft had been engaged in log lifting operations when he noticed a minute long change in noise level. After adjusting the power he determined that everything seemed normal… .at that time the noise level changed again and he than began to maneuver to return to service landing site.”
The disassembly showed: “…the forward bearing had disintegrated…the roller bearings were observed to be heavily deformed and the forward (geared) end of the accessory drive shaft had separated from the remainder of the shaft…“
Comments: This example shows that even damage to high-RPM shaft systems, especially bearing damage, can make itself known well before the engine fails completely.
Excerpt: The pilot was instructed to go-around on his first approach due to preceding traffic on the runway. After the second approach, he allowed the airplane's descent rate to become excessive, and it touched down firmly and bounced…During the landing roll-out the airplane vibrated, but was controlled with brakes and engine reversing. A post-flight exam. revealed damage to the nose gear, and ingestion of tire rubber in the no.1 engine.”
Comments: This description does not indicate to what degree the vibrations could be traced back to the damaged landing gear and/or the foreign object entering into the engine.
Excerpt: “The incident airplane experienced an APU-Fire shortly after landing when the APU was started. The airplane had been deiced at its point of departure with a glycol and water mix. Company policy called for the flight crew to shut off the APU during deicing. The APU exhaust outlet is on top of the right wing near the fuselage, and will accept deice fluid into the exhaust and subsequently the APU combustion chamber if deicing fluid is directed into the opening. Glycol is highly flammable once the water is vaporized/evaporated. The APU-start following the landing…ignited the residual glycol in the APU, causing the exhaust stack fire.”
Comments: It is astounding that sufficient glycol remained in the engine during the flight, that it could ignite after landing.
Example "Smoke in the cabin" (Ref. 4.2-6):
Excerpt: “After arriving at the gate…, fumes & smoke were noted in the zone C of the cabin, then the cabin LGTG went out & the emerg LGTG came on….the no. 2 APU had an auto-shutdown due to malfunction. The APU was placarded as inop & the aircraft was returned to service. As it was climbing, shortly after takeoff smoke became noticeable & the crew elected to return to the airport and landed….suspecting only an air conditioning problem, the crew elected to continue to the gate. As they began taxiing, a flight attendant reported the smoke was increasing, so the captain stopped the aircraft & directed an evacuation….An investigation revealed the smoke was caused by oil from a failed APU bearing which leaked into the air conditioning system.”
Comments: Smoke development in the passenger cabin indicates a problem with the air conditioning system and therefore also with the APU.
Figure "Diagnosis": Today, commercial and military engines are monitored by modern diagnostic systems (Ref. 4.2-7).
These are in contact with ground stations that receive, collect, and process data. The results of this processes data permit an overview of the engine conditions and temporal changes. These are the basis for maintenance measures to be taken.
In case of damage and accidents, these data can provide important clues as to the causes and damage sequences.
The diagram depicts a modern diagnostic system for a new generation of fighter aircraft.
The monitoring device supplies data for analyzing trends, problems, and damage.
Figure "Monitoring and control": Engines have many monitoring and control systems. Much of the data that is registered by these systems is necessary for regulating the engine or as information for the pilot.
However, there is also a considerable number of devices that do not continually transmit data, but only indicate anomalies. These devices include magnetic chip detectors, but also components such as oil filters, which actually have no monitoring function at all. However, they can still give important clues as to the condition of engine parts in case of a high incidence of swarf.
Naturally, the number, type, and configuration of sensors differs depending on the engine type, but
there is certain “standard equipment”. For example, in some modern engines, the temperature relevant to the regulators is pyrometrically measured in the most highly-stressed section of the turbine rotor blades. Older engines, especially, tend to have several stationary thermal elements in the hot gas flow.
A special problem is failure or malfunction of sensors (Example "Faulty connectors") and components of the data processors. These cases are not rare and can lead to fatal misinterpretations.
The following text discusses typical parameters and findings and what they indicate:
Modern engines have up to three shaft systems. In engines that power mechanical systems (e.g. propeller turbines and helicopter engines), the third shaft (of the low-pressure turbine) usually has the task of transferring power. The RPM can indicate the condition of the engine. Small changes with a trend over longer time spans indicate an operation-conditional efficiency decrease or internal leakages, e.g. due to wear processes. In case of rapid, unintended RPM changes, it is important to determine whether the RPM increased or decreased. For example, a decrease in RPM can occur due to a compressor stall or blade failure in the turbine or compressor.
If turbine RPM increase rapidly and compressor RPM decrease simultaneously, then the shaft has failed (Chapter 4-5, ). In extreme cases, when the usual safety mechanisms are not sufficient, the turbine can reach overspeed (runaway turbine) and disks may burst.
Depending on the configuration of the engine speed sensors in certain older shaft engine types, it is possible that power take-off shaft failure may be registered as an RPM decrease, causing the regulator to respond by increasing the fuel supply, resulting in the separated turbine rotor reaching dangerous overspeed. This type of dangerous RPM increase can occur in a fraction of a second.
Control instruments for monitoring temperature are also standard equipment, as are RPM monitoring systems. These are usually made up of several sensors to ensure redundancy.
Fast, unintended temperature changes indicate an unusual change in operating behavior. A large temperature increase indicates the failure of individual components in the hot gas flow (e.g. turbine blade failure). Extreme temperature peaks can occur in case of an internal fire (e.g. oil fire due to bearing damage or a titanium fire in the compressor blading).
Dangerous overheating resulting from air starvation or fuel oversupply after a compressor stall can completely destroy the turbine blading. Usually, the actions to be taken during and after specified temperature limits are exceeded are precisely determined in order to prevent dangerous damage to the hot parts or to facilitate proper inspections. Dangerous overheating can be caused by improper operation, such as overly fast engine start-up.
Dangerous temperatures or fires outside of the engine are detected by a fire warning system.
The processes that are monitored are the ignition of the fuel/air mixture in the combustion chamber and the ignition of afterburners in engines with afterburners.
The pressure in the gas flow is a characteristic indicator of the condition of an engine. The pressure is determined primarily by the efficiency of the components, and are therefore useful for determining component efficiency. Gas pressure is also an indicator of the condition of components (e.g. wear, erosion, leakages, pressure losses due to large clearance gaps).
The pressure of other media, especially liquids such as oil and fuel, indicate the condition of the relevant system. Unusual decreases in pressure can indicate leakages and a possible fire risk.
Pressure fluctuations indicate damage in the pump systems. Pressure increases indicate blocked filters or nozzles, which in turn indicates that some parts in the system are giving off swarf and wear products, i.e. that damage is developing.
Pressure vibrations can indicate air in the material being pumped. In fuel systems, these vibrations may also indicate interactions between the combustion chamber and the fuel injection system.
Speed and pressure of gases and liquids are physically connected by Bernoulli`s Law. In liquids such as fuel and oil with a known flow cross-section and independent of pressure, the flow speed is a measure of the mass flow rate and therefore also the consumption.
In the framework of flow-relevant processes in engine development, measuring local gas speeds is a necessary aid for analysis and optimization (e.g. with the aid of lasers, affixed sensors, and conventional flow probes).
Deposits are important indicators and evidence of the condition of a system with flow-through. Because of this, magnetic plugs (Example "Spectral analysis I") and filters in oil systems and filters in fuel systems are necessary. Often, unusual deposits or pressure increases (the bypass to the line is activated) are reported to the cockpit. Inspection of the deposits can give detailed information about the affected components in the system (e.g. bearings).
Development of gases:
Oil fires can increase the proportion of gaseous burn products such as CO and CO2 in oil systems. These can be detected by Lambda probes (such as in cars with regulated catalytic converters). This type of probe is only used in engine development.
Electric output of the starter generator:
The output of the (starter) generator indicates the functioning of this important aggregate.
Future monitoring technologies:
Efforts are being made to improve current technologies as well as development of new ones (e.g. pulsed pyrometers). These require continual analysis of the gas flow and any particles it carries.
These and similar methods are intended to make it possible to detect processes within the engine in real time. This is targeted towards, for example, unusual types of rubbing that can be recognized by wear products in the gas flow.
A further possibility for future monitoring may be the implementation of electronic memory in regulators and monitoring systems. The evaluation of this memory is valuable even after a flight accident or failure of the external power supply. A prerequisite is that the stored data are “not transient” over the relevant time span.
Example "Faulty connectors" (Ref. 4.2-8):
Excerpt: (The aircraft ) ”… experienced a fire warning light illumination on no.1 engine as the aircraft was climbing through 6000 feet. The pilot declared an emergency and returned…The examination of the fire warning system disclosed that a duct connector in the system had failed.“
Comments: This is a typical case among many where the sensors contribute to uncertainty. Unreliable warning systems not only cause unnecessary actions during false alarms, but also diminish the faith of the pilot in the system in case of a real problem, thereby promoting false actions or preventing necessary actions from being taken.
Excerpt: “Typically, accident investigators would develop spectral analyses of background sounds on CVR tape. Such analyses can detect distinct noises from the movement of controls and switches in the cockpit and some flight control surfaces on aircraft. Spectral analyses also could isolate the resonant signatures from the rotation of the first-stage fans (Fig. "Ice buildup")in each of flight… two (high bypass turbofan) engines. That could provide important clues to engine performance. Investigators for instance, can compare the last recorded fan-rotation rates (see Fig. "Typical water damage" and Fig. "Complete blade row destruction") with the physical impact damage to the turbine and compressor sections of each engine, which provides a rough indication of the power setting on which each was running at impact.”
Excerpt: “While in cruise (with a one engine helicopter)…. the engine chip detector illuminated. A few seconds later it extinguished…about three miles afterwards, the chip light re-illuminated, and the pilot began looking for a landing site. Then the low rotor speed warning horn sounded and the pilot began an autorotation….During the post accident examination of the engine, the number one, three, and four main bearings were found to have failed because of lack of lubrication.”
Comments: The relatively early detection of the damage can be seen as positive. However, the illumination and extinguishing of the warning light is a negative sign, since it promoted improper action by the pilot by not giving a definite warning. Flickering or extinguishing warning lights should generally not be seen as a malfunction of the warning system. However, warning system “instabilities” are frequently reported and can seriously compromise the value of the warning system.
Figure "Ice buildup": This description is based on Example "Available engine thrust". The occurrences during an accident are recorded by two crash recorders (black boxes) with different tasks. One is a flight data recorder (FDR) and one is a cockpit voice recorder (CVR). An improved FDR has been required since Sept. 1969, i.e. a digital flight data recorder (DFDR). Large commercial aircraft are required to be equipped with approved versions of both devices.
Analyzing the CVR:
The CVR contains a closed loop of tape that records a 30 minute time span.
The recorders are light yellow in color and are usually located at the back of the aircraft. One
exception may be aircraft with engines mounted on the back of the hull. The recording media have the highest possible protection against fire and impact forces.
Analysis is done by an authorized agency (e.g. NTSB) with consultation of further centers (e.g. FAA, pilot organizations, operators, nacelle manufacturers). Analysis of the tape from the cockpit voice recorder can take between several days and several weeks, depending on the quality of the recording. The result of this detailed process is a written document with the necessary time notations.
Examples "Lubrication oil starvation", "Available engine thrust", "Spectral analysis II", and "Risk elimination by early detection" show how a CVR analysis can contribute to solving engine damage.
Analyzing the FDR:
The FDR records the temporal progression of the following flight parameters for at least 25 flight hours:
- Flight altitude
- Flight speed
- Magnetic compass heading
- Vertical acceleration
- Crew microphone.
The DFDR records additional data such as:
- Position of the control levers
- Engine thrust
Analysis of the DFDR is done with the aid of a computer. It creates a list of the desired flight data (engineering units print-out) and a print-out in diagram form. FDR data usually only result in the graphic version.
Excerpt: ,,…fortunately the cockpit microphone recorded the sound of the engine during start-up clearly. These sounds could be recognized since the engines of the affected aircraft are mounted on the wing. The safety inspectors conducted extensive frequency analyses. The results were compared with measurements from the nacelle manufacturer during test starts and indicated the engine thrust during start-up. The proper thrust level is an engine pressure ratio of 2.04. However, the cockpit voice recorder recorded an engine frequency corresponding to an engine pressure ratio of 1.70. Tests showed that with a blocked intake pressure probe and deactivated deicing system, the cockpit indicated a pressure ratio of 2.04, not the actual pressure ratio of 1.70.
The safety investigators concluded that the aircraft had less available engine thrust than was indicated to the crew, resulting in a collision with the 14th Street Bridge, near Washington National Airport, Washington DC.”
Excerpt: “Typically, accident investigators would develop spectral analyses of background sounds on CVR tape. Such analyses can detect distinct noises from the movement of controls and switches in the cockpit and some flight control surfaces on aircraft. Spectral analyses also could isolate the resonant signatures from the rotation of the first-stage Fans in each of Flight… two (high bypass turboFan) engines. That could provide important clues to engine performance. Investigators for instance, can compare the last recorded Fan-rotation rates with the physical impact damage to the turbine and compressor sections of each engine, which provides a rough indication of the power setting on which each was running at impact.”
“A military turboprop transport aircraft suffered heavy damage to a propeller reduction gear unit immediately after landing. Analysis of the flight recorder showed that before the damage, an extreme sound was present during the measurement of the power take-off moment of the affected engine. The low recording frequency of one Hertz made it impossible to determine the frequency of the sound.
However, the recorder of the same aircraft had been routinely played back one month earlier. At that time, a less pronounced sound was noticed at the same RPM moment indication. Additionally, a similar weak sound had been registered during RPM measurement on four other engines on different airplanes. The Directorate of Flight Safety in the Department of National Defense was notified, and the following disassembly of the reduction gear unit showed the early stages of the same damage in each case. This eliminated the risk of four engine damages during flight as well as the corresponding hazards and costs.”
Figure "Cockpit voice recorder" (Example "Risk elimination by early detection"): Aside from engine sounds, noises from other components can also give important clues as to potentially damaging processes. Gearing, especially propeller gearing, usually has a pronounced characteristic sound that is picked up by the cockpit voice recorder. From the outside, this sound is obscured by engine noise.
Figure "In flight damage symptoms": A fundamental guideline for investigation of accident engines is the realization that all damage symptoms that could not have been caused by the crash are important for understanding the damage process and its causes. The diagram shows several typical damage symptoms:
Streaks of soot from FOD:
If notches and burrs caused by foreign object damage have streaks of soot leading from them (Fig. "Soot coating"), then it is a clear sign that the notches were created while there was still considerable a air flow in the engine.
Dynamic fractures (pitting) in roller bearings:
Typical pittings in roller bearings require considerably more time to develop than it takes for a flight accident to occur. Therefore, this type of damage is certainly not caused by the crash.
Drops of fused metal on hot parts:
If the fused drops or splashes of metal are high-melting materials such as steel, titanium alloys, or nickel alloys, then it must be assumed that these particles traveled through the operating combustion chamber. It is the only place where the temperatures are high enough to melt these materials. These drops of melt often stick to the cooler blade surfaces and are clear indication of damage that occurred during flight.
Pronounced dynamic HCF fractures require a great number of load cycles, and even at high vibration frequencies of the kind common in bladings, it is highly unlikely that a dynamic fatigue fracture could be consequential damage of a different causal damage or the impact itself, especially if the dynamic fracture has signs of oxidation or corrosion. It can be safely assumed that this type of fracture is a cause of engine failure. This is not true for LCF fatigue fractures, which require considerably fewer load cycles and can occur in a few seconds in extreme cases (Fig. "FOD damage mechanism").
Titanium fires require sufficient flow speed and pressure in the surrounding air to obtain the oxygen necessary for burning. These conditions are only present in a running engine. Ignition occurs at such high temperatures that it is not possible for a fuel or oil fire to be the cause. Experience has shown that ignition temperatures are usually reached due to rubbing. This also means that titanium fires do not result from an impact. Therefore, if there are signs of a titanium fire, it must be assumed that it occurred during flight while the engine was running and therefore played a pivotal role in the accident-causing damage process.
Overheating damage to hot parts indicates that there must have been a sufficient air flow containing fuel for combustion. If there is pronounced oxidation, then the process can be assumed to have taken longer than the time available during an accident.
Oil fires can heat rotating engine parts such as disks and shafts until they reach extreme overtemperatures with characteristic deformations and fractures. This is clear evidence that the fire occurred while the engine was running.
Swarf in the oil:
Unusual swarf in the oil collect on magnetic chip detectors and in the filter (Fig. "Deposits"). They are a clear indication that the damage occurred while the oil was still flowing.
Conclusions concerning the affected parts can be made from the shape and chemical composition of the swarf.
Figure "Soot coating": Deposits on engine parts can hold important clues regarding the damage process.
This includes especially coatings on the compressor blades of the damaged engines.
These coatings can consist of, for example, soot deposits from oil and fuel fires or other burning components such as lacquered or synthetic parts.
One sign for the presence of an air flow during the depositing process is streaks of soot. If these are found behind notches from FOD, then one can assume that a directed air flow was still present for a sufficiently long time after the foreign object struck the engine.
The appearance of the deposits, such as shape and formation, can also give important clues concerning their origin. Comparing the damaged blades with normal blades after long operating times is always helpful. The normal deposits then indicate, for example, the point in time when the soot was created, when the engine was still operating normally. The structure of the soot deposits indicates whether the material struck the surface as a liquid and then burned up or coked up, or if it had already burned up and was deposited only as soot.
Analyzing the deposits can indicate the origin of the soot. For example, oil soot from the synthetic oils normally used in engines contains large amounts of phosphorous and sulfur. Soot from burned synthetics (PVC) can contain chlorine, whereas burned lacquers can be recognized by the pigments they contain.
Of course, there are many other coatings apart from soot deposits. The following are examples:
- Organic remains after a bird strike
- Splashed rub coatings
- Wear products from rubbing parts
- Deposited contaminants sucked in with the air
- Deposits from foreign objects sucked in during impact (e.g. dirt and plants)
During operation, deposits created by the extreme heating up of deposited foreign material (e.g. coking, burning, melting) can be expected to form on the hot parts. These include, for example:
- Splashed metal from overheated hot parts (fused), from rubbing, as well as melted particles from the compressor
- Remnants of rub coatings
- Deposits of contaminants in the air flow (e.g. in a eddy of the dust-carrying flow)
- Fuel deposits
On the other hand, conclusions can be drawn from deposits that were not very influenced thermally:
Deposits that were sucked in upon impact are not usually thermally altered in the combustion chamber to the degree they would be in a running engine with a normally operating combustion chamber. If, for example, organic remains are found in the turbine of a crashed engine, it must be assumed that they are a result of the crash.
Figure "Postcrash markings": When an airplane crashes, extremely high accelerations affect its components. These cause permanent changes such as:
These changes can indicate the operating conditions of the engine at the time of impact.
Markings on components of the control systems and indicator instruments are especially important
Control systems: In older aircraft, especially, the control systems are largely mechanical. The regulators are also connected to peripherals by cables. These may be actuator cables connected with the cockpit as well as cables connected the components to be regulated (e.g. thrust nozzles). Response cables report the position of the components being regulated to the control system. These parts of the control system can strike against one another due to elastic and plastic deformations during a crash, which creates markings that can be used to determine the engine operating conditions at the moment of impact. Typical markings include deformed teeth (“B”) or notches on regulated cross-sections such as vanes (“C”).
Indicator instruments: The hands, cover glass, and faces of mechanical indicator instruments can come into contact with one another due to the accelerations during impact. This will usually result in the fluorescent paint on the hands leaving an imprint or depositing on the glass and/or the face, showing the position of the instrument at the time of impact (“A”).
Naturally, important information can be gathered from markings on many other components. For example, deformation of the flaps of an adjustable thrust nozzle can indicate its position at the time of the crash/impact.
Deformation of the rotor blading is an important indicator of the operating conditions at the moment of impact ( ) and is usually the first analyzable outer characteristic for this type of conclusion.
Figure "Postcrash rotor condition": One of the standard first steps of any flight accident investigation that deals with the engine is an inspection of the rotor blading, in order to gain an initial idea of the operating conditions at the moment of impact.
The deformation of the blading indicates the rotor RPM and therefore also the output of the engine at the time of impact:
If the entire circumference of the blading is seriously bent against the direction of rotation (Fig. "Typical water damage"), it indicates high RPM at the moment of impact. Depending on the blade material, there may be many broken blades , and in extreme cases a “haircut”.
Minor bending of the blades against the direction of rotation indicates low rotor RPM at the moment of impact (Fig. "Large fan postcrash").
If the engine rotor RPM were in the idling zone, then it may be difficult to differentiate the corresponding blade deformations from those of a rotor being spun by the air stream (windmilling). Naturally, the windmilling RPM depends on the air stream speed (i.e. flight speed) and can vary greatly. In military engines with an airfoil, the windmilling RPM are relatively low, about 30% (with 100% being the rated speed). These 30% must be reached in order to make a restart possible. Without the airfoil, the windmill RPM during flight are over 60%. The high-pressure area reaches at least this value. If the restart RPM can not be reached through windmilling during flight, then air from a parallel engine can be used to power the rotor. This can bring the RPM of the shut-down engine close to the level of the flight idle RPM (flight idle RPM > 50%, ground idle RPM > 20%).
In fan engines with a large bypass ratio as are usual on modern commercial aircraft, the windmilling RPM of the fan is roughly ca. 20%, and that of the high-pressure core is roughly 30%. If the deformation of the blading is limited to one section of the circumference or is in an S-shape, then the rotor was standing still at the moment of impact.
Any abnormal appearance in the deformed blading can be especially important. For example, an abnormal fracture location (in the root rather than the leaf) is grounds for further investigation, since this may well be the cause of the engine failure.
The angle of deformation of the blading in axial direction can indicate the angle of impact. In extreme cases, where the impact was in the direction of the engine axis, the entire engine well be compressed, but the blading will not be as seriously bent against the direction of rotation as it would be with a flatter angle of impact.
The conditions at the point of impact are important for the damage symptoms of the compressor. Heavy damage can also be expected from a crash into water (Fig. "Typical water damage").
Figure "Large fan postcrash": A large passenger aircraft crashed over water (Ref. 4.2-14). The diagram depicts one of the fan engines. The fan rotor blading is hardly bent against the direction of rotation, which indicates that RPM were very low or that the engine may have even been stopped at the time of impact. Despite this, the forces acting on the rotor were powerful enough to separate it from the fan disk.
Figure "Typical water damage": In this case, the engine struck the water at idle RPM. Even though the RPM was far from the maximum, the blades were wound around the rotor. The lost front compressor housing was evidently blown apart due to the incompressibility of the water that was forced in, momentarily creating extreme pressure levels in the compressor.
Figure "Complete blade row destruction ": Comprehensive blade fractures on compressor rotors are not always caused by the impact. The top diagram shows the compressor rotor of a helicopter engine after an ice strike (see Figure "Compressor blade damage"). The titanium blading of this engine failed catastrophically in this case.
The lower diagram depicts the rotor of an engine after a crash. Here, as well, the blading is mostly destroyed. In this case, however, the damage symptoms can be attributed to the high rotor RPM at the moment of impact.
4.2.1 Recommendations for Determining Operating Data
These recommendations are a more detailed supplement to the recommendations for general investigation given in 4.1.1.
- Along with the standard analysis of all recorder data, any available external records (e.g. through radio contact during a test flight) should also be processed at another center.
- Analyze all signs of a fire (oxide formation, undeformed holes, coke and soot development, creep, bulges, etc.) and check their coherence with operating conditions (e.g. titanium fires in the compressor, oil fires in bearing chambers). For example, titanium fires are only possible under certain operating conditions.
- Analyze and evaluate the damage symptoms of the compressor blading for evidence regarding the RPM, angle of impact, location of impact, and chronology.
- Pay extra attention to blades with abnormal fractures (e.g. fracture location, fracture progression, fracture initiation zone, deformations, color). These blades must undergo a detailed inspection. They can give valuable information as to the causes of the damage and unusual operating loads.
- Inspect impact marks on instruments (e.g. in the cockpit) and parts of the mechanical engine control that, for example, might indicate the position of the regulators at the moment of impact.
- The type and location of foreign objects that were sucked into the engine during impact can indicate the location and speed of the impact, as well as the functioning of the combustion chambers.
- Analyze plastic deformations that indicate pressure differences (e.g. bulging of an overheated combustion chamber housing) or unusually high RPM (e.g. expansion of disks indicates overspeed). Note the stress direction of plastic deformations, and also if they easily fit into the (hypothesized) total damage process.
- With fatigue damage to roller bearings, the type and distribution of the damage can indicate the bearing forces and the load directions, therefore also providing information about the air system, for example.
- The condition of fuel and oil filters may indicate unusual pressure differences. Also, collapsed fuel filters may indicate that the fuel might have frozen, which in turn suggests earlier temperatures in that section of the engine. The same holds true for the condition of elastomer seals, the failure behavior of which varies depending on the temperature.
- The analysis of the deformation or fracture appearance of forcefully broken (e.g. as consequential damage) hot parts can supply important clues regarding the temperature of the parts and therefore also regarding the operating temperature at the moment when the damage occurred.
- The location and distribution of deposits such as soot, dust, and/or oil remnants may indicate abnormalities in the flow that have been acting over a long period of time. This can, for example, indicate a malfunction of a bleed valve.
- A follow-up examination of hot parts allows conclusions concerning whether or not a certain temperature limit was exceeded, which would lead to characteristic structural changes (e.g. phase decomposition, new secretions, fused phases). Any changes in protective coatings can also be helpful in determining the temperatures.
- Inspection of the structure of oil coke or soot deposits for evidence as to the temperatures when it was created and the chronology of the damage.
4.2-1 NTSB Identification NYC86FA076, microfiche no. 33858A.
4.2-2 NTSB Identification MIA941A043.
4.2-3 “Airbus warns A300-600 users after airborne reverse”, periodical “ Flight International” 9, 15 December 1998 page 8.
4.2-4 NTSB Identification ATL941A097.
4.2-5 NTSB Identification CHI91IA041, microfiche no. 43586A.
4.2-6 NTSB Identification CHI87MA101, microfiche no. 37190A.
4.2-7 S. Budrow, “System Analysis & Integration of Diagnostics & Health Management for the F119-PW-100”, AIAA-Paper 98-3545, page 7.
4.2-8 NTSB Identification ATL86LA090, microfiche no. 30982A.
4.2-9 NTSB Identification ATL94LA180.
4.2-10 J.T. McKenna, G. Thomas, “Recorders, ATC Tapes, Key to SilkAir Crash”, periodical “Aviation Week&Space Technology”, January 19, 1998, pages 47,48.
4.2-11 C.A. Roberts, “Flight Recorder's Role in Accident Investigations”, periodical “Flight Digest”, Vol.2, No. 3 March 1983, page 6.
4.2-12 NTSB Identification SEA96FA076.
4.2-13 B.Caiger, “The Recovery and Analysis of Accident Data from Flight Recorders in Canadian Transport Aircraft”, pages 1-15, Flight Research Laboratory, National Aeronautical Establisment, National Research Council, Ottawa, Canada K1A OR6.
4.2-14 J.T. McKenna, “Recovery of Swissair Wreckage Accelerates”, periodical “Aviation Week&Space Technology”, October 26, 1998, page 38.