Table of Contents
11.2.4.2 Damage to Afterburners, Tailpipes, and Thrust Reversers
Problems with and damages to these components were already described in the framework of Chapter 11.2.4-1. Therefore, this chapter is merely a supplement to the preceding chapter. Of course, the damages and examples described in this chapter only cover a fraction of the whole palette of possible damages.
In turbofan engines the afterburner and fan influence each other directly via the bypass duct. This means that a wrong combination of the tailpipe position and the fuel inflow in the afterburner have a direct influence on the fan. The ignition of the afterburner is especially critical with regard to stalls in the fan and compressor, since it creates an unavoidable pressure increase. For this reason, it is vital that ignition occurs throughout the entire flight envelope without any explosion-like effects (Fig. "Dmage by afterburner 'screetch'"), even though maximum thrust must be available within 1-2 seconds.
When the afterburner is lit, no dangerous combustion instability (pressure vibrations) must be allowed to occur anywhere in the operating sector (Fig. "Indicating values for combustuion vibrations"). Otherwise, this could lead to dynamic fatigue in the engine (Fig. "Gas vibrations inafterburner cause stalls in the fan") and afterburner. High-frequency vibrations with transverse modes (vibration forms, diagram) can result in hot gases escaping sideways from the afterburner pipe (Ill. 11.2.3-13). Low-frequency vibrations are capable of causing stalls at the fan (Fig. "Compressor damages due to missing 'bell mouth'").
Fig. "Fracture of after burner mantle pipe" depicts the consequences of high mechanical loads on an afterburner liner pipe, which fractured completely.
The ignition systems of afterburners are not without problems. One type of igniter is a darting flame from the combustion chamber, while another variant is a torch igniter (Fig. "Damages at after burner 'torch igniter'"). These igniters are subjected to special thermal loads and exhibit corresponding extensive damages.
The tailpipe is an adjustable engine part with many individual parts that move in relation to one another. Their life span is determined partly by the wear behavior of their contact surfaces (Figs. "Damages on inner thrust nozzle flap" and "Fighter thrust nozzle weak points").
Thrust reversers in the cool air flow of fan engines (Example "Missing cold stream nozzle"), as well as those in fighter engines (Fig. "Hot gas thrust reverser problem areas") are subjected to powerful mechanically-induced dynamic loads. Thrust reversers that redirect the hot gas flow are additionally subjected to thermal loads. Damage to and failure of the blocking system are considered especially dangerous (Examples 11.2.4-3 and 11.2.4-5). In extreme cases, they can cause aircraft to crash (Ref. 11.2.4-6).
Figure "Gas vibrations inafterburner cause stalls in the fan" (Ref. 11.2.4-13): Buzz is a low-frequency pressure vibration in the form of a standing wave (also see Volume 2, Ill. 7.2.2-17). During fuel combustion, a longitudinal vibration mode is created in the gas flow. This vibration is similar to that in an organ pipe or flute. The afterburner pipe is similar to these instruments in that it is only closed at one end. The fan (via the bypass) or turbine (top diagram) act on the sonic vibrations like lids. A supersonic zone can also act as a lid. The standing vibration always forms a node on the closed side. On the open side, its transition is continual. The wave decays completely at the distance “s” from the end of the pipe (Helmholtz`s end correction). The acoustic pipe length in a pipe opened at one end is “L”+“s”. The frequency of this type of combustion-dependent sonic vibration in the afterburner is between 50-150 Hz. This frequency also depends on the flight conditions and the amount of afterburner fuel. The axially acting buzz can excite dangerous vibrations in turbine and/or compressor rotors. The pressure pulses can cause stalls in the fan and core compressor. The effect of pressure pulses from the afterburner on the compressor is also evident during failed ignition attempts (Fig. "After burner triggered compressor surge").
High-frequency transverse vibration modes are usually of a higher order and radially oriented (see Fig. "Dmage by afterburner 'screetch'"). They are extremely dangerous. If they are sufficiently strong (Fig. "Indicating values for combustuion vibrations"), they can cause serious damage within seconds (Fig. "Dmage by afterburner 'screetch'").
Figure "Indicating values for combustuion vibrations" (Ref. 11.2.4-13): In most cases, the pressure amplitudes of low-frequency pressure vibrations are less than 2% of the total pressure. These vibrations are not relevant to damage. They are below the typical strength reserves. Despite the high noise levels of afterburner operation, pressure amplitudes between 2-10% can easily be detected. Their volume can distract the pilot. At pressure amplitudes above 10% of total pressure, there is a possibility of dangerous vibrations being excited (e.g. in the rotor).
Figure "Dmage by afterburner 'screetch'": During the combustion process in afterburners, transverse modes of pressure vibrations can form (bottom diagram). These are radially oriented and high-frequency (screech, Fig. "Excitement of combustion vibrations"). In the depicted example, this type of vibration occurred during a run on an altitude testing rig in the development phase of an engine. The vibration evidently parted the protective cooling air film on the inner side of the inner liner. A hole was burned through the liner pipe (top diagram), allowing a flame to escape and cause extensive damage outside of the afterburner.
Figure "Fracture of after burner mantle pipe" (Ref. 11.2.4-1, Example "Malfunctioning thrust reverser"): This diagram is a reconstruction made by the author based on the literature. The description can be found in the below example.
Example "Malfunctioning thrust reverser" (Fig. "Fracture of after burner mantle pipe", Ref. 11.2.4-1):
Excerpt: “……An engine-related class-A mishap happened…in early February…the engine suffered an augmentor weld failure, which caused the entire augmentor and engine nozzle to separate from the aircraft. About 1.5 mi. away from touchdown the aircraft suffered an explosion and caught fire. The pilot safely ejected….there have been about 10 previous weld failures like this `but non ever resulted in a class-A mishap. For now, maintenance technicians are conducting fluorescent penetrant inspections of the augmentor welds as a field control. Additonally, one set of 96 augmentors, all made over a specific time period, are being inspected immediately. A longer-term fix is to retrofit a chemically milled augmentor duct into the fleet.”
Comments: Fractures of the afterburner liner are evidently specific to the engine part. Even in an older engine type in a fighter aircraft, frequent cracking and complete separation occurred in the region of a circumferential roll seam (spot welded) at overlapping metal sheets (Volume 1, Ill. 4.4-5). In this earlier case, flexural vibrations caused fatigue cracking. In the above case, there is no mention of the type of cracking, but the culprit is also probably fatigue cracking. Other sources indicate that this case also involved a spot weld. In this case, the fact that all welded parts, even those without welding flaws, are to be replaced by unwelded parts (chemically milled parts), indicates that flaws in the weld seam were not believed to be the primary cause of the damage. The determining factor is probably the combination of high loads with roll seam welds. Even resistance welds that meet the specifications (roll seam, spot welds) typically have low dynamic strength (notch effect due to jumps in stiffness) relative to base material that has not been influenced by welding. These connections are hardly suitable for the absorption of high dynamic loads.
Figure "Damages at after burner 'torch igniter'": If the afterburner is not to be ignited with a darting flame from the combustion chamber, then there are other possibilities (Ref. 11.2.4-12). One alternative is a torch igniter. This is installed near the flame holder (top diagram) and functions like a small combustion chamber. Its flame is used for ignition (bottom diagram). Experience has shown that torch igniters are subject to high thermal loads. They are heated by both their own flame and the flame of the afterburner. Because of this, they exhibit a great deal of overheating damage such as thermal fatigue cracks, heavy oxidation (burning), and warping (Fig. "After burner problem zones"). Hot gas corrosion at the exit opening is probably promoted by coke formation and/or the impact of fuel droplets.
Example "Missing cold stream nozzle" (Ref. 11.2.4-23):
Excerpt: “..the crew of the aircraft `heard an audible bang and felt a jolt through the airframe' while the …(4 engine civil aircraft) was climbing through approximately 3,050 m after take-off …No.1 engine parameters fluctuated momentarily and the engine pressure ratio dropped…An inspection …(on the ground after flight) found that `most of engine's cold stream nozzle was missing, a number of the outboard leading edge flap panels were damaged and outboard flap canoe was holed'…A fleet-wide check by the operator found a further six nozzles cracked in the same area…and the cracks varied in length from about 2 cm to 21 cm'.
Comments: Thrust reversers in the cold flow behind the fan are evidently also subjected to damaging loads. One may speculate that the damage mechanism was dynamic fatigue fractures.
Example "Unfavourable afterburner conditions at high altitude" (Ref. 11.2.4-2):
Excerpt: ”…(the OEM) is planning to incorporate a new, angled igniter fuel tube into all …fighter turbofans following evaluations that show the new design offers `dramatic' cuts in the number of afterburner no-lights to slightly more than one in every 1,000 afterburner cycles…(the OEM) engineers started the process in the mid 1980s when spotty reports of no-lights in remote corners of the flight envelope were being reported…
The straight igniter tube used at the time sprayed fuel downward into the after burner pilot can, where it had to be caught in the swirling air. But because the JP8 fuel being used in those aircraft wasn't very volatile, and the fuel was being sprayed away from the ignition source, no-lights would occasionally happen. The straight tube worked best in warmer temperatures, lower altitudes and with more volatile JP4.
…Engineers came up with the angled design and used it, coupled with a higher fuel flow rate, to make the fuel system generate a richer fuel/air mixture-ideal for JP8…planes with angled tubes, flying at low altitudes and fueled with more volatile JP4…reduced the flow design…“
Comments: This example shows the considerable difficulty of ensuring reliable ignition of the afterburner in all operating conditions. It especially shows how apparently marginal changes in the fuel can have serious effects, and the important influence of the proper location and angle of the igniter tube (Fig. "Damages at after burner 'torch igniter'").
Figure "Damages on inner thrust nozzle flap": The hot gases from the afterburner strike the inner tailpipe flaps (primary flaps, top diagram). The overlapping covers the edges of the flaps. This creates large temperature differences with extreme gradients, which in turn leads to thermal fatigue cracking (bottom diagram, also see Fig. "Fighter thrust nozzle weak points"). Another problem is wear on the sealing surfaces. The outer flaps (secondary flaps) are cooled by the bypass flow or the airflow around the engine, and are affected primarily by wear and vibrations.
Illustration 1.2.4-17: Thrust reversers are very rare in fighter engines. They are used in cases where runways are very short. Unlike thrust reversers in civilian engines, those in fighters are located behind an adjustable nozzle at the end of the afterburner. Of course, thrust reversers are used only when the afterburner is shut down.
Older engine types in commercial aircraft also use thrust reversers behind the engine core, often in combination with a thrust reverser in the bypass duct behind the fan (Ref. 11.2.4-12).
Thrust reverser flaps that are struck by hot gas are subject to additional loads in comparison with afterburners located behind the fan (compare with Fig. "Adjustable thrust nozzle problem zones"):
Thermal fatigue: Cracking at jumps in stiffness and welds on the flaps.
High-frequency vibrations: The typically pulsating gas jet can dynamically stress the thin metal sheeting of the flaps and cause fatigue cracking.
Corrosion: Thrust reversers are exposed to the environment. Marine environments have a corrosive effect on the steel parts of the actuation system (e.g. joint bearings, spindles, and nuts), especially when the aircraft is standing. This can cause the joints to jam, usually in combination with unsuitable lubricants and fretting (see Volume 2, Chapter 6.1).
Fretting: Contact surfaces, bearings, and especially the mechanical locking (Volume 2, Ill. 6.2-15) of thrust reversers are subjected to fretting. Wear is especially a cause for concern if it occurs between locking system components, i.e. a bolt and an eyelet. If a step is created, the bolt can catch and prevent the flap from unlocking. The flap can no longer be opened. If wear notches cause a bolt to fail or prevent the flap from being secured because a bolt is no longer flush and blocks the edge of the opening, then the thrust reverser can unintentionally unlock and open.
Improperly lubricated, high-RPM flexible synchronized shafts can cause dangerous wear to and/or plastic deformation of their cladding tubes (Volume 2, Ill. 6.2-14).
Overloads: Tightness in the joints or jamming can overload the actuation mechanism (e.g. a spindle and/or its nut). In extreme cases parts may fracture.
Example "Afterburner buzz" (Ref. 11.2.4-18):
Excerpt: ”..Although afterburner buzz is usually associated with fairly recent turbofan engines, it is by no means confined to this type, and, as early as 1961, the phenomenon was encountered in a …turbojet engine…during bench engine investigations into a quite separate engine instability…It was known that the possiblity of buzz arose (in this engine type) because of the nature of its fuel control system. This introduced the fuel in consecutive stages, with each fuel injection stage feeding one of the afterburner's annular flameholders. The control system was designed to deliver the scheduled fuel irrespective of the back pressure from the fuel injection system and this meant that, if one fuel injector failed to be selected, due to faulty valve, for instance, its intended share of the fuel would be automatically divided between the injectors already operating. As a consequence their flameholders would receive significantly more fuel than their scheduled flow and it was this circumstance which, it was found, could generate buzz.“
Comments: These engines were designed for a supersonic passenger aircraft. In this aircraft, cruising flight involved several hours of afterburner operation. This example shows the importance of the regulators for preventing pressure vibrations (also see Fig. "After burner triggered compressor surge").
Example "Importance of thrust reverser locking system" (Ref. 11.2.4-22):
Excerpt: “The US Federal Aviation Administration plans to require operators of over 400…airliners…in service worldwide to complete thrust reverser system safety modifications…The work includes installing new wiring and an additional thrust reverser locking system …A service bulletin in February described procedures for installing the additional thrust reverser locking system.”
Comments: This example demonstrates the importance of reliable thrust reverser locking systems. Evidently the required safety levels could only be reached with an additional locking system. This indicates that the original system alone was not considered sufficiently reliable.
Example "Broken thrust reverser drive link" (Ref. 11.2.4-24):
Excerpt: “While descending …from 25,000 feet the pilot experienced a violent lurch in the airframe. The flight continued without incident. An aircraft examination disclosed that the right upper thrust reverser drive link was broken. Further examination of the drive link disclosed a fatigue fracture.”
Comments: It is interesting that this incident occurred during flight, and not during deployment of the thrust reverser.