Table of Contents
11.2.4 Afterburner, Tailpipe, and Thrust Reverser
Afterburners (AB) are primarily used in military aircraft such as fighters and bombers. Only in special cases (e.g. supersonic passenger aircraft) does one encounter civilian aircraft outfitted with engines with afterburners. Afterburners roughly double the amount of engine thrust relative to the “dry thrust” (with no AB). The high fuel consumption during AB operation limited its activation to operating phases that require high power, such as takeoff, supersonic flight, and rapid acceleration (e.g. combat missions). However, supersonic passenger aircraft and newer types of fighters now fly for extended periods using the afterburner (super cruise).
The afterburners of modern turbofan (bypass) engines are very short in length. This is achieved by arranging the fuel injector and the flame holder close to the turbine exit, where the bypass flow and the hot gas flow from the core engine come together. Thorough mixing of these two flows must be ensured in order to achieve maximum thrust even when the afterburner is idle. For afterburner operation, combustion must be reliable despite the cold bypass flow.
The exhaust jet has a typical appearance during afterburner operation. This is caused by the supersonic flow inside the jet. These are periodically glowing pressure zones, called diamonds (above picture and Ill.11.2.4-2, Ref. 11.2.4-9). The heat emission in an afterburner is similar to that of a combustion chamber, and requires a heat shield (inner AB liner). Cooling air flows past the outside of this liner, and passes through to the hot gas side via a large number of openings, forming a cooling air film on the inner wall.
A typical malfunction of afterburners is delayed or failed ignition due to insufficient oxygen. This risk is especially high at high altitudes and low flight speeds, i.e. in the top left area of the flight envelope (Fig. "Afterburner operating limits by a flight envelope"). If the unburned fuel ignites after a delay, it can deflagrate and create extremely high pressure levels. This pressure wave can travel forward through the bypass duct to the fan, causing a fan surge. This, in turn, can initiate surges in the compressor aft of the fan, and in many cases leads to the engine shutting down (Example "Unfavourable afterburner conditions at high altitude"). If both engines of older fighter aircraft types with “stable” aerodynamic nacelles are affected, then the engines can usually be restarted following diving flight with sufficient windmilling.
The tailpipe regulates the speed of the exhaust gas, thus providing the necessary thrust. In military engines with afterburners, the tailpipe is adjustable and is opened during AB activation to accommodate the large volume of hot gas. The loss of thrust resulting from unintentionally opened tailpipes (e.g. malfunction of the control system due to electrical problems; Volume 1, Ill. 5.1.3-5) has caused many single-engine fighter aircraft to crash in cases in which the emergency closing mechanism was not activated in time.
Tailpipes of civilian engines without afterburners have mixers that reduce sound by combining the cold fan air with the hot exhaust gases from the core engine.
In fighter aircraft, thrust reversers are considered to be engine components. This means that the OEM is responsible for their design and function. In fan engines with high bypass ratios, such as are used in commercial aircraft, the thrust reverser is located in the nacelle behind the fan. These thrust reversers are generally apportioned to the manufacturer of the engine nacelle, who also assumes technical responsibility during operation (e.g. in case of malfunctions).
If the thrust reverser acts on the entire hot gas flow, then it redirects the hot gases and is subjected to high thermal loads. This is not the case with thrust reversers behind the fan.
Thrust reverser malfunction is a safety-relevant occurrence. While only a small number of unintended thrust reverser activations lead to critical situations, malfunctioning does represent a potential threat to the aircraft (Example "Malfunctioning thrust reverser"). If the thrust reverser opens during flight (Refs. 11.2.4-6 and 11.2.4-7), then its effect (especially its moment) on the aircraft (one engine pushes, one brakes) depends largely on the location of the engines. Fighter aircraft with engines in the fuselage experience relatively weak moments since the engines are very close to the central axis of the aircraft. This makes the moment arm very small. If the engines are located on the wing, then the moment arm is relatively large (Fig. "Danger by falsely thrust reverser activation "). The moment becomes larger as the engine thrust at the point of activation of the thrust reverser increases. This makes the takeoff and climbing phases especially critical. Further influences include the flight speed and aerodynamic effects such as the influence on the flow at the wing. Unintended thrust reverser activation can even cause fighter aircraft to crash, despite the weaker moment on the cell. High speed flight at low altitudes is especially critical, since the necessary high power levels create powerful moments, even if the moment arm is small. If one of two thrust reversers fails during landing, it may be impossible to keep the aircraft on the runway.
Example "Malfunctioning thrust reverser":
Excerpt “1” (Ref. 11.2.4-6): “..the cockpit crew was aware of an in-flight thrust reverser deployment problem and apparently was trying to cope with it just prior to crash.Thrust reversers have deployed on several occasions on four…(four engined) transports and at least once…on a (two engined transport), but in every instance the crews have returned the aircraft to stable flight.”
Excerpt “2” (Ref. 11.2.4-7): “..thrust reversers (of the concerned aircraft type) had good service experience, with only one known in-flight reversal and that was caused by a maintenance error on a deactivated unit…Most …thrust reversers are commanded by mechanical cables from the cockpit, but newer aircraft that have engines with Full Authority Digital Electronic Control (FADEC) have electrically commanded reversers…
…an attempt to evaluate accident scenarios…found it was `no problem' to fly with a reverser deployed and the throttle snatched to idle. However, the only thorough reverse thrust testing was conducted on the ground at low speeds, and there is little test basis for the thrust levels and where dynamic effects used in simulators in flight.”
Comments: As long as the engine immediately returns to idle following unintended thrust reverser activation, it seems that the pilot is able to control the problem. The higher the engine power and flight speed, the more dangerous this occurrence becomes. Evidently there are flight conditions under which even experienced pilots are overwhelmed by sudden deployment of the thrust reverser.
Figure "Danger by falsely thrust reverser activation " (Example "Malfunctioning thrust reverser", Refs. 11.2.4-6 and 11.2.4-7): If only one thrust reverser on a two-engine aircraft is deployed (top diagram), it will result in one-sided deceleration. This causes a powerful yawing movement (bottom diagram), which slows the flow on the wing on the side with the engine with the malfunctioning thrust reverser. The flow on the other wing accelerates. This creates a difference in lift between the two wings, resulting in a rolling movement that makes the aircraft difficult to control.
In one case involving a fighter aircraft, the aerodynamic forces created by the high flight speeds became powerful enough to tear off the afterburner flaps.
Example "Unfavourable afterburner conditions at high altitude":
Excerpt “1” (Ref. 11.2.4-3): “…stall stagnations occur when afterburner is selected, and most of these in the upper lefthand corner of the flight envelope - high altitude, low speed. A typical incident might occur when the aircraft is flying at 430 kt, 40,000ft. These conditions do not favour stable afterburner combustion. When the pilot attempts to light the afterburner it blows out.The cloud of unburnt fuel left in the afterburner by the blow-out is ignited a moment later by the hot turbine gases. A powerful pressure pulse travels forward through the fan duct. The fan stalls.”
Excerpt “2” (Ref. 11.2.4-4): “..Stall/stagnations, which have occurred 755 times on the ..(two engine `standard fighter') while not yet afflicting the newer… .(single engine `standard fighter'), have been approached with three fixes. The first two change afterburner settings upon sensing a stall by reducing afterburner fuel flow to a minimum and opening the afterburner nozzle, thus decreasing afterburner pressure pulses from sustaining the compressor stall…The third fix…is called the `proximate splitter' fix and consists of reducing the clearance between the fan exit and the structure, which splits airflow between the compressor and the compressor bypass duct, thereby reducing the tendency for afterburner pressure pulses traveling up the duct to stall the compressor.”
Comments: In this case the afterburner is the source of the problem/damage, but is not damaged itself (see Fig. "After burner triggered compressor surge"). Hundreds of aircraft of a two-engine “standard fighter” type were affected.
Figure "Afterburner operating limits by a flight envelope" (Example "Unfavourable afterburner conditions at high altitude", Refs. 11.2.4-4, 11.2.4-4 and 11.2.4-25): This is the typical flight envelope of a fighter aircraft with design points and operating limits for the afterburner. Attempted ignition or operation above the limits can cause compressor surges and engine flame-out (Fig. "After burner triggered compressor surge").
Figure "Effects ('diamonds') by shockwaves in the exhaust jet" (Refs. 11.2.4-9 and 11.2.4-10): The light areas (top diagram) are created by shockwaves that form at the tightest point of the nozzle. These shockwaves are repeatedly reflected from the transition zone of the hot exhaust gas flow to the surrounding atmosphere (bottom diagram, Ref. 11.2.4-10, Fig. "Effects ('diamonds') by shockwaves in the exhaust jet"). The shockwaves pass through the gas jet almost vertically at Mach 1 and become increasingly angled at higher Mach speeds. The gas speed in the jet can be estimated using the distances between the bi-conical jet regions (“diamonds”). Pressure differences concentrate soot particles in the diamonds. This soot is responsible for the luminescence. The available literature does not indicate the degree in which the exhaust gas jets of parallel engines influence each other and/or act with the nacelle in a damage-relevant manner (Ref. 11.2.4-8).
In the boundary area between the jet and the atmosphere, a mixing process occurs that creates many small swirls (bottom diagram), which are responsible for the intense high-frequency noise development. The sound increases with the 8th (!) power of the gas jet speed.
Figure "Exhaust jet interference with components of the aircraft" (Ref. 11.2.4-8): The hot gas jet thickens in the direction of flow. In a process similar to the action of a water jet pump, it sucks air from the surrounding area (mass flow entrainment, top diagram). At subsonic fight speeds, this effect can act forward against the direction of flight and affect the aircraft nacelle. The design of the rear of the aircraft should be seen in this context, since its contour interacts with the jet. During supersonic flight, shockwaves from the gas jet influence (bottom diagram) the steering gear flow. Therefore, they must be taken into account when designing the steering gear configuration.
The exhaust gas jets of the tightly spaced parallel engines in two-engine fighter aircraft can act on one another through interference. This necessitates a sufficient distance between the engines.