Table of Contents
5.2.1.2 Intake of Foreign Objects
Engines can ingest foreign objects in the air. Fairly common foreign objects include objects on the runway thrown up by the landing gear or parts of the landing gear itself, such as tire pieces (Figs. "Engine configurations" and "Nose wheel position"). There are attempts to minimize this risk through suitable constructive measures. There is also a risk when using thrust reversers, since these may throw up particles and objects that get dangerously close to the engine intakes (Fig. "Thrust reverser").
It seems less natural that even large compact objects such as rocks or metal objects (bolts, etc.) can be sucked up from the ground. The powerful suction needed for this is created by (ground-) vortexes (Figs. "Vortex II", "Maximum ingestion size", Size ratio" and "Influence of wind"). In order to minimize this risk, modern commercial aircraft with large fan engines on the wings with their intakes relatively near the ground (Fig. "Size ratio") are outfitted with constructive measures to prevent vortexes from forming (Fig. "Vortex prevention").
Some Russian aircraft have a very effective method of preventing the ingestion of foreign objects during takeoff and landing (Fig. "Inlet duct design"). They are outfitted with a shutter mechanism in the intake duct that can be closed when the aircraft is near the ground.
The trend towards higher efficiency and more concentrated performance necessitates not only RPM increases, but also optimization of the blade shapes. This puts greater mechanical (static and dynamic) and aerodynamic loads on the blades. The trend towards supersonic shapes leads towards sharper and thinner edges. This means that, compared with older engines, smaller damages (roughing, profile changes, deformation) already have an unallowably strong effect and can no longer be tolerated (Ref. 5.2.1.2-2).
Example "Ingestion of tire fragments" (Ref. 5.2.1.2-1):
Excerpt: “…The take off roll progressed normally until a point 4,200 feet from initiation. The crew reported a vibration and the left engine power diminished. The take off was aborted, but the aircraft speed at the time of the abort and runway distance remaining was insufficient to avoid departing the end of the runway. The aircraft became mired in the rain soaked terrain up to the axle. Post-incident examination of the left main gear tires indicated a failure of the carcass….The cause of the initial power loss on the left engine was from the ingestion of the tire debris.”
Comments: This case involved a small cargo aircraft with two engines with small bypass ratios on the rear fuselage (similar to Fig. "Engine configurations" top). Experience shows that this engine configuration seems to promote the ingestion of foreign objects that have been thrown up or broken off (Example "Increased FOD risk due to larger inlet diameters").
The below shown case is connected with a tire failure which was triggered by a sharp foreign object on the runway,.and caused in the critical start phase (Fig. "Issues during takeoff II") the loss of thrust. Additionally it came to a fuel fire, obviously because a large piece of the tire sprang a leak at the wing tank. Nobody in the aircraft survived the following catastrophic crash.
Example "Increased FOD risk due to larger inlet diameters" (Ref. 5.2.1.2-2):
Excerpt: “…Two obvious contributors to the increased FOD-rate in the (transport aircraft with two engines at the end) were increased airflow and approximately 30% larger air inlet compared with (a former type).”
Comments: The probability of an increased FOD risk with larger inlet diameters is also taken into account in the rating regulations (see table in chapter 5.1.1).
Figure "Engine configurations": The location of the engines affects the probability of foreign objects entering the engine (also see Fig. "Fatigue fractures"). In general, the probability of foreign objects being sucked in by the engine from the ground increases, the lower the engine inlet is to the ground. The probability also increases with the size of the inlet (Example "Increased FOD risk due to larger inlet diameters").
However, experience has shown that this is only true for engines that are located on the wing in a similar fashion. Engines located on the rear of the fuselage at a relatively high position have a high rate of FOD (Ref. 5.2.1.2-4). Ice breaking off from the fuselage or broken parts of the fuselage (fragments of the radome, windshield wipers, etc. Example "Short timespan") can easily be sucked up by the engine. Even foreign objects thrown up by the main landing gear can threaten engines located at the rear of the aircraft (Example "Ingestion of tire fragments").
Figure "Nose wheel position": In fighter aircraft, the front wheel is often the cause of foreign objects being thrown up into the engine. The tendency to ingest foreign objects depends on the relationship between the inlet size (diameter, diagonal, cross-section) and the height of the inlet above the ground (Fig. "Maximum ingestion size"). The greater this relationship is, the more probable foreign object ingestion becomes. For this reason, the engine inlets underneath the fuselage of modern fighter aircraft must be critically considered (see Figs. "Vortex II", "Maximum ingestion size", Size ratio" and "Influence of wind").
Figure "Protective cover" (Ref. 5.2.1.2-2): Fighter planes that are intended to take off from unpaved runways are commonly outfitted with deflectors or protective covers on the front landing gear (the diagram shows a Russian design).
Example "Poorly adhering sealant" (Ref. 5.2.1.2-3):
Excerpt: „…….(The aircrafttype with two rear engines) aborted a take off…after having a compressor stall on the right engine. The aircraft returned to the terminal and deplaned normally…The aircraft received only minor damage…The examination of the engine revealed it had ingested bituminous-polymer sealant that had been recently applied on the runway.“
Comments: Evidently the poorly adhering sealant was thrown up by the landing gear and ingested by the engine. It is surprising that the persons responsible for the condition of the runway overlooked this flaw. Probably it was not a stall at some blades, but a surge.
Example "Debris deflector" (Ref. 5.2.1.2-2):
Excerpt: ”…the following modifications released in the 1988 to 1990 timeframes:
- repositioning (rotation) the thrustreverser doors („A“)
- improved main wheel water/debris deflector („B”)
- blocking plate between flap and wing trailing edge („C“)
- debris curtain on wing aft spar („D”)
- improved nose gear water deflector („F“)
In addition the engine manufacturer… introduced new improved fifth stage (compressor) blades („F”) and a new sixth stage anti flutter bleed system to reduce seventh stage blade flutter and -induced stresses.
…all modifications mentioned (were implemented) without seeing the anticipated improvements on the engine deterioration rate.
The only aircraft modification that has proven to reduce the amount of debris entering the engines are the improved debris deflector on the main wheels, even though the improvement is marginal.
Comments: This example shows the difficulty of preventing the ingestion of foreign bodies when the basic principle of the engine location is unsuitable for this requirement. Even effective deflectors on the wheels require a great deal of knowhow. Russian fighter aircraft landing gear have been the subject of much successful pioneering work in the area (Fig. "Protective cover").
Figure "Thrust reverser": Activation of the thrust reverser can throw up foreign objects from the runway that are then ingested by the engine (Refs. 5.2.1.2-2 and 5.2.1.2-3). This is also dependent on the direction of the gas jet, i.e. the deflection system. If possible, the gas or air jet should be directed away from the engine inlet. The location of the engines on the side of the fuselage at the rear limits possible options more than the location of engines on the wing does.
Formation of an inlet vortex in front of engines
A vortex consists of rapidly whirling air that creates a tube-like underpressure zone. In nature, vortexes occur as tornados. A comparison can also be made with the whirlpool formed by fluid draining through a hole (Fig. "Vortex I"). In aircraft, vortexes often form around wing edges and can be recognized by condensation in these areas.
The larger the inlet of an engine, and the closer its center is located to the ground, the higher the probability of a vortex forming which could suck up dangerous foreign objects. This risk increases with higher bypass ratios and/or inlets that are located to the ground (Refs. 5.2.1.2-5 and 5.2.1.2-6).
Ground vortexes are undesirable in aircraft. The underpressure in the airflow due to the high rotation speed can throw up dense objects such as bolts and stones from the ground and carry them through the tube of underpressure that is the vortex (Ref. 5.2.1.2-6). The throwing up of the foreign objects is impulsive. They may include objects from inside cracks on the runway, from the edge of the runway, or from soft ground. Vortexes do not always form. Instead, they require specific conditions such as sufficient air flow speed and prevailing wind (strength and direction with regard to engine axis; Fig. "Influence of wind"). For this reason, taxi and takeoff procedures are often important factors for the frequency of FOD. If these conditions are met, for example, through an increase in engine performance, then a vortex can suddenly form (Ref. 5.2.1.2-6). It spreads from the engine inlet down to the ground. The larger the diameter of the vortex, the heavier the foreign objects it can suck up (Fig. "Maximum ingestion size", Ref. 5.2.1.2-6). The power of a vortex is related to the cube of its diameter. The following example from Ref. 5.2.1.2-5 illustrates the strength of vortexes:
An engine inlet with a two meter diameter, the center of which is about two meters off the ground, is capable of sucking up a concrete sphere with a diameter of up to 30 cm!
A vortex can be recognized as a hose-like shape if there is condensation around it due to temperature or pressure drops. Unlike the outside of the vortex, the inside is clear without condensation (Ref. 5.2.1.2-7). This gives the vortex its typical hose-like shape. This shape is created by the lower relative humidity inside the vortex, a corresponding tendency for the moisture to evaporate (steam is clear!), and the centrifugal force which acts on condensed droplets, moving these out of the inner zone. On damp runways the underpressure zone near the ground can be seen in splashing water if the vortex is over a puddle. The vortex and its end can move very quickly on the ground and also in other directions. This means that maintenance personnel near running aircraft are at risk. This is especially true for fighter aircraft on aircraft carriers.
Engines with sufficiently high performances can show pronounced vortex formation around the inlet (Fig. "Vortex II"). If the engine intake (dependent on the inlet diameter) is sufficiently near to the ground, then this creates conditions for ingestion of dangerous foreign objects from the ground. The trends in both commercial and fighter aircraft are towards smaller height/diameter ratios and therefore towards greater sensitivity to FOD (Fig. "Influence of wind").
Figure "Vortex I" : The top two diagrams explain the concept of a vortex with the aid of commonly known natural phenomena.
The bottom diagrams depict phenomena that were observed during tests (Ref. 5.2.1.2-5). These phenomena are important for understanding the various factors in the surrounding environment that can have an influence on vortex formation. These factors should be understood by maintenance crews and testing rig operators in order to prevent accidents or damage.
For example, it was observed that vortexes became stronger when holding one`s hand between the inlet and outlet of the fan of a testing assembly, thus affecting the intake flow. Merely opening the door to the testing chamber or movement by the testing personnel near the test assembly can create vortexes that suck up foreign objects.
Therefore it must be assumed that interferences such as people (A), doors (B), and machines (C) in an enclosed testing rig will influence the ingestion of foreign objects. This may also be the case when maintenance personnel are active near an engine on an aircraft or field testing rig.
Figure "Vortex II": The top diagram depicts obvious vortexes formed by a military (testing) STOL cargo aircraft (Ref. 5.2.1.2-7). It is surprising that the engines, which are located high on the wing, can create underpressure tubes that reach all the way to the ground and increase the risk of FOD. This is especially disconcerting in the depicted case, since this aircraft is designed for especially short runways, which are not usually as debris-free as longer standard runways.
The lower diagram shows the parameters that influence vortex formation as explained in Refs. 5.2.1.2-6 and 5.2.1.2-12. The values indicated by “0” are for the surrounding wind conditions, those marked “1” refer to the flow conditions in the inlet. The mass of the foreign object that could potentially be sucked in is proportional to the cube of the vortex diameter “a”.
Figure "Maximum ingestion size": The top right diagram shows a test assembly described in Ref. 5.2.1.2-5 with a H/D of 1.2, with an inlet diameter (D) of 30 cm sucking up 25 mm diameter glass spheres.
The lower diagram is based on Ref. 5.2.1.2-8, and refers to the ratios shown in the top left diagram, giving the maximum ingestible foreign object sizes (granite spheres) and the corresponding (Russian) aircraft engine types. As the diagram shows, in extreme cases rocks with diameters of considerably more than 10 cm can be sucked up, depending on the air flow rate.
Figure "Size ratio": The ratio of the height of an inlet above the ground to the inlet size (i.e. air flow rate) is important for the probability that an engine on the ground will suck up foreign objects. The smaller this ratio is, the greater the probability of FOD occurring. The recent trend for both commercial aircraft and fighter aircraft is towards smaller H/D ratios. This has led to, for example, the following special measures in inlet of the fighter aircraft depicted below: a shutter mechanism (Fig. "Inlet duct design") closes the inlet during start-up, and the engine takes in air through openings on top of the wing during this phase.
Example "Improper procedures" (Ref. 5.2.1.2-9):
Excerpt: “…..engines that power the carrier's reengined (Aircraft)…have encountered sand and particle ingestion problems.
Changes are being considered seriously by the carrier and the manufacturer… in a continuing analysis of the engines…Aim of the redesign and the vortex dissipater would be to reduce particle ingestion that has been causing high levels of erosion and has necessitated a higher engine removal rate `than we would like to see'…
The vortex dissipater under consideration would be a tube inserted at the bottom of the cowling that would blow the engine bleed air downward to disrupt vortices that develop under the engine.
… the ingested dirt was clogging high-pressure turbine blade cooling holes.
..taxiing procedures…probably helped cause more vortices and contributed to the particle ingestion problem.
The procedure borrowed from the carrier's previous practices….involved the activation of thrust reversers on outboard engines to slow taxiing speeds.
(The carrier)… has revised the procedure to use the inboard engines only, one engine at a given power setting at idle and the other at varying speeds.”
Comments: The affected aircraft type is an older four-jet commercial aircraft type, outfitted (reengined) with modern fan engines with large bypass ratios. It is interesting that improper operation of the thrust reversers changed the flow direction and promoted vortex formation (see Fig. "Influence of wind"), leading to an increased risk of FOD.
Figure "Influence of wind" (Ref. 5.2.1.2-5): The wind conditions around the inlet play an important role in vortex formation. This effect increases with smaller H/D ratios and weaker side winds (top diagram). The range for vortex formation lies below the curves for headwind (top curve) and tailwind (bottom curve). Therefore, with small H/D ratios, vortexes can form even with considerably stronger side winds. The most dangerous situation is to operate a high-performance engine in a windless environment.
Since the most powerful suction effect of the vortex is at “A” in the range of the stagnation flow line, the strength of the headwind is especially important in determining its location. With strong headwind, such as during taxiing, the stagnation flow line is shifted backwards (“1” in the bottom left diagram) and loses its contact with the ground. When there is no wind, it is located in front of the air inlet (“2”).
During operation of thrust reversers while taxiing, the forward-directed flow can reduce the vortex-preventing headwind and additionally contribute to the throwing-up of any foreign objects on the ground, thus increasing the risk of foreign object intake (see Example "Debris deflector").
The side wind gradient affects the direction of rotation of the vortex (bottom left diagram).
Figure "Vortex prevention": There are various possibilities for suppressing the formation of a dangerous vortex. These include air jets around the inlet that blow air radially outward (Example "Debris deflector", Refs. 5.2.1.2-9 and 5.2.1.2-10) or small wings that mechanically prevent foreign objects from entering the engine. However, evidently none of these measures is completely reliable (Ref. 5.2.1.2-5). Because sufficiently strong headwind prevents vortexes from forming and thus minimizes the risk of FOD, rapid forward movement seems the safest measure at high engine performance. This requires progressive power increases in the engines during takeoff.
The degree to which this can even be realized depends on the specific takeoff conditions.
Figure "Inlet duct design": This Russian fighter aircraft type has a special shutter mechanism in the inlet duct (also see Fig. "Inlet screen as safety precaution" and Fig. "Various FOD grill designs") of the engine (Refs. 5.2.1.2-12 and 5.2.1.2-13) because the large inlet openings are located very low on the underside of the fuselage (also see bottom of Fig. "Size ratio"), which would otherwise increase the probability of foreign objects being sucked up from the runway. Because this aircraft is designed for takeoff from makeshift runways, this risk is especially high. The function of the complex air intake system is depicted in the two bottom diagrams:
From start-up on the ground (top diagram) until the landing gear leave the ground, the main air inlet “C” is closed except for three permanently open vents in the main shutter, which are protected with blind-like metal strips. The necessary air flow is drawn through a secondary inlet “D” on top of the wing “A”, which is opened through a shutter system.
During flight at subsonic speeds (bottom diagram) the main air inlets are opened and the secondary inlet “D” is closed by the shutter system “C”.
During supersonic flight the shutter system “C” is partially closed as an inlet ramp, and the excess air is blown off through the overflow openings “E”.
The shutter system closes again during landing as soon as the oil shock struts of the main landing gear are compressed. This system has evidently proven itself during operation and makes possible the extremely low air inlets that are advantageous in flight situations with a high angle of attack.
Illustrations 5.2.1.2-11: The air inlets of supersonic aircraft, especially modern fighter aircraft, have complex adjustable shutter and ramps systems. These systems consist of components such as hinges, actuators, and parts made from metal sheeting attached with fastening devices such as bolts and rivets. These present many sources for possible FOD to the engine. Internals such as those described in Fig. "Inlet duct design" should further increase this risk.
Testing of these systems during inspection and maintenance work may require tools such as flashlights that may accidentally be left in ducts and inlets with poor visibility, later resulting in extensive damages.
Figure "Pop rivets": During surge of the aeroengine a sudden deceleration of the air flow in the inlet duct can occur. Thereby develop intense shock waves ( „hammer shock“, see Ill. 11.2.1.1-17 and Ill. 11.2.1.1-18). This pressure shocks load the whole structure of the inlet duct very high. As especially dangerous has shown the loosening and separating of so called „pop rivets”(blind rivets) in provisional inlet ducts of test rigs in the development phase. These foreign objects produce very expensive and dangerous failures/damages at the blading of the compressor.
References
5.2.1.2-1 NTSB Identification MKC89IA024, microfiche number 37058A, Index for Nov. 1988.
5.2.1.2-2 P. Stokke, “Erosion, Corrosion and Foreign Object Damage Effects on Gas Turbines”, AGARD-CP-558, Proceedings of the Conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands, 25-28 April 1994, Chapter 7.
5.2.1.2-3 NTSB Identification SEA98WA086, microfiche number 37058A, Index for June 1998.
5.2.1.2-4 T.L. Alge, J.T. Moehring, “Modern Transport Engine Experience With Environmental Ingestion Effects”, AGARD-CP-558, Proceedings of the Conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands, 25-28 April 1994, Chapter 9.
5.2.1.2-5 D.E. Glenny, “Ingestion of debris into intakes by vortex action”, U.D.C. No. 621-757: 533.697.2:532.527, N.G.T.E. Peyestock, G.P. 1114, December 1968.
5.2.1.2-6 C.L. Bore, “Scaling Laws For Vortex Induced Debris Ingestion Into Air Intakes”, Research Note BAe-KRS-N-GEN-274, March 1983.
5.2.1.2-7 “AMST: Tactical Airlift Into the 21st Century”, Boeing Co. and not ascribable literature source of a contribution to the proceedings.
5.2.1.2-8 A.K. Ivanyushkib, E.V. Pavlyukov, “Aerodynamic Problems of Propulsion System Operation Safety”, Proceedings of the “ Aircraft Flight Safety Conference”, Zhukovsky, Russia, August 31-September 5, 1993, pages 148-162.
5.2.1.2-9 J. Jeffrey, M. Lenorovitz, “CFM56 Powerplant Fixes Based on DC-8 Operations”, periodical “Aviation Week & Space Technology”, February 14, 1983, page 32.
5.2.1.2-10 “Delta Weighs Changes to CFM56-2”, periodical “Aviation Week & Space Technology”, February 14, 1982, page 31.
5.2.1.2-11 V.I. Vasiliev, “Vortex Ingestion of Foreign Objects in Gas Turbine Engines of Aircraft”, Proceedings of the “ Aircraft Flight Safety Conference”, Zhukovsky, Russia, August 31-September 5, 1993, pages 163-167.
5.2.1.2-12 MIG-29/Fulcrum, periodical “Soldat und Technik, 8/1988, page 480.
5.2.1.2-13 periodical “Military Technology, MILTECH”, 4/87, pages 124-128.