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Chapter 1 – INTRODUCTION

(by J. Warren)

1.1 INTRODUCTION AND DESCRIPTION OF THE FOD HCF PROBLEM

First we need to define what “FOD” is and how serious is the threat. In the aviation industry, there are two generally accepted meanings for the acronym FOD. One definition is “Foreign Object Debris”, which includes substances, debris or any articles that could potentially cause damage to a vehicle or engine (Figures 1 and 2). It is anything – large or small – inside or around aircraft and flight-line operations that does not belong there – which could create a hazard to equipment or personnel. Typical items that make up the list of types of debris are extensive. The list includes animals (birds, deer, moose, humans), hand tools, ballpoint pens, screws, rivets, tie-wire and other aircraft small parts, plastic, paper, wood, rocks and pebbles, ice – the list is nearly unlimited [1]. For the purposes of this technical manual, we will only be concerned with “hard body” foreign object debris – this category includes rocks, small metallic objects etc., but excludes “soft body” debris such as ice, cloth rags and animals. However, limited soft-body FOD prevention information is presented in Chapter 6 and in Annex D.

Figure 1: Typical Runway Foreign Object Debris.

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Figure 2: Typical Runway Foreign Object Debris, as Picked up by FOD BOSS System Runway Cleaning System.

The second definition for FOD is “Foreign Object Damage”. This includes any damage done to aircraft, helicopters, launch vehicles, engines or other aviation equipment by hard body (usually) foreign object debris entering the engines, flight controls or other operating systems. In this document, FOD will mean foreign object damage. When a reference is made to foreign object debris, the entire term will be used.

The primary focus of this manual is air breathing aircraft propulsion systems. However, FOD issues are not unique to just those systems (reference Annex B). Land-based propulsion systems typically experience FOD from dirt/sand debris in inlets, seals and joints, primarily due to improper operation of inlet screens or improper cleaning procedures. Small gas turbine engines operating in dusty and dirty desert environments (e.g., rotary wing aircraft or tanks) are exposed to severe erosion, especially the compressor and turbine components, causing engine performance deterioration. Sea-based propulsion systems can suffer similar effects from dust and sand if inlet screens are not properly fitted. Rocket propulsion systems are also vulnerable to FOD. For these systems, FOD may occur externally in the form of extremely fast moving debris, or internally from degradation of propulsion system flow path components or ice from cryogenic propellants. Additional details about land-, sea- and space-based propulsion system FOD issues are addressed in Annex B.

Maritime aircraft operations present a unique hazard from foreign object debris coming off the deck of an aircraft carrier during aircraft arrested landings (Figures 3 and 4). This foreign object debris source consists of pieces of “non-skid” (a carrier deck-surfacing material similar to asphalt), and the steel shot used to clean the deck when repairs are required. The steel shot can become imbedded in nearby non-skid material, and can be knocked loose by the fast moving arresting cable during aircraft landings (Figure 5).

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Figure 3: US Navy Aircraft Carrier Deck “Non-Skid” Material – Foreign Object Debris Source includes Small Shot Peen used to Clean and Repair Flight Deck.

Figure 4: US Navy Foreign Object Debris – Small Shot Peen used to Clean and Repair Flight Deck (magnified views, units in right photograph are inches).

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Figure 5: US Navy Foreign Object Debris Source – Arresting Cable Dragging along Flight Deck during Arrested Landings Kicks up Foreign Object Debris from the Deck Surface.

The costs attributed to FOD are significant, encompassing billions of dollars per year for maintenance and repair for the commercial aviation industry alone. For the military, the cost is comparable. Typical military depot repair costs in the US for a foreign object damaged engine range from $100K to $400K each, depending on severity. Other NATO members experience similar maintenance costs. And since FOD is a primary degrader for military aircraft, there are other impacts, most notably a loss in readiness. Lastly, if the foreign object-induced damage becomes catastrophic, primarily due to HCF, the loss of the aircraft and the pilot can result (Figure 6).

Figure 6: UK RAF Harrier Crash due to FOD and HCF Interaction.

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Figure 7: Compressor Blade Tip Damage due to HCF, and Subsequent Domestic Object Damage (DOD).

1.2 HCF-FOD INTERACTION

FOD, cracking, erosion and corrosion in the airfoils of compressor and fan blades can change the vibration characteristics, particularly natural frequencies of the components (mistuning), which can resonate at high enough stress levels to cause HCF (Figure 7). The occurrence of FOD in fan and compressor blades (Figure 8), and fretting-fatigue at blade-to-disk attachments, can significantly reduce the HCF resistance of advanced turbine engines. In both cases, the reduction in fatigue resistance is related to highly concentrated stresses, tensile residual stresses and microcracks resulting from the FOD impact at the surface of the component. If the FOD site is at or near a vibratory nodal location at a blade resonant frequency within the engine’s speed regime (i.e., a crossing on the Campbell diagram), the cyclic stress can be sufficient to initiate and propagate a crack. The steep stress gradients that are often associated with such stress concentrations represent technical challenges for predicting HCF resistance. Moreover, the extent of the stress concentration is strongly dependent on the geometry of the notch and contact condition, and these can vary significantly in service. Engine manufacturers often conduct coupon, spin, rig or engine tests on simulated foreign object-damaged components to assess the effect of FOD (Figures 9-10). Proper characterization should provide critical mechanistic insight into the influence of the stress concentration and residual stress state on fatigue behaviour.

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Figure 8: Foreign Object Damaged Fan Blades at Tip with Subsequent HCF Failure.

Figure 9: Simulated FOD on Fan Blade Leading Edge, Near Root.

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Figure 10: Close-up of Simulated FOD Notch on Fan Blade Leading Edge.

The current trend to optimize engine performance induces important technology changes. The primary concern relates to recent development of integrally bladed fan disks (referred to as blisks, integrally bladed rotors or IBRs) to reduce stage flow leakage, component mass and part count. For these rotor designs, the blades are generally thinner than the previous generation and are potentially more susceptible to FOD. Also, mechanical damping is critically reduced. For the same configuration of resonance, response levels are much higher which can lead to HCF problems. Furthermore, maintenance issues arise since blisks are much more expensive to repair than a classical bladed-disk.

A second area of concern regarding current technology trends in engine design relates to the reduction of blade tip clearances. Some improvement in engine performance (efficiency, operability) can be obtained by the reduction of tip leakage, and the global trend is to reduce the blade tip clearances. Physical contact between the rotating blades and the engine casing can become more severe, especially in the case of blisks (where low inherent damping is present). Moreover, after a FOD event, the induced rotor unbalance, even though small, can lead to more violent blade/case impacts due to the tip clearance reduction.

1.3 FOD PREVENTION

Elements of good FOD prevention include training, assigning responsibility to all flight-line personnel, ensuring maximum participation in FOD walk downs (Figure 11), good housekeeping “Clean-As-You-Go” procedures, effective runway cleaning operations (Figure 12), accurate record keeping, submitting incident reports and investigating the sources, compiling data and recognizing trends. FOD prevention ensures the safety of the aircrew, ground personnel and equipment assets.

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Figure 11: Runway “FOD-Walk” to Remove Foreign Object Debris.

Figure 12: FOD BOSS and Typical Runway Foreign Object Debris being Picked Up.

Procedures to eliminate FOD must address two fundamental requirements: (a) the avoidance of debris and (b) the removal of debris from airport pavements. Consideration should be given to the potential of all airside activities as being a cause of FOD. During aircraft maintenance, one must account for and dispose of nuts, bolts, washers, safety wire, and hand tools used in repair jobs. Aircraft engines should be inspected for potential FOD to prevent in-flight failures. A new “blending borescope” device has been developed which can both inspect and repair a damaged blade in-situ (Figure 13).

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Figure 13: Blending Borescope for On-Wing Blade FOD Repair (Machida and Pratt & Whitney).

In tarmac areas, there is a high potential for blowing debris from aircraft engine exhaust streams. Inspections of operational areas should be conducted frequently. Additional inspections should be performed in construction areas, and immediately after any aircraft or ground vehicle accident or incident, or after any material spill. Another key focus area is pavement repair. Spalled or cracked pavements, while structurally sound, may require expedited repair to minimize the ingestion of pavement fragments. Special attention should be given to the cleaning of cracks and pavement joints as tests have shown that these are the main sources of foreign objects that are ingested. One must consider using specialized brooms, magnets, and vacuum-type machines to clean aircraft operational areas, as well as runway and taxiway safety areas [2,3].

A crucial feature of an effective prevention strategy is the positive identification of the nature and origin of foreign objects. Unfortunately, the investigation of FOD is a particularly difficult task especially for highly damage-tolerant engines that are not routinely subjected to detailed after-flight compressor inspections. FOD incidents abound in the databases of most airlines and the military (Figures 14-16). Even the best preventive measures don’t eliminate them; hence, good engine design practices are important to provide damage tolerance. It is the intention of this best practices manual that NATO members can share information to improve aircraft safety and readiness, and minimize costs associated with FOD.

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Figure 14: Foreign Object Damage to Nacelles and Engine Front Frame Structures due to Impact with Ground Equipment.

Figure 15: Foreign Object Damaged Propeller from In-Flight Impact with another Aircraft (USN P3 vs. Chinese fighter, over Hainan, China).

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Figures 16a & b: A Chinese 747 airliner, whose destination was Paris, had landed at Frankfurt Germany for an “unscheduled” refuelling stop. Engine #3, shown above, had previously been shutdown due to excessive vibration caused by FOD, yet the Chinese carrier continued to use the aircraft for passenger service by strapping the fan blades in such a way that the #3 engine

would not windmill, thereby reducing vibration in flight. Flying from China to Paris, the 3 remaining operating engines consumed more fuel than expected, hence the aircraft needed

to land at Frankfurt to refuel, only a few hundred miles from its final destination. The German air controllers, after inspecting the engines, grounded the aircraft. A total of 3 engines

were subsequently repaired on the aircraft before it was permitted to fly again.

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1.4 HCF-FOD DESIGN CONSIDERATIONS

Design practices have already been dictated in a general sense by the current Joint Aviation Requirements, Europe (JAR-E) specification which states that “The engine shall be designed so that the strike/ingestion of foreign matter likely to affect one engine will not hazard the aircraft as a consequence of fire, burst, generation of loads greater than the ultimate load capability of the engine mount, inability to shutdown, etc. Ingestion of foreign matter likely to affect more than one engine in any flight will not preclude the continued safe flight and landing of the aircraft”. In the US, an effort was initiated in 1995 to better understand HCF and its relationship to FOD. That program, entitled the “National Turbine Engine HCF Program”, has focused on several technical areas of research to prevent HCF – forced response, passive damping, component analysis/probabilistics, materials damage tolerance, component surface treatment, instrumentation, aeromechanics and test/evaluation. Several of these technical focus areas relate to good design practice regarding FOD. The HCF program has found that key contributors to engine FOD susceptibility include: inlet location, airflow, bypass ratio, blade aspect ratio, chord thickness, fan/compressor speed, and engine operating tolerances. Operational needs, however, dictate military design requirements. Several new design initiatives for FOD prevention and mitigation include damage tolerant design methods, enhanced blade coatings, inlet particle separators, laser shock peening, sensors to improve FOD detection capability, inlet doors, inlet screens, and borescope blade blending repair systems. Some of these new technologies have applicability for legacy systems. The National Aerospace FOD Prevention Incorporated (NAFPI) FOD Prevention guideline also contains a general list of design considerations.

1.5 CHAPTER SUMMARIES

Subsequent chapters in this best practices manual discuss important FOD-related topics in more detail. Brief chapter summaries are included below.

• Chapter 2, FOD Data Mining and Investigation, provides information concerning FOD Reporting and Investigation. A template is provided for the collection of FOD arising data in a standard NATO format, with a list of common terminology and pictorial representative damage in order to aid reporting. To improve reporting of FOD, investigation methods are discussed to provide the greatest possible information concerning the cause of damage. Finally, the benefits of FOD data collection are discussed with examples provided from a number of NATO member nations.

• Chapter 3, Experimental and Numerical Simulation of FOD, discusses the methods for laboratory FOD analysis. The chapter begins with a comparison of specimen designs, followed by a description of the processes that can be used for mechanical evaluation. A discussion of the benefits and shortcomings of numerical impact analyses supplements the experimental section. The chapter concludes by describing several methodologies that can be used to incorporate long- and small-crack effects in post-FOD component life prediction.

• Chapter 4, Method for FOD/HCF Interaction Evaluation, focuses on design rules to deal with the complex mechanism of interaction between FOD and HCF. Engine manufacturers must provide users with a maintenance book giving some guidelines to avoid HCF during engine operation in service. Formal design practices dedicated to FOD and HCF interaction are immature, and most approaches are based on experience. However, as new technologies are introduced, the extrapolation of experience from one engine generation to the next may involve significant risk. The objective of this chapter is to propose a more formal way to write maintenance books, using simple design rules to predict HCF margins after a FOD event.

• Chapter 5, FOD/HCF Resistant Surface Treatments, discusses component surface modification methods used to improve high cycle fatigue life of aerospace components. Shot peening, laser shock peening, as well as low plasticity burnishing are described in this chapter, and the effects of

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surface compressive stresses introduced by these processes on high cycle fatigue properties are reviewed.

• Chapter 6, FOD Prevention, describes basic methods for prevention of foreign object damage at the different levels of aircraft maintenance and operation. The FOD prevention methods used at the engine design and manufacturing levels were treated only marginally here. The major part of the chapter is concentrated on tool control methods, hardware accountability, housekeeping procedures, personnel training program and lost tools and item procedures.

• Chapter 7, Conclusions and Recommendations, provides a summary of the major issues raised in this best practices manual, drawing out the important conclusions. It also identifies where the principal opportunities are in preventing FOD-induced failures and suggests the best routes forward that aero-system design, manufacture, operation and maintenance organizations may take in achieving that aim. Finally, it summarizes the main recommendations for implementing the content of this report.

1.6 ANNEX SUMMARIES • Annex A, FOD Terminology and Acronyms, contains a list of terminology, definitions and

acronyms to enable users to understand and apply the terms correctly when describing FOD events.

• Annex B, Air, Land, Sea and Space FOD Issues, provides reference documents that describe FOD-related issues of concern for Land-, Sea-, and Space-based propulsion systems.

• Annex C, Engine Blade Damage Definitions, includes detailed descriptions of FOD damage including diagrams and photos. Accurate documentation will enable better data collection to maintain consistency across the international community, and to guide analyses in identifying prevalent FOD hazards and routes of entry. Specific FOD impact location identification on the engine airfoils will dictate specific remedies to improve damage tolerance and design.

• Annex D, Soft-Body FOD Issues, contains information pertaining to other FOD sources including ice, small animals, and cloth rags etc., which are important issues for maintenance and airport operations personnel.

• Annex E, Additional FOD Reference Material/Websites, provides a listing of various additional websites and printed reference material of interest to the FOD prevention community.

• Annex F, Member Nation Maintenance Personnel, includes a listing of Points of Contact for personnel who have military FOD prevention, maintenance, data documentation and investigation responsibility within their respective member nations.

• Annex G, Terms of Reference (TOR), describes the purpose, goals and objectives for the RTO AVT-094 technical team to support NATO member nations by providing guidance on how best to focus their efforts to monitor and mitigate FOD-induced HCF problems in their aircraft propulsion systems.

REFERENCES [1] “FOD Prevention – It’s Up to You!”, June 1999, Gary Chaplin, Pres., The FOD Control Corporation,

Tucson AZ, 1-800-425-8363.

[2] U.S. Department of Transportation Federal Aviation Administration Advisory Circular 150/5380-5B, Debris Hazards at Civil Airports, 7/5/96, Initiated by: AAS-100.

[3] The National Aerospace FOD Prevention, Inc. (NAFPI), FOD Prevention Industry Guideline, David L. Bennett, at 1-800-FOD-1121.

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