Related Glossary Terms
Way of displaying real-world objects in a natural way by showing depth, height and width. This system uses the X, Y and Z axes.
1. Heating to a low temperature to remove gases. 2. Curing or hardening surface coatings, such as paints, by exposure to heat. 3. Heating to drive off moisture, as in the baking of sand cores after molding. Often used after plating or welding, or when the presence of hydrogen is suspected, to prevent embrittlement.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- electrical-discharge machining ( EDM)
electrical-discharge machining ( EDM)
Process that vaporizes conductive materials by controlled application of pulsed electrical current that flows between a workpiece and electrode (tool) in a dielectric fluid. Permits machining shapes to tight accuracies without the internal stresses conventional machining often generates. Useful in diemaking.
Conversion of an ingot or billet into lengths of uniform cross section by forcing metal to flow plastically through a die orifice.
Phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.
- fatigue resistance
Ability of a tool or component to be flexed repeatedly without cracking. Important for bandsaw-blade backing.
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- lean manufacturing
Companywide culture of continuous improvement, waste reduction and minimal inventory as practiced by individuals in every aspect of the business.
- quality assurance ( quality control)
quality assurance ( quality control)
Terms denoting a formal program for monitoring product quality. The denotations are the same, but QC typically connotes a more traditional postmachining inspection system, while QA implies a more comprehensive approach, with emphasis on “total quality,” broad quality principles, statistical process control and other statistical methods.
- wear resistance
Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.
On a rotating tool, the portion of the tool body that joins the lands. Web is thicker at the shank end, relative to the point end, providing maximum torsional strength.
From simple bushings to critical engine components, producing replacement aircraft parts requires unwavering commitment.
The “spare parts” market for aircraft is anything but spare. Airlines that once purchased parts to maintain their aircraft and engines almost exclusively from OEMs such as Boeing and GE Aviation now have the option of buying from third-party manufacturers approved by the Federal Aviation Administration. It’s a fast-growing market for shops that can meet strict FAA regulations regarding the design, manufacture and QC of the parts.
OEMs hold FAA “type certificates” authorizing production of complete aircraft or engines. The FAA also has “parts manufacturer approval” (PMA) rules for other suppliers wishing to make replacement parts for aircraft and engines.
Courtesy of HEICO Parts Group
At the HEICO Parts Group, a 5-axis laser finishes the complex geometry of a T56 Hastelloy X jet-engine combustion chamber component.
PMA regulations cover two areas: approval of the part design and definition of the QC systems governing part production. The design process usually begins with reverse engineering, or what the FAA calls “test and computation of the original part.”
Federal part-replacement standards initiated after World War II were generally applied to simple parts for legacy aircraft. As economic pressures on air carriers grew, interest in PMA parts for more sophisticated applications increased. In 2009, the FAA clarified regulations to make it clear that PMA parts and OEM parts are held to the same quality standards. Today, PMA parts are authorized for critical parts, including engine components.
Images courtesy of PMA Products
PMA Products Quality Inspector Jeremy Kimrey performs receiving inspections on 6.325 "-long, 0.624 "-dia. landing-gear bolts (below) made from aerospace-grade alloy steel.
The main advantage of PMA parts is they can cost 20 to 50 percent less than OEM parts. As a result, interest in PMA parts is growing. At the Modification and Replacement Parts Association (MARPA) annual conference in October, consulting firm ICF SH&E, Cambridge, Mass., estimated that the global PMA market will grow from $410 million in 2011 to $750 million by 2018, with engine parts being the largest category.
Learn from Experience
One PMA focus includes components for general aviation. Causey Aviation Service Inc., Liberty, N.C., provides general and charter aviation management, maintenance and avionics sales. Experience gained from years of aircraft maintenance and parts replacement led the owners to form PMA Products Inc., a PMA replacement parts company also based in Liberty.
“We reverse engineer a lot of simple parts, such as bolts, bushings and landing-gear parts,” said Charles Causey, president of PMA Products. Although simple, the parts are thoroughly analyzed. “For example, if it is a steel part, we send it to a lab and have the material analyzed to determine what alloy is used,” Causey said. “If it is plated, we determine what type of plating it has. Hardness is always checked and heat treatment must be a consideration. Then we have to establish the dimensions of the part and a range of tolerances on the dimensions.” Part design details are documented and reported to the FAA.
Courtesy of Jet Parts Engineering
Jet Parts Engineering provides PMA components for airlines and independent MROs worldwide.
Causey said most of the parts the company produces are for aircraft from Piper Aircraft Inc. and Hawker Beechcraft Corp. Causey Aviation previously was a Piper sales and service center and operated Piper aircraft, and so is familiar with typically replaced parts.
When the company began to operate Beechcraft King Air planes, it was shocked by replacement part prices. “We came up on a 5-year landing-gear overhaul and were just floored by the price of some of the bolts and bushings,” Causey said. “We were already doing PMA parts for Piper aircraft, so we decided to PMA some of the Beechcraft parts. Now we are selling a lot of the Beechcraft parts to landing-gear overhaul shops.” While OEMs continue to supply parts at higher prices, Causey noted that OEMs have reduced some of their prices.
PMA Products subcontracts parts manufacturing after designing the parts and obtaining the approvals. The FAA oversees design and production. “They have to approve our QC systems and inspect them on a regular basis,” Causey said. “For parts considered critical by the FAA, we are required to go into our subcontractors’ facilities and inspect them.”
Word of mouth sometimes determines the choice of a subcontract shop. “We talk to other people in the PMA business that are having similar parts made and learn about shops they have confidence in,” Causey said.
Because airlines and aerospace OEMs operate globally, there is global interest in PMA parts. Jeff Dark, vice president of sales and marketing for Jet Parts Engineering Inc., Seattle, said its customers are major airlines, including Delta, Lufthansa and United, as well as independent airline repair stations worldwide.
Many PMA companies, Dark said, focus on one area of the aircraft or aircraft systems. Jet Parts Engineering, for example, is mostly involved in the component and accessories field, which includes hydraulics, pneumatics, flight controls and fuel systems. “We recently purchased the assets of an engine hardware company, so we are into that now as well. As the market is maturing, PMA companies are branching out to find opportunities where they can.”
The company first produced PMA parts for Boeing airframes, then moved into supplying parts for regional aircraft makers and Airbus. Except for some assembly, Jet Parts Engineering subcontracts its manufacturing work.
Courtesy of Star Novak, McFarlane Aviation
McFarlane Aviation President Dave McFarlane (right) and Machine Shop Supervisor Brad Price at the shop’s dual-head, CNC MultiCam router where 2024T3 Alclad aluminum control surface skins are being vacuum fixtured for trim-out.
A number of factors influence the choice of parts to replicate. “We get a lot of usage downloads from different airlines and maintenance and repair organization (MRO) shops,” Dark said. “That lets us vet parts ourselves through our database. And customer requests are a major driver. When one feels an OEM part is too expensive, we will look at it.” If the customer is a big airline, its usage alone can justify a project. Jet Parts Engineering holds 570 PMAs, with a goal of adding 120 each year.
In addition to providing lower-cost replacement parts, PMA companies may redesign parts to improve performance. “We have made some minor improvements to parts that affected their life,” Dark said. “If you make a change like that, you have to explain your reasoning to the FAA and get it approved.” For example, an upgrade might involve changes in the way a gear fits on a shaft, with the intent of minimizing the likelihood of rubbing that could accelerate wear.
Dark said the company’s choice of reverse-engineering methods for a part depends on the application and critical dimensions. Most of the reverse-engineering processes, including measurement and metallurgical analysis, take place in-house. However, he said, “Our engineers may choose to outsource the reverse engineering on some parts with a high level of geometric complexity or special material features.”
Responsive service is a key element of the PMA business. Almost all of Jet Parts Engineering’s parts are in stock and can be shipped within 24 hours, compared to typical lead times of 2 to 12 weeks for OEMs, according to Dark.
In contrast to PMA providers that subcontract most manufacturing, McFarlane Aviation Inc., Baldwin City, Kan., designs and manufactures many PMA parts. Most of the PMA parts McFarlane manufactures are for general aviation, although the company is also a Tier 2 or Tier 3 supplier for Boeing, Airbus and the U.S. military. The shop’s specialty is components that require swaging (metal reduction) to fabricate assemblies, such as push-pull controls for throttles and auxiliary control systems.
Dave McFarlane, president, said the company generally chooses parts for PMA that can be improved. “Many times we use more modern materials; we try to bring the technology up to more recent standards,” he said. “Some of the parts we replace were designed 60 years ago. Our biggest driver is customers who are dissatisfied with original part life. Of course, price plays into it too.”
Courtesy of HEICO Parts Group
PMA parts undergo processing identical to their OEM counterparts. This automated shot peen equipment at HEICO Parts Group creates a compressive residual layer to increase fatigue resistance in jet engine components.
To improve a part, McFarlane determines its failure mode. A good example of relatively simple but flight-critical products are seat rails for light aircraft, machined from heat-treated 2024 aluminum alloy extrusions. The rails are subject to cyclical loading as the pilot’s weight moves in rough air, McFarlane said. “They’ve had trouble with the seat slipping because of hole wear and cracking in the rail.” In a worst-case scenario, a shifting seat could cause a pilot to lose control of the aircraft.
Upgrading the parts was complex. “We thickened the web of the extrusion where we could,” McFarlane said. “We put small radiuses where there were none in the blends of the metal, and we changed the profile of the cap on the rails so that it had more metal around the holes.”
To improve wear resistance, the shop added a light anodized finish. “We use specific anodizing processes to ensure they don’t aggravate the cracking problem,” McFarlane said. The shop also straightens the extrusions to assure a precise fit. The result is longer service life and fewer cracks.
McFarlane said the FAA is involved from the start of the reverse-engineering process for any PMA part. “Sometimes we can justify a new design by calculation. We also use a comparative analysis to the original component.” In other cases, testing of the replacement part is required. “We generally have to coordinate with the FAA and agree on the testing required. Then, of course, everything has to be documented,” he said.
McFarlane does much of its testing in-house. For example, the shop fabricated a test rig to subject an improved fuel-level transmitter to vibration like that encountered in flight. The shop has other test equipment as well, but will outsource specialty testing, such as spectral analysis or mapping case hardness.
Establishment and approval of production QC systems follow part design and testing. “We must have our QC system for a component approved through the Manufacturing Inspection District Office of the FAA,” McFarlane said. “We have to meet the same regulations on the aftermarket parts that Boeing, Cessna or Piper does on the original airplane.” The FAA visits and audits the shop several times per year.
Courtesy of Star Novak, McFarlane Aviation
Quality Control Inspector Pete Streb checks superficial hardness of an aircraft component at McFarlane Aviation.
Although making PMA parts may appear to be an attractive way for a general machine shop to diversify, McFarlane said the transition has its challenges. “The difficult part is the initial design phase,” he said. In addition to understanding the FAA design and QC requirements, the shop must understand the application of the part on the aircraft. “For example, when we apply for a PMA, we have to provide a safety analysis describing what happens to the airplane if this part fails. You need a lot of background knowledge in the actual function of the airplane to show compliance with the regulations,” McFarlane said.
McFarlane Aviation uses Swiss-style machines, mills, EDMs and a high-speed router to machine its extrusions. “The Swiss process has dramatically improved our production of quality products,” McFarlane said, noting that some bearing tolerances are ±0.0002 ".
The shop picks the most efficient manufacturing method for each part. “Where the OEM might have stamped a part, we may find it more cost effective to laser cut, plasma cut, waterjet or, in some cases, EDM a small quantity of parts,” he said.
The small-volume nature of PMA part production requires careful planning. “One challenge for our industry is to make a smaller number of products cost effectively,” McFarlane said. Quantities of 25 to 50 units are common for aviation parts in general, and especially so for legacy aircraft replacement parts. “We use lean manufacturing processes for tool design, quick setup and quick changes in order to manufacture cost effectively,” McFarlane said.
When the shop does outsource manufacturing, assuring full participation in the PMA QC system requirements is required. Generally, when parts are machined outside the shop, they go through McFarlane Aviation’s “total-quality system” for inspection upon receipt. However, if a machined feature is hidden or results of a process—such as a post-plating baking operation—are not measurable when the part is returned, McFarlane must verify the subcontractor has a QC system it can rely on for inspection of that phase. The subcontractor may also be subject to an FAA audit.
A few PMA part makers now handle critical components that until recently were produced only by OEMs. The Aerospace Parts Group of HEICO Corp., Hollywood, Fla., is reportedly the world’s largest independent supplier of FAA-approved engine components and other parts, holding more than 6,000 PMAs and delivering over three million parts per year to the world’s major airlines, aircraft leasing firms and independent MROs.
HEICO focuses on making a part equal to the original, according to Patrick Markham, vice president of technical services. Improved performance is an option, but “especially for more complex systems, you don’t want to make something better in one aspect because you may make it less capable in another,” he said. “Your primary goal is to try to duplicate what currently exists so you can get the same performance.
Images courtesy of HEICO Parts Group
HEICO’s PMA high-pressure compressor blades (below) are subjected to high cycle fatigue testing to prove their equivalency to OEM parts.
“If a customer asks for improved performance, you need to do it very carefully,” he continued. “Each part requires case-by-case assessment so you are not changing something that could adversely affect the part.” Increasing the wear resistance of one part, for example, might unintentionally make a mating part wear more quickly.
Over the last 15 years, HEICO has developed a highly successful proprietary test and computation method of gaining PMA design approval, according to Markham. The method is proprietary because it is more detailed than other companies’ PMA procedures, he added.
One of the details involves determining the best way to reverse engineer a part. Analytical tools include 3-D white light scanning, coordinate measuring machines and optical comparators.
Markham said HEICO uses subcontractors in some production steps based on their experience and QC record. Shops seeking to become subcontractors for PMA parts “have to realize they are going to have to make very high-quality parts and all the documentation and paperwork must be there. It is not an option,” he said. CTE
About the Author: Bill Kennedy, based in Latrobe, Pa., is a contributing editor for CTE. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or email@example.com.
Causey Aviation Service Inc./PMA Products Inc.
HEICO Corp., Aerospace Parts Group
Jet Parts Engineering Inc.
McFarlane Aviation Inc.
Modification and Replacement Parts Association
OEM vs. PMA? The debate is on
Because OEMs and PMA part makers compete for the same replacement-part orders, the relationship between them can be and has been contentious. However, that may be changing, according to Jason Dickstein, president of the Modification and Replacement Parts Association (MARPA), which represents the PMA industry. The association’s members include part manufacturers and commercial air carriers.
“Neither OEM or PMA is a four-letter word; we are all one industry,” Dickstein said. “It is a complex area. A mark of the maturity of the PMA community is that we have gone from litigating against our [OEM] competitors to supplying them with parts. I tell our members you can’t hate the OEMs because without them we wouldn’t have a platform to put our parts on.”
The different business models of aerospace OEMs make it difficult to generalize about the relationship between OEMs and PMA companies. Dickstein pointed out that large airframe builders like Boeing profit from aircraft sales and service as well as sales of some replacement parts, but earnings from parts are not crucial to the companies’ success. As a result, Boeing has licensed some PMA companies to produce parts.
However, engine manufacturers’ business strategy is more of a “razor blade model,” where the engines are sold at close to cost, with profits dependent on sales of replacement parts, according to Dickstein. As a result, engine builders “have to be much more cutthroat in their competition with potential competitors, and you won’t see them licensing PMA companies,” he said.
But particular circumstances can change even those assumptions. In one case, a major engine manufacturer is manufacturing PMA parts for another large manufacturer. In another, industry consolidation has resulted in some OEMs acquiring PMA part makers.
Machine shops fall into and move about a variety of niches within the PMA industry, according to Dickstein. Some PMA companies are essentially distributors that hold the PMA and do final inspection, with fabrication performed by outside shops. Others maintain their own fabrication infrastructure and outsource little.
Dickstein said shops can move from one manufacturing niche to another. Shops that acted as subcontractors for PMA holders have “gotten interested enough in the business that they’ve decided to develop their own engineering departments and get their own PMAs.” PMA holders themselves can grow to effectively OEM status. “Today’s PMA company can be tomorrow’s blockbuster OEM,” he said.
Dickstein pointed out that PMA companies are investing in the advanced manufacturing equipment needed to further reduce part costs while increasing quality. The latter is a key consideration because tolerances on PMA parts are becoming tighter, reflecting an FAA effort to drive the industry toward higher levels of reliability.
PMA companies serve their customers by “saving them money, improving their parts’ reliability or solving problems for them,” Dickstein said. “But that just gets the PMA part maker in the door. They don’t actually get on the aircraft until they can prove to the air carrier that they have a safe product as well. Yes, PMA companies offer better prices and improved availability and reliability, but every part has been fully reviewed and approved by the FAA. PMA companies are committed to quality.”