The ABCs of PMAs

By By Mark Robins | June 1, 2011
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Able Aerospace Services mechanic Nick Baca checks the torque on the main rotor hub. Able Aerospace Services

Fillers, seals, bearings, plastic components, rotor blades, electrical components and the radio are commonly replaced rotorcraft parts. Any dynamic mechanical and hydraulic part that moves and wears is frequently subject to repair, overhaul and/or replacement. Helicopter drive train components such as rotorheads, transmissions and gearboxes generally have specified overhaul intervals or service-life standards. When parts meet reach their pre-set thresholds they must be removed, inspected, repaired/replaced and sometimes their clock reset.

Corrosion, vibration, normal wear-and-tear, UV damage, stress, improper lubrication, heat and incorrect installation cause part replacement. Not replacing rotorcraft parts can impact aircraft performance, making it more difficult for a helicopter to perform its stated mission. Ignoring the signs of distress can lead to component failure, collateral damage and possible rotorcraft loss. The rotorcraft parts aftermarket got its start after World War II to address the shortage of replacement parts of older military equipment. The aftermarket grew rapidly in the 1980s, as rotorcraft manufacturers sought to reduce the cost of premium-priced spares by finding alternative sources of parts.


“Any critical flight safety part manufacturer must show the ability to consistently produce a part that meets required specification,” said retired Colonel Sam Evans, research associate at Penn State University. “It must demonstrate and certify all the processes to produce that part while maintaining the part’s positive chain of custody from ‘birth to death.’” A parts manufacturer approval (PMA) part is a replacement or modification part manufactured by a company other than the holder of FAA Type-Certificated original product. PMAs are governed under federal aviation regulation 14 CFR 21.3030. It is generally illegal in the United States to manufacture replacement or modification rotorcraft parts without a PMA. There are exceptions to this, including parts manufactured to government standards and parts manufactured under technical standard order authorization (TSOA).

There are essentially two halves to a PMA and both elements are needed to acquire the PMA. The first half is design approval from FAA. “The applicant has to show that their design meets all the applicable FAA regulations,” said Jason Dickstein, president of the Modification and Replacement Parts Association (MARPA), a nonprofit organization dedicated to promoting the replacement parts industry. “That’s going to include some very specific FAA regulations concerning engineering standards out of the airworthiness standards and it’s also going to include some broader regulations as broad as ‘it needs to be safe.’”

Generally, a PMA’s design approval will be issued in the context of the particular PMA product. The applicant must show that in addition to meeting the safety standards associated with the part. The part also has to be demonstrated to be appropriate for installation into the product for which it is intended. If you are designing a part to go into a 737, you are going to have to demonstrate that the part is appropriate for the 737, Dickstein says. There are several strategies to do that.

Testing can generate empirical evidence to demonstrate how it works. “Testing-on-wing” actually introduces PMA parts into the environment and then tests the environment. There are tests for various physical features, like tests that push or pull on something. “If you are going to design a new overhead compartment to replace a passenger service unit on an aircraft and its going to include the actual overhead compartment area, you need to do pull-and-push tests to simulate both the weight put in there and the effects of that weight in an environment,” Dickstein said.

Any PMA part that goes into the interior of a rotorcraft has to meet certain flammability standards involving flammability testing on it. Before the various tests, computations and analysis necessary to demonstrate design compliance are performed, it’s important to identify which regulations and physical features apply to this particular PMA part in order to identify what aspects need to be tested to.

“The laws of physics don’t change,” Dickstein said. “As a consequence, if you have a design that is already approved, and you’re essentially copying that design, the analysis may be minimal on the design side. For example, it is not uncommon for a PMA manufacturer to have a relationship with a design-approval holder. Boeing has licensed out its data to a fairly significant number of PMA-holding compliers. They have licensed that with the ability to use Boeing data in order to apply for a PMA. One of the valuable elements of that data is that the data has already been approved by the FAA.”

The second element necessary is production approval. The design approval is generally done by FAA’s sub-directorate aircraft certification offices (ACOs). When an OEM develops a component, all design aspects related to that component are subject to the approval of the ACO.

At manufacturing inspection district offices (MIDOs), inspectors will inspect the quality assurance systems that have been developed to ensure they meet such a standard so that everything that comes out of the system meets the accepted design. From FAA approval perspective, PMA components are subject to the same regulatory scrutiny as the OEM.

Once you take the quality system, together with the design approval—because the quality systems ensures that it meets the approved design—and the approved design’s approval has ensured that it meets the requirements of the regulations, those two pieces together should ensure that everything coming out of that system should meet the requirements of FAA’s safety regulations, Dickstein says.

Because FAA requires design assurance, many PMAs do their own independent research and parts analysis. Reverse engineering is the first step. Once completed, PMAs frequently test both the OEM part and their part, trying to develop tighter manufacturing tolerances than their competitors. With this testing, they are looking for potential flaws and creating reliability improvements.

Dynamic mechanical and hydraulic parts that move and wear are frequently subject to repair, overhaul and/or replacement.  Timken

To fully comply with FAA regulations, Timken, a PMA rotorcraft parts supplier evaluates multiple samples of type-certified (TC) parts from a functionality, dimensional and metallurgical standpoint. It evaluates the field service history of the TC part and includes design improvements where applicable. “A drawing and specifications are then developed from this evaluation, and we provide substantiation to the FAA demonstrating our design is as ‘good as or better’ than the TC part,” said Rich Ritter, product sales manager at Timken. “Depending on the complexity of the part, we will propose a test plan that may include both bench testing and engine testing. Once the engineering aspects of the design are approved, the FAA audits our manufacturing/quality system to ensure we have the capability to ensure the produced parts meet the approved design data.”

Able Aerospace Services has customers who recommend candidates for PMA, specifically parts that are most expensive, wear out often, are only available from the OEM, and consequently provide the greatest cost and value to operators. “We inspect and analyze these parts to understand what is wearing and why, and most importantly if there is significant value to the operator to have a PMA replacement part,” said Ed Mongiovi, vice president of marketing at Able.

Is the concept behind PMAs ethical? Most PMA companies will say yes. They believe what they are doing, at least from a macro-perspective, is the same thing every company in every other industry everywhere else in the world does. PMA companies take something that is the public domain and provide it on a more cost-effective basis or in many cases, identify reliability issues associated with the OEM part, and then modify the design to improve upon those reliability issues.

“Reverse engineering is a legitimate development technique used in most markets including the electronics, automotive and pharmaceutical industries,” said Peter Zimm, product sales manager at Timken. “As long as another company’s proprietary data is not used contrary to established law, we do not believe there is an ethical issue. We are responsible for the integrity of the parts we produce.”

OEMs have questioned PMA ethics, especially when it comes to the design rights of their rotorcraft parts. In their defense, many OEMs have accused PMAs of not looking at the full picture and not respecting the parts’ design limitations, weight, form or engine-effecting fuel-consumption rates.

However, many OEMs complaints are motivated by economics, not ethics. PMA parts are 25 percent to 40 percent less expensive than OEM parts. Because of this, many OEMs have been forced to bring down their prices on parts. “Many operators have complained to me ‘why is it I used to get a PMA part at 50 percent of the cost of the OEM part and now I am only getting a 10 percent saving?’” said Dickstein. “I heard that from a fixed-wing carrier. Then he showed me that the PMA part stayed the same price and the OEM part had come down in price—exactly what we have learned in economics. Competition forces competitors to find pricing that represents true market pricing.”

Able Aerospace Services claims that when it opened in 1982, helicopter operators demanded repair solutions for expensive parts that the OEM required be scrapped and replaced only with new OEM parts. “We repaired those parts, returned them to the OEM and charged $300 each,” said Mongiovi. “The OEM inspected them and charged the operators $10,000 each. That is an ethics violation that led us to enter the business of safe, approved component repair, overhaul and replacement parts. This is why monopolies are prohibited in the United States.”

FAA published a significant revision to the U.S. manufacturing regulations on October 16, 2009 and the new rule goes into effect April 16, 2011. This new rule eliminates some of the legal distinctions between forms of production approval issued by FAA, further demonstrating FAA’s support of PMA manufacturers’ quality systems.

Specifically, instead of having a separate body of regulations for a PMA Fabrication Inspection System (FIS), as was the case in prior regulations, the PMA regulations now include a cross reference to 14 CFR 21.137. This is the quality assurance regulation defining the elements of a quality system for all production approval holders.

In practice, all production approval holders were held to the same production quality standards before the rule change, this will now be more obvious in FAA regulations. “Most of the major PMA companies have investigated this very thoroughly and have reported back to me that they aren’t doing anything to change because they are ‘already there,’” said Dickstein. “Much of this was already either required by FAA order 8110.42, or if not required, voluntarily implemented by companies who want to meet high levels of safety. We are in an industry where safety sells and there is no surer way to go bankrupt than being accused of being unsafe.”

Dickstein continued, “The advantage of merging them all in the same regulation is it is easier for the FAA to put out one set of guidelines and have it apply to everyone uniformly. The advantage to the PMA industry is now they don’t have to go to customers and explain why its system is the same as an OEM’s system. Now, they just point back to the regulation and say we’re both meeting the same regulatory requirements. The hypothetical disadvantage could be if a PMA company can find a reason why the safety quality system should be different, now it is going to be much more difficult to impose such a difference. However, the practical reality is both PMA and production-certification (PC) companies have been meeting the same basic quality regulatory standards, which is the reason why the transition to the new system is not really much of a transition for most companies.”

Not only are cost savings and safety assurance advancing the rotorcraft parts aftermarket, several other factors also are pushing its growth. OEMs are concentrating on newer products and often do not support older products. Without PMA parts, many aircraft would be grounded, Zimm says. Also, PMAs are growing in acceptance and popularity because of better just-in-time delivery and shorter lead times over OEMs.

Improved part performance/durability is one of PMA’s best drivers. If a part needs to be replaced every 1,000 hours and a PMA can come in with a different material that achieves the same safety ends and only needs to be replaced every 10,000 hours, you are looking at something that will only need to be replaced three times in the rotorcraft’s lifetime, instead of 30 times, Dickstein says. There are numerous examples of OEMs copying the design improvements originally introduced by a PMA company, Ritter says.

PMA parts are largely considered to be a U.S.-sourced item. FAA has devoted a lot of resources ensuring that PMA manufacturers live up to the same standards as their PC-holding peers. Not only is the aftermarket thriving here, but PMA regulatory structures and approvals are being granted in other countries. Australia and China have been developing and issuing PMAs. To ensure safety, Dickstein stresses that as PMA propagates into other countries and other regulatory authorities, it’s important for potential installers of PMA parts to look at the differences in the regulatory systems. “I am hoping that other countries will follow FAA’s model,” he said.

FAA currently allows foreign manufacturers to export modification and replacement parts to the United States, if an applicable bilateral agreement exists. Foreign manufacturers are expected to include documentation of airworthiness with their parts.

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