Are service documents mandatory? No. Are they a good idea to implement? It depends, but for the most part, yes, and you’ll soon discover why.

Corrosion comes in all shapes and sizes. It is a natural process in which metals deteriorate due to chemical reactions with environmental elements, such as oxygen, moisture, and pollutants. In aviation, corrosion can manifest in various forms, including surface, pitting, intergranular, and stress corrosion cracking.

Each type poses significant risks, potentially weakening the structural integrity of the aircraft and leading to safety hazards. We’ve recently seen how corrosion affects wing spars like with the Piper PA-28 and Cessna 177 and 210, for example. These started as Service Bulletins (SBs) to inspect for corrosion and escalated to mandatory compliance in the form of Airworthiness Directives (ADs).

• READ MORE: How Do I File a Pilot Weather Report Online?

The implications are severe. Corrosion can lead to structural failures, increased maintenance costs, and, in worst-case scenarios, catastrophic accidents. Once you have the basics and understand the theory, you are ready to progress to the next level.

That’s right, it’s aircraft maintenance time, and here’s one example.

Learjet 45

The Learjet is legendary among corporate aircraft—produced from 1964 to its unfortunate scuttling in 2021. Learjet became synonymous with business aircraft in the early days of private business travel.

“Learjet models are known for their exceptional performance, speed, and range,” said Sky Aircraft Maintenance. “Airframe issues can be a common maintenance concern for Learjet aircraft. Due to the high speeds and stresses placed on the airframe during flight, wear and tear can occur over time, leading to a need for structural repairs. This can include corrosion.”

On July 13, 2007, the Australian Civil Aviation Authority released AWB 57-004 Lear Jet Industries 45 Wing Corrosion. The agency said this correspondence was needed because “recent reports have been submitted indicating that corrosion has been found on the lower skin of both wings fitted to the Lear 45 aircraft. This corrosion resulted in the replacement of the entire lower-wing skins.”

Years later, in February 2019, Learjet, now owned by Bombardier, released a series of Service Bulletins addressing “wing spar inspection.” The reason? Corrosion had been observed on the lower-wing splice plates, requiring a more frequent interval to detect and correct protective coatings.

Remember our chat earlier about adhering to the manufacturer’s recommendations? 

• READ MORE: What Does It Take to Transition From a Jet to a Piston Airplane?

Fast-forward a bit and the series of Learjet bulletins are now the FAA’s AD 2021-23-08.

What prompted this escalation? According to the AD, exfoliating corrosion was found on a particular Lear 45 upper surface of the lower center-wing, midspar splice plate during unrelated maintenance. The corrosion appeared to extend halfway through the thickness of the splice plate. Since the initial report, the FAA has received 23 additional accounts of corrosion from Learjet.

Jerel Bristol, owner of SEAL Aviation in Hollywood, Florida, was not surprised when the call came in. Bristol is aware of the trouble that Learjets have with wing-spar corrosion and knows the AD well. His team deploys to aircraft or ​AOG situations for mobile fuel leak repair, nondestructive testing, and structural repairs anywhere in the world.

During the center-wing inspection, a SEAL technician identified corrosion on the center-wing splice plate. I spoke with Bristol, and he said that it is a common area to find corrosion past repairable limits, which requires the replacement of the forward and aft splice plates.

After pulling the affected parts off the airframe, the SEAL team quickly repaired the area and replaced the damaged parts. The pictures reveal a sea of cleco fasteners. 

The guys buttoned up the Lear, and the owner was wheels-up again. 

The Cause

The big question remains: What causes corrosion?

One follower commented on a SEAL Aviation webpage post about the Lear 45 repair, stating that brine used for deicing could have contributed to the corrosion. He is not far off. Environmental elements can contribute to aircraft corrosion.

These factors include:

• Humidity and moisture, particularly in coastal regions.

• Temperature fluctuations which can cause condensation.

• Exposure to deicing fluids and other chemicals.

• Poor maintenance practices and infrequent inspections.

The environment is not the only player in the corrosion game. According to Aviation Devices and Electronic Components (Av-DEC) in its article “Causes of Corrosion,” industrial pollutants are equally harmful and can be difficult to protect against.

These include several contaminants such as:

• Ozone (exposure from high altitude, motors, and welding)

• Carbon compounds (exposure from combustion engine exhaust)

• Sulfur dioxide (exposure from engine exhaust, smokestacks, and acid rain)

Operators and GA aircraft owners alike are well advised to take heed when an SB shows up in the mailbox. A recommended inspection, especially when it can be coordinated with other scheduled or unscheduled maintenance, may help ultimately reduce the total cost of ownership and down time if/when an issue becomes an AD.

Perhaps the best reason to take a closer look is personal safety and peace of mind through identifying a problem before it manifests in something tragic.

This column first appeared in the September Issue 950 of the FLYING print edition.

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The FAA has issued an airworthiness directive (AD) for certain Robinson Helicopter models: R22, R22 Alpha, R22 Beta, R22 Mariner, R44, R44 II, and R66 because of problems in the field involving the tip cap of the tail rotor blade.

According to the Notice of Proposed Rulemaking (NPRM), the AD (FAA-2023-2232) was developed after three field reports about TRB tip caps coming loose due to corrosion at the bond on Robinson Helicopter Co. (RHC) Model R44. The incidents occurred during a run-up check, after landing, and during landing on different helicopters.

The similarity of the TRB caps on other Robinson Helicopter models resulted in the expansion of the AD beyond the R44 as a precaution.

According to RHC, there have also been reports of TRBs that have “corroded to an unserviceable condition, including severe leading-edge pitting and degradation of the bond at the tip cap.”

Affected TRBs were factory-installed or shipped as spares before November 2022. 

Compliance with the AD requires visually checking and inspecting certain part-numbered and serial-numbered TRB tip caps for evidence of corrosion and, depending on the result, removing the corrosion. The AD necessitates removing all affected TRBs from service and prohibits the installation of them on any helicopter.

The FAA estimates the cost of compliance with the AD will be $85 per hour. Visual checks of the TRBs will take approximately 0.25 work hours for an estimated cost of up to $22 per helicopter cycle.

The FAA also estimates the AD will affect about 2,701 helicopters in the United States.

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When installed, this type of altimeter provides information critical to situational awareness, including the aircraft’s altitude in absolute terms above the ground. This is particularly important during approaches and departures in inclement weather. 

In the published ADs, the FAA states that radio/radar altimeters (RadAlt) cannot be relied upon to perform their intended function if they experience interference from wireless broadband operations in the 3.7-3.98 GHz frequency band, also known as the 5G C-Band.

The ADs, published on the agency website, were created “to provide a framework and to gather more information to avoid potential effects on aviation safety equipment.

“The FAA is working closely with the Federal Communications Commission and wireless companies, and has made progress toward safely implementing the 5G expansion.”

The ADs also require the revision of the limitations section of existing airplane/aircraft flight manuals, where they reference the use of radio or radar altimeter data when in the presence of 5G C-Band interference as identified by NOTAMs.

• READ MORE: Pressure Mounts on Feds to Delay 5G C-Band Deployment

The NOTAMs will be issued at specific airports where radio/radar altimeter performance is determined to be unreliable, thereby prohibiting certain operations such as: ILS approaches, required navigation performance (RNP) procedures, and automatic landing operations, as well as use of enhanced flight vision system (EFVS) or a heads-up display (HUD), which are used to guide an aircraft from approach all the way down to the runway.

The ADs were not unexpected. In November, the FAA released a special airworthiness information bulletin providing initial information about possible interference from 5G C-band wireless transmissions.

How We Got Here

In March 2020, the FCC adopted final rules authorizing flexible use of the 3.7-3.98 GHz band for next generation services. This includes 5G and other advanced spectrum-based services. Pursuant to these rules, C-Band wireless broadband deployment was permitted to occur in phases with the opportunity for operations in the lower 100 MHz of the band in 46 markets. 

The deployment was slated to begin as soon as December 5, 2021; however, upon request of the FAA, telecommunications providers AT&T and Verizon voluntarily agreed to delay their plans for deployment of 5G until January 2022, as the technical details are worked out among communications and aviation industry leaders across the world. The ADs (AD 2021-23-12 and AD 2021-23-13) can be read in their entirety here. Comments are encouraged.

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In March 2020, Flying reported on an airworthiness directive focused on corrosion concerns on most models of the Cessna 210. On Tuesday, May 11, the FAA published a Notice of Proposed Rulemaking (NPRM) that may broaden the scope of that original AD to include both the Cessna 210R and N models, as well as all models of the Cessna 177 Cardinal. An NPRM is a preliminary announcement of the FAA’s intentions that could lead to a new AD. The agency will base its final decision to proceed onto a new AD after reviewing the user comments that the NPRM is requesting.

This proposed AD was prompted by the May 26, 2019, in-flight break-up of a Cessna T210M in Australia—the result of fatigue cracking that initiated at a corrosion pit, as well as subsequent corrosion reports on other Model 210-series and Model 177-series aircraft. The T210 experienced a wing separation after the carry-thru spar failed, and a subsequent loss of control in-flight while performing low-altitude aerial survey work.

The proposed AD to address the unsafe condition on these aircraft would require visual and eddy current inspections of the carry-thru spar lower cap. Corrective action, if necessary, calls for the application of a protective coating and corrosion inhibiting compound (CIC), as well as reporting the inspection results to the FAA. If a crack is identified, the carry-thru spar must immediately be removed from service. The FAA requests to receive comments on this NPRM no later than June 25, 2021.

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The FAA recently published an airworthiness directive—2020-26-16—covering many single-engine Pipers that carry either a PA-28 or PA-32 model number. “This AD was prompted by a report of a wing separation caused by fatigue cracking in a visually inaccessible area of the lower main wing spar cap,” the directive said. “This AD requires calculating the factored service hours for each main wing spar to determine when an inspection is required, inspecting the lower main wing spar bolt holes for cracks, and replacing any cracked main wing spars. The FAA is issuing this AD to address the unsafe condition on these products,” adding that failure to comply with the directive could lead to failure of a main wing spar.

The AD will require the installation of inspection panels close to the main wing spars to give technicians access to that area when searching for corrosion. “After the NPRM was issued, Piper revised its service information to add a minimum thickness dimension for the top inboard wing skin and to include procedures for reapplying corrosion preventive compound if removed during the inspection,” the AD continued. “Also, at the request of some commenters, the FAA replaced the proposal in the NPRM to install access panels for the visual inspection with optional access methods: The use of existing access panels, installation of access panels, accessing the area during a concurrent inspection, or using a borescope through existing holes or openings. In the SNPRM, the FAA proposed to inspect the left and right main wing spar for corrosion, and, if corrosion is found, take all necessary corrective actions.”

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On August 28, the FAA published an emergency airworthiness directive (AD) 2020-18-51 for certain Sandia attitude indicators with a part number (P/N) 306171-10 or 306171-20 installed, as well as those marketed as BendixKing Model KI-300 or Sandia Model SAI-340A.

This emergency AD was prompted by an initial report of three failed attitude indicators (P/N 306171-10) and, after a thorough investigation, a total of 54 failed attitude indicators with the same part number. The failure of these units can create erroneous attitude data provided to the pilot and autopilot, if equipped. In some instances, the pilot may be unaware the data is erroneous or unreliable. In other instances, where the aircraft is equipped with multiple displays, the pilot may be provided with conflicting information, but will have no way to determine which display contains the correct data.

If not addressed, these failures could result in aeronautical decision-making based on erroneous attitude information, which may result in loss of control. The emergency AD requires, before further flight, revising the existing airplane flight manual for each affected aircraft prohibiting operation under IFR or night VFR, as well as prohibiting these units to couple with the autopilot. As it relates to this AD, attitude indicator P/N 306171-20 is identical to P/N 306171-10 and is also affected by the same unsafe condition.

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As announced on August 4, general aviation avionics OEM Aspen Avionics has been acquired by The AIRO Group. The new owner has a fairly deep bench in aerospace, holding a portfolio of companies in a wide range of aerospace segments, including commercial and military, and piloted and remotely-piloted vehicles.

Flying spoke with Aspen’s CEO John Uczekaj about the acquisition, the state of the company’s general aviation business, and a recent airworthiness directive aimed at an inflight reset issue involving the Evolution MAX series flight displays.

“Our business has been focused in the GA market, driven by the technological changes and the competitive nature—that’s the main draw for tech companies” in the space, says Uczekaj. With the Part 23 rewrite giving more access for technology into certified aircraft—all along Aspen’s core business—the company has been poised to respond. “The customer base is demanding of new technology—the GA pilot is expecting a lot now from their aviation platform.” Aspen has survived the downturns of 2008, 2014, and now the COVID-19 pandemic, and though it has benefited greatly from accessing funds via the PPP loan program via the CARES Act, it needs new investment dollars to continue to grow.

“We saw a major downturn in April and May,” says Uczekaj, especially in the international business, but people are now showing signs of emerging from the uncertainty a little bit more comfortable spending the money on panel upgrades—balancing a use of the downtime with a desire to get back into flying. Uczekaj’s also an active pilot, and he estimates that, though his own activity is off for the year like so many of us, the local airports in Albuquerque are getting busy again. With business slowly coming back out of the darkness since May, Uczekaj notes that the company has seen the same number of open queries as before the pandemic.

Seeking a good partner for those needed investment dollars—and to demonstrate a higher growth rate to feed more into the loop—Aspen found a like-minded collaborator in The AIRO Group. Uczekaj has watched the UAV and urban air mobility segments “explode” in terms of growth—and an avionics company with deep experience in regulatory compliance, tech development, and systems integration makes them an attractive partner for AIRO. It’s in Aspen’s DNA to innovate forward.

One lingering issue Aspen has addressed stems from a trend of in-flight reboots in certain Evolution MAX series installations—which the company alerted the FAA to and issued a service bulletin for back in March. On July 31, the FAA came out with an AD to cover the issue, though Aspen had released a software fix (2010.2) earlier this spring. According to Uczekaj, the reboot is rare and only raises a concern with multi-tube installations, with a backup battery, for those lacking full redundancy. For more information, contact Aspen if you require clarification on the text of the AD.

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