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Going deep on C172 maintenance

The widely popular Cessna 172 series have proven to be very reliable and comfortable as a training aircraft and remain the most familiar model in the Brantford Flying Club fleet to this day. The 172 Skyhawk is undoubtedly the most well-known single engine production aircraft of all time making it the perfect choice for flight schools and fair weather fliers alike. The venerable N model (as used in our fleet) was produced from 1977 until 1980 and followed by the similar P model up until Cessna temporarily suspended production of its single engine line beyond 1986. Cessna were also moving into a new era around 1980 with a timely upgrade to 28 volt electrical systems. The two-place C-150 evolved into the C-152 which continued from 1978 until 1985. They were also busy with the introduction of specific model variations from the long-established 172 template to become known as the Hawk XP and the Cutlass RG. Both of these variants had a limited production run but many are still active today.

One notable drawback of the C-172 N model was its new factory installed (and soon to be troublesome) Lycoming O-320-H2AD engine. It would be this engine that was blamed for a dark cloud over the 172N prompting a valve train upgrade known as the T-Mod. Even with the proper fix established for existing H series engines, it seemed that buyers were still shy with this model airframe due to its unfortunate engine reputation. As more time passed and subsequent engine design improved, the stigma over the N model eventually subsided and buyers were just reminded to verify incorporation of the T-Mod, the oil additive AD, or consider an STC for a newer bulletproof model to be known as the Lycoming O-320-D2J that became standard on the C172P from 1981 to 1986. The 180 HP O-360 also became a great option.

Maintaining the Cessna 172 has become a commonplace activity for the hangar crew at the Flying Club as we are tasked with performing a detailed inspection every 100 hours of flying. We also perform a simpler 50 hour check in order to change the oil, check tire and brake condition, check operation of lights, and other regular servicing items. A typical 100 hour inspection for a commercially operated aircraft is performed in accordance with a pre-approved maintenance schedule which references a standardized check sheet of inspection items. This routine is repeated at every 100 hour interval year after year looking at the same targeted areas while dealing with other unscheduled work along the way. As a result, these training aircraft could almost be considered over-maintained if not for the fact that this ongoing attention includes key preventative actions, defect monitoring and regular cleaning. These aircraft will continue to provide many years of service provided that we stay on top of the required inspections as well as the out-of-phase items, applicable Airworthiness Directives, Service Bulletins and at some point, specific structural inspections to address fatigue and corrosion in higher time airframes. This is where we get into the inevitable territory of the not-so-new aging aircraft requirements.

Cessna had introduced their Continued Airworthiness Program (CAP) for the 100 series legacy aircraft back in the mid 1990’s. This program includes very specific inspections that were deemed necessary in order to ensure the continued airworthiness of aircraft based upon specific calendar time and/or hours in operational service. The CAP program supplemented the standard aircraft service manual and is recommended for aircraft that meet the documented criteria. Not only did the targeted aircraft remain strong enough for continued service several decades after manufacture but they were also being utilized in ways that were not accounted for within the basic inspection program. Aircraft were being operated in various climates throughout the year, they were often being used for consistently low level aerial survey, and a growing number of flight schools were adopting the simpler 152 and 172 series as practice machines for students to use every day as much as possible. These and other extreme operating environments prompted the need to study newer structural concerns based upon years of data research and testing of what they designated as Structurally (and System) Significant Details (SSD). These newly incorporated inspections steered maintainers beyond their usual suspect areas and into the deeper nooks and crannies of the vulnerable aluminum structure.

Meanwhile, Cessna also encouraged an open exchange of the structural findings generated from these special inspections so that the industry could meaningfully affect the data collection process associated with their field reports. Many of the CAP items also reference Service Bulletins which already contain specific instructions for previously known issues. The strict regulatory requirements (from the CARs) pertaining to commercial operators prompted many to gradually adopt CAP inspections into their approved schedules while most private owners responded solely at their own discretion.

Over time, the manufacturer continued to expand the technical database for aging aircraft since the implementation of the CAP program and went to work evaluating further useful criteria. Cessna then began incorporating a more comprehensive aging aircraft program known as Supplemental Inspection Documents (SID) which were integral to the service manual through the established revision process. The company identified their known subject areas as Principal Structural Elements (PSE) which implied that any structure whose failure, if remained undetected, could lead to more serious issues or even loss of the airplane. These PSE references have been influenced by the susceptibility of a structural area, part or element to fatigue, corrosion, stress corrosion or accidental damage. While some of these areas can be easily detected through basic inspection, the emphasis of the SID is to detect fatigue damage whose probability increases over time.

The evaluation criteria used by the SID program is further explained through a better description of the prevalent operating environment as being either Typical or Severe. A severe operating environment may include regular low altitude aerial patrols. A corresponding description of the corrosion environment can be described as Mild/Moderate for average climates or as Severe for regular operation in extreme coastal areas. A Corrosion Prevention and Control Program (CPCP) baseline must then be established to determine a starting point for this program. The aging aircraft theory has simply built upon the time-in-service model to now include key operating factors as well. The SID program shall be initiated only after the aircraft has reached a specific number of operating hours or calendar time in service. The program therefore begins with an initial or baseline inspection to determine the condition of specific and/or possibly unfamiliar areas and then repeat the inspections at a frequency determined by the document. While structural integrity and service longevity is determined by each scheduled inspection, Cessna has specifically recommended that their airframes be retired upon reaching 30,000 flight hours as continued airworthiness at that stage can no longer be assured.

The Brantford Flying Club had previously adopted the applicable CAP requirements for the fleet and have recently also adopted the SID items as part of their revised maintenance schedules. Our first application of the SID program was on our C-172N model C-GBSL as it became the prime candidate for an in-depth assessment after recently passing the milestone of 20,000 total flight hours. The aircraft has also been looking a little worse for wear in terms of paint, plastics and other visual aesthetics.

Besides tearing down the aircraft in order to gain access to all inspection areas, one of the most significant exercises within this process would be complete removal of the wings and struts to facilitate Non-Destructive Testing (NDT) in the form of Eddy Current methodology on all wing and strut attachment points. This step of course aided with other related inspections of the fuel system, flight control systems and wing structure. The NDT service was provided by a sub-contracted organization who are certified for the type of NDT inspection required by the inspection document. That was our first big win as the failure of any of these attachment points would require some serious work to have them replaced. The results were all good and the time it took to perform the tests was only a fraction of the time it took to actually prepare the area for inspection.

In order to gain complete access to the interior of all airframe (cabin) skins, all of the carpet, side upholstery, plastic trim panels and headliner would be removed from the aircraft. All exterior access panels were also removed so that certain areas of the aircraft could become accessible for the various inspections. The empty cabin allowed us to also clean up the instrument panel, eyebrow and dash material. The open cabin area also permitted easy access to the fuel venting system and the rubber hose connectors which were also replaced at this time. A fuel supply line was found to be chafing on the structure and had to be replaced. These kinds of tasks simply represent one of the clear advantages of going deeper into the inspection process.

Many of the interior plastics were repaired and others had to be replaced. Most of the Club aircraft have been fitted with a new style of cabin air vents (at the forward interior wing root) so that we could get rid of the frustrating old style pop-can contraptions that were forever leaking air or simply falling out. The interior upgrade also included a new carpet and two new seat tracks in the right seat position.

Early in the planning stages of this work, it was determined that the rudder, elevators, and ailerons would all be removed as much of the paint had been weathered off leaving the bare metal exposed to the outside elements.  This work would also be sub-contracted to a properly equipped paint shop once we had stripped much of the existing paint ourselves to cut a few hours off of this labor intensive task. We deliberately took advantage of these items going to the paint shop for the white base color only and included a few other pieces, including the top engine cowling for a fresh coat as well.  Once all of the necessary NDT and paint work was completed, we re-attached the wings and flight controls with new hardware and bushings as necessary.

Many of the exterior plastic tips and fairings were also touched up and/or replaced as needed. We have become more proficient at repairing the plastic trim pieces used on our aircraft so that the cost of replacement can be reduced to a more reasonable cost for simple yet effective repairs. Some of the acrylic cabin windows were re-worked with a product called Micro-mesh which removes scratches to restore good visibility and cleanliness. The front windscreen was replaced with a new one altogether and it looks quite remarkable upon completion with absolutely no marks in the surface whatsoever. All windows are finished with PRC sealant around the perimeter to prevent moisture from seeping into the cabin area. The proper application of approved paints and sealants are good examples of small details that could go a long way in the prevention of future problems.

All of this work to apply the CAP and SID items was being done as part of a 100 hour inspection which made sense considering everything was being opened up anyways. We also performed many of the upcoming out-of-phase items from our maintenance schedule. Attention was also given to the main landing gear, attachment structure, and especially the nose gear oleo assembly which was completely rebuilt with new seals and hardware as needed. The previously recurring AD’s and SB’s were also completed as detailed in the corresponding technical log entry.

Performing these types of inspections on club aircraft requires good planning and patience as certain tasks are prioritized for efficiency within the planned process. It is also an opportunity to not only complete these special requirements but to have a good look at all areas of the aircraft and to better understand its construction and systems. Even the overall presentation and cleanliness of the aircraft is considered with the customer in mind. Each aircraft on the BFC fleet will be subject to these same initial inspections as we move forward and many of the cosmetic details will also be addressed according to time available and operating schedule considerations.

I often hear people within the industry talk about certain work required on small aircraft as a burden on our time and money. While I agree that the work we do needs to be balanced with realistic financial considerations, I also feel that we should willingly take ownership of the required items as well as any work that potentially enhances the overall safety of the product. There has always been a persistent undercurrent of thinking that resists doing the extra work unless it is absolutely mandated. The safety-first mentality is often reduced to a mere sentiment that is not taken literally unless it’s convenient. The AME must take the opposite approach to ensure safety at all times. We are not only looking at compliance for the sake of compliance but consciously using our skills and resources to promote continued airworthiness - and we are already looking forward to the next project!

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