Flying Friday: Carmen goes to the doctor

A couple of weeks ago I dropped Carmen off at the airplane doctor for an annual inspection. “What’s that?” said no one. Well, I’ll tell you what it is, since you didn’t ask…

The FAA requires regular inspections of most airplanes. That sounds vague, but the truth is that the requirement varies according to what the airplane is used for, whether it’s an experimental or amateur-built (E/AB) airplane, and so on. For my purposes the FAA regulation that applies is FAR 91.409:

…no person may operate an aircraft unless, within the preceding 12 calendar months, it has had (1) An annual inspection in accordance with part 43 of this chapter and has been approved for return to service by a person authorized by § 43.7 of this chapter

That seems straightforward enough. Back in February, I hired a mechanic in California to perform what’s commonly called a “pre-purchase inspection”, and then I had the excellent experience of taking Carmen to one of the service clinics sponsored by the American Bonanza Society, where master mechanic Wayne Whittington did a very thorough checkout using the ABS clinc checklist. However, neither of these are considered to be an “inspection in accordance with part 43,” as the FAA says above.

FAR part 43 annual inspections are required every 12 calendar months; the last one on this plane was done 4 November 2021, so a new one is required before 30 November 2022– you get until the end of the month when the last annual was signed off. It’s bad to let a plane go “out of annual,” because legally it is considered unairworthy and can’t be flown without a special permit at that point. It’s easier and cheaper just to make sure that doesn’t happen.

One part of the annual ritual of inspection prep is to make a comprehensive list of all the things you might want to have the mechanic inspect, fix, or change. This is just a starting point, because more often than not a thorough inspection by a trained professional will find things that the pilot’s missed. Here’s the list I gave to Jon, with my notes to you in italics:

  1. Annual inspection per the Beech maintenance manual (this manual is what FAR part 43 says you have to use to make the annual legal)
  2. 3 common items that seem to have been done each preceding annual. (I didn’t check to see if these were in the manual or not)
    • Check and service unfeathering accumulators (these accumulators are small tanks of nitrogen that apply pressure on demand to push the propeller out of its “feathered” position)
    • Check and service shimmy dampener (think of this dampener like a horizontal shock absorber mounted on the nose gear to keep it from wobbling)
    • Borescope inspection of all cylinders
    • Compression test of all cylinders. (If any are low, we’ll talk about whether to replace the cylinder or try another way to fix the underlying problem, depending on whether it’s the rings, the intake valve, or the exhaust valve)
  3. Fix a small crack in the skin of left aileron. (Although this sounds scary, it’s not; the crack is in a little piece of cosmetic metal that sits over a bracket to hide it, not part of the structure of the aileron)
  4. Replace the instrument air filter (this filter keeps crap from the carbon vanes of the vacuum pump from getting into the vacuum-driven flight instruments)
  5. Verify compliance with AD 2007-08-08 and AD 91-17-01 (more on this below)
  6. Oil change both engines 
  7. Replace main tires 
  8. Service brakes, using new pads; replace discs if necessary 
  9. Replace fuel cap O-rings (each fuel cap has two O-rings to help seal it, and these wear out over time)
  10. Install fluorosilicone washers on 2 main tank fuel caps
  11. Check front seat inertial reels; fix if needed (the pilot side seems awfully slack)
  12. Fix socket/bulb/wiring of leftmost glareshield panel light
  13. Fix “hitch” in electric trim in “Down” mode—it is binding or hanging. “Up” mode works OK, manual trim in both directions works ok.

A good mechanic will help you identify things that are legitimate airworthiness or safety issues and prioritize those. In the above list, probably the most safety-critical item is the seat belt reels– the accumulators, dampener, oil change, air filter, and so on all need to be done eventually. I’m thankful that the prior owners maintained Carmen to such a high standard so there aren’t any known lurking horrors– but Jon may find some, which is the whole point of doing annual inspections.

One key part of the annual is that the mechanic will verify compliance with airworthiness directives, or ADs. These are important maintenance actions that must be complied with– for example, AD 2007-08-08 requires periodic inspection of a part that locks the landing gear in the “up” position to ensure that it will work properly when needed. This inspection must be done every 100 hours; there are other ADs that must be performed periodically, so Jon will check all of the ADs that apply to this plane (and its engines, and propellers) and make sure that they’re complied with.

I also decided to have the propellers overhauled. The manufacturer recommends that these particular props be overhauled every 6 years or 2400 hours. These props were installed in 2008 and have close to 2400 hours on them, so it’s time– but, as with engines, the “time between overhaul” (TBO) is a recommendation– for ordinary general aviation operations under part 91, owners aren’t required to observe that TBO. It’s legal for me to keep flying engines and propellers as far past TBO as I want to. With engines, you can do compression checks, borescope inspections, and oil analysis to get a good idea of the engine’s health– but with propellers, you can only see the visible parts of the prop, and that’s it. There’s no good way to non-destructively inspect the propeller’s components, other than the blades. That’s because the propeller hub isn’t visible when the propeller’s mounted, not to mention that it contains a bunch of seals, springs, and other parts. That’s because this plane uses constant-speed propellers, where the propeller governor changes the blade pitch so that the propeller maintains the commanded RPM even as the engine power changes. (For more on how constant-speed propellers work, this explainer is pretty good). With all that in mind, I decided to go ahead and have the props overhauled since the plane would be down The overhaul process can take a while– the shop I chose, First Flight Propellers in Mississippi, is currently estimating about 4 weeks. This seems like a long time, but the overhaul process is quite involved, plus once the overhaul’s done I’m good for another 6+ years.

Once the propellers have been overhauled, Jon will reinstall them, run the engines for a leak check and general test, Then I’ll pick the plane up when the weather is good and do a test flight of at least 30 minutes, staying in the vicinity of the airport just in case something is amiss. Assuming that goes well, I’ll fly Carmen back to the hangar and enjoy another year of being in compliance with 91.409. I’m looking forward to it!


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