Thanks to the Federal Aviation Regulations, we Part 91 operators have had it pretty firmly pounded into our heads that aircraft inspections happen mainly one way: once a year, like filing tax returns and actually flying an NDB approach. About every 12 months, we open up our birds and see how they're ticking. Sure, there are other inspections, based loosely on oil changes, signs of mechanical distress, or airworthiness directives. For the most part, though, the airplane stays buttoned up for a year at a time.
Not every operator tends to maintenance matters this way, though. Some go the 100-hour-inspection route. According to the FARs, a 100-hour inspection carries the same weight as an annual; in fact, the terms are used interchangeably in the regs. The main difference is that the Airframe and Powerplant mechanic doing the work can sign off on the 100-hour, while an annual requires the signature of someone with inspection authorization.
There's still another tack, called progressive maintenance, where certain systems and components are inspected at varying intervals, coincident with their importance in overall airworthiness, reliability, and accessibility. Most airframe manufacturers publish a specific progressive-maintenance plan. In order to switch from annual or 100-hour inspections, you must contact your local Flight Standards District Office for prior approval. Then, usually, the airplane must undergo a standard annual inspection, just to make sure the progressive method starts out with an airworthy airplane. Progressive maintenance tends to work well for high-usage airplanes, but isn't usually as effective or economical as the annual for low hours-per-year birds.
Just because you are operating your airplane under the annual- inspection rules, doesn't mean that it should only see a mechanic once a year. Depending upon your type of airplane and the kind of flying you do, it might be prudent to schedule ad-hoc inspections between annuals, and we don't mean the "change the oil; yep, the engine's still attached to the airframe" type, either.
Fortunately, for the maintenance-minded owner, these interim inspections tend to include items listed under FAR Part 43, Appendix A, as preventative maintenance, which may be performed by the aircraft's owner or operator without supervision. For those of you unfamiliar with the tasks outlined in this appendix, or how to perform them, contact your local friendly A&P/IA for some supervision.
To give you some idea of how this works, consider the maintenance schedule I have been implementing on the Mooney 231 I fly. The airplane, operated under Part 91 rules, is thoroughly inspected annually (see " Airframe and Powerplant: Helping Hands," August Pilot), generally with owner Mark Reno and I doing the majority of the grunt work.
Between annuals, something different takes place. Essentially, we drag out the check list for the annual/100-hour inspection requirements vis-a-vis the engine and its related systems and perform those items. This happens as a rule every 50 hours — conveniently at the oil change — although many items could probably be deferred to 100-hour intervals, given the airplane's 200-hour years. If you fly less than that, consider making this inspection every six months. (Though oil-change intervals vary by engine, oil, and filter type, most mechanics agree that the lubricant ought to come out at least every six months, even if the recommended hourly figure has not been reached.)
Also, apply the following guidelines according to the complexity of your airplane. You may only need to make cursory inspections on something simple, like a Cessna Skyhawk, but you might want to take a full day to complete the checks on an airplane like the Beech Duke. Turbocharged airplanes, especially those routinely operated at high altitudes, should come in for more thorough inspections. Finally, go into the interim inspection thinking about how the airplane has been running. Are the oil and fuel consumption normal? How about speeds and climb rates? If the airplane is a turbo, is the critical altitude in line with book figures? Does the wastegate work correctly? If it's the only airplane you fly, these items should be a snap to identify; if you use several mounts, consider writing down some of the variables for reference on the flights preceding the inspection.
The vast majority of the checks can be performed without removing much more than the cowlings and spark plugs — both of which are covered under preventative maintenance — and so don't require the mechanic to be present. (He is, however, on call should we find something that needs to be fixed.) Now, there is no mention in the logbook as to the airplane having been inspected and returned to service as though it had just undergone an annual or 100-hour.
We start, just as with any annual, by uncowling the engine and giving it an eagle-eyed look-see. We check the condition of the cowling and its attachment hardware, as well as examining the cooling-system baffles. Should we espy excessive oil leaks — I say excessive because this 1,200-hour turbo Continental just about sweats oil; if someone out there knows how to keep one perfectly clean, let me know — we'll stop and try to determine the cause. Ditto for the fuel system; any obvious leakage or stains are cause for calling in the A&P.
Should nothing appear awry, we'll clean the engine and begin a thorough, flashlight-guided check for cracks and general condition. We know that this particular model tends to crack cylinders, so we'll spend extra time with a hand mirror and a powerful flashlight looking for these. You probably already know where your engine's weak spots are, and that's always a good place to begin the general inspection.
With the oil change under way, we'll remove the spark plugs to check on their condition as well as to prepare for the compression check. By itself, the leak-down test is only a marginally enlightening bit of information, but when linked with ongoing spectrographic oil analysis and a compression-test history, it offers an up-to-the-minute glimpse of the top-end's health. Should, say, a cylinder go a bit off song, we'll usually run the airplane for five or 10 hours and check compression again; should it go way soft, we'll call the mechanic and investigate further.
Turbocharged airplanes should have their turbo and exhaust systems inspected at this time. The exhaust stacks should be free from cracks, and the slip joints should show little, if any, exhaust stains; some soot is typical on some installations, so it pays to know your airplane.
Once the engine compartment has been tended to according to the maintenance manual's inspection recommendations, we turn our attention to the airframe. The Mooney is relatively simple systems-wise, with durable and trouble-free gear, so we usually just have a careful eyeballing of the exposed components, including the brakes. The hydraulic reservoir is topped off and the battery examined while we're meddling about in the tailcone. Much of this simply constitutes a very thorough preflight; unless there's some compelling reason, none of the airframe inspection panels come off. Your airplane will, of course, have other needs; Cessna 210 owners might want to pay special attention to the landing gear and its associated plumbing, as just one example.
Finally, we finish up in the engine compartment with fresh oil and a new filter — double-checking the safety wire, thank you — and perform a ground check, looking for leaks and listening for expensive sounds.
That done, we put the cowling on — trying mightily to remember the landing-light connectors — and fly the airplane locally for an hour or more. Doing so confirms that all the work has been performed correctly and that we have not broken anything while we were at it. (I'm amazed at the number of owners I've chatted with who plan business trips right after maintenance work and don't get the chance — or take the time — to do an in- flight operational check. It's time very well spent.)
So just what is the purpose of all this? In 1992, while performing one of these between-annual inspections, eagle-eye Reno found cracks in two cylinders. The airplane had been flying regularly and had shown no signs of distress; the next annual was seven months away. Would the cylinders have held together until the next oil change or, worse, to the next annual? Maybe. Do I or Reno want to let something like that go unnoticed for six months or more? Not in the slightest.