Every general aviation, FAR Part 91, aircraft has to have an Annual or Progressive Inspection every 12 calendar months. The Progressive Inspection is used for aircraft that fly more than 200 hours every year and will not be addressed at this time. The Annual Inspection must be accomplished by an FAA certified mechanic holding an Inspection Authorization. The manufacturer of the aircraft and an FAA Certified Repair Station may also perform the Annual Inspection. The inspection must be done in accordance with FAR 43 Appendix D-Scope and Detail of an Annual or 100 hour Inspection. The 100 hour inspection is identical to the Annual Inspection but can be done by an Airframe and Powerplant (A&P) Mechanic. More about the 100 Hour Inspection at a later date. Now that we have all of the requirements listed, lets see what it takes to do an inspection on a typical single engine airplane.
After gathering the paperwork together(always the paperwork!) and doing a walk-a-round, the engine is started and warmed up. During the warmup, the aircraft is taxied to a safe place, systems are checked for operation, instruments are checked, radios are checked, windows are checked for minute cracks called crazing and on and on.
When the engine is warm enough, sometimes up to 20 minutes in the winter, it is run
to maximum power to check the tachometer, oil pressure, oil temperature, instrument air system pressure(vacuum), alternator, fuel pressure and on and on.
The aircraft is towed (or pushed) into the hangar where the work begins. First the engine cowling is removed enough to gain access to the cylinders where a compression test is done to assess cylinders leakage. This tests the cylinders for cracks, valves and piston rings for condition. The test involves pressurizing each cylinder with 80 psi of shop air and testing to see how much leaks out. It is a procedure that is a bit dangerous as the pressure in the cylinder can make the propeller spin pretty quickly. If the cylinders check good, it makes the owner of the aircraft much happier. Any cylinder that leaks excessively must be repaired. Cylinders can be removed individually for repair.
My next step-each IA has his or her own approach to accomplish the inspection checklist-is to drain the engine oil while it is hot and to remove the oil filter. The oil filter is cut open and the element is exposed to reveal any material that is circulating through the oil system. Normally the filter will have carbon in it and may have tiny pieces of different metals. With experience, the general aviation maintenance technician knows what is acceptable and what is not. If any ferrous metal is suspected, then a magnet is drawn across the filter element to grab that iron for further inspection. Many owners request an oil sample be taken from the drained oil to be sent to a laboratory for spectrometric trend analysis. (more about oil analysis at a later date)
My reason for starting the inspection the way described above is: if a cylinder is bad, it can be removed and be shipped to a cylinder repair shop for repair while the remainder of the inspection is being done; if there is any unusual material inside of the oil filter, the engine may be defective and it does not make any sense completing an inspection and servicing on an engine that may have to be replaced.
If the engine appears to be OK, an inspection of the engine and all of the components in the engine compartment can now be done. Components may include: exhaust system, starter, alternator, carburetor, ignition leads, vacuum pump, magnetos, fuel pumps, hoses, control cables, cowling and on and on. Servicing includes: cleaning engine, clean, gap & test spark plugs, fuel filter, oil suction screen, etc. In our shop, we have more than one person working on the aircraft at once to reduce the amount of time the aircraft will be unavailable to fly.
The next step I take is to research the aircraft records for component times (some have to be replaced after a certain calendar or time in service), Airworthiness Directives compiance (more about AD's at a later date), Pilot's Operating Handbook (POH) currency & completeness, Weight & Balance information current, major repairs forms are present, Supplemental Type Certificates and Flight Manual Supplements are correct, etc.
Next comes a visual inspection of the airframe. This portion of the inspection is a look, touch and feel event. Experience with the particular type of aircraft is very beneficial as each aircraft has it's own areas of continuing problems.
The airframe requires servicing also and this includes: cleaning the whole airplane, cleaning wheel bearings & repacking with grease, putting air in the tires, servicing the battery and brake reservoir, lubing all control systems bearings & hinges, etc.
During the inspection, a list of anything found to be unacceptable (unairworthy) is noted as a "discrepancy". The inspection and servicing checklists are completed by the person accomplishing the work. When this is all done, the owner is contacted with a report and is asked to come to the airplane to review everything that has been found and done. This is always the most difficult part of the Annual Inspection as it is necesssary to tell the owner, his "perfect" airplane is not quite so perfect. We never want anything to be "bad" on the airplane when we do the inspection but if anything is "bad", it is our job to find it. The word "airworthy" is not defined by the FAA, but the word is used to judge if the aircraft is safe to fly. There is not enough space here to express my feelings about "airworthiness" but I plan to address it at a later date.
After making repairs to "unairworthy" items and any additional "cosmetic" items requested by the owner, the aircraft is put back together, the engine is run again and all of the systems are checked for operation again. The engine is then checked for any leaks and if everything is OK the aircraft is almost safe for another year's flying. Almost ready, because the required certification in the aircraft records as being "airworthy" needs to be done. The statement "I have inspected this aircraft in accordance with an annual inspection and have found it to be in airworthy condtion" with a proper certificate number and signature make it safe to fly.
I have found many unusual things during an annual inspection including giant spiders from a South American based airplane (luckily the spider was dead), mice, nests, birds, bee hives with live bees (when you look inside of the wing, nothing should be moving around and buzzing), tools, bucking bars and notes written on the metal from whomever built the airplane. The general aviation maintenance technician has a very interesting job.