For a plane to gain 10 kts with a new mag, the plane has to have been flying 10 kts to slow in the cruise power settings and be no where near normal takeoff and climb values. A good pilot would recognize this issue and have a mechanic involved long before annual.
Almost everyone are not flying unairworthy airplanes. Just those who chose to do so.
It would take a serious increase in power to gain ten knots, alright. But getting the engine to run properly and produce its rated power will indeed improve cruise speed a little.
Power required to increase cruise speed is, I think, a function of the square of the increase in speed. To get a 10% increase you'd need a 21% increase in power. That's a lot of power.
As far a unairworthy airplanes, I am an experienced mechanic and have been a thorough inspector and a director of maintenance. I have inspected airplanes that have been looked after by other mechanics and have found many glaring deficiencies that put the airplane far out of airworthiness, and those defects have been there a long time. Many years. It makes one very cynical about either cheapskate owners or lazy mechanics. My generation has known for a long time that just about any airplane will have airworthiness defects if you look deep enough.
A chief place to look is the AD record for any airplane and check it against the ADs that pertain to it. You can often find that an ACS ignition switch is installed, but the mechanic has been doing the Bendix ignition switch AD for 25 years without ever getting under the panel and seeing what make it is. The ACS has a completely difference AD issue. A whole bunch of aircraft appliances have ADs against them: wheels and brakes, radios, seat belts, altimeters, vacuum pumps, magnetos, carbs, injector systems, and so on, and I have often found these completely overlooked. They don't normally come up in the typical FAA AD search function. You have to know what's in that airplane and go looking though the AD database. That takes time.
Then there are the defects inside closed-up places like wings and tailcone. Defects like seized control cable pulleys, worn and fraying cables, cracks in bulkheads, leaking fuel fittings, cracked and rotten fuel hoses. Control systems way out of rig. Corroded wheel bearings. Plenty of stuff that gets ignored because the mechanic didn't take the considerable time and effort to pull all the inspection covers off and get a good look inside. It's false economy. You know what a seized cable pulley does? Besides making the controls stiff, it abrades the cable as it skids over the pulley. Dirt and grit get embedded in the pulley and accelerate the damage. Eventually you're replacing both the cables and the pulleys, and replacing cables is an expensive proposition. All because nobody relubricated the pulley bearings.
Look at what the regs require. I've deleted some stuff for brevity and added a few comments:
Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections
(a) Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling. He shall thoroughly clean the aircraft and aircraft engine.
I have found inspection cover panel screws rusted solid in their anchor nuts. Haven't been out in a generation.
(b) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the fuselage and hull group:
(1) Fabric and skin—for deterioration, distortion, other evidence of failure, and defective or insecure attachment of fittings.
(2) Systems and components—for improper installation, apparent defects, and unsatisfactory operation.
That takes a lot of looking. There are numerous systems that run through the airframe. Pitot and static, all of the flight controls, fuel lines, hydraulic lines, and so on.
(c) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the cabin and cockpit group:
(1) Generally—for uncleanliness and loose equipment that might foul the controls.
It's often horrifying what one finds under the floors of some airplanes. Incredible debris packed in by mice, junk left by avionics technicians. Such junk can get between a cable and its pulley and jam it. Accumulated oil and dirt and so on. Corroded cables and other parts. Leaking fuel fittings, leaking so slowly that there's little odor, but a huge blue stain.
(2) Seats and safety belts—for poor condition and apparent defects.
Frayed seat belts, or belts that are 40 years old. They're made of nylon that UV and other factors slowly destroys so that if the airplane crashes they might not restrain the occupants.
(4) Instruments—for poor condition, mounting, marking, and (where practicable) improper operation.
Frequent United Instrument altimeters affected by ADs. Also frequent decaying vacuum and pitot-static lines. Sometimes find wiring and hoses being abraded by the control columns.
(5) Flight and engine controls—for improper installation and improper operation.
(7) All systems—for improper installation, poor general condition, apparent and obvious defects, and insecurity of attachment.
"All systems." Is your airplane up to snuff with its annual inspection of all its systems?
