Bits and Pieces
Aircraft Inspection Techniques for Homebuilders - Part 2
Principles of Inspection B - Priorities
By Bill Evans, President - EAA Chapter 266, EAA 794228
Flight control failures are often fatal. Even seemingly unrelated things, such as a frozen fuselage drain, can result in frozen controls.
On my homebuilt, the elevators have two hinges each but are joined by a torque tube. Hence, one piece with four hinges. One hinge failure may be survivable. However, the rudder is designed with just two hinges. A hinge failure, especially with the upper hinge, would probably result in a loss of control. Since it's only neutrally stable to begin with, a failure here is most serious.
Over the years I found that better rudder and yaw control were desirable. The short version is that the size of the rudder was increased, and I took the opportunity to add two more hinges at the same time. How safe is the design of your control system?
Most factory-built aircraft have sufficient control hinges. Cessna has its hinges made in halves. Thus a crack in one half should still allow some control. As an inspector, you check those halves for cracks. Further, there are three of those hinge sets.
Trim systems, if adequate, can allow some control. I've seen a video where a test airliner landed on just the stabilizer trim when the elevator hydraulics had a simulated failure.
My aircraft began life with just an elevator trim. If the stabilizer had exactly the right spacers for the loading, then the elevator trim was adequate. Add a passenger and it's not adequate to trim for climb, for example. Thus a second trim system was added, a stabilizer trim system, now electrically operated. It's amazing.
If you add a system and components, then you also need to add them to your inspection procedure.
What servicing/test should this stabilizer trim actuator have?
You'd think that this would solve all the problems. In the last part of this article I mention a small flutter I'd had in the elevator trim tab, which was found in time. As an inspector you take your time to be sure you have found all the problems and potential problems.
There are three fatalities noted in the NTSB records for my aircraft type, one each for aerobatics, carbon monoxide poisoning, and the canopy opening in flight. The canopy may be involved in two of these.
On one flight the pilot failed to notice that the canopy lever was creeping toward the open position. I have tested this. That lever creeps one centimeter aft every 10 minutes. So all the owners have installed locks on that lever. The preflight list calls for the three hinges and three locks to be checked. After entry there are three more items; the lever is closed, the lock is in place, and the lanyard is in place, to limit canopy movement to three inches.
When you inspect your aircraft, inspect all these for wear and defects since a canopy can result in loss of control and loss of life - yours.
Carbon monoxide is insidious, gets into places silently. Many pilots have a CO detector patch on the instrument panel. It only helps if you check it and if it's fairly new. Some fuselages have a negative pressure inside. The idea is to bring in sufficient fresh air to prevent engine compartment air from seeping in. In climb, exhaust gases may wash over the fuselage. If the cockpit air smells hot or oily, you need to rethink your aircraft and rethink how you fly it.
Where can engine compartment air enter your cockpit?
Tires and Tubes - If you suffer a flat tail wheel tire, once the minimum control speed "rudder" is reached on landing, you may be looking at a forced ground loop. If you suffer a main tire flat, then the runway excursion may be more abrupt. What are your life and aircraft worth? According to Van's annual inspection documents, tubes have a life of five years. In my world that's maybe two tire lives.
I hope that you will look at your tires and tubes differently when you inspect your aircraft. Are they good enough? (It depends - good enough for what?) Are they good enough for a hard landing? You'd want your tires and tubes to be new if you had a hard landing. Make it so.
Many pilots buy six ply tires, not six ply-rated but six real plies for that reason. They are tougher at speed and at touchdown.
Limits or "What will you risk your life for?"
Limits in the world of aviation control the extent your aircraft may operate with defects. They control the time it may fly and circumstances under which it may be flown.
On airliners, there is a book that stipulates those limits and conditions, called the Minimum Equipment List (MEL). I've never seen one for a light aircraft, but you can read all about it in the FAA document 43.13 or Transport Canada Airworthiness Manual, Chapter 571.
Examples of Limits
Tires - I've seen tires where the cord had started to show, and more than once, where the cord showed around 360 degrees - well beyond the limits. Think about the tires and tubes on your aircraft.
Cracks and Corrosion - You will often see fuselage skins and Plexiglas stop-drilled to prevent cracks from progressing. If the cracked area is load bearing, then it should be repaired before flight. For our purposes in homebuilding, if a skin or window has been stop-drilled once, then progresses further, it's time to replace or install a repair. Why? Limits are not only about how far you can go, but it's also about the effect of all the defects and repairs on an aircraft. Quite a bit of ink has been spilled on this subject.
Similarly, deferring repairs to corroded parts may result in deeper and perhaps irreparable corrosion. Or if not irreparable, then not economically repairable. Many light aircraft have fairly light spars. The main spar on my aircraft is 0.04 inch. That's 40 thousandths. How much corrosion can it withstand and still bear the loads of a steep turn?
Let's say that locomotive boilers will withstand 12 times the rated pressure and hold. However, aircraft must fly. Perhaps you have a safety factor of 2 or 3. Think about that when you are inspecting your structure.
Leaks - A leak may result in consequences beyond the leak itself. Even small oil leaks may cause CO if the oil drips on the exhaust. Leaking brakes may result in loss of braking as the fluid gets on the lining. It can also be a real nuisance in the flight compartment. If the vents are closed in winter, the fumes can be offensive if not harmful. Fuel leaks often cause damage, will certainly burn if ignited, and are always toxic. Some designs incorporate fuel shrouds and shields to pipe leaks overboard or at least to prevent fire.
One of my close friends lost power this summer past. In the end it was shown that an exhaust gasket came adrift and allowed exhaust gases to contact a fuel line. The line was insulated but soon percolated such that the engine power was reduced. He was forced into a mountain landing and was not injured...apart from the three weeks of shock.
Had be been able to use an electric pump or carb heat or fuel choke circuit to regain engine power, then the fuel would have either burned or exploded. Fuel leaks are all potentially dangerous.
What is the condition of the fuel lines and components in your engine compartment? How are they routed? As an inspector, you should ask what would happen if there was a fuel, oil, or exhaust leak. What are your limits?
Engine Operation - For my type of aircraft, we say that you need 40 psi oil pressure, 2850 rpm, and so on before you release the brakes on the runway to start a takeoff. You should know what is your limit for mag drop. For many aircraft it is 200 rpm. At the run-up and leak-down test you perform before the inspection begins, you should have some limits in mind.
Those limits should be strict enough to allow you a little margin for, say, mag drop so that you can get home without exceeding your operating limits.
My mag drop is presently 25 rpm. At, say, 50 rpm I would start checking for plugs. What sorts of limits do you have at annual inspection and on the run-up pad?
These controls are shown on an engine test stand. What improvements
are required for engine controls used for flight?
Inoperative Components - I think most readers would agree that if your Firex is empty or unusable, then your required equipment is below what the regulations require. In fact every component of your aircraft may similarly be required. If you have a certified glass cockpit, then your attitude and navigation displays and computers are required for IFR flight. Likewise lights and your VOR, and gyro horizon may be required equipment for night VFR.
Many components are not that obvious. In the years ahead my aircraft will have tip tanks, on wings with very little dihedral. If I take off on a flight that requires that fuel to be pumped, the transfer of that fuel is required by the MEL. It has to be operative.
Canadian-designed aircraft usually have fuel selection from either the
main or aux tank, which this valve illustrates.
You should have some understanding of what your components and systems do. If the defects on your aircraft cause it to fall below the MEL, you should know that. At inspection time, those defects should be front and centre in your mind. For our purposes, let's say that no real defect can be deferred beyond annual inspection. That is your practical limit.
Worn or Frayed Parts - It has been generally allowed that many standard bolts, screws, and fittings may be worn 15 percent and still be operable. However, there are limits to that. Axles, engine mount bolts, and wing and control attachments have no permitted wear at all. Especially axles. These become the top of the list for key points because failures here are shown to cause accidents or incidents.
Limits Do Not Just Refer to Hardware - Limits also control flight time. You can see that if your engine (like mine) contains 2.5 litres of oil, then the leak rate does not need to be more than a seep to ground the airplane. Let's say the engine will run on 1.5 litres of oil, because 0.5 litre is in the oil cooler. Below that there will be time gaps in lubrication and cooling. Further, even small leaks can cause smoke and fire.
Fuel leaks may seem less serious, but because of the tetraethyl lead and benzene compounds in our fuel, any fuel leak can be considered significant.
Quite a few suppliers carry fuel tank sealants. I am thinking of the Pro-Seal line of products, not the Creem that the motorcycle shops use on fuel tanks. However, even the butyl and nitrate rubber sealants need to be applied to properly repaired surfaces and to structurally sound fuel tanks. If you tested your fuel vents with compressed air and cracked the tank, the best sealant in the world is not going to seal the leak for long. How is your tank structure?
Effects on Flight - It may be fair to say that on some flights you do not need a working autopilot if the system has been rendered safe, as in properly deactivated. There are many effects on flight: weather, icing conditions, heavy traffic areas, and perhaps most important the length of the flight. I've known crews that hand-flew a jet across the ocean as the autopilot was out. When you are deciding what is to be fixed or removed or permanently deactivated, think about the mission of your aircraft. Think about your medical state, age, and how long you can fly your aircraft, and also deal with the issues resulting from the component that doesn't work.
If you really have done something that affects pilot workload, then there should be some placard or notice in the cockpit to remind pilots of that. If this item placed limits on flight, then that information should be posted as well.
Hazards - Some equipment when inoperable poses a hazard. I once flew from Sault St. Marie to St. Lazare, single pilot IFR. Partway home the ADF quit. It was at IFR minimums, so I just left it. Ten minutes later smoke appeared in the cockpit on a back course ILS. I was fortunate that it was smoke and not fire. The power supply had failed, and a vibrator stuck and then smoked. Now ATC gives me a final approach fix to begin my final descent, into minimums and heavy rain.
If that smoke had contained even a little carbon monoxide, my landing could have been very different.
This is here to say that limits may also involve hazards. A landing in Ottawa might have been a better choice.
If your aircraft can be safely flown home, then perhaps that should be your flight-time limit. I would estimate that rather few experimental aircraft fly more than 25 hours per year. For things that cannot be readily seen as endangering flight, may I suggest a limit of 25 flying hours?
When you inspect your aircraft, have its limits in mind.
Part 3 - In the next issue of Bits and Pieces, I will cover the topic of preparing yourself, and your equipment, to inspect your aircraft. While the necessary equipment is not at all expensive, it is essential.