Constant Speed Propeller Maintenance

This cut-away shows the interior workings of a constant-speed prop hub.

Over the years, I’ve noticed that pilots tend to give insufficient attention to two critical airframe elements: tires and props. I’ve already covered tires, so today let’s look at the perils of improper maintenance on a constant-speed propeller.

On January 23, 2003 at about 4:20 p.m., Rob Cable — the grandson of Cable Airport founder Dewey Cable — took off from that airfield to perform a post-annual test flight in his twin-engine Beech 95 Travel Air. Six minutes later he was killed when the Beechcraft crashed in Rancho Cucamonga.

This accident was big news in the Southern California flying community. Cable Airport bills itself as “the world’s largest family-owned public-use airport” and anyone who’s been there can tell you what a scrappy little place it is. From the friendly people to the quirky Maniac Mike’s Cafe to the gently rolling terrain that seems to encompass every bit of the airfield, a trip to Cable always reminds me of what general aviation can — and should — be.

Set against the San Gabriel mountains just north of Ontario Airport, family-owned Cable is going strong long after so many other airports have fallen victim to the ravages of time and development.

Set against the San Gabriel mountains just north of Ontario Airport, family-owned Cable is going strong long after so many other airports have fallen victim to the ravages of time and development.

The NTSB investigation soon found that the cause of the accident was a mechanical failure. This alone made the crash significant. Statistics point to pilot error outweighing mechanical failure as the root cause of fatal accidents by a ratio of about 9-to-1.

In this case, it was determined that a 2.5 foot-long portion of one of the right engine’s propeller blades had failed. Think about that for a moment. This aircraft was equipped with two-bladed props, each of which had a diameter of about six feet. Therefore each blade was about three feet long. Losing 2.5 feet of a blade meant that the hub was now attached to a three-foot blade on one side and a broken 6″ stub on the other. Can you imagine the difference in weight between the two sides of the propeller?

According to the NTSB, the resulting imbalance cause a vibration severe enough that it overstressed the engine mount and tore the right engine off the airframe. A witness reported “reported observing the right engine hanging straight down toward the ground with the propeller stopped”. At that point the center of gravity would have rendered the aircraft uncontrollable.

It should be noted that while this was a massive failure, I’ve seen cases of props shedding just an inch or so off a blade tip causing such severe vibration that instruments in the cockpit were shattered, cowlings were torn away, and other serious damage was created.

One of the most famous constant-speed prop failures occurred during a test flight of the the Rutan Voyager in 1986. A blade broke off the rear engine near the prop hub. Voyager was equipped with composite propellers with blades which were much lighter than the metal Hartzell unit on Rob Cable’s Travel Air. Dick Rutan later wrote that after figuring out which engine had the problem, he moved the mixture control to the cut-off position. As the rear engine slowed down, the amplitude of the vibration increased, eventually tearing the powerplant completely off it’s mount. Those engine mounts were designed to handle 10g, so you can imagine the forces at work. Rutan said that after landing at Edwards Air Force Base, they found the engine lying on the bottom of the cowling, attached only by a safety cable they had installed for just such a purpose.

The famous Rutan Voyager in the hangar.  Note the composite MT Propeller assemblies on the front and rear engines.  They were replaced a few months later with metal props after losing a blade in flight.

The famous Rutan Voyager in the hangar. Note the composite MT Propeller assemblies on the front and rear engines. They were replaced a few months later with metal props after losing a blade in flight.

In Voyager’s case, the MT propellers were so troublesome that they were soon replaced with more traditional metal props specially manufactured by Hartzell (in record time — something the folks at Hartzell are still proud of) with specially shaped blades. The increase in aerodynamic efficiency more than made up for the increase in weight, and the program went on to successfully circumnavigate the planet on a single tank of gas.

Anyway, back to our story. The NTSB delved into the Travel Air’s maintenance records and found that, rather than being neglected as one might expect, the props had just been overhauled! Their next stop was the FAA-approved Repair Station that did the work, T&W Propeller in Chino. This is where things got particularly interesting for me, as I owned an aircraft with a constant-speed prop that had just been overhauled by that very same shop.

You can read the full report if you’re so inclined, but here’s just a partial list of what was found on the accident airplane’s propellers:

During the Hartzell participant’s teardown examination he made a series of observational findings. He observed the following discrepancies between the overhaul procedures specified by Hartzell in its maintenance manuals and the physical evidence found in the propellers:

1. The blade internal bores were clearly not in compliance with overhaul requirements for inspection, rework, and finishing. There was no paint and there appeared to be no chemical conversion coating in the bore area. There was extensive corrosion in the internal bearing bore area A, as defined by Hartzell Service Bulletin 136H. The participant stated that a proper overhaul requires removal of the blade bronze bushings in order to accomplish rework and inspection.

2. The hub arm of the right propeller had cadmium plating on top of deep corrosion pits. Such corrosion is required to be removed during overhaul.

3. A blade clamp in the right propeller had cadmium plating on top of deep corrosion pits. Such corrosion is required to be removed during overhaul.

4. Blades from the left propeller were too long. The aircraft is approved for installation of a propeller having a diameter of 72 to 70 inches. The length of blade L1 was measured to be approximately 32-5/8 inches long, which corresponds to a 74-inch diameter. Blades from the right propeller were measured to be approximately 31-5/8 to 31-3/4 inches, which is the correct length and corresponds to a 72-inch diameter.

5. Blades from the left propeller were impression stamped 8447-4 and 8447-12, and should have been stamped 8447-12R. Blades from the right propeller were impression stamped 8447, and should have been stamped 8447-12A.

6. Remnants of phenolic washers were found in the left propeller. The washers were approximately 1 to 2 inches in diameter and installed over the hub pilot tube, between the hub arm and blade butt of both blades. These were not Hartzell parts and such usage is not authorized.

7. Small particles, which appeared to be plastic cleaning media, were found in the grease in the blade balance hole.

8. The cadmium plating on the blade clamps and hubs was unusual. While much of the surfaces had bright cadmium plating, there were numerous spots that had no plating, areas of dull gray appearance, and areas that appeared worn. Portions appeared to have either deteriorated plating or had not been plated. Given the report that the propeller had only 5 hours of operation since overhaul, the general condition of the cadmium plating was considered very poor.

9. One O-ring, used as a seal between the clamp and hub was severely deteriorated. It had many cracks around the circumference of the outside diameter. The other three blade clamp O-rings were in good condition. It appeared that the deteriorated O-ring had not been replaced during overhaul.

In conclusion, the Hartzell participant made the following statement regarding the observed overhaul procedure discrepancies: “The most significant discrepancy was the presence of obvious, significant corrosion in the internal bearing bore area of the blades. This, plus the absence of required corrosion protection (chemical conversion coating and paint) in this area, clearly indicates that proper overhaul was not accomplished.”

Even if you don’t speak “A&P”, the gist is undoubtedly clear: T&W Propeller was criminally negligent in the performance of their work and it resulted in a fatal accident. The FAA quickly issued Airworthiness Directive 2003-13-17, which required another overhaul of my improperly zero-timed constant-speed prop. I believe the price tag for the two overhauls was nearly $6,000. Welcome to the world of aviation! It reminds me of an old joke where a prospective student pilot asks a grizzled veteran how much money it would take if he wanted to learn to fly. The answer: “All of it.”

It was about this time that I realized that the “FAA Certified Repair Station” designation means absolutely nothing. I sent the prop to a small, non-CRS shop in Bakersfield called Johnson & Sons and got a better result for less money. Caveat emptor.

I also started researching propeller-related failures and realized that most of them are a direct result of neglect on the part of the owner or operator. Just like an engine, props have a recommended Time Between Overhaul (TBO). For most constant-speed props, it’s 2400 hours or six years, whichever comes first. Not many us of put 400 tach hours on our planes each year, so the six year calendar interval will almost always be reached first. And for reasons I’ll never understand, it’s the calendar limit which is most likely to be ignored. Inside the hub are seals, bearings, and other parts which age with exposure to the thermal cycles, humidity, and so on. But time and time again, you’ll find aircraft with 500 hours and 10 years on the propeller assembly and the owner claiming it’s not anywhere near TBO.

The recommended TBO is not mandatory if you’re flying under Part 91, and as a result it’s not uncommon to see aircraft with 10, 20, or even 30 years since the prop and/or governor were overhauled. Personally, I’d much rather fly behind a 30 year old engine than a 30 year old prop. Why? I know how to fly an airplane without an engine (and not just because I fly gliders)! If the powerplant takes the day off, I can still control the aircraft quite nicely. But losing a blade? That’s likely to create a problem no piloting skill can rescue you from. The more I learn about propellers, the more convinced I am of this. At the very least, I’d have the prop hub opened and inspected by a (hopefully) trustworthy shop for what’s called a “re-seal” job.

I visited the Hartzell factory in Piqua, Ohio about ten years ago and took this photo of an actual constant-speed propeller which had been cut-away and turned into a display model. (Extra credit if any of you can tell me what type of constant-speed prop this is. Clue: look at the relationship between the spring and the piston in the hub.)

This cut-away shows the interior workings of a constant-speed prop hub.

This cut-away shows the interior workings of a constant-speed prop hub.

You can see that the blades are individual pieces held in the hub by a beefy retention bearing. With the prop spinning at 2600 RPM, there are more than 20 tons of centrifugal force trying to rip that blade out of the hub. As I mentioned, even if a shed blade didn’t hit the airframe as it departed, the resulting imbalance would almost certainly tear the engine off and shift the center-of-gravity to an uncontrollable location.

Suddenly, skimping on that prop maintenance doesn’t seem like such a hot idea, does it?

A spinning prop also exhibits gyroscopic properties, so every time the aircraft is pitched or yawed, immense forces twist and bend those blades. You can see an extreme example of that in a slow-motion video of a helicopter main rotor blade that I posted a while back. Rotorcraft airfoils are far less rigid than any constant-speed prop, but the principal is similar.

Aerobatic pilots know all about gyroscopic effect. If you’ve been amazed by scenes like this at an airshow and wondered how they do it, most of the spectacular maneuvers like tumbles are produced with gyroscopic effect.

The aircraft is largely being thrown about the sky from forces generated by the prop. But you pay for it with high stress on the item the prop is connected to: the crankshaft. My Pitts S-2B once broke a crankshaft due to high stress imposed on it from a two blade metal Hartzel prop after repeated snap rolls. After that, the owners elected to spring for a new light-weight, 3-blade composite MT propeller.

The takeaway is this: propellers are under high stress in flight, and although they’re quite reliable, due to their nature when things go bad they are more prone to an unrecoverable failure than a reciprocating powerplant and thus deserve even more respect than the engine they are attached to.

Aircraft Tire Pressure

Jet aircraft nose gear

Every aviator has their soapbox issues, and when it comes to maintenance, my top two are constant-speed propellers and aircraft tires. I may touch on the former in a future article, but for now let’s focus on the latter.

Tires are one of the most vitally important — yet frequently ignored — parts of an aircraft. It’s easy to see why: they’re relatively simple elements which work day in and day out without problem, and as such are taken for granted. In addition, some of the typical pilot’s attitude toward tires is transferred from the way they treat their automotive counterparts. Be honest, how often do you inspect your car’s tires? When was the last time you checked the pressure on all four wheels? I’m about as anal as a person gets when it comes to car maintenance and upkeep, and I might check the tires once every couple of months at best.

In a light GA aircraft, tire failure on the takeoff or landing roll can lead to loss of directional control, runway excursion, and/or a ground loop. These things are unlikely to be fatal but are frequently embarrassing and inconvenient as they’ll shut down the runway for a while. I’ve had several of those in my career. Ironically, it always seems to happen during a student’s softest, smoothest 3-point landing in the Decathlon. Well, almost always.

Not what you want to see on a deserted runway in the middle of nowhere as the sun is going down.

I once lost a tire — literally — while taxiing a Pitts S-2C. I had stopped for cheap fuel in Limon, Colorado, a paved but little-used strip on the edge of a small town near the Nebraska border. After a great landing in a 25 knot crosswind, I refueled and started to back-taxi on the runway when the left main tire slowly deflated. I shut down and opened the canopy, but as soon as my feet came off the brakes, the airplane weathervaned into the wind, taking the tire right off the rim as it pivoted. Now I couldn’t even move the plane.

Oh, and did I mention the sun was setting soon and the runway had lights? Suddenly the flat tire was less important than ensuring some wayward pilot didn’t attempt a night landing with my disabled, unlighted biplane sitting on the runway.

Light aircraft flats are more often caused by the failure of the tube than the tire itself. The Pitts incident taught me the value not only of proper tire inflation, but also of alighting at airports with maintenance services when flying cross-country. Making that trip today, I’d at least carry a spare tube.

Of course, that won’t always save the day. I once had to rescue a friend from Death Valley when his underinflated aircraft tires melted into the tarmac on a 120+ degree day. Tire condition can be difficult to judge on fixed-gear aircraft on account of the fairings which often hide 90% of the rubber from view.

Jet aircraft nose gear

In turbine aircraft, improperly inflated tires are more likely to lead to expensive damage, if not outright catastrophic consequences, due to the higher speeds and heavier weights of those aircraft. There’s simply a lot more kinetic energy for the tires to absorb. This is why turbine aircraft tires are stronger and more advanced than those found in their lighter brethren. It also explains why those tires are filled with nitrogen instead of air. Nitrogen doesn’t expand at altitude the way air does. It has a low moisture content so it doesn’t freeze, and it will not support combustion.

The FAA recently issued Safety Alert for Operators (SAFO) bulletin 11001, “The Importance of Properly Inflated Aircraft Tires”, which notes:

Research has shown that transport-category airplanes can lose as much as five percent of tire pressure per day under typical operations. At a pressure rate loss of five percent per day, it would only take a few days before they require servicing.

Tires not serviced within an acceptable range may require tire replacement due to under inflation limitations specified in the maintenance manual. Additionally, servicing of underinflated tires without proper protection such as a tire screen or other protective devices may cause damage to the aircraft or injury to the individual servicing an underinflated tire.

The FAA’s not alone in their crusade to get us to pay more attention to our tires. Last week, Gulfstream reviewed tire safety “best practices” in their weekly Breakfast Minutes publication. It referenced the Goodyear Aircraft Tire Care and Maintenance publication. It’s an excellent read.

The Antonov 225. How long would it take to check the tire pressures on this aircraft??

A bit of research revealed that tire failure has caused a variety of high-profile jet accidents, including Air France flight 4590, Nigeria Airways flight 2120, Mexicana flight 940, and most recently, the 2008 crash of a Lear 60 in South Carolina.

That last accident was cited in their SAFO bulletin. In fact, the FAA recently issued an Airworthiness Directive for the Lear 60 which requires a tire pressure check no less than 96 hours before any flight.

Obviously it’s not possible to prevent every instance of tire failure, but we can skew the odds in our favor by paying more attention to them. That means checking the tire pressure at appropriate intervals. Mounted tube-less aircraft tires lose significant pressure every single day, and underinflated tires cannot necessarily be detected by simply looking at them. If nothing else, proper inflation leads to longer tire life and better ability to survive FOD damage should it be encountered. Remember too that the landing and takeoff distances listed in the Aircraft Flight Manual are predicated on proper tire inflation!

To show the importance of proper tire pressure, consider that as a tire leaves the deflected area (aka the ground) as it turns, it attempts to return to its normal shape. Due to centrifugal force and inertia, the tread surface doesn’t stop at its normal periphery but overshoots, thus distorting the tire from its natural shape. This is called a traction wave. Assuming the tire is turning at 250 mph:

At this speed, it takes only 1/800 of a second to travel 1/2 the length of the footprint (CX). In that same time, the tread surface must move radially outward 1.9 inches. This means an average radial acceleration of 200,000 ft./sec./sec. That’s over 6,000 G’s! This means the tread is going through 12,000 to 16,000 oscillations per minute.

Tires are designed to withstand traction waves… but only while inflated to the appropriate pressure. Under or overinflation will magnify the effect of traction waves. Suddenly the AFM recommendation on tire pressure seems pretty important, doesn’t it?

This Hawker was severely damaged after tire failure caused by repeated takeoff aborts.

Speaking of the AFM, the landing gear may have other limitations which must be observed. In 2007, I witnessed first-hand what happens when those limitations are exceeded.

Whatever you call them — tyres, hides, skins, rolling stock, stickers — aircraft tires are certainly one of the most highly-stressed yet least respected parts of an aircraft. Next time you fly, think closely about the punishment they take and whether you’re sure those babies are truly airworthy.

Aviation Myths, Part 2

Obtaining a pilot certificate in only 40 hours is virtually impossible in today's complex aircraft.

[For the first five myths, see Part 1]

Myth #6: Only an FAA-certificated mechanic can perform maintenance on an airplane.

This myth can cost you — big time. A typical GA maintenance facility can charge $100 per hour, and aircraft spend far more time in the shop than even the most maintenance-prone automobiles. Do the math and you’ll see that, especially if you don’t fly your airplane at least a couple hundred hours per year, maintenance can easily top all other ownership costs combined. Why pay that much when you can do much of the work yourself?

Experts agree: Aircraft owners who studiously and routinely do some basic maintenance themselves, rather than waiting for the 100-hour or annual inspection, not only might save money in the long run by averting major repairs, but also reduce the aircraft’s down time, fly more safely, and learn valuable information about their airplane, which makes them better able to detect and troubleshoot problems that arise during the preflight.

Appendix A in Part 43 of the Federal Aviation Regulations includes a long list of major alterations and repairs reserved for certified mechanics. Also listed there are 32 preventive-maintenance chores that certified pilots can tackle themselves as long as they own the airplane, it isn’t flown commercially, and the maintenance doesn’t involve “complex assembly.”

These chores range from changing tires, servicing shock struts, and simple lubrication, to repairing broken landing-light wiring circuits, cleaning and replacing spark plugs, servicing and replacing batteries, and making simple repairs to cowlings and farings. If you do perform any such tasks, you must have the appropriate maintenance and service information at your fingertips.

Just do it.

The 32 preventative maintenance tasks cover the vast majority of everyday jobs you’d be paying a mechanic $100/hour to do. The full list includes some surprisingly critical items: fabric skin repair, replacing windows, changing out fuel lines, hoses, and filters, servicing wheel bearings, and painting the airframe. Better yet, if you can find a certified Airframe & Powerplant mechanic who is willing to supervise your work, there’s virtually no task you cannot legally perform.

I used to participate in the annual inspection of my aircraft, and it not only saved me money, but the “hands on” aspect of swinging wrenches on the plane taught me more than any book or class ever could about what goes on under the cowling.

Myth #7: Shock-cooling an air-cooled piston powerplant causes premature wear, engine damage, IRS audits, and the defeat of your favorite sports team.

When I was working on my commercial pilot certificate, I was taught that power reductions of more than 2″ MP per minute were verboten due to shock cooking, the concept that if the engine cooled too quickly, the hottest part of the top end could warp or crack. Myth or reality?

The bottom line on shock cooling is that yes, it does exist. It’s scientifically provable. If you heat a thin slice of metal to several thousand degrees and then plunge it into a tub of sub-zero water, it will warp if not crack. But aircraft engines do not operate at such temperature extremes and generally cannot be cooled quickly enough to cause damage of that severity.

Shock cooling: myth or reality?

The closest thing we’ve got to those extremes might be a skydiving operation where an aircraft departs at heavy weight and climbs to altitude, drops the skydivers, and then makes a long fast descent at idle power. They’ve been doing it for decades and you don’t see any jump planes falling out of the air.

Aerobatic aircraft powerplants are probably the most highly stressed and badly abused engines in the sky. Slamming the throttle from full power to idle, over and over again. Rapid shifts from high power/no airspeed to low power/high airspeed. Heavy G loads, odd stresses on the crankshaft from propeller-induced gyroscopics. In my experience, those engines don’t seem to suffer from shock cooling any more than they do from other forms of hard living.

This one has been thoroughly debunked by people who are far better versed in the care and feeding of reciprocating aircraft engines than myself. I recommend the following reading:

Kas Thomas: Shock Cooling, Myth or Reality?
John Deakin: Pelican’s Perch #36

Myth #8: You can become a pilot with just 40 hours of flight time!

Flight training providers are for-profit companies who rely, in part, on advertising to find their customers. It’s understandable that they want to make flight training as appealing as possible. However, it’s short-sighted to advertise a pilot certificate based on the legal minimum of 40 hours.

Obtaining a pilot certificate in only 40 hours is virtually impossible in today's complex aircraft.

Can it be done? Yes, it’s possible, but only a miniscule percentage of aviators complete their training in that time. The national average is now over 70 hours.

There are several reasons for this. For one thing, the regulatory minimum of 40 hours has been in place for decades. Back then, airspace was simpler, there were fewer regulations, no TFRs, and society in general was less litigious. Today, we have ballistic parachutes, wake turbulence procedures, computerized flight displays, additional training requirements, more complex aircraft, and a far lower tolerance for risk than they did fifty years ago.

The 40 hour minimum remains on the books, but don’t think that you’ll be a properly trained pilot in that time unless you come to the table with a high level of aptitude, plenty of drive, and can train intensively. Oh, and you’ll want to fly a simple airplane (Citabria, anyone?) out of a quiet airport.

Myth #9: Stalls and airspeed are related.

To my mind, this is one of the most dangerous misconceptions in aviation. Airline accidents like Air France 447 and Colgan 3407 can be traced to it, as can hundreds of GA crashes.

I wish this one was relegated to students or flight simmers, but it’s not. Most pilots equate stalls with low airspeed, but in reality the two are unrelated. The pilot most likely to have a proper understanding of the relationship between stalls and airspeed isn’t the professional airline pilot with 20,000 hours in his logbook, it’s the guy who flies competitive aerobatics, because they see the extremes of the envelope again and again.

It's all about angle-of-attack, not airspeed!

An aircraft’s stall speed will vary — sometimes dramatically — with load factor, weight, CG location, and other factors. What does not change is the relationship between stall and angle of attack. Any airfoil will stall at the same angle of attack. When you exceed that AoA, it stalls regardless of your airspeed. Aircraft can be flown at any airspeed without stalling (even 1 knot!). Likewise, any aircraft can be stalled at any airspeed up to and beyond Vne.

Think about that the next time you’re looking at that red radial line on the lower end of your airspeed indicator. We may refer to that as the plane’s “stall speed”, but it’s only valid on a clean, new airframe flying at very specific weight and CG location under a 1g load. Change any of those factors and the airplane will stall at a different airspeed.

Myth #10: Tailwheel airplanes are not worth the difficulty and hassle.

Tailwheels — airplanes with the main landing gear located in front of the center of gravity and a small third wheel under the tail — make great pilots because they have to be better in order to operate them safely. That’s a good thing.

Sure, they have plenty of negatives: they’re highly unstable on the ground, suffer from limited forward visibility (if they have any at all!), typically have weak brakes, and they’re more vulnerable to wind due to the built-in angle of attack when on the ground.

The advantages? There must be some reason they keep building them, after all. How about better prop clearance, shorter takeoff and landing rolls, lighter weight and less drag than a tricycle gear configuration, tighter turning radius, and simpler, less expensive, more durable construction?

Beautiful, historic, and fun to fly. What's not to like?

However, the big bonus a tailwheel provides is the serious upgrade in good old fashioned stick-and-rudder skill you get from flying one. No more dropping the plane onto the runway with the nose pointed who-knows-where and rolling out more as a passenger than a pilot. With a tailwheel, you learn to keep it straight, stop any drift, pay serious attention to where the wind is coming from, and most of all keep flying the airplane all the way to a full stop.

It’s worth pointing out that you needn’t fly a conventional gear aircraft to become proficient at landing an airplane. Truth be told, the exact same technique is used to land GA aircraft regardless of landing gear type. But the tricycle gear configuration is far more tolerant of sloppy technique, and people tend to use only as much skill as is necessary for the aircraft they’re flying. As an instructor I’ve been guilty of it myself, demanding a higher level of performance from a Pitts student than one flying a DiamondStar. In an ideal world, I’d require the same high quality landings from both candidates.

According to the Air Safety Foundation, takeoff and landing phases of flight are where the vast majority of aircraft accidents occur, and the skill developed by taming a taildragger can be put to use flying any airplane regardless of size. They can be a handful, but I was convinced years ago that these wonderful aircraft can do more than anything else to eliminate takeoff and landing accidents, not cause them.

[... continue reading in Part 3]

Aviation Myths, Part 1

faa_charts

Over the past decade and a half I’ve been keeping a mental list of frequently encountered misconceptions about flying. For some reason, I recently Googled “aviation myths” and found quite a few articles on the topic and it inspired me to finally set my own list to virtual “paper”.

This list is not exhaustive, but it does represent the myths I encounter most frequently. Some of these are misconceptions held by non-pilots, others are more common among student aviators or even experienced professionals. I’ve written about a few of these in the past, but thought it might be worthwhile to throw the whole list out there for others to chew on. I’m planning to make this a three-part series, with five myths per post.

Have you encountered any of these before? Do you disagree with any of them? If so, I’d love to get your feedback. OK, here we go!

Myth #1: Logging “actual IMC” is only allowed when flying in clouds or low visibility.

Some aviation myths and misconceptions are absurd while others are entirely understandable. This one falls into the latter category. Even a non-pilot would find it logical to assume that logging flight time in the “actual IMC” column would require one to actually fly in instrument meteorological conditions (IMC). Thankfully for those of you who are attempting to build instrument time, it ain’t necessarily so.

14 CFR 61.51(g) states that “A person may log instrument time only for that flight time when the person operates the aircraft solely by reference to instruments under actual or simulated instrument flight conditions.” In other words, any time conditions are such that maintaining control of the aircraft by outside visual reference is in serious doubt and the instruments are used in lieu of those references, one may log actual IMC flight time.

The classic example of this situation is flying on a dark, moonless night over unlighted terrain (desert, ocean, mountains, etc). If John F. Kennedy, Jr. had realized this, he might be alive today. He took off from New York and headed toward the island of Martha’s Vineyard on just such a night. The reported and actual visibility was far above VFR minimums. In fact it was a CAVU night. Unfortunately, without any discernible horizon to look at, his situation required flying on the instruments. It’s not something primary or instrument instructors often pass along to their students, but we should.

If my word isn’t sufficient on this issue, here’s an excerpt from an FAA legal opinion issued by the agency’s Assistant Chief Counsel.

As you know, Section 61.51(c)(4) provides rules for the logging of instrument flight time which may be used to meet the requirements of a certificate or rating, or to meet the recent flight experience requirements of Part 61. That section provides in part, that a pilot may log as instrument flight time only that time during which he or she operates the aircraft solely by reference to instruments, under actual (instrument meteorological conditions (imc)) or simulated instrument flight conditions.

“Simulated” instrument conditions occur when the pilot’s vision outside of the aircraft is intentionally restricted, such as by a hood or goggles. “Actual” instrument flight conditions occur when some outside conditions make it necessary for the pilot to use the aircraft instruments in order to maintain adequate control over the aircraft. Typically, these conditions involve adverse weather conditions.

To answer your first question, actual instrument conditions may occur in the case you described a moonless night over the ocean with no discernible horizon, if use of the instruments is necessary to maintain adequate control over the aircraft. The determination as to whether flight by reference to
instruments is necessary is somewhat subjective and based in part on the sound judgment of the pilot.

Note that, under Section 61.51(b)(3), the pilot must log the conditions of the flight. The log should include the reasons for determining that the flight was under actual instrument conditions in
case the pilot later would be called on to prove that the actual instrument flight time logged was legitimate.

I have logged actual IMC this way. Once you leave the Los Angeles basin, flying over the desert southwest on moonless nights can necessitate being on the gauges every bit as much as flying in a cloud. Even if there is some moonlight or a small town out there, the ambient light put out by today’s glass panels can obliterate the view out the windscreen. In those cases it’s completely legitimate and proper to claim that time in your logbook.

Myth #2: Flying without appropriate charts is illegal.

In my experience, this is one of the most pervasive myths out there. As with logging actual IMC, it makes sense. Why wouldn’t the FAA require pilots to carry current versions of whatever pertinent charts applied to their route of flight?

Answer: 14 CFR 91.103 already requires pilots to becoming familiar with “all available information” concerning a flight. How an aviator obtains that information is up to them. Simply requiring a person to carry a large folded piece of paper isn’t going to necessarily familiarize them with anything. Believe me, as an instructor, I see that truism put to the test every day. I’ve seen pilots with a 14″ color moving map display have absolutely no idea where they were or where they were going.

As far as the charts are concerned, the FAA details their policy on chart carriage on their web site.

The subject of current charts was thoroughly covered in an article in the FAA’s July/August 1997 issue of FAA Aviation News. That article was cleared through the FAA’s Chief Counsel’s office. In that article the FAA stated the following:

“You can carry old charts in your aircraft.” “It is not FAA policy to violate anyone for having outdated charts in the aircraft.”

“Not all pilots are required to carry a chart.” “91.503..requires the pilot in command of large and multiengine airplanes to have charts.” “Other operating sections of the FAR such as Part 121 and Part 135 operations have similar requirements.”

…”since some pilots thought they could be violated for having outdated or no charts on board during a flight, we need to clarify an important issue. As we have said, it is NOT FAA policy to initiate enforcement action against a pilot for having an old chart on board or no chart on board.” That’s because there is no regulation on the issue.

…”the issue of current chart data bases in handheld GPS receivers is a non-issue because the units are neither approved by the FAA or required for flight, nor do panel-mounted VFR-only GPS receivers have to have a current data base because, like handheld GPS receivers, the pilot is responsible for pilotage under VFR.

“If a pilot is involved in an enforcement investigation and there is evidence that the use of an out-of-date chart, no chart, or an out-of-date database contributed to the condition that brought on the enforcement investigation, then that information could be used in any enforcement action that might be taken.”

If you, as an FAA Safety Inspector, Designated Pilot Examiner, Flight Instructor, or other aviation professional are telling pilots something other than the foregoing then you are incorrect.

From a practical standpoint, some airplanes like the Pitts S-1 are so small that there’s no place to carry a chart. Even if you wanted to use one, how would you do so when the airplane is about as stable as an Robinson R-22 in a hover? Can you imagine the pilot of a Cri-Cri or BD-5J trying to use a chart while in flight?

I’m not discouraging chart usage. Quite the contrary, I carry them myself. In fact, there are times when it is legally required. The aforementioned Part 121, 135, and 91 Large Airplane rules call for it when flying under those regulations. Some Class B VFR airspace transitions require a current terminal chart (the LA Special Flight Rules Area comes to mind). But for the most part, they are not legally required for Part 91 operators, even when flying under IFR!

Myth #3: Perfect eyesight is a requirement to be a pilot.

This one is a holdover from the days when most pilots came from the ranks of the military, which did require perfect eyesight. Even today most branches of the military require 20/20 vision (or better) for pilot candidates (helicopter requirements are occasionally a bit less stringent). But even they will allow for corrective lenses in many cases once they’ve invested the seven figure sums that it requires to transform a person into a mission-qualified aviator.

The FAA’s vision requirement for civilians is — and has been for many years — that a pilot’s eyesight be correctable to 20/40 for non-professional aviators. Those requiring a first- or second-class medical certificate must be correctable to 20/20 for distant vision and 20/40 for near vision.

Color blind? No problem. You can still fly with virtually no restrictions. In fact, you can obtain a medical certificate even if you’ve only got one eye. Pilots can get medical clearance after major brain surgery. While on anti-depressants. After heart and other organ transplants. You can even fly if you’re completely deaf! I’m aware of at least one pilot, a woman named Jessica Cox, who has no arms and still flies her aircraft solo. She demonstrated that she could do everything necessary to safely operate the aircraft using only her feet.

These days, you can fly gliders and Light Sport aircraft without any medical certificate at all. Old airport codgers may complain about how things ain’t the way they used to be, but in this case that’s a good thing.

Myth #4: TBO is mandatory.

Time-between-overhaul intervals are not well understood by most aircraft owners. For one thing, while most pilots understand that manufacturers establish a recommended hourly interval between major overhauls, they are often unaware that overhaul is also recommended once it reaches 12 years of age. This is important because most mechanics will tell you that the greatest enemy of piston aircraft engines is lack of use. One of the easiest ways to maximize engine life is to simply fly the plane frequently. This ensures the oil is brought up to operating temperature, any water in the system is boiled off, and the internal parts of the engine are coated in a protective layer of oil.

For non-commerical operators, TBO intervals are simply recommendations. There is no legal requirement to overhaul an engine at any time. Nor does exceeding TBO void insurance or warranty coverage. Plenty of people take published TBO intervals with a grain of salt, preferring instead to allow oil consumption, spectrographic oil analysis, borescope inspections, and other such metrics dictate when the engine is ready for overhaul.

Even commercial operators don’t necessarily have to overhaul at TBO. The FAA often grants extensions to those intervals by as much as 50% or more.

Myth #5: Repairs must always be accomplished using FAA-approved parts.

Let’s say you’re fortunate enough to fly an original 1917 Sopwith F-1 Camel — one of the preeminent fighters of the first World War. Where are you supposed to go for parts? They stopped manufacturing them nearly a century ago.

Okay, that’s an extreme example. But there are plenty of orphaned aircraft types still flying. Even among those that are still supported, parts can be exorbitantly expensive, even to the point of rendering an otherwise fine aircraft economically unfeasible to maintain.

Thankfully, 14 CFR 21.303(b)(2) and 21.9 allow owners of an aircraft — any aircraft, not just a vintage warbird — to manufacture parts for their airplane or pay someone to make them as long as the replacement part is identical to the original. The only caveat is that the owner must participate in the manufacture of the part by providing specifications, design information, quality control, materials, and/or supervising the fabrication of the item.

A personal example: a Pitts S-2B in which I share ownership needed a new seat back for the pilot’s seat. The old one was cracked and slowly failing after years of hard aerobatics. Now this is literally a flat rectangular piece of half-inch plywood with wood blocks attached to the back side to hold it in place. No fancy curves, shapes or fasteners. Just a plain old piece of wood. As I recall, the manufacturer of the Pitts series of biplanes, Aviat, wanted something close to thousand bucks for that part. We were able to manufacture one for a few dollars.

If you’re the kind of person who’s handy and has access to the proper tools, you can manufacture any part for your aircraft. A wing spar, a new crankcase, a propeller, and anything in between. If you’re not so handy? You can still hire a machinist, friend, or virtually anyone else to make the article as long as you materially participate in the process and create a part that is identical to the original in form and function — in other words, “airworthy”.

EAA posted an 85-minute video last August entitled “Owner Produced Parts for Certificated Aircraft” which covers this topic in great detail.

[... continue reading in Part 2]