3 Things Every Pilot Needs to Know About… Inspecting the Prop

There are only a few airfoils on each aircraft… The wings, the horizontal stabilizer/elevator or stabilator, and the propeller blades. The majority of small general aviation aircraft operate with a two-bladed proper. However, for a variety of reasons, a good number of aircraft have more than two propeller blades too… On Mooney aircraft, the common 3-bladed props provide much better ground clearance. A variety of propeller designs also consider other factors such as noise, efficiency, etc… A small number of aircraft actually exist that operate with one propeller blade! (Seriously!)

We’re not here to discuss how a propeller works, the difference between a fixed pitch and variable pitch propeller or even the difference between cruise and climb propellers. Today we are looking at how we inspect the propeller. This may seem simple on the outset, but the following “3 Things” are points that many pilots miss! So here we go…

3 blades

3 prop blades get the job done for this Pitts! Spiffy paint scheme too!

The prop is the only airfoil that moves at incredibly fast speeds (on general aviation aircraft), on some aircraft the propeller tip travels near super sonic, hence the loud noise you hear as these aircraft zip past. Seaplanes typically have longer/thinner propeller blades, which as you would expect, make a great deal of noise. To this day I’ll never forget the bright orange Cessna 185 on floats that shared a lake with the place where I learned to fly float planes up in Alaska all those years ago! The noise was unmistakable, very overpowering, yet it always brought a smile to my face.

Every time you preflight an aircraft there are some things to remember about your propeller, and while some of these may seem trival, you’d be surprised how many expereince pilots and even (gasp) aircraft owners forget…

1. Never use your hand/fingers sliding along the leading edge of the propeller to check for nicks/cracks…

Even the smallest nick in the leading edge of the prop can cause increidble damage to the human skin sliding along the edge. Think of a steak knife, you can slide you fingers on the backside (smooth edge) without incident, but the seraded edge is a different story! Think of a “clean” prop as the backside of the knife… The reason you should avoid using your fingers is obvious, you are literally checking to ensure that the prop is smooth, do you really want to find out there is an anomoly with your soft, smooth skin? I didn’t think so…

I’ll never forget, years ago shortly before a group of my friends and I were about to embark on a day-long flight from Massachusetts to Ohio with a variety of stops along the way, one of the guys learned this lesson the hard way. He was sliding his hand along the edge at a typical “prop check” speed when he hit a fairly minor nick in the propeller. He managed to cut all four of his fingers! The bloody mess that ensued was incredible and certainly not the way he hoped to start our 3-aircraft exploration of the Northeast United States…

Changing this habit isn’t that difficult either. Most pilots have a checklist handy in their aircraft, whether or not they are carrying it with them for the preflight (which is a topic for another post)… Using the edge of a checklist will do the same job and allows you not to risk your own flesh and blood, literally, to check the propeller.

The last thing about nicks in the prop, is any time you find a nick in a prop, have a mechanic take a look. It doesnt take that long to have the local A&P drive over in his/her golf cart and take a quick look and maybe do some filing work to even it out. It’s a heck of a lot smarter to have it checked, even if it is no big deal, than to take off and find out you have a serious issue while in flight. Remember my golden rule, there is no emergency situation that doesn’t become non-life threatening when you get on the ground…

2. Knock, knock, who’s there?

Damage to the leading edge of the propeller as a result of debris being pulled in by the propeller spinning so quickly over the ground isn’t the only thing that can cause “prop issues”. This one, I’ll admit may seem trivial, but once you see something happen you never forget.

While checking the propeller, knock on the propeller a few times at various points across the blades. You’ll notice a “thawng” sound, like a bell. Then knock on the cowling, you’ll hear a hollow sound and even the ground for a “thud”. The bell-sounding “thawng” is an indication that there is likely no internal damage to the propeller. Of course the only way to know for sure is to bring the propeller to a “prop shop” and have them inspect it… But if the bell sound isn’t there then it is possible there is some type of internal issue with the propeller, maybe nothing serious, but maybe not. Definitely worth getting checked out… Over time (months) if the sound you hear while knocking changes, then an internal issue could be developing…

How often do propellers fail in flight? In reality, it is very rare. However I have flown with two different people who have experienced this phenomenon.  One of the gentleman, a former flight instructor student of mine, had a few inches of propeller blade depart his Tobago in flight. With some vibration, they were able to land safely, and have the propeller replaced.

The second story was a bit more dramatic! The date was June 6, 2006, written differently, 06/06/06. Take that for what it’s worth… A friend of mine was providing a pilot an Instrument Proficiency Check, when on the missed approach, the majority of one side of the Cessna 172’s propeller decided it had had enough and departed the aircraft. The massive imbalance created a dramatic vibration. My quick-thinking CFI friend (who had previously dealt with a safe impossible turn due to the complete loss of oil from the engine of a Cessna 152 he was flying) quickly pulled the mixture to stop the vibration, a move that would prove crucial to their safety. Upon executing a turn around, my friend landed the aircraft safely on a runway. It wasn’t until they disembarked the aircraft that my friend and his fellow pilot saw the magnitude of what had happened. As it turned out the Cessna 172 has four (4) engine mount points. Due to the massive vibration, three of these were compromised, two completely broken… His decision to “kill” the engine when he did, likely saved their lives.

They took

For post-event analysis/investigation another inch or two was cut off, but this certainly isn’t where you’d expect the prop to break, in the (seemingly) “bulkiest” spot…

Cessna/McCauley determined that a manufacturing defect inside the propeller caused the issue, roughly 3 years into the aircraft’s life as a trainer… Whether or not a knock would have found the issue, I don’t know, but every time I fly, especially in a new (to me) aircraft, I always make sure to here the bell sound when I knock on the prop!

3. Always double check that there is nothing obstrcuting your propeller before you crank the engine!

This may seem like common sense, and it is, but that doesn’t mean that a lot of pilots don’t make the mistake… I could easily name more than half a dozen, very smart, very professional and very intelligent pilots who I have witnessed do this and more I’ve had confess to me that they have made this common error, but I don’t want to name any names…

Starting an aircraft with cowling plugs still in the cowling openings may not immediately cause your aircraft not to operate correctly, but it is embarrassing. Many aircraft, such as the common Cessna 172 has a separate air intake below the spinner/propeller where the air used for combustion in the cylinders, and thus power, enters the aircraft. Having the cowling openings blocked simply means you’re going to run very hot, very fast and that, of course, is not good…

But typically, the error is noticed much earlier than that. My favorite story is of a pilot who completed an extremely thorough preflight before any flight. This pilot finished his near 20-minute inspection of his own aircraft, then climbed aboard and went through his preflight checklist procedure inside the cockpit. He was in the second row of aircraft from my vantage point, which prevented me from seeming his cowling plugs still in the cowling openings as he yelled “clear”.

As an prudent pilot, I turned to see which aircraft was starting and immediately upon his cranking the engine, the turn of the prop sent his cowling plugs flying through the air, landing in front of an aircraft parked three spots down the line from him! I went over and retrieved the plugs for him as he, in a very embarrassed state, climbed out of his aircraft. I handed him the mostly, intact plugs and he thanked me. He told me he always wrapped the connection strap between the two plugs around the propeller so that when he made this mistake they would come off… He knew it would happen eventually. I don’t believe he ever did it again though, I think he learned his lesson. Even on a cold morning, when leaving the plugs in after preheating his aircraft, he never again forgot what happened to him that day. After witnessing that, neither have I!


Cowling plugs are a great way to keep critters out of the engine cowling, though not all of them are designed like these ones with little warming flags, visible form the pilot’s seat to alert the pilot they are about to start up with the plugs still installed!

It is very common for pilots to get distracted… Leaving the cowling plugs in during the preflight is a common occurrence too, often to keep heat in the engine cowling or to prevent birds form flying in while the pilot completes their personal preflight trip to the restroom… Even the smallest distraction, a passenger question or text message can cause a break in the mental process and result in the embarrassing instance of starting the engine with the cowling plugs still installed. Usually this is a non-event, however damage to the plugs and even the sinner/cowling openings is not uncommon. Replacing the cowling plugs isn’t the worst thing (you probably needed a new set anyways, right?), but damage to the aircraft can be a little more costly…

Remember, these are just some tips… The definition of learning is a change in behavior as a result of experience. I am a firm believer that the experiences you learn from don’t always have to be as a result of your own actions. I whole-heartedly believe that I learned to always double check things from seeing others commit the acts. I always double check the cowling plugs are removed because I remember watching that set soar across the ramp at Mansfield on day many, many years ago. Similarly, when flying an aircraft that gets plugged in for the night during the winter to run the oil pan heater, I would triple check that the extension cord was unplugged. I watched too many instructors taxi aircraft out of the ramp with the 25′ or 50′ extension cords trailing behind them and saw as they all very embarrassed shook their heads, when the dispatcher noticed them taxiing by the FBO’s windows.

Maybe you won’t learn as well from just stories, but hopefully these stories gave you something to think about, and hopefully you’ll imagine that set of cowl plugs arching over the ramp, and maybe some day you’ll catch it before you commit a similarly embarrassing act…

-Fly Safe, @MTElia1B9


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