Preflighting your prop: your airplane's propeller is one of its most neglected components. Use these tips to help make inspecting it easy and its lifespan long.
If you have a constant-speed or full-feathering version, you also have a small collection of very expensive and specialized parts regularly subjected to massive forces. Contrast all that with what many pilots seem to think: A prop is a poorly designed handle with which to help move the airplane back in its hangar. In fact, according to the pros, treating your propeller like the critical component it is and lending it a little TLC every now and then can go a long way toward preventing costly maintenance. Or worse.
THE USUAL SUSPECTS
The average propeller's main enemy? It's not the wet-behind-the-ears private pilot who insists on using it to muscle the airplane in and out of the hangar. It's not even the guy who taxis over runway lights and into potholes while talking on a cell phone--more about him in a moment. Instead, prop shop managers and manufacturers' reps tell us it's aviation's oldest bugaboo: corrosion. Look at just about any metal prop out on the tiedown line. You'll probably find its leading edge is rough, with small pits and--if it hasn't been painted recently--some whitish discoloration. That's corrosion, and it's slowly eating away at the prop.
A prop's other main enemy is the nicks and gouges picked up in normal operation. Along with the pitting from corrosion, these create stress risers, weakening the prop blade. Nicks and the resulting stress risers can result in cracked blades. In extreme cases, part of the blade can break off in flight, creating a severe out-of-balance situation. If you catch it quickly enough and reduce power or shut off the engine altogether, the engine might not shake itself to death and the prop might stay on. Either way, you're going to get some real-world engine-out practice and probably pay for an engine teardown.
In really extreme cases, the prop simply departs the aircraft. Not only are you finished producing thrust for the day, but the prop's departure can drastically alter the weight and balance situation, shifting the airplane's center of gravity aft.
Your IA will smooth the lesser nicks and gouges at the annual inspection. Larger ones are problematic: The temptation is to smooth them out, too, but they might be large enough to condemn the blade. To make the call, your mechanic must consult the prop manufacturer's literature.
CARE AND FEEDING
"Props don't usually break," admits Ron Porciello of New England Propeller. But, he adds, "We have a problem out in the field. There's a lack of knowledge on how to check a prop and what to look for." And if most pilots' preflight inspections are any guide, he's right. Sure, run your fingers along the leading edge, checking for abrasion and nicks. But what about corrosion or, in the case of a constant-speed prop, blade security and what's going on underneath the spinner? The box at lower left summarizes Porciello's suggestions.
Again, according to Porciello, finding someone knowledgeable may mean the local prop shop. "The average mechanics don't know a thing about modern propellers. They were trained on 1940-50s technology and a lot has changed since then."
Some prop-related specs are readily available. For example, check your airplane's type certificate data sheet (TCDS) for maximum and minimum diameter specifications, plus pitch settings for a constant-speed model. If your airplane/prop combination is fixed-pitch, the TCDS also will include static rpm limits. Other specs, like minimum blade width, static balance and the like can only be found in the prop manufacturer's literature. Few owners have gone to the trouble to obtain that material, and even their mechanics usually defer to the local prop shop.
WHAT YOU CAN DO
Keeping your prop happy between visits to the shop isn't rocket science. Keep it clean and free of corrosion with an oily rag, and have a competent technician smooth out any nicks or pitting. Keep it painted. If you insist on polished prop blades, you'll be out at the hangar quite often, keeping corrosion away.
Pay attention when taxiing and during other ground operations, too. "If people knew how to fly and knew how to taxi, we'd be out of work," offered Porciello. Based on prop strikes alone, owning a prop shop is a "wonderful business," he adds, since his shop sees three or four of them a week. And no aircraft type is immune: "We see Aerostars and Piper Cubs" getting involved in prop strikes, Porciello says. H&H Propeller's Chris Yo agrees: "We had one guy in a twin who was trying to taxi and talk on his cell phone at the same time. He needed two new props, plus engine work."
If there's a universal complaint among prop shops, it's that owners basically ignore the manufacturer's recommendations on maintenance intervals. In fact, New England Propeller's Porciello says it's not uncommon to receive props "25 to 30 years out of overhaul." Of course, those are the extremes. Still, aircraft owners tend to their props last.
Of the "big four" propeller manufacturers--Hartzell, McCauley, MT and Sensenich--only Sensenich lacks a calendar-based maintenance requirement. That, according to General Manager Ed Zercher, is because his company only manufactures fixed-pitch propellers, where corrosion is the greatest long-term threat to airworthiness. Sensenich in 1999 revised upward its service interval requirements, from 1000 hours to 2000. One reason, Zercher admits, was to bring them in line with TBO limits of popular engines mounting Sensenich props.
For other manufacturers, most of which concentrate on constant-speed propellers, 72 months is the upper calendar limit and 2400 hours the maximum time-in-service before a prop mounted on a piston engine needs to see a shop. For some propellers--those Hartzells used in aerobatics, as an example--the TBO recommendation is 1000 hours or 60 months, whichever comes first. Another thing about Hartzells: Many hub models incorporate grease fittings. The company recommends greasing the hubs every 100 hours, which is something an owner with at least a private certificate can do.
For the most part, properly maintaining a propeller merely requires keeping it clean and corrosion-free, while ensuring it sees a professional every now and then. Be careful when inspecting it, of course, and pay attention when taxiing, especially on anything other than clean, dry pavement.
RELATED ARTICLE: CHECK YOUR SPINNER
A number of years ago, while sitting in ground school on the C-46 Commando, the instructor, John Deakin, all of a sudden blurted out, "Wake up and pay attention, now. We're going to talk about the props. Of all of the things on an aircraft that can kill you, the prop is the only thing that can kill before you know what happened. Everything else will kill you slow."
He was right. A separated blade can jerk the engine right off the airframe. Fortunately for us, propeller failures are rare, but they can and do happen. While I haven't suffered one, I may have come very close.
Last April, the weather was keeping my partner from picking me up on the way to Sun 'n Fun. Instead, I pulled my A36 out of the hangar and was fueling up to make the last-minute trip to Lakeland.
As I walked around the front of the airplane during the pre-flight, I touched the prop spinner's nose. I was rewarded with a funny little "click" noise. "That's weird," I thought as I walked past it. I stopped after a step or two and went back to touch it again. "Click."
I wiggled it back and forth and it seemed solid and not loose at all, but it went, "click, click, click" each time I pushed slightly on it. I really needed to get to Sun 'n Fun.
I opened the cowl and looked at the back of the spinner and backing plate and all looked fine. I could see nothing unusual.
The lineman was about finished topping her off and I was about to get in to fire up when something told me to look again. Did I mention I really needed to get airborne and get on down to Sun 'n Fun, and I was already late?
Frustrated, I got out of the airplane, turned the prop 180 degrees and reopened the cowl. I looked at the back of the spinner and backing plate again. I could see what looked like either a tiny crack or a small, thin rivulet of grease coming out from behind the crank flange. I tried to talk myself into the grease theory.
But the doggone spinner ain't supposed to go "click," a voice kept telling me, and this looked so small. Plus, I really needed to get to Sun 'n Fun.
The line guys at my home field, the Natchitoches (La.) Regional Airport, are all aviation program students at the local university. I called two of them over to take a look. I asked them if they would go flying and both said that they would.
Instead, I suggested I was a weenie and I wasn't going flying until we removed the spinner and took a closer look at the backing plate. When we did, it was clear that it was already cracked completely around one side and about one inch from a complete separation from the aircraft on the other side!
I seriously doubt it would have made one more flight without coming loose and ending up with either the spinner going through the windshield or worse. "Worse" could include severely damaging a blade and/ or destroying the propeller, which might have jerked the engine off its mounts.
That would have been a very bad day and all of a sudden I was looking pretty smart to these young aviators. Inside, I was frustrated at not being able to go. Deeper inside I was thinking about how glad I was that I had listened to that little voice.
The bad news is I missed Sun 'n Fun. The good news is I can try again this year.
A SIMPLE TEST
Using the thumb and first two fingers, New England Propeller's Ron Porciello advocates a four-direction test as part of a prop's preflight:
1. Try to move the prop blade fore and aft.
2. Attempt to rotate the blade in its hub (fixed-pitch need not apply).
3. Push and pull the blade toward and away from the hub.
4. Try lifting and pushing down on the blade. Does any blade react differently than the others? If so, the prop has a problem and someone who knows something about them should look at it.
AN AVIATION AAFETY MAGAZINE STAFF REPORT
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|Title Annotation:||AIRCRAFT SYSTEMS|
|Date:||Apr 1, 2009|
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