Avoiding the Sounds of Silence: A Dive into a Healthy Engine Mayday, Mayday, Mayday, N666FM, engine failure. 4300 feet, 2 miles Northeast, Coyle VOR, attempting to land MJX, 2 souls on board. Thankfully, this is something most pilots will never experience in a lifetime of flying. Why is that? How have we gotten such reliability out of these 4- and 6-cylinder piston engines? The easy answer is redundancy and required aircraft maintenance. What happens in between those 100-hour and/or annual inspections, however, is where the pilot plays the critical role of observation and awareness, followed by reporting and continuing education.
These aircraft engines are like good friends. They talk to us all the time, albeit, through inuendo and the occasional subtlety. It’s the rare case that our friends tell us directly what’s on their minds. The JPI engine monitoring unit did not tell the pilot of N666FM that the engine would fail that day, however, what did it say in the leadup to this emergency? Let’s be honest, there is a lot going on under the cowl of a single engine airplane. None of us can know everything, but hopefully the following will encourage us to get to know our aircraft just a little bit better. We’ll explore the main causes of complete engine failure as well as partial power loss. Fuel, air, and spark are the three essentials to combustion. No fuel, no power. High oil temperature and low oil pressure and / or high cylinder head temperatures should be considered “Act Now” items. When you takeoff with fuel less than a known quantity, such as full or to the tabs, how much do you have? For the high-wing Skyhawk in our fleet, you can simply stick the tanks and read the remaining Avgas to know for sure.
Our low-wing Pipers are not that transparent. We are always guessing, in their case. Our Mooney and Saratoga have site gauges on the wings, but like flight deck gauges, they’re not to be relied upon. We should be able to ascertain when the last fueling occurred if we inquire with line service. That’s a start, but how many hours were flown since that time? We can check the tach logs for tach time only, which is generally 30% less than actual block time, or how long the engines been operating and sucking in fuel. What’s the average endurance of our aircraft with fuel to the tabs or fully topped off? How many gallons are you typically burning per hour? The only time the fuel gauges are required to be accurate is when they’re empty.
The self-service fuel farm at Lakewood has been out of operation for a while now. If you have a late evening flight, ask the pilot before you to top it off before their mission, if they’re able. If you have an early morning flight, be ready for the possibility of less than sufficient fuel. BLM’s self-service fuel farm is possibly the best and closest alternative if you’re too low for your planned flight. Fuel starvation is the most egregious act we can star in, if in doubt, take fuel or cancel your mission.
If fuel is the suspected problem, switch to your other tank and turn the fuel pump on, if installed. Be sure your primer is in and locked. At the JAC, we’ve had engine roughness reported due to an unlocked primer. Even with the primer slightly open and unlocked, pressure could suck that needed fuel to air mixture from the engine and result in engine roughness or failure.
There is a report of a pilot in a C-152 who lost engine power due to a carb issue and was able to limp back to an airport by continually feeding their engine with this needed fuel by priming all the way to the ground. The reason this worked is that the primer delivers fuel to the intake valve or valves of the cylinders themselves and beyond the point of that aircraft’s carburetor. Primer installations vary, with some only feeding one cylinder and others feeding all. Therefore, some aircraft are easier to start when cold.
Carburetor ice is another easy one. Our carbureted engines are susceptible when outside air temperatures are between 50-70 degrees Fahrenheit, and the relative humidity is more than 60%. So, even on that 90-degree summer afternoon with some humidity present, be mindful of this possibility. Signor Venturi proved that as air moves through a constriction, it speeds up and the pressure decreases. As pressure decreases, the temperature will drop, possibly to the freezing point. This is the process occurring at the throat of our carburetor while the engine is running. Cessna’s are much more susceptible of this and therefore require carburetor heat on every landing or with prolonged low power settings. Piper says that you only need the heat on if you suspect carburetor icing. The difference may be in the proximity of the Piper’s exhaust gasses and the carburetor itself. Check the carburetor heat on the ground and look for a drop in RPM as hotter air is mixed in with your fuel. In the air, any engine roughness should consider fuel supply and carburetor ice simultaneously. If ice is suspected, use the carburetor heat, and expect a momentary additional loss of RPM and then an increase to almost normal as the ice is melted. You’ve introduced hotter, unfiltered air with the carb heat and therefore may need to lean the mixture a little further if extended use of the carb heat is planned.
If fuel and carburetor ice is not the culprit, maybe your induction or air intake system is to blame. The best preflight check for this is to assure your air inlet filter isn’t fully soaked and impacted with dirt, oil, or debris and cowling inlets are unblocked. Check that your baffle seals seem to be meeting the top of your engine cowling and directing the air properly over your cylinder heads as well as proper operation of cowl flaps in the case of our Mooney. While these last few items seem more cooling related, it’s all intertwined. A properly sealed cowling will provide for an important small pressure difference between the top of your engine and the lower cowl area and aid in a proper fuel to air mixture.
Inflight, if alternate air is a pilot selectable action, your checklist should direct you towards this item. This is the case in our Saratoga. In our Mooney, a blocked air intake should bypass automatically. Either way, or If in our other aircraft, start by leveling your altitude and
increasing airspeed to allow more air into your aircraft’s induction system. This is the second most critical piece of the puzzle that is combustion it’s on par with the fuel supply.
Mayday, Mayday, Mayday, Ocean County Traffic, N666FM, engine failure, 2-mile final Runway 06, this is a real emergency.
While your throttle controls how much power you seek from your engine, the mixture control aids in that balance between air and fuel and lends to a smooth running powerplant or a rough one, to no power at all. The mixture lever allows us to start the aircraft and shut it down, but what happens in between full rich and full lean?
This is a great time to consider that JPI or other such engine monitoring unit. Beyond checking for your oil temperature, pressure, and fuel being in the green, The CHT’s, and EGT’s are the real story of our engine and it’s one being continuously written. Get to know your engine and each of their cylinders. What are their normal readings? On the 4-cylinder Lycoming engine, cylinders 3 and 4 are typically a little hotter than 1 and 2 due to their placement closer to the firewall of the aircraft and therefore receiving less immediate cooling from your cowl inlets. Snap a picture of your JPI or engine monitoring unit at various phases of flight to learn your aircraft’s baseline.
Cylinder head temperatures are our most critical values. Lycoming says the upper limit or redline is 500 degrees F on our fleet’s engines. For longer engine life, 420 degrees F is a number we should stay below. Even at 400 degrees CHT, the aluminum piston and piston head are only 50% as strong as when cold.
However, keeping your cylinders too cool and you’re likely running too rich and building up carbon and lead deposits that will block the intake and exhaust valve seals. When those valves don’t operate just right and form a tight seal at the point of combustion, that cylinder will operate less than desired.
The runup check, or ignition check as it relates to not just magnetos, but, wiring harnesses, spark plugs, and the ignition control switch is more important than we know. Let’s see how we can learn more of this phase of pre-flight.
A bad mag check may mean your spark plugs are to blame. As mentioned before, redundancy is a key factor in the reliability of these engines. Each cylinder has two plugs, fired from separate magnetos. If a plug is fouled, failed, or cracked you’ll experience an uneven detonation during the combustion stroke in the cylinder itself, causing a certain roughness in your engine’s operation. Fouled spark plugs will usually present themselves during this pre- takeoff run up check. A greater than normal drop in RPMs or a rough sounding or misfiring engine is your chance to take pause. Also present could be a colder than normal CHT on your engine monitoring unit.
Although our mag RPM drops can be as high as 175 RPM, the electronic magnetos in 40JA, 41JA, and 44JA should be significantly less than our engine driven magnetos. On 41JA, a drop of more than 50 RPM is likely telling us something.
The following is a plug from our Cessna Skyhawk, N73335 on May 06, 2021. The buildup of lead extended all the way to the side wall of the plug and was shorting itself out while the engine was being run up. To the members doing the run up, it was a rough magneto check on one side. They attempted to burn off the potentially fouled plug or plugs to no avail. After a second mag check, the engine was misfiring and clearly wasn’t safe to fly.
To burn off the occasional carbon or lead buildup on your spark plugs, select both magnetos, lean the engine aggressively, or just before any RPM drop is identified and heat them up for one minute. After that, try the magneto check again with the mixture back at full rich and see if that corrects the issue. If not, cancel your flight and call the trustee. Only attempt a burn-off once. Sometimes a plug is sufficiently fouled that the carbon deposit extends from the gap of the plug where the spark occurs to the side of the plug wall itself, grounding it out and therefore failing it fully as seen above.
The following scenario occurred more recently at the JAC on N41JA. While a few members noticed a higher-than-normal EGT reading on cylinder #1 over the course of a couple flights, it was the lower-than-normal EGT and CHT on cylinder #4 that was talking to us.
The problem was a cracked plug on the colder cylinder #4. There was obviously less than optimal combustion in that cylinder. In the lead up to this discovery, one member reported an intermittent RPM drop in the pattern and then a return to normal. Another member squawked a slight vibration in the engine while airborne and that’s where the JPI image above came from. If safe to do so, always snap a picture of your engine gauges and JPI display when the engine just doesn’t seem right. This will hasten a solution by enabling maintenance to target a particular cylinder or magneto, for instance.
Although the JAC recommends leaning aggressively on the ground, this is only minimally helpful as until our power is stabilized above 1500 – 1800 RPMs for an extended period, there isn’t exactly sufficient heat to prevent spark plug fouling. This is still deemed useful for our operations. The aircraft POH’s and Lycoming engine manuals typically state that until you’re at 3000-5000 feet or higher, leaning isn’t required. This doesn’t mean we shouldn’t seek to take care of our friend. If conducting maneuvers with altitude changes and varying power settings, it’s best to leave the mixture full rich. However, if heading for that $200 hamburger, considering inflation, pick a desired power setting from your POH and lean the engine.
Leaning can be simple and leaning can be exact. Simply put, to lean your aircraft engine, level off at cruise altitude, set your power to 55%, 65%, or 75% power as desired and pull the mixture back slowly until you experience a slight engine roughness and associated power RPM drop. Then enrichen the mixture back to the original RPM and a smooth-running power plant. This should be close to peak EGT or exhaust gas temperature. Peak EGT will provide our lowest gallons per hour or highest endurance for our flight. The more we pull the mixture back, the less fuel we are using. Our other typical option is to run 50 degrees on the rich side of peak ROP, or a best power setting. Best economy will yield your lowest gallons per hour fuel flow and provide the greatest endurance for your mission. Best power will get us there quicker but use more fuel.
Leaning our engine can also be much more specific with an engine trend monitor. 4 of our 6 aircraft have a JPI engine monitor installed. This gives us precise engine health and monitoring values for us to target and observe. Become familiar with your aircraft’s engine monitoring system and watch a couple videos on how to lean properly. Lycoming publishes maximum CHT values of 500 degrees that we should never approach in any of our flight regimes. Several of our JPI instruments are set to a redline value of about 400-degrees to stay well away from those upper limits. To understand the importance of this, you’d need to know that even at 400 degrees CHT, the aluminum cylinders are now only at 50% of their cold strength. That makes the metal much more fragile at higher temperatures. EGT is another matter and fine indicator of other trouble in these engines. Lycoming doesn’t publish upper limits for these values; however, 1650-degrees is very hot. While CHT is the temp at the top of the cylinder head itself, EGT is the temp of the gasses being expelled from the exhaust valves of their respective cylinders. Higher than normal exhaust gas temps could be an indication of a sticky exhaust valve on that cylinder. Lead builds up in there over time and then gums up the valves normal opening and closing operation.
Ocean County Traffic / Unicom, N666FM is disabled on the approach end of Runway 06. Request a tug to your maintenance shop. We made it. If you’ve read this far, I’d like to think we’re all part of the maintenance team responsible for keeping our fleet of friends happy and healthy. The items addressed above are all those items you can see or hear as the pilot. As many of us aren’t flying around with the maintenance technicians responsible for our fleet’s upkeep, it’s up to us to report the abnormalities and anomalies, however slight, to our Trustees and subsequent members flying your aircraft. As many of us don’t have an A&P license, the best we can do is report anything that seems out of character with our airplanes. Many larger mechanical issues often give us some fair warning ahead of time. Usually over the course of several flights, dozens of hours, or possibly over the span of weeks.
However small an item may seem to you; we need to send it up the ladder in order to allow our maintenance staff the time to put the full picture together and plan a proper proactive diagnosis. Our first step is squawking the plane in AircraftClubs.com and calling the Trustee. If you wouldn’t fly it again, then recommend it for grounding. There is absolutely no shame in taking this step and letting the professionals have a look. The next thing you can do as a courtesy and second level of protection is to notify the next few members scheduled to fly that aircraft and advise of the uncertain nature of its airworthiness.
As pilots, we’re all continually learning. We invite you all to take a deeper dive into your aircrafts engine and systems alike. Some of the resources listed below would be a great place to start. JAC Safety Committee
Sources and available resources: https://www.aviationpros.com/home/article/10381828/aviation-spark-plugs The internet and YouTube Videos Airplane Engines – A Pilot Friendly Manual from Pilot Workshops Lycoming Engine Manuals O-320, O-360, IO-540.
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