Why Aircraft Engines Quit

let's look at an engine this is a mazda cx-5 which in automotive tax omni is called a crossover cuv or if you prefer a compact suv among the japanese manufacturers mazda is a niche company and their thing is sporty performance definitely off the mainstream this car has a really nice six-speed manual automatic transmission that i really like anyway the engine it's a four-cylinder inline 2.5 liter about 150 cubic inches and 184 horsepower four valves per cylinder with dual overhead cams and direct not port fuel injection direct fire coils for ignition and variable valve timing it has drive by wire for the throttle body control as automotive engines go it's not at the bleeding edge but it's pretty sophisticated mazda makes about a million and a half cars a year that's small potatoes by toyota and honda standards but still a lot of engines keep that number in mind so if you look at this level of mechanical sophistication it's logical to wonder how reliable is this thing i mean that's a lot of moving parts and electronics how often does it just up and quit how about never this never worked for you it sure works for me now let's compare the mazda engine to a typical airplane engine so this is about 40 grand on the hook here this is a continental i o 550n this is arguably the best or one of the best piston engines in general aviation today it's smooth kind of derated in this application which is a cirrus sr-22 and pretty efficient as airplane engines go it's a six-cylinder horizontally opposed air-cooled engine 550 cubic inches and 310 horsepower so that's 70 percent more horsepower on three and a half times the displacement compared to the mazda engine we just looked at big-ass cylinders with 5.2 inch bore and pistons the size of beer kegs a measly two valves per cylinder but what they lack a number they make up for in tendency to stick and burn this is a push rod engine in case you're not familiar push rods first appeared on buicks in 1904 and they haven't changed a hell of a lot since then the 550 has port fuel injection not direct injection and continental has a pretty good system for that it too has throttle by wire a big steel one that runs directly from the pilot's mids to a mechanical throttle body no mass air sensor here but it does have an air filter ignition is by a pair of magnetos magnetos were invented by bosch in 1897 and they're not too much different today although it took bendix a few years to figure out how you could really irritate the hell out of owners by requiring an inspection every 500 hours and by the way this engine has a time between overhauls of 2200 hours you might get that much service out of one it might even go 3000 hours the one behind me and the cirrus went 2400 hours or it might tank at 900 hours with soft cylinders or spun bearing either way it'll cost you north of 40 000 bucks to overhaul or about 10 grand more than the cost of a new mazda cx-5 in the whole of the known universe there are about 225 000 engines like this flying around give or take between continental and lycoming under 10 000 new engines a year are manufactured plus a bunch of overhauls keep those numbers in mind too so nothing wrong with old technology right the less complicated it is the simpler it is the less that can break right so that means an airplane engine ought to be a lot more reliable than a car engine and you'd want it to be for reasons i'm sure i don't have to explain but is an airplane engine actually more reliable than a car engine it's kind of hard to answer although you know i'm going to try but we can look at why they quit first of all define quit for my purposes quit means this the airplane loses thrust and the fight ends either as an accident or a reportable incident this is a quit or a lot closer to home this or even way closer to home this this is our mooney that ended in a muddy swamp in south carolina engine failures for various reasons initiated all of these accidents but how often does this happen well it's not quite an everyday thing but engine failures kill and injure people every year and damage or destroy a lot of airplanes the good news is that both fatal accidents and overall accidents in aviation have trended downward for the past 10 years but we seem to have reached a plateau one thing that hasn't changed much is that mechanical issues stuff that breaks on the airplane account for about 15 percent of all accidents every year but only about one percent of fatal accidents that's a small number but not exactly zero either it's not statistically insignificant neither is the fact that of those 15 percent of accidents caused by mechanical or maintenance problems more than half it's about 60 percent are the results of power plant problems if not total failures then serious enough to cause an accident that nibbles the wrists down some more but it's still not zero also not all engine failures make it into the database this one a cub engine stoppage few weeks ago probably won't make it into the data nor is it required to we don't know how many engine failures are never reported it's certainly some in the imaginary world i might have lived in when controlled dangerous substances were fashionable i could crunch numbers and come up with a failure rate based on hours but the available flight hour data is not very accurate for this report i'm relying on ntsb raw accident data which is itself kind of sketchy reports often lack detail sometimes reach no conclusion and are probably wrong sometimes but you play the hand you're dealt so we'll have to make some estimates so for this exercise i'm using 2018. that year there were 1 224 accidents all together with 117 listed as power plant failures according to the null data that's a little over two a week using the faa's rough hack on flight hours real rough that works out to 0.45 engine accidents per hundred thousand flight hours that's a little more than half the fatal rate but a tenth of the overall general aviation accident rate another way of looking at this is to survey owners and ask them how often their engines quit nasa did this very kind of survey in 2001 after they posited this stunner the current reliability of complex ga aircraft systems is unknown no kidding lots of data on airline level equipment but next to nothing on general aviation piston aircraft so the national researchers constructed a statistical model based on an imaginary six hour flight they used this complicated formula to shuffle things around and they calculated some probabilities exceedingly low probabilities as it turned out on this list of stuff that breaks on airplanes engines rank third of six not great maybe if you're flying over the rockies at night in a stose norm but pretty good odds so why do they quit what causes engines to just all of a sudden stop this may be a disturbing surprise but a third of the time no one knows 29 percent of engine stoppages have no provable cause the accident report explains what happened but doesn't say why there appear to be several reasons for this the biggest one is that in a lot of accidents the airplane lands the investigator shows up and the engine starts right up and runs just fine thus dashing the hopes and prayers of the hapless pilot who desperately wanted to be vindicated by a tank cylinder or fractured fuel line quite a few of these happen to this airplane the venerable cessna 150 whose continental o200 is famous for suffering carburetor icing so the airplane lands in a muddy field after the engine quits the still warm engine melts the ice and like magic the airplane starts right up the ice ferry strikes again more than a few of these have happened with an instructor on board forgetting to use the carb heat is a good way to turn a practice emergency landing into a real one also many of these investigations are pretty minimal and that leads to an interesting story about our mooney here recall in this video i explained that the engine quit on takeoff from an airport in south carolina and one of my partners landed at an assault marsh no injuries the ntsb didn't send an investigator which is not uncommon for no injury accidents instead the faa sent a dar a designated air worthiness representative he dragged the airplane out of the swamp bent the prop straight with a crowbar and started the engine right up no problem a couple of months later after we cashed the six-figure insurance check the airplane showed up in pieces on ebay really i called the guy who bought it and asked about the engine pretty sure he was an amp hey yeah nice engine runs great shame about your crash further research revealed that this wasn't an isolated incident i found some other accidents mainly moonies and piper arrows that used the same lycoming i o 360 with a similar pattern that made me wonder why wouldn't the insurance companies investigate these as part of a loss prevention program well they don't at least not very often one executive explained it to me this way if your claim was one hundred thousand dollars and that's about what ours was we might spend a third that much trying and not succeeding and discovering why the engine quit so if the insurer is making its quarterly numbers it's all lost in the wash it's all about the benjamins and that's just the harsh reality of it then there's the ntsb many of these investigations are too cursory and don't produce the usable data that they should the safety board is sending its investigators to the fa's engine school to improve this but they've got a long way to go unknown as a cause in the third of engine accidents is unacceptable in my view back to the pie chart here for my purposes i'll lump fuel exhaustion fuel starvation and fuel contamination into one category even though these technically aren't engine failures but they cause engine failures and they're all preventable taken together these account for the largest known slice of engine failures and most of those are aviation's favorite stupid pilot trick running the airplane out of gas as recently as a decade ago that used to happen twice a week but now we're down to less than once a week so we're getting better about half the fuel related engine stoppages are mismanagement say selecting a tank with only air in it and being surprised when the engine quits i'd like to say i've never done that i'd also like to say i'm dating jennifer lopez both are equally true one reason for mismanagement is this it's the fuel valving setup in a mid-1970s twin cessna easy to make a mistake and select the wrong tank another reason is panicking after choking the engine and not being able to switch to a tank that has fuel in it before the glide intersects the planetary surface fuel contamination accounts for about five percent of engine stoppages most of these are undetected water in the tanks see this video i did last year on this topic and remember to always sump the tanks always miss fueling is part of that five percent too that's what put this 421 into the grass it was topped with jet a same advice sump the tanks and smell the fuel if you have doubts use the method of dumping the suspect fuel onto a paper towel and look for a faint straw colored stain when the fuel evaporates that signifies the presence of jet a if the fuel is mostly avgas this method might not work well however carbice accounts for six percent of all engine stoppages some engines are more susceptible than others mainly continentals lyco means less so you know what to do engines have carburetor heat for a reason so use it when the engine is pulled to idle or at low power and make sure the linkage is in good shape and that it actually works sometimes that cable has come loose and isn't connected to anything now this is where things get serious get rid of all that other stuff and the two slices of pie left are structural failures and maintenance cause failures the two are sometimes related first structural failures when a connecting rod blows out the side of the county and the windshield is smeared with oil that's nature's way of telling you you've had an engine failure structural failures include broken crankshafts fractured magnetos burned or broken valves and camshafts crumbling rocker arms cracked cylinders failed oil pumps and sundered crank cases among a list that's not too much longer than that thankfully these are not common but they're not rare either numerically 17 accounts for about a half a dozen a year but the real number is probably quite a bit higher because not everything is reported remember our mooning why do the big parts fail well let's consider crankshafts in an airplane engine it's a big heavy beefy part that also happens to carry a lot of load not that long ago crankshaft failure seem to be caused by manufacturing defects in 2002 lycoming had a multi-million dollar recall of crankshaft after a series of in-flight failures related to metallurgical issues continental went through a spate of these two crankshafts also failed due to prop strikes that don't get reported and that caused cracks that later fail the shaft none of these are common but they do happen in the typical engine overhaul shop these days up to a third of all the work done as prop strike inspections and for some shops it's more like half some crankshafts fail because of lubrication failures either low oil or blocked oil galleries camp shafts also fail most commonly due to corrosion or lack of lubrication which causes spalling these pop up in the accident reports from time to time and that yielded this rarity from the ntsb files an actual runway turn back that didn't work out it's a cessna 172 turning back to the runway after a partial engine failure caused by a worn cam the airplane appears to be in a stall mush but obviously it didn't spin it landed hard and although everyone survived there were injuries this is what the cam looked like this is what a really badly spalled cam looks like and this is a set of valve lifters that were within hours of failing fortunately these are often caught before an accident happens either through an oil filter inspection occasionally oil analysis or just loss of power valves also tank due to poor fitting and guides or substandard materials lead nav gas doesn't help this and no lead is not a valve lubricant when rods fail the engine rather spectacularly comes apart and sheds parts and oil this often happens because the big end bearings fail due to lack of lubrication they spin on the crank journal and just come apart sometimes these are hard to categorize for example i found a few rod bearing failure reports that were caused by under-torquing of the bolts when the engine was overhauled so is that a maintenance or a structure failure well it's both a noticeable number of failures occurred after the engine had cylinder work done which as any owner will tell you is a routine fact of airplane ownership what appears to happen in these failures is well let's just look at an engine and for that i'm at my favorite engine shop zephyr engines in zephyr hills florida let's start with a basic look at an aircraft engine this happens to be a continental o200 small engine four cylinder but it's fairly typical of how aircraft engines are built two big castings with these heavy-duty studs holding the thing same thing together crankshaft goes in here the cam goes up here and the cylinders bolt in from each direction and this is what the connecting rods look like and as you can see they're pretty heavy-duty and when an engine is built up it's done from the crankshaft outward and because this entire structure is light and highly stressed all the torque values on these fasteners are critical especially the rod bolts and case studs when an engine is partially opened up for cylinder work those rod bolts sometimes don't get re-torqued correctly especially on continental engines and they end up breaking or coming loose and that is an ugly really noisy failure if the case bolts aren't properly torqued the case halves begin to work leading to fretting and that can cause the crankshaft bearings to spin in their mouths resulting in rapid wear and a progressive failure it's common for the front nose bearing to fail first this can produce a rough partial failure not the engine exploding thrill of a thrown rod cap but the engine will eventually come apart while we're here a look at cylinders cylinder failures aren't that common but when they do come apart it sometimes cracks around the spark plug holes or radial cracks where the cylinder head mates to the barrel or sometimes at the base of the barrel these also tend to be partial failures since the piston and rod remain attached rather than blowing a hole in the cowling and treating innocent bystanders to an oil shower now which flavor of engine fails more often continental or lycoming neither it's rotax surprised well i was let me explain how i arrived at this we don't have reliable hours flown data so we can't calculate a rate per flight hour that way but we do know how many registered engines are in service so this data represents failures per 1000 registered engines the caveat here this data is based on small numbers only 13 total accidents over the six year period we studied so a few accidents can swing the conclusion one way or another i couldn't determine why rotax is higher than the other two but it may be small numbers effect some of those rotax accidents were fuel related but there were mechanical failures in there too but why do continental engines appear to have a higher failure rate than lycomings i have a theory the continental engine population skews more toward high output large displacement engines like the i o 550 series and the cirrus i showed you at the beginning of this video those engines are more highly stressed and often get midstream cylinder work this probably makes them more susceptible to failure hardly a month goes by when we don't report a spectacular fatal crash on takeoff leading to the conclusion that engine failures on takeoff are more deadly than in other phases of flight but is that really true let's blow out the pie chart again according to this accident sweep failures occur about equally on takeoff and in cruise as a percentage of all engine failure accidents it's in the mid 30 percentile range but takeoff represents a smaller portion of the total flight time so yeah on a per hour basis the risk during takeoff is definitely higher but what about the fatal accident risk this is where it gets interesting for both takeoff and cruise engine failures fatalities occurred in one-third of the accidents so on an occurrence basis take-offs are no more deadly than crews the eye-opener is that an engine failure on takeoff can be deadly for sure but two-thirds of the pilots survive it same is true of cruise engine failures two-thirds survive them too but engine failures and crews ought to be far more survivable because you have more time and more options to find a place to park the airplane you might bend it a little but it ought to be survivable either way you should regularly practice both emergency landings out of cruise flight and responding to a power failure immediately after takeoff i've posted a couple videos on this subject and the links are down in the description wrapping it all up we know this much about engine failures at least half of them and probably more than that are caused by pilots or mechanics doing something wrong or stupid out of the blue no mechanical failures are under 20 of the total engine failures so that's a small fraction of the one percent of fatal accidents caused by mechanical failures overall you can reduce it further by doing a few things and i really shouldn't have to say this but put enough gas in the airplane check the tanks for water always never skip it put enough oil in the airplane and consider using oil analysis that might detect a failure trend before it is a failure same applies to engine monitors modern ones record engine data that can be analyzed for trends that's how the airlines do it know how the fuel plumbing works and how much fuel the tanks actually hold not what the poh says they hold use the carburetor heat when you're supposed to don't skip engine maintenance including mag inspections and once every couple of months i like to pull the cowls off the airplane and spend 15 minutes inspecting stuff looking for loose fasteners shaped hoses leaks that sort of thing i once found an oil filter that hadn't been safety wired and was starting to come unscrewed and a loose alternator cable it's worth looking otherwise don't stress too much about an engine failure on the long list of stuff that will kill you and flying it's actually pretty far down the list thanks to l.j warren here at zephyr engine for letting me paw around the scrap pile bravo i'm paul bertorelli thanks for watching oh and by the way my mazda quit yesterday the battery went belly out [Music]

Channel: AVweb

Views: 253,315

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Id: mwpzTnLC8BY

Length: 24min 24sec (1464 seconds)

Published: Thu Apr 08 2021

Reddit Comments

[u/Detraxa]

Literally watching that right now! Great video