Stats for Multi vs Singles- Engine Failure rate

[Jaybird180]

I'm not trying to start a shouting match here, but I'd like some real-world data to backup the assertions I've heard that twins are safer than singles.

I hypothesize that the failure rate for an engine in a twin is higher than the failure rate of singles. Given that however, unfortunately I theorize that given the same failure rate we are likely to see higher injury rate in singles due to training and lack of options that the pilot of the mutiengine may have at his disposal.

I hypothesize that engine maintenance in personally owned singles tend to be deferred more often because of the idea that they have two.

 

[dmccormack]

oh boy....

 

[Fearless Tower]

oh boy....

There isn't enough accurate real world data because many engine failures in twins never get reported since the airplane made a successful OEI landing.

About the best you can do is some kind of comparison on single vs multi-engine hours flown vs accidents reported, but that alone won't necessarily give you the straight engine failure comparison that you seek.

 

[tonycondon]

there is no hard data on successful multi engine engine failures that result in safe landings

 

[sdflyer]

Here is a math... In single engine you have 50% chance of failure - engine either fails or doesn't. Nwo imagine two engine each has 50% failure rate ..50%+50%=100%

 

[Art VanDelay]

Well, here's the big deal in this argument that everyone seems not realize and that is - When you have an engine failure in a single you are ALL but guaranteed to read about it in the paper and NTSB. Conversely, there are NO good numbers that record the number of times a twin driver has safely returned or continued on to an UNEVENTFUL landing. For some odd reason the general consensus among many general aviation pilots is that twin engine airplanes INSTANTLY just flip over and crash when they have an engine failure or that they are only manageable by super-pilots with Yeager-like skills.

 

[Inverted]

Yep, a stat for a multi engine airplane is that you are twice as likely to have an engine failure. Thats about the only stat thats real when it comes to this stuff.

All I fly are twins but it is true...

 

[bobmrg]

No agency collects these data.

Bob Gardner

 

[Bob Noel]

there is no hard data on successful multi engine engine failures that result in safe landings

The same is true for ASEL engine failures that result in a safe landing.

However, I think I'd rather be flying a twin if my aircraft has an engine failure.

 

[Bob Noel]

I'm not trying to start a shouting match here, but I'd like some real-world data to backup the assertions I've heard that twins are safer than singles.

I hypothesize that the failure rate for an engine in a twin is higher than the failure rate of singles. Given that however, unfortunately I theorize that given the same failure rate we are likely to see higher injury rate in singles due to training and lack of options that the pilot of the mutiengine may have at his disposal.

While you are asking for data (that probably doesn't exist), you might want to weed out twins that can't maintain altitude on one engine (the 150hp? Apache comes to mind). An engine out in that kind of twin is pretty much like an engine out in a single, but maybe a lower sink rate and possibly a higher crashing speed.

A complication is the fact that failure rates vary from engine to engine, right? Some engines are a bit more tempermental than other engines.

Another complication is the engine monitoring that might or not be installed on the aircraft. You might expect a twin to be better instrumented so that impending engine problems might be detected *before* an actual failure occurs. otoh - someone with a single might be more motivated to detect symptoms of an impending failure.

I hypothesize that engine maintenance in personally owned singles tend to be deferred more often because of the idea that they have two.

um, huh? Do you mean "twins"?

 

[Mtns2Skies]

While you are asking for data (that probably doesn't exist), you might want to weed out twins that can't maintain altitude on one engine (the 150hp? Apache comes to mind). An engine out in that kind of twin is pretty much like an engine out in a single, but maybe a lower sink rate and possibly a higher crashing speed.

As well as Mountainous and high altitude terrain.

 

[bob_albertson]

I've had three OEI landings.... one was because we couldn't get the prop to windmill after an intentional shut down (mechanical failure)... Two have been in the SAAB. One was due to a coupling failing on a high pressure oil line, the other was an intentional shutdown because we couldn't reduce the torque below 80%.

I've never had an engine failure in a single, so that scientifically proves you have a 33% less chance of an engine failure in a single

 

[Ted]

There's really no good data on the failure rates. But if you have two engines instead of one, that does give you twice the probability of an engine failing.

As to which is better, there as also too many variables. A Seminole in Colorado is going to be less safe, because when the engine fails, you will be going down. Conversely, an Aztec in Florida will be very safe, because if an engine fails, you can easily make it to an airport. Single engine service ceiling and all.

My summary (and what I tell my students) is that a twin is safer if you're proficient in it. If you aren't proficient, then you'll likely turn a not-so-bad situation into a really bad situation.

 

[poadeleted20]

A Seminole in Colorado is going to be less safe, because when the engine fails, you will be going down.

...but not as fast as an Arrow whose engine has failed, giving you more options, and thus perhaps making it safer (as long as you maintain control). That's the problem with these single v. twin arguments -- too many variables.

 

[wabower]

Objection, speculation. How is engine failure defined? Big bang, everything immediately stops? Other? Is a failing engine different than an engine failure? If oil is pouring out of an engine that's still running, is than an engine failure? If there's any question on this subject, you can assume that I'm treating this as an engine failure and landing on the closest airport available. And did.

Well, here's the big deal in this argument that everyone seems not realize and that is - When you have an engine failure in a single you are ALL but guaranteed to read about it in the paper and NTSB. Conversely, there are NO good numbers that record the number of times a twin driver has safely returned or continued on to an UNEVENTFUL landing. For some odd reason the general consensus among many general aviation pilots is that twin engine airplanes INSTANTLY just flip over and crash when they have an engine failure or that they are only manageable by super-pilots with Yeager-like skills.

 

[Jim Logajan]

I'm not trying to start a shouting match here, but I'd like some real-world data to backup the assertions I've heard that twins are safer than singles.

I hypothesize that the failure rate for an engine in a twin is higher than the failure rate of singles. Given that however, unfortunately I theorize that given the same failure rate we are likely to see higher injury rate in singles due to training and lack of options that the pilot of the mutiengine may have at his disposal.

I hypothesize that engine maintenance in personally owned singles tend to be deferred more often because of the idea that they have two.

I believe this article addresses the points you bring up:

http://www.avweb.com/news/usedacft/182809-1.html

He discusses safety, operating costs, maintenance, reliability, and so on of twins versus high performance singles; here is what he says about safety (but read the whole article) :


"I recently finished editing a Cessna 310 safety review for the AOPA Air Safety Foundation. In the course of this project, I took an in-depth look at the safety record of the Cessna 310 and a group of comparable aircraft (Aerostar, Aztec, Baron, Commander, Crusader) during the eleven year period from 1982 through 1992. Some interesting statistics emerged from this study.
The overall accident rates of high-performance singles (like Bonanzas or 210s or Mooneys) and light twins (like Aerostars or Barons or Commanders or Cessna 310s) are astonishingly close. Twins have a slightly higher accident rate per 100 aircraft and a slightly lower accident rate per 100,000 hours, but for all practical purposes the accident rates are the same. The same is true if you consider only "serious" accidents that involve death, serious injury, or substantial damage. For both high-performance singles and light twins, approximately one-third of all accidents are classified as serious.

For both singles and twins, roughly three-quarters of all accidents are classified as "pilot caused". While weather-related accidents dwarf all other pilot causes in the single-engine accident data, the pattern for twins seems to be significantly different. Weather is still the leading cause of pilot-caused twin accidents, but a variety of other non-weather-related causes are quite significant: botched takeoffs and landings, controlled flight into terrain, improper IFR procedures, fuel exhaustion, and gear-up landings, just to name a few.

About one-fourth of all accidents are classified as "machine caused" for both singles and twins. Only a small fraction of those are engine-failure accidents. But it's interesting to look at the impact of that second engine on engine-failure accident statistics.

For the group of light twins we looked at, mechanical failures of the engine or propeller were responsible for One about 3% of all accidents. Breaking that down, 15.3% of all accidents were due to mechanical failures, and 20.8% of those involved the engine or propeller.

In contrast, roughly 8% of all accidents in high-performance singles were attributed to engine or propeller failure: 17% of accidents were mechanicals, but nearly 50% of those involved the engine or prop.

The statistics showed that a light twin is about equally likely to have a mechanical-caused accident as a high-performance single. But the twin's mechanical problem is most likely to be gear-related while the single's is most likely to be engine/prop-related. A single is about two-and-a-half times more likely to have an accident due to engine/prop failure than a twin (8% versus 3%). And if we assume that a twin is twice as likely to have an engine/prop failure (since it has twice as many to fail), then we can conclude that an engine/prop failure in a single is five times more likely to result in an accident than an engine/prop failure in a twin.

So are you any safer flying a light twin than a high-performance single? In terms of the overall and serious accident rates, the answer seems clearly to be no. But your risk profile changes somewhat: in the twin, you're less likely to be hurt by an engine failure, and more likely to be victimized by something else."

 

[Mtns2Skies]

Excellent article Jim

 

[Ted]

...but not as fast as an Arrow whose engine has failed, giving you more options, and thus perhaps making it safer (as long as you maintain control).

Potentially - unless you're in a situation where you're still going down off-airport. In the Rockies, that's more probable.

That's the problem with these single v. twin arguments -- too many variables.

Correct. You have to make your choice based on your preferences and the needs of your mission.

 

[Let'sgoflying!]

AOPA has a good supply of articles on the topic;

From 1998,

1991,

and 1973.

.

 

[Mtns2Skies]

Also don't discount that non-pilot Passengers would certainly feel safer and better in a twin over a single.

 

[Ted]

Also don't discount that non-pilot Passengers would certainly feel safer and better in a twin over a single.

I notice that without question.

My mother's responses to various events...

Obtain Private: "Well, you need your instrument rating. JFK Jr died because he didn't have an instrument rating."

Obtain IR: "Well, you should fly twins so you have a spare engine and are safer."

Fly twins: "Ok, I'm happy... as much as I'll ever be."

 

[Jaybird180]

Correct. You have to make your choice based on your preferences and the needs of your mission.

Then based on this statement (which I agree with), when prospective purchaser looking at aircraft, the number or engines should not be a choice in the selection matrix (or website search/toggle field). The options should be more like, useful load, range, speed, etc

 

[Mtns2Skies]

Then based on this statement (which I agree with), when prospective purchaser looking at aircraft, the number or engines should not be a choice in the selection matrix (or website search/toggle field). The options should be more like, useful load, range, speed, etc

Not necessarily, If the purchaser will primarily doing long legs over water then the number of engines does infact matter.

 

[Bob Noel]

Not necessarily, If the purchaser will primarily doing long legs over water then the number of engines does infact matter.

um, maybe not. If the article referenced earlier indicated that the serious accident rate is roughly the same. Unless a lot of those serious accidents due to engine out in singles over land would be survivable, then I don't think it really matters whether you have a twin or a single.

 

[COFlyBoy]

Then based on this statement (which I agree with), when prospective purchaser looking at aircraft, the number or engines should not be a choice in the selection matrix (or website search/toggle field). The options should be more like, useful load, range, speed, etc

As far as safety goes maybe it should not be part of the criteria, but for cost of ownership the number of engines has a big impact. Mx, overhaul, and fuel burn costs are higher for twins that provide equivalent performance to their single engine counterparts.

A good example is the piper Seneca which is a twin version of the PA-32. The fuel burn is roughly 16gph for a PA-32-300R at 75% and 24gph for a PA-34-220R where I'll bet the final cruise speed is identical. Even if the cruise speed was 10-20kts higher it wouldn't make up for the 33% increase in fuel burn.

 

[Jaybird180]

AOPA has a good supply of articles on the topic;

From 1998,

1991,

and 1973.

.

I don't believe one article quoted $2.25/gal
But the sentiment is that my intuition was correct. Singles are safer (for people - not necessarily so for insurance companies).

 

[Ted]

Then based on this statement (which I agree with), when prospective purchaser looking at aircraft, the number or engines should not be a choice in the selection matrix (or website search/toggle field). The options should be more like, useful load, range, speed, etc

Those are some of the choices for sure, but then there's more into it with a single vs. twin. It's not just about engine failures, it's about other failures - things like magnetos, alternators, vacuum pumps - that in a twin are benign and in a single can cause bigger problems. Reliability is definitely part of it, and what failure modes you can accept.

um, maybe not. If the article referenced earlier indicated that the serious accident rate is roughly the same. Unless a lot of those serious accidents due to engine out in singles over land would be survivable, then I don't think it really matters whether you have a twin or a single.

Sure it does. Remember that virtually any twin can maintain sea level. Being over water is the best place to be in a twin in the case of an engine failure.

Then remember that not all failures are engines. If an alternator goes out over water (or over wilderness areas) you may be in a world of hurt to find where you're going without your navaids.

However, Wayne has said on multiple occasions to ask his Pilatus-flying friend about the value of a single PT-6 over the Gulf of Mexico when it fails...

 

[Bob Noel]

Sure it does. Remember that virtually any twin can maintain sea level. Being over water is the best place to be in a twin in the case of an engine failure.

Then remember that not all failures are engines. If an alternator goes out over water (or over wilderness areas) you may be in a world of hurt to find where you're going without your navaids.

However, Wayne has said on multiple occasions to ask his Pilatus-flying friend about the value of a single PT-6 over the Gulf of Mexico when it fails...

I understand that a good twin can maintain altitude down low. But don't forget that the overall serious accident rate is pretty much the same for high-performance singles and twins. So whatever extra safety is provided by the 2nd engine is lost due to other failure modes.

 

[Ted]

I understand that a good twin can maintain altitude down low. But don't forget that the overall serious accident rate is pretty much the same for high-performance singles and twins. So whatever extra safety is provided by the 2nd engine is lost due to other failure modes.

The primary failure mode that I observe is poor airmanship or a poor understanding of systems. This, of course, is highly variable, and you have the power to change that through instruction and studying.

When people told me that the second engine only gets you to the scene of the crash, I always dismissed their arguments. Some twins in some conditions, yes, that is true. However the pilots who screw up and crash typically needed better training. So, I chose to get better training and learn the systems better. It's worked well.

 

[poadeleted20]

Keep in mind that light piston twins (other than the 4-banger trainers like the Seminole, which underperform singles of even less total power, e.g., 400 HP Seminole vs 300 HP Beech 36) have two engines not for redundancy, but because they need more power than one air-cooled horizontally-opposed engine will produce in order to carry the desired load at the desired speed. If we could make 500+ HP air-cooled horizontally-opposed engines, you'd probably see fewer light twins in the Aztec/Baron/310 class. However, since it takes that much power to haul six people with bags 500nm at 170 knots, the only way to do it is with two 250+ HP engines (radial engines being undesireable for many reasons). The downside is that if you lose one, you lose basic performance, not just redundancy. The only areas where you get real redundancy in those light piston twins are accessories like alternators, vacuum pumps, and hydraulic pumps, and with planes like the SR20/22, you can get that redundancy with only one engine.

 

[alaskaflyer]

there is no hard data on successful multi engine engine failures that result in safe landings

There is no hard data on single engine airplane engine failures that result in safe landings either.

 

[Ted]

Keep in mind that light piston twins (other than the 4-banger trainers like the Seminole, which underperform singles of even less total power, e.g., 400 HP Seminole vs 300 HP Beech 36) have two engines not for redundancy, but because they need more power than one air-cooled horizontally-opposed engine will produce in order to carry the desired load at the desired speed. If we could make 500+ HP air-cooled horizontally-opposed engines, you'd probably see fewer light twins in the Aztec/Baron/310 class. However, since it takes that much power to haul six people with bags 500nm at 170 knots, the only way to do it is with two 250+ HP engines (radial engines being undesireable for many reasons). The downside is that if you lose one, you lose basic performance, not just redundancy. The only areas where you get real redundancy in those light piston twins are accessories like alternators, vacuum pumps, and hydraulic pumps, and with planes like the SR20/22, you can get that redundancy with only one engine.

I don't think anybody claimed that you'd continue to your destination with the same performance if you lost one. However, being able to continue is the big thing. When I'm over Canadian wilderness or the Gulf of Mexico, knowing that I can make it to the nearest airport even if I lose one (which may be 200 nm away) is very nice.

 

[alaskaflyer]

Objection, speculation. How is engine failure defined? Big bang, everything immediately stops? Other? Is a failing engine different than an engine failure? If oil is pouring out of an engine that's still running, is than an engine failure? If there's any question on this subject, you can assume that I'm treating this as an engine failure and landing on the closest airport available. And did.

One data point for anecdotal reckoning: my engine failure event evolved over ten minutes or more (where I was aware of a problem) and resulted in a total power failure and emergency landing. I'm glad I was in a light single and not an underpowered twin where I landed because though it was at an airport it sure as heck wasn't on the runway. No damage other than the cylinder and manifold. Not captured in any official statistic.

 

[poadeleted20]

I don't think anybody claimed that you'd continue to your destination with the same performance if you lost one. However, being able to continue is the big thing. When I'm over Canadian wilderness or the Gulf of Mexico, knowing that I can make it to the nearest airport even if I lose one (which may be 200 nm away) is very nice.

If you lower the performance you demand of the aircraft appropriately, then the second engine will become redundant, but you'll have to adjust your gross weight significantly, especially over the Canadian wilderness with its high elevations. But then the plane doesn't need 500 HP to fly, and it's no longer a 6-passenger/500nm airplane, and that's my point.

 

[Ted]

If you lower the performance you demand of the aircraft appropriately, then the second engine will become redundant, but you'll have to adjust your gross weight significantly, especially over the Canadian wilderness with its high elevations. But then the plane doesn't need 500 HP to fly, and it's no longer a 6-passenger/500nm airplane, and that's my point.

To clarify, the Canadian wilderness in the eastern half that I fly over is very flat and low elevation. No mountains.

However at gross, the Aztec will still fly on one engine up to about a standard hot day, and maintain sufficient altitude for where I fly it mostly.

The Chieftain is a good example of a plane that needs all 700 hp, and is marginal on one engine. Again, in the eastern half of the country you'll probably be alright, but in the Rockies, not so much.

 

[poadeleted20]

However at gross, the Aztec will still fly on one engine up to about a standard hot day, and maintain sufficient altitude for where I fly it mostly.

IIRC, book single-engine service ceiling is something over 5000 feet, although my nearly 1000 hours in type suggests that a well-worn Aztec doesn't make book performance. The good news is that it seems better at holding altitude than making book OEI climb. So, losing one in cruise leaves you in a better place than losing one after takeoff, but that's probably no surprise. The beauty of a Part 25 twin is that as long as you don't violate the book, no matter where you are in the flight envelope from brake release to full stop, you can lose one and still fly away. Part 23 light twins don't have that requirement.

 

[Everskyward]

The beauty of a Part 25 twin is that as long as you don't violate the book, no matter where you are in the flight envelope from brake release to full stop, you can lose one and still fly away. Part 23 light twins don't have that requirement.

But I would say a Part 25 twin is way out the price range of most people.

Statistically it seems that it would be twice as likely for one of the two engines to fail in a twin vs. a single. I would say that if you can maintain control and avoid hitting the ground in your twin, the twin is safer. If you hit the ground either because you lose control or don't have enough performance a single is probably safer given that singles are slower and lighter on average so there is less energy to dissipate. But I think the biggest factor for most people is that a twin is going to be more expensive to operate than an equivalent single.

 

[Ted]

Statistically it seems that it would be twice as likely for one of the two engines to fail in a twin vs. a single. I would say that if you can maintain control and avoid hitting the ground in your twin, the twin is safer.

Exactly.

 

[Ted]

The good news is that it seems better at holding altitude than making book OEI climb. So, losing one in cruise leaves you in a better place than losing one after takeoff, but that's probably no surprise.

Which has been my point in the first place. Sometimes, fate is just against you.

The beauty of a Part 25 twin is that as long as you don't violate the book, no matter where you are in the flight envelope from brake release to full stop, you can lose one and still fly away. Part 23 light twins don't have that requirement.

However, a Part 23 aircraft in many situations does still give you the potential of making it safely back to an airport. That's what my 750 hours in my Aztec have told me, anyway.

 

[Bob Noel]

But I would say a Part 25 twin is way out the price range of most people.

Statistically it seems that it would be twice as likely for one of the two engines to fail in a twin vs. a single. I would say that if you can maintain control and avoid hitting the ground in your twin, the twin is safer..

Yet what about that avweb report that accident rates and serious accident rates were the same for high performance singles and twins?