Why would someone prefer to own a single-engine vs. a twin-engine airplane?
What are the pros and cons of each?
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asked Nov 7, 2014 at 2:26
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1$\begingroup$ This old AvWeb article might be of interest. $\endgroup$– voretaq7Commented Nov 7, 2014 at 2:55
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$\begingroup$ No-one going to do the old saw about when the first engine fails on a twin, the second will get you to the site of the accident? Me neither! $\endgroup$– Dave GremlinCommented Mar 14, 2017 at 23:38
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$\begingroup$ hahaha (9 more to go....) $\endgroup$– Walrus the CatCommented Mar 18, 2017 at 23:54
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$\begingroup$ That may be true for Part 23 aircraft, performance on 1 engine does not need to meet the much stricter requirements that Part 25 imposes. $\endgroup$– KoyovisCommented Sep 15, 2017 at 5:21
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$\begingroup$ I own a twin and a single. A single is far safer as shown by every NTSB study done since the 1960’s. $\endgroup$– user959690Commented Sep 27, 2018 at 3:41
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3 Answers
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In the light aircraft world there are a few trade-offs to consider. The short version:
Single-Engine Pros:
- Simplicity
One engine means fewer controls, a simpler fuel system, simpler vacuum system, simpler electrical system, etc. - Operational cost
Only one engine to feed, so you spend less on - fuel & oil per flight. - Maintenance cost
Only one engine to maintain - One set of spark plugs & cylinders, One engine to overhaul.
Single-Engine Cons:
- No redundancy: You have one engine, if it fails you're a glider.
- Singles generally tend to have less spacious cabins than twins.
Twin-Engine pros:
- Redundancy
If you lose an engine the other will give you a chance to get to a landing site. - Cabin space
If you step up beyond "twin trainers" you can get a very nice cabin in a piston twin. - Equipment
Again, if you step up beyond the "twin trainers" twin-engine planes tend to be "traveling aircraft", so they often come with nice features like deice/anti-ice equipment.
Twin-Engine cons:
- Operating costs
Two engines to feed (fuel & oil costs double). - Maintenance costs
Two engines mean twice the cylinders, twice the spark plugs, etc. - Complexity
The redundancy you can get in a twin comes at a price: Synchronizing the propellers, a more complex fuel system (crossfeed), paralleling alternators, vacuum system failover, etc. - Short and Soft field performance can be lacking.
- Failure of an engine can be problematic.
In addition to the operational considerations there are some other practical items to consider: twin-engine aircraft require a multi-engine rating (which means more training time). Twins will also generally be more expensive to purchase/rent (and may be more difficult to rent when away from home), and will likely cost more to insure (particularly for low-time pilots).
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$\begingroup$ This may sound naive, but is there no power / speed difference? $\endgroup$ Commented Nov 8, 2014 at 0:52
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$\begingroup$ @WalrustheCat There is absolutely a power difference, and usually a speed difference, but it might not be what you expect: the Seminole is a twin with 360HP that cruises at 162kts with a full-fuel payload of 543lbs, the Arrow is a 200HP single and cruises at 137kts with a full-fuel payload of 528lbs, and a Cirrus SR20 (also a 200HP single) will do 155kts with a full-fuel payload of around 550lbs. $\endgroup$– voretaq7Commented Nov 8, 2014 at 3:27
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$\begingroup$ @WalrustheCat Continuing from the example in my other comment, if you expand the list of singles to include High-Performance aircraft you can find planes like the SR22 (310HP - 50 shy of the Seminole's combined horsepower) which will leave a Seminole in the dust in terms of both speed and useful load, and do so while burning less fuel. $\endgroup$– voretaq7Commented Nov 8, 2014 at 3:34
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$\begingroup$ My granddad always said multi-engine aircraft were difficult to trim...that's not mentioned in this answer though. Is his opinion valid or..? $\endgroup$– elrobisCommented Mar 14, 2017 at 14:28
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1$\begingroup$ @elrobis In my experience multi-engine aircraft as a class are no more difficult to trim in the pitch axis than single engine aircraft as a class as long as both engines of the twin are operating. Of course, if you've lost an engine on a twin, trimming then becomes more of a problem in all three axes. Is it possible your granddad was referring to the need to keep the rpm of the engines in synch? That is difficult to do on older twins that do not have automatic propeller synchronization. $\endgroup$– TerryCommented Sep 15, 2017 at 4:33
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Insofar as the safety issue that's been mentioned in the previous answers, there's a historical perspective on the question. Engines are far more reliable than they used to be, and turbine engines are typically more reliable than piston engines.
As I remember, most pilots coming out of WW2 (including my father), considered twins safer than singles, certainly for over-water operations, but with the caveat that the pilot of a twin losing an engine had best be proficient in handling that.
Engine technology changed. For example, the Cessna AT-17/T-50/UC-78 Bobcat, with two radial engines, was a 5-place twin in use through and after the war. Certainly an argument could have been made that it was safer than single-engine planes with radial engines in use just after the war. I certainly felt that way, an opinion gotten from my father. However, by the time the six-seat Cessna 210 came along with it's horizontally opposed engine and flying higher and faster than that old twin, my guess is that most pilots familiar with both aircraft would prefer the single. I certainly did, especially since I saw two engine fires that happened when starting the T-50 radial engines.
Now bring on the turbine-powered singles. My guess is that most pilots would prefer a single turbine-powered aircraft to a horizontally opposed twin. Having spent a lot of time in twin-engined, horizontally opposed, Cessna 310s (including one engine failure), personally I would opt for a single turbine, especially if it was a PT6 or Garret TPE331.
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1$\begingroup$ I dig your reflecting on the experience and opinion of the WWII flyers in this respect. This is wildly naive I'm sure, but I always romanticized the idea of bringing home a crippled, shot up B17 and landing it against the odds amidst the other-worldly flight characteristics of odd engine configurations, weird drag, and non-functioning control surfaces. But I recently read A Higher Call and got a reality check. The idea of saving the day is compelling, but there were alot of other forces to contend with, especially early in '43 when the Luftwaffe was at the top of their game. $\endgroup$– elrobisCommented Sep 15, 2017 at 13:54
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There are single engine, long range, large cabin aircraft. A Flying Magazine article takes the example of the PC-12 in discussing single engine benefits:
All of the size, range and payload capabilities of the PC-12 flow from the fundamental design choice of using only one engine. Carrying the fuel to feed a second engine, plus the weight and drag of the engine itself, all cut into range and payload. The second engine and its costs also mean the twin-engine turboprop that comes closest to the PC-12 in cabin size and full fuel range falls hundreds of miles short when the same payload is aboard, and costs at least a million dollars more to buy.
The con of having just the single engine is of course the consequence of an engine fail. If the single engine is very reliable this may be considered a moot point, especially since:
- Single engine aircraft are certified under CFR 14 Part 23: it may not have the ability to climb and the operational redundancy after engine fail that Part 25 aircraft must have. So having a twin engine Part 23 aircraft may make the owner feel good about redundancy, but the aircraft's certification rules never demanded this to be demonstrated.
- A twin engine has twice the chance of engine failure that the single engine has!
An aircraft with a very reliable single engine therefore seems to be the best choice under Part 23.