An affordable twin???

[IO540]

It is the engines, mostly.

Which is more than sufficient to deter serious usage.

[Fuji Abound]

Ah yes IO, you are right.

You dont really want to go anywhere in a twin if you are going to spend the whole time wondering when the engine is about to quit on you. I was at FL95 from Ireland having just got on top after a climb up through solid IMC from about 1,100 feet, and I recall thinking I would prefer to make my way down with the same number of engines as on the way up.

[wsmempson]

In answer to the original question, how about Gabriela Irimia?

[liam548]

Originally Posted by LowNSlow

Bringing it back to the realms of affordable VFR day-only twins I can see why Chris Heintz isn't putting any effort into developing the Gemini. The extra cost of the 2nd engine isn't offset against the potentially increased risk of an engine failure. The reason I looked at this initially was that then I had the potential to be flying across to Ireland regularly and for that I would like two engines. I know the Rotax 912 or Jabiru engine that the Gemini would potentially use seem as reliable as your average car engine so the actual risk is relatively low, but half way across the cold Irish Sea I would feel greatly reassured looking past my 10-year old daughter at the spare "get out of jail free" card sitting on the wing!

how do aircraft engines compare to car engines for reliability generally?

[SNS3Guppy]

I tend to agree with IO540 in supposing that theoretically you have double the chance of engine failure with a twin.

That's really not the case. The truth is you have the same chance with everything, every time. Either it will, or it won't. Period. This doesn't increase with engines, with numbers of takeoffs, with anything. Either it will, or it won't. Either it will run, or it will fail.

One would imagine with four engines we'd be having failures right, left and center with odds and chances increasing in proporition to the number of engines...but suprisingly, no.

In a light piston twin, the second engine isn't there for safety. It's there for performance. Specifically, climb performance. Many light piston twins won't maintain altitude on one engine, let alone positive climb performance, and have very low single engine service ceilings. While this is often cited as a big disadvantage for the light twin...it's a given that singles don't have many options during a power loss, either.

One of the big advantages of a twin is redundancy of systems; additional hydraulic pump, additional vacum pump, additional generator, etc. Learning to fly a multi engine airplane, or any advanced or complex airplane for that matter, is learning to fly it with engines and systems failed...learning to handle it in less than ideal conditions.

[Mr Grimsdale]

In answer to the original question, how about Gabriela Irimia?

Cheeky boy!

[Fuji Abound]

In a light piston twin, the second engine isn't there for safety. It's there for performance.

Yet another fallacy, we are on a roll.

Singles have been around that can match the performance of nearly every light twin for as long as I can remember.

Even today, you would have to go a long way to find a twin that can out perform a Mooney or even an SR22 in every practical sense. I can go faster in the SR22 than I can in either the 42 or an Aztec (two generations of twins in regular use), I can fly as high as makes no difference, and whilst I might get 200 feet more of climb performance out of the Aztec it as good as makes no difference.

I doubt the second engine was ever fitted for pure performance.

However passengers and regulatory authorities (and even some pilots) take comfort from the extra engine. The CAA has never approved single engine turbines (or anything else for that matter) under JAR-ops or before for IFR ops. Passengers are on the whole horrified when you let slip there is only the one fan.

Even the multi world started out with the 60 minute rule, that evolved into any transoceanic aircraft being required to have four engines. Whilst this has evolved again into ETOPS and will shortly evolve into LROPS the second engine is considered a prerequisite for safety, and even then, under ETOPS a second engine is still not quite considered as stacking the odds sufficiently in the passengers favour, hence the ETOPS requirements which are not, as many think, restricted to only transoceanic crossings even if in practice this is effectively the case.

No body wants to carry around an extra engine for performance, even if their was any truth in the fallacy, and everyone is desperate to see ETOPS evolve in LROPS but I don’t think even the commercial world will ever persuade the general public to get in an aircraft with one engine unless perhaps they think there is half a chance of landing it in a friendly field about the size of their back garden!!

[Pace]

I think the idea that you have twice the chance of a failure in a twin has grown from the conception that as you have two you have two engines to potentailly fail which doesnt mean twice as likely to fail.

One safety factor of a twin which is often missed is that the engines are on the wings rather than infront of you as in a single.

Aircraft design is attrocious regarding crash protection and a head on with a single on ground contact inevitably means that giant lump of metal in front of you joins you in the cockpit!

Normally aspirated twins do have a poor single engine ceiling but something like a Seneca Five turbo charged intercooled unit has a service ceiling single engine of 16500 feet which is pretty good.

Pace

[Fuji Abound]

I think the idea that you have twice the chance of a failure in a twin has grown from the conception that as you have two you have two engines to potentially fail which doesn’t mean twice as likely to fail.

Is this yet another fallacy? I thought we were on a roll.

If an engine fails once in every 1,000 hours, and never goes beyond a thousand hours, and the failure rate is linear, then there is twice the probability of one engine failing on twin in a given time frame because both engines eventually must fail on the twin whereas the single has only got one engine to fail.

In reality there are other factors involved that even where the same engine is concerned the probability of a failure in a twin will be different from in a single. Moreover engine usually dont fail before they become either time expired or have adequate work to enable them to continue to operate on extension where this is permitted.

In reality, if you were to plot hours against failure rates (or at least problems identified requiring remedial action) would you end up with a bell curve? If so is that also why it is common to see twins with engine hours that are not the same?

[Chuck Ellsworth]

However passengers and regulatory authorities (and even some pilots)


Hmmm let me think about this for a while......

.......I am flying over water at night far from land and the engine fails....

.......would I want to be in a SR22 or an Aztec?

[Fuji Abound]

Chuck

You have had enough thinking time .. .. ..

[Chuck Ellsworth]

The SR22?

[smo-kin-hole]

The Safest Twin?

I have a friend with a Cessna 337 Skymaster. Under certain conditions, he does shut down the front engine and rides a good tailwind to his destination. These things are old, hard to work on, noisy, cramped,but really are quite slick in concept. Adam tried to build one and there are a few Rutan Defiants out there. Its a great idea that might be executed on a cheaper level if somebody had the time/money.

What about having the nose engine much smaller than the tail one? Use the front just for takeoff and loitering,but otherwise shut it off? Maybe make it water cooled so there is no thermal shock? Solid-fuel booster fired by a big red panic button for takeoff emergencies? What about a belly mounted micro-turbine with just enough power to maintain altitude?

Do I have too much time on my hands or what?

[vanHorck]

Could anyone enlighten me on the service ceiling of singles when their first engine fails?

[Duchess_Driver]

IIRC

The performance calcs for the Duchess, IIRC, give about 15 or 16,000 as the maximum 'cruising' line. Can't recall what the 'service' or absolute figures are - would need to check the books - but it's not really relevant for this discussion.

What is important here is that mid channel when one donkey goes tilt - I've got another one! That's where the benefits are. A single may be faster, easier to manage, cheaper to operate but here's the real benefit - when my engine fails my DRIFTDOWN is significantly higher than yours!

Thats what I call a performance benefit.


(And I'm not too sure I want to go long distance over water or over mountains in a Cirrus at the moment!)

[IO540]

how do aircraft engines compare to car engines for reliability generally?

This is not possible to compare.

The reason is that car engines spend most of their time at around 10% to 30% of max rated power. A typical 2 litre saloon, 130mph top speed, is running at about 30% power at 70mph.

Whereas an aircraft engine is running at 65% or maybe even 75% power continuously.

When one sees normal car engines used in real competitions e.g. rallying they tend to fall to bits really quickly. I gather the rally teams have a pile of spare engines.

Car engines have the big advantage of water cooling, which keeps temperatures under close control, but I think they are way less reliable than the old Lycos.

And they are not more efficient, either. They are much more efficient over a wide range of power settings, which is what one needs to sell cars, but if one compared constant operation at say 65%, the two would be very close.

[SNS3Guppy]

You can't really make a comparison based on percent; the power percent for a horizontally opposed aircraft piston engine is based on propeller limitations, not what the engine can do. Aircraft engines typically operate in the 2000-2500 rpm range in cruise. So do cars, give or take. Aircraft engines are capable of far more power and far higher RPM's...the truth is that aircraft engines, while exposed to thermal stresses, generally operate far less harder than car engines.

[madlandrover]

I would be interested to know what specific reliability issues people have had with 42s?

Only seen a few so far, but... Head gasket problems seem fairly rare, unlike Thielert installations on other aircraft. Some aircraft have the odd ECU problem which seems to affect original and replacement ECUs on just one engine - possibly due to faulty injectors. Other problems are relatively minor stuff like worn autopilot gyros, faulty KAP140 autopilots, and the odd minor avionics issue, often related to the plugs & power supplies for the factory supplied Sennheiser headsets. The only other grounding fault I've seen recently was a couple of props either missing 1 of the 3 rubber protection strips, or 1 blade delaminating. I haven't got anywhere near the experience of many on here, but of the 50-odd DA42 hours I've done both in training and solo ferrying I haven't had a single heart stopping moment, only the odd "Hmmm, I wonder where this might going" query!

They are quite sensitive to decent maintenance, so can be a bit fragile if poorly maintained, but in general the work I've seen so far from a certain centre of excellence in the south west results in very few faults - and better still no new scratches on the gel coat.

Given the option of the simple and normal complex singles that I teach on, or the few light twins I've flown (DA42 and PA44) I would take the twin for pretty much every mission, for the confidence as much as anything else! The average school PA44 can be a bit limited in single engine performance at higher levels, but simultaneous twin engine failures are still reasonably rare...

[Fuji Abound]

how do aircraft engines compare to car engines for reliability generally?

Most car engines will make over 100,000 miles without any mechanical rectification. That is about 2,500 hours of operation, ignoring periods at idle, which adds to making a comparison complicated. Most aero engines will do well to get to 1,600 hours without a top overhaul, and a major overhaul by 2,000 hours. On that basis the car engine is vastly more reliable.

The engines are, as others have said, operating in very different environments.

However, my knowledge of performance cars would leave me to believe that lubrication and heat are the two most important factors in determining the life of a well built engine. While a road car may operate a great deal of its time at low rpm the aim of a race car is to operate continuously in its maximum power band. That will almost certainly mean between 5,000 and 8,000 rpm. In the case of a car add a few G in corners and it is all to easy for the engine to be starved of oil. For this reason we fit dry sumps, in some ways not dissimilar to the Christen systems on aircraft. Without these systems it is quite possible to destroy an engine in a single race. In a similar way light weight alloy car engines are hugely susceptible to excess heat. A loss of coolant will destroy a hot engine in a minute or two. We cringe when you watch novices starting the engine from cold and immediately applying throttle. Not only has all the oil found its way to the sump but it takes 5 seconds or so for oil pressure to have established even with a dry sump. In this 5 seconds a great deal of damage can be done.

The engines we use day to day in our cars are way short of the power the engine is capable of producing. The tolerances are large and the stress on the components is kept to a minimum. Despite what the advertising agencies may like to tell us the name of the game is to deliver an engine that is no where close to its peak performance capability, because by doing so it will be reliable. My 1600 engine in its usual road going guise develops less than 100 bhp, in some sportier cars that is pushed to 120 bhp. The same engine with hot cams, ported head, forged pistons and properly mapped will produce over 200 bhp - twice the power.

In some respects aero engines are no difference. On the whole the mapping is rudimentary and the engine is not designed to operate close to its potential. However, there is an equally big difference. I can easily apply +6-4g to my aero engine. I can easily chop the power, and without any thermal buffer from the water cooling system in my car engine expect the air flow to dissipate all the heat from beneath my tightly cowled engine. I can easily expect the engine to sit idle for two or three or four weeks, start the engine, and expect it to provide maximum power within 10 minutes of startup whilst also expecting it to reach its full operating temperature in the same time. In short even when we are kind to aero engines we give them a load of abuse. As to the original topic of the thread this is one of the reasons twin engine aircraft should be more kind to their engines - it is far easier to dissipate excess heat in a twin than in a single.

I reckon if you used your aero engine every day, always allowed at least 5 minutes to fully establish oil pressure, avoided shock cooling and any manoeuvers likely to restrict oil flow aero engines would do nearly as well as car engines.

[IO540]

The subject of "why we don't have car engines in a plane as they would obviously be so much more reliable" has been done to death on every pilot forum going

As Fuji suggests, one cannot do a direct comparison for a number of reasons.

But there are also others.

Car engines are designed to deliver their rated HP at high RPM, 5000-7000, while direct drive aero engines deliver a similar power at much lower RPM, say 2500 (my car and my plane are both 250HP, funnily enough) so the aero engine has to deliver an appropriately much higher torque (HP=rpm*torque). On an engine, everything incl crank stroke being equal, torque comes from piston surface area which is why my IO-540 is 8.8 litres whereas my car engine is only 3 litres.

But there is no way one could make the IO-540 rev at 7000rpm. It would shake itself to bits, I reckon. They have enough problems with dynamic stress on the crank at 2500rpm and have to add movable balance weights to reduce this.

So, the engine designs are very different.

I reckon every IO-540 would make TBO without any work, if it ran constantly at say 65% power, with good airflow. There are plenty that do make TBO in normal operation.

But I just don't believe that a car engine would run for 2000hrs at 65% power without something breaking. Not mechanically in the engine but on the ancillaries e.g. cooling hoses.

The basic mechanical reliability of some car engines is awesome. I gather Toyota spent USD 400M developing a certain 4 litre V8 engine for the U.S. market (as used in e.g. the V8 Lexus Soarer) and one just doesn't see any investment like that in aviation. But one cannot escape the fact that 99.9% of these engines will still spend 99% of their lives doing 10-30% of rated HP and that is bound to massively slant the figures.