I'm strongly considering a Seneca II or III, and I'm trying to wrap my head around the powerplant differences (EB vs KB engine). According to the internet they are the same engine, the KB just runs higher RPM to give extra power. Fair enough.

Where I'm getting confused is that I hear mentions of throwing a lot of extra fuel at the KB in order to keep CHT's in check. Is this done through an actual mechanical adjustment to the fuel injection system? Or is it simply the POH saying to run it richer by specifying a lower TIT limit? (Ie the pilot just doesn't pull the mixtures back as far.)

In cruise, does any of this matter or do you effectively run the same power settings and fuel flow?

Can either of these be flown lean of peak? (Not trying to start an endless LOP/ROP debate. Just trying to understand if its possible.)

And yeah, I know this is a Beech forum. But I don't feel like joining yet another forum (I have 3 airplanes) yet and already learned a lot of good Seneca stuff on another beechtalk thread so thought I'd give it a shot.

For a flight school?

For a personal plane they seem more like a trainer twin, personally I’d look at something more capable of you’re going to feed and maintain twice as many engines

Aero star, 310, beech 18, etc

I owned a Seneca III. I was able to run LOP in my -KB engines where no one I knew with a Seneca II (-EB engines) could. I ran 9-9.5 GPH per side for 165 KTAS at 10,000 ft. The zero fuel weight also favors the III over the II.

Still own part of a Seneca II. Have had no issues running lean of peak once the gami-jectors were installed.

For those advocating things with more capability, I also owned a 601P at one point and the maintenance costs are 25% of the aerostar or less. True, an Aerostar 600 would be a better comparison but if the logic follows that the 600 is half the cost of a 601P then it's still 100% more than the Seneca! The aerostars fly at ~20-40% faster.

For a big, comfortable 4+1 twin that cruises at 165kts on 21gph (LOP) and not costing buckets more than the equivalent single, its a great aircraft. We have club seating which is amazing and worth it.

Only caution is that the Seneca II (don't know about the 3) is very payload loaded and our MZFW is such that we can't legally load 6 adults even for a trip around the block.

When I bought my 11 one of the reasons was safety. It had the lowest fatal accident rate of any of the other light twins. I has a VMC of 66 kt. amd single engine service ceiling of 13,000 plus.

When I bought my 11 one of the reasons was safety. It had the lowest fatal accident rate of any of the other light twins. I has a VMC of 66 kt. amd single engine service ceiling of 13,000 plus.

The Seneca II is a solid twin. Very easy single engine ops. Mine runs LOP without much trouble. I remember reading that the III engines are just higher HP for around 5 min. Should look in the POH of the two and see what the operating difference is. I think you can convert a EB engine to KB with some minor changes. There is an STC out there somewhere for that.

Michael

I don't think 4x maintenance costs would hold up statistically, while they may have been such in your case.

If you compare a ratty Aerostar maintained by an expensive and/or unskilled shop to a queen of the fleet Seneca those numbers could be easily replicated.

A good 601P engine operated properly is a pretty bomb-proof engine. I have seen them go past 2000 hours quite a bit. I know of a pair of 601P engines that went to 2700 hours or so without so much as a cylinder being overhauled. Most people seem to get too scared to run them as far as they can go due to the number since major getting so large.

In 30+ years immersed in aviation business I know of only one engine in the IO-360 family of engine that made it significantly past TBO. It was normally aspirated and based here at 5000 ft, which I think may well have been the key. I think luck was a huge factor. It was a well built engine from the start.

With the longevity of the TSIO-360 Continental, I would think that alone gives an advantage to the Aerostar on the hourly cost for the engines. The Aerostar has two turbos per engine, which will cost a bit more than one turbo per engine on the Seneca. 601P's tend to have much better longevity with regard to the turbos due to being turbo-normalized.

An Aerostar maintained by a shop that does not know them well will cost a premium, sometimes a huge premium.

I have one current Aerostar client flying a Superstar that has repeatedly had multiple major Seneca II engine issues on low time quality engines by well known /reputable shops. His experience has included constant cylinder issues. He displays good leaning technique in training on the Aerostar.

The TSIO-360 is just not known for being the heartiest of engines. They have lots and lots of parts (leading to expensive overhauls), typical Continental cylinders, and statistically not a great longevity factor between majors, aside from the top end issues.

I have had some feedback that later factory remans (1990's and 2000's) have done quite a bit better than the early engines. Continental FRM engines lately seem to be problematic.

Neither of these engines top ends will put up with being run too hot.

I believe a good 600 Aerostar will run every bit as inexpensively per mile as a Seneca II/III , likely cheaper. They ran those in the check hauling game and made lots of money with them. You don't see the Senecas doing those types of commercial ops due to operational costs, in my opinion. You do see Seneca I's in training because they are cheap to buy and have Lycoming IO-360's.

My recommendation is that you make sure to get one with the Merlyn Wastegates and intercoolers, or factor in the cost of those on the purchase price. The fixed wastegate engines did not do well.

Graphic engine monitors with digital CHT's are a must, with knowledge and training of how to lean properly. Cylinders don't last all that long on these even if treated properly. It gets really ugly and expensive real fast if you abuse them at all.

Be careful with turbo times and condition, as the Seneca and Twin Comanche turbos were in very short to no supply (since the covid debacle) as of the last time I checked (several weeks ago).

I had a TSIO360 on my Mooney 231 many years ago, and it was a good, reliable engine that ran very well LOP with GAMI’s. The Seneca II and on are, as compared to other light twins better in short field operations and PAX comfort, and they are worse in range and cruise speed. If what it does well is important to you, and wheat it does poorly is not, it is a good choice.
-dan

I owned a Seneca II for about 10 years. I didn't have a lot of luck running it LOP, for a variety of reasons. But on the other hand, the engines don't really burn that much fuel in the first place.

There are other differences between the II and III that would be the deciding factors, mostly airframe improvements and some weight and balance improvements.

The EB and KB engines are actually the same exact thing mechanically. They just change the dataplate and limitations in the POH to allow higher power operations for a 5-minute limit at takeoff and a little bit higher power at rich mixture in cruise. A number of people with Seneca III and Turbo Arrows use the more conservative power setting tables from the Seneca II for improved longevity.

Despite internet claims to the contrary, my experience with the TSIO360 was very positive. Generally, trouble free, no problems with cylinders or valves nor other significant maintenance issues. They just pretty much worked fine. I bought it with timed-out field overhauled engines that had overhauled cylinder put on them, and in 1800 hours smoh, only 3 had been replaced out of the 12. That's pretty darn good actually.

Both have non-tuned induction and more importantly "fixed orifice wastegate". That means there's actually NO wastegate at all. The amount of exhaust that bypassess the turbo is set by a bolt, that is set on the ground, that restricts the amount of exhaust that goes to the turbo. The mechanic does the initial setting on the ground, then you fly and do the critical altitude test, then land, adjust more or less, fly again, repeat as necessary until the engines setup correctly to reach 13000' under ISA conditions for critical altitude.

It is a simple system and once set, generally is maintenance free.

The downside is there is no feedback loop to control the turbocharger at all. It is completely static. So, if you adjust the mixture, the manifold pressure changes. If you change the prop RPM, the manifold pressure changes. If you climb or descend, the manifold pressure changes. If you sneeze, the manifold pressure changes.

You get used to it, but I found it hard to manage the -50f LOP, when anything you touch, including the yoke, alters everything else and watch the TIT's all the time. It's just easier to fly rich, keep everything cool and within limits. Fat dumb and happy. At least, in this case, that worked out to ~20gph total (about 10-11 gph a side). On the upside, my Seneca II had more useful load than I ever needed, full FIKI and generally was reliable.

I sold the Seneca II about 7 years ago and got the Columbia 400, with a TSIO550, which is really easy to fly LOP, a good bit faster on less fuel. I gave up FIKI and useful load, but it suits my current needs better.

Thanks everyone for the feedback. Sounds like the Merlyn wastegates are a must have. I'm leaning towards a III for the ZFGW issue, and because the seem to have the highest useful load. (I almost bought a V a couple years ago, but it had like 1200 lbs useful. In a 6 place twin....really?)

I would consider a deiced Aztec as well. I've flown one a bit and they fly nice. The engines are loafing, and the plane is a truck as far as useful load. The only real problem is that they are so freakin' ugly! I just don't know if I can own an airplane that ugly.

That and they are generally older. I've decided I want my next plane to actually be younger than me. (I'm 47.)

Yep, ugly. With room for six full-sized people, all sitting upright in full sized seats, all facing forward, and with headroom and legroom to spare. Useful loads as high as 2100 pounds. Ability to carry from 850 to 1200 pounds in the cabin, and 800 pounds of fuel. Two large external baggage compartments that will hold Montague 26" folding bikes. Reliable Lycoming engines. Super strong landing gear. Ability to safely handle 2700 foot strips at full gross.
Yep,, they're ugly. Better off squishing yourself into a Seneca and keeping the bank account ready for the next engine problem.

The second one here cruises at 174 knots on 24 GPH, in air conditioned comfort. Too bad it's ugly too. Might be due to the flushing potty in the back row, for the grandkids.

The parallel valve Lycoming in the Aztec is one of the two best engines ever in general aviation. Start with a quality overhaul and 4500 hours is not uncommon to get between overhauls for anyone willing to go that far.

The glider tows in Boulder abuse those engines all day every day and still get 4500-5500 hours out of them. 24” mp at takeoff is the best they can do, and under those circumstances you just can’t kill them!

The Merlyn wastegates are really not much of an improvement. While some people like them, I knew of several operators of Seneca II and III who removed them. You just get used to the turbo system after a while. It is what it is.

Same goes for club seats. While everyone seems to want the club seats, in practice, they're not all that great. Mine had the all-forward facing seats, which actually was more practical.

Shop around on the II's and III's. On my particular II, it was a particularly light one, and it had the "standard" fuel tanks. Useful load was amazing, well over 1500lbs. But it was configured in such a away from the factory, that there was no way to actually load it to full gross weight!

A prior owner had added vortex generators which increased the ZFW by 180lbs, allowed up to full gross loading, and improved low-speed handling a lot.

Generally, you want a twin you can fly all your normal missions well below gross takeoff weight. That 100lbs or more of margin makes a big difference in low speed, one-engine-inop conditions.

The other Seneca II/III STC that is worth getting are installing access panels in the lower engine nacelles to allow changing the oil filter without decowling the entire engine. That one was worth every penny.

All in all, I found it to be a good airplane, though my particular one had lot of wear on it and required a lot of undoing dumb-mechanic tricks done to it in the past. The TSIO-360's despite internet lore, turned out to be very good for me anyway.