View Full Version : Long range flight with rear engine only

01-14-05, 07:58 AM
Dear Gentlemen

I had a C337P for years in the 80's and then I stepped up to the bigger cessnas and TP.

Recently I aquired a additional aircraft: The '68 T337C for one reason - to fly to Africa. The aircraft is actually in his annual check and will be equipped with a ferry tank.

Here the story: One owner of a similar aircraft told me, that the range can be incresed dramaticaly flying with the rear engine only. It means to feather the front prop and to fly with a speed of some 130 KTAS. That guy did this to fly to Lebanon in the 90's.

Now to my question: I did not find any notice in the manual. On the other side, the proposal make sens, since in level flight one engine can do the job. Has any one the answer ?

Thank you


larry bowdish
01-14-05, 09:15 AM
It's not legal. OK, you know that. And you probably know that the O-2's were ferried to VietNam, Okland to Hawaii, with big ferry tanks. They flew at reduced power settings, with big ferry tanks, but they flew with both engines. If I remember correctly, big ferry tanks don't require anything special, as long as you are less than 30% over gross. That would be an additional 1300# for your plane. The plane won't get off the ground with one engine at that weight. I think it would not maintain level flight at that weight on one engine.

If your plane has the aux tanks, you won't be able to feed the front engine aux to the rear engine.

Brian Von Herzon has crossed the Atlantic several times in his Super Skyrocket. He has flint wing tip tanks, which provide some additional fuel.

The longest stage, across the atlantic, assuming you are going over the northern route, is about 670 nm. You plane would be able to do that with out any difficulty, without ferry tanks.

MaryAnn, from Riley, said she had a customer with a Super take off from Goose bay with out one engine. Crossing went well, and he apparently flew for a while with only one engine. not to save fuel, only because the engine was broke.

What route are you going to fly?? Brazil to Africa?

01-14-05, 10:31 AM
Hi Larry

thanks for the rapid answer !

Certainly, i can imagine that the procedure 'one-engine-cruise' is maybe not legal. Can be. But we are are looking for max. efficiency. !

In any case, my aux-tank is certified (has his paperwork) - and the weight is in the limits ;-). I know the limits of the skymaster...

You say: ^^ If your plane has the aux tanks, you won't be able to feed the front engine aux to the rear engine.^^

My answer: Right ! But for climb and final approach, i'll consume the aux-tanks. Ah - the skymaster's aux-tanks are famous...

You say : ^^Brian Von Herzon has crossed the Atlantic several times in his Super Skyrocket. He has flint wing tip tanks, which provide some additional fuel. ^^

My answer: The flints are ok, but not in my budget - normally I d'ont use so much fuel, so this modification is not required.

You say: ^^The longest stage, across the atlantic, assuming you are going over the northern route, is about 670 nm. You plane would be able to do that with out any difficulty, without ferry tanks.^^

My answer: The atlantic is one thing - but I fly to south - africa. There are crazy and corrupt people to OVERFLY. I have some experience - so I need a max range of + 1600 nm.

Of coarse, the Skymaster may fly perfectly with one engine - but my question is very simple: IS IT REALLY MUCH MORE ECONOMIC WITH ONE ENGINE FEATHERED ?

Thanks very much.


PS. Routing is Geneva - Morocco - Senegal - an then ??? depends...

larry bowdish
01-14-05, 10:43 AM
In the '60's, I spent some time on that continenent. It isn't any better today.

Since I don't have a T-, you'd would just have to get up to altitude, and watch your fuel burn. Currently, my plane won't fly on the rear engine only. It's out!!! The new one isn't here!! I think my weight and balance is really out of wack right now!!

In theory, of course, it seems that you would get much better mileage with only 1 engine.

Mitch Taylor
01-14-05, 12:07 PM
I'm not sure you can hold altitude on one engine with the ferry tank full. Also, I'd be more concerned about how much harder you're running the rear engine.

01-14-05, 12:47 PM
I agree with Mitch. Running the Skymaster on one engine will require a lot of power to maintain altitude, and I would worry about what you would be doing to the engine.

Also, on that year aircraft, you cannot burn from the aux for climb. You have to burn from the main for an hour before using the aux. If you don't, you will vent the return flow from the engine (something like 5 gph I think, but I am not sure) overboard.

Run 'em both at 55% power or so. You will get much better efficiency, much easier on the engines. And please look into the fuel system carefully.


01-14-05, 08:39 PM
Hey Larry,

Where did you get the notion that "It's not legal" to fly the Skymaster on one engine in cruise, or for that matter, to perform single-engine landings... keeping in mind, of course, that single-engine takeoffs are placarded as a no-no?

Our '77 P-model POH speaks specifically of single-engine operation and single-engine cruise flight.


01-14-05, 09:00 PM
One other thing, even the "Performance - Specifications" on sheet number "ii" in the front of the POH specifies "Certificated Maximum Operating Altitude" as being rated as "Single OR Twin Engine" @ 20,000 feet, although the SE Service Ceiling is 18,700 feet.

Obviously, one of the great redeeming features of the Skymaster with its center-line thrust, is its abillity to fly on a single-engine. Why not take advantage of it, especially on a long-distance flight when time is not a factor?

As far as the Skymaster requiring a lot of power to maintain altitude, or needing to "worry" about what you would be doing to the engine, that would seemingly depend on how the airplane is loaded, and using proper piloting technique with the necessary amount of mixture and cowl flaps to keep things in the green. I just don't see how you'd 'hurt' the engine, all things considered.


01-15-05, 06:27 AM
Thanks for your competent answers, folks. I'll read all that carefully that (misty) weekend.

Greetings from Switzerland


Ernie Martin
01-15-05, 09:16 AM
GMAs, a Skymaster expert who writes in another Message Board, says that he has data which shows that range is not extended by single-engine operation, when compared to running both engines at the lower power levels (e.g., 2300 RPM, 18 in of Hg) which result in maximum range.


01-19-05, 11:11 AM
Hy folks

I calculated the whole thing and I found that the one-engine operation brings you 15% longer range.

Instead 2 engine producing twice 33% PWR, you fly with the rear one with 66% PWR. It works! 22 versus 26 gal !

Actually, we have to replace a wing-boot, so I can't check it in flight, but there are numbers...

With that trick you can fly up to 1500NM with the 128 gal. tanks! Sounds good!?

All is legal - the only problem is that the TBO-time is also running on the feathered engine. This procedure you can apply just for very long ferry-flights with no additional tanks.

Calculate and try !

01-19-05, 11:27 AM
Try flying it, I think you will find that speeds, fuel flows, etc do not work out as you expect.


01-19-05, 12:26 PM
Yeah, i'll try this. I am just woundering that no one did this before here.



01-19-05, 01:26 PM
Many of us have flow Skymasters on one engine.

Speaking only for myself, I really question whether a Skymaster at gross weight will maintain altitude on 66% power on one engine, as your calculations require. It has been a couple of years since I did it, but I remember it requiring more than that.

If it *will* maintain altitude, it will be at or near best rate of climb airspeed, with a high angle of attack. That would be a very uncomfortable way to fly the airplane for long periods, the air flow to the rear engine would be compromised, and it just doesn't make sense to me that a higher angle of attack would be an efficient way to fly the wing.

So that is why I suggested you fly it in a closed course to eliminate wind effects, and see what works the best overall. I think you will find that low power settings on both engines will serve you better.

By the way, 100 knots is the best glide speed with both engines feathered, so your best range should occur by reducing power to maintain that speed. This is about 10 knots faster than the best single engine rate of climb speed.

Lastly, if you are flying over water at cold temperatures, keep in mind that if you cold soak the front engine by flying it shut down at cold outside air temperatures for a while, you will very likely find it difficult to restart if you need it (because the other engine failed).


01-19-05, 07:51 PM
i think someone should do and report the results. my money is on single engine. could be wrong but never in doubt. JCH

Kevin McDonnell
01-20-05, 02:18 AM
Someone had to bring up this heresy ....

Just get a set of Gami balanced injectors and run LOP. Apart from installing bigger tanks, this is the easiest way to increase your range. You'll get 15% or better decrease in fuel burn.

Let the flames begin ....

Ernie Martin
01-20-05, 10:37 AM
No, no flaming. You do have a point.

I happen to believe, as others above, that 2 engines running at lower power levels and properly leaned for economy per the POH will give more range than one engine. But you can certainly extend the range even more by leaning further and operating lean of peak (LOP), if you have the proper equipment and conditions.

There's another thread on this message board about operating LOP at www.337skymaster.org/messages/showthread.php?s=&threadid=1208 but here's an excerpt from there on what you need:

"Those who are technically astute (you don't have to be an engineer or a mechanic, just someone who is technically inclined and understands the subject), who have installed GAMI injectors for smooth operation across all cylinders at LOP, who have cylinder-by-cylinder engine management/monitoring equipment, who are willing to to give a bit more attention to engine management in flight, and whose engine is no longer under warranty can run LOP at cruise."

Of course the cost of the GAMIs and the engine monitoring equipment, if you don't have it, is substantial.


Nick Bailey
01-20-05, 07:30 PM
I'd have to say with my recent experience running single engine that the skymaster will not hold altitude at 65% power and full gross, even at SL. Lighter, maybe but not full gross. From a safety standpoint, I agree with Kevin that the single engine operation offers too many potential problems. A recent experiment with extreme leaning ( Gami jectors) I was able to get about 130 knots with each engine using just over 7.5 GPH. Don't know if that is better than single engine, but will let the rocket scientists do the math.

Jim Rainer
01-21-05, 10:10 AM
This has been an interesting thread to keep up with. One comment: there have been numerous "won't hold altitude" comments - what altitude?? I had the front engine of my normally aspirated 337G feather itself (governor blew a seal ) over Greenwood, MS and after the dust settled, I held 6,000', lost about 20K didn't change the rear engine setting (FT and 2400 rpm) and flew on to Memphis - about 100 n mi. I cracked the rear cowl flaps but didn't need to and this in the summertime. Medium load of fuel and folks.

When taking a mountain flying course in the middle of the summer my instructor and I were over Leadville, CO at 12,000 and he wanted to see how well it'd hold altitude, if at all. We reduced the front engine to zero thrust and held the 12,000' with about a 20K loss. Two people, 2/3 fuel (148 gallons when full) and HOT. The change in angle of attack in each case was minor.

The airplane in question (fly to Africa) is a turbo model. I believe it'll hold any altitude below at least 10,000 without a struggle. I've done no testing but my gut says a normally aspirated 337 will go further on the rear engine alone than on both (but I probably wouldn't push one thay way!)

03-24-05, 01:57 PM
Our aircraft is still in maintenance (due to a change of the de-ice boots)

I promise that I'll check the numbers with just one engine running at altitude. It seams that this test will be a subject of interest !!

Thanks to be patient.


Walter Atkinson
03-26-05, 10:12 AM

The answer is in the science. There are two things required to achieve max range.

1) Attain the best L/D airspeed (someone said that's 100 knots in a Skymaster. I cannot confirm that, but suspect that's very close.)

2) Set the mixture to BSFC (min). This will be at a mixture of about 15dF LOP at the very low power setting required by having two engines running and at 100 knots. With one engine running BSFC(min) will be 20-25dF LOP since the single engine will be putting out more power.

BOTH of these actions will result in max range AND max efficiency.

Personally, I would choose to attempt this with both engines running and I'll be quite surprised if the single engine scenario were better. Running on only the rear engine is likely to alter the angle of attack at the slow airspeed to a less efficient drag condition. Testing would be necessarly at the expected flight weights.

03-27-05, 06:55 AM
I have a pair of tip tanks. $3000.00 if interested.
Jaws83@optonline .net
Walt Simendinger

Paul Sharp
03-27-05, 03:48 PM
I did a precautionary shutdown of my rear engine coming back to SLC from Montana one day. Full load of fuel, 3 passengers plus luggage. Of course I had burned off about an hour of fuel before I saw the engine CHT start to rise.

My plane is a 1967T model. I was at 16,500 feet at the time. After getting over the mountains, I let it down to about 10,000. It held just fine and I feel quite confident that I could have neld the higher altitude had there been a reason. This was in November if I remember correctly.

Anyway it was an NBD (no big deal), and I could have started up the engine again had there been a need. I flew on in to Idaho Falls, did some checking, and found that it was just a bad gage. Fired 'em both up and went on our way home.

03-28-05, 02:20 PM
Dear Walter

you're talking physics. Very good. I am sorry, our T337 is still not ready - SO WE MAKE ESTIMATIONS !

I just tell you that a Conti - Engine takes a minimum fuel of about 4 gal / hour in IDLE (NO THRUST). If you fly with one engine, you save this amount of fuel at least!!

There is no problem to maintain this bird in the air with one engine. I think, we can even fly with 14 gal/hour (LOP) with one engine at FL 120. Speed > 120 KTAS..

In that cas we can fly nearly 1200 nm without a ferry tank and reserve, since we have the 4 tanks. This distance will be ok for our mission.

What do you think ?


03-28-05, 04:43 PM
I don't know if this is a true story, or a tall tale, but it was presented as a true story:

A fellow was ferrying a skymaster back from Europe on the North Atlantic route. He feathered the front engine and was cruising only on the rear.

There was a low overcast over the atlantic, and he was flying very low to stay under it.

As he was approaching Greenland (I think), he felt a huge shudder through the whole airframe and heard a loud noise. But he could see nothing wrong. He continued on and landed.

Upon inspecting the airplane after landing, he found that the tip of one blade on the front prop had been bent backwards.

Pilots in the area advised him that he probably clipped a large ice ridge that had developed some distance south of their location.

Talk about close calls!

Walter Atkinson
03-28-05, 05:07 PM

**There is no problem to maintain this bird in the air with one engine. I think, we can even fly with 14 gal/hour (LOP) with one engine at FL 120. Speed > 120 KTAS..**

If that's true, then the question is, "will it fly on two engines at the same TAS on less than 7 gph each when LOP?"

That answers the question.

04-03-05, 09:35 PM
I'll try to contact one of the pilots who ferried the O2's across the pacific. As was stated they did it on 2 engines at lower power settings. If there was an advantage to single engine cruise, I'm sure they would have used it. GMAS's data showed they was an efficiency advantage with both engines over the theoretical SE cruise. None of the FACS I've talked with flew SE to lengthen their patrol times or time over target. The military fleww the wings off these birds and I trust and use the procedures they did. Personally, my O2-A is a dog on one engine.

larry bowdish
04-05-05, 09:35 AM
The 0-2's ferry flights are more analagous to what Proto wants to do than simply shutting down one engine and flying on one. In his initial post, he says he intends to install a ferry tank. Everything I have read indicates that you can take off at 30% over gross, with out a special permit. Now, take the average skymaster at 4400, add 30% and you are at 5720. Make it 20%, and you are at 5280. If I was going to go a long ways over water, I'd want all the fuel on board that I could get away with. In other words, I'd be at 5720. If the average usefull load is 1500, that means you could carry 2800 of people and fuel, or 2600 of fuel. To get to the point where you have burned off enough fuel to get down to normal max takeoff, you will have flown for a while. Quite a while. I don't think a normal, or turbo, skymaster can hold altitude at 20% over gross. Maybe 10%, but I doubt that too.
So, my initial premise was that for a long time, he's going to have to run with 2 engines. It would make sense to me to have them leaned out as much as possible. I would also, no science here, just an opinion, think that 2 engines running at reduced power, could generate more thrust than one engine running at some reduced power setting.

There are a lot of opinions, we'll just have to see what happens when proto gets the bird in the air.

Good luck.

Walter Atkinson
04-05-05, 09:49 AM

** It would make sense to me to have them leaned out as much as possible.**

No, you want BSFC(min) for max engine efficiency. That would be at about 15-20 LOP at the reduced power you would want for best L/D for the airframe. That will give you max range. Leaning further will actually decrease the range as the engine will be less efficient. BSFC => Brake Specific Fuel Consumption => pounds of fuel required to produce one HP per hour => efficiency. You want to be producing the most HP per pound of fuel possible.

Max range is found at BSFC(min) and best L/D airspeed.

04-12-05, 06:34 AM
Hi Larry

Additional tanks are expensif, they take to much space (hangar etc.) and in Europe is a hassle with the authorities to get a permit - even with a STC. If you have to ditch your aircraft with a full aux-tank (interior), there is dead-penalty - so forget it!

So we try to fly with one engine shut down for max. endurance - (I know, you are all waiting for the results) - and then we will see.

I bet with one engine we can go 10% or more farther !


PS. End of the week I am in Lakeland ( Sun n' Fun) - is there any C337 people around ??

Guy Paris
04-12-05, 09:22 AM
However the 747 that left the west coast of the USA and had an engine fail and continued on to England, (bad decision), could not make it to it's destination... guy, the old 72 driver...

04-12-05, 09:33 AM
I just got back from the CPA Skymaster Systems and Procedures course in New Orleans (It was great, by the way!)

The instructor related a story about a fellow who had the engines overhauled. Around 100-200 hours later, the fellow came back to the shop because he was loosing compression on the front engine. The rear engine was fine. All of the front cylinders were replaced as a warranty repair. He came back again after another 100-200 hours with the same complaint. Again, the front cylinders were shot. Everyone was scratching their head and wondering why. The rear engine was still fine.

Then, it came to light that the fellow was in the practice of taking off, climbing to altitude, and shutting down the front engine. He would make a long cross country flight, then restart the engine, and land.

According to CPA, air-starts are very hard on these engines, especially if they are allowed to become cold soaked at altitude before restart.

If you are considering following this practice, you must factor in the the cost of dramatically shortened cylinder life.

As to the legality of the practice, there was general agreement that it would be perfectly legal under FAA regs. Attempting takeoff on a single engine, however, would not be legal.

Walter Atkinson
04-12-05, 10:57 AM

Ya, know if the guy had simply left the front engine off through the landing, he probably wouldn't have toasted the cylinders so quickly. As you said, in-flight restarts are hard on an engine that has been allowed to cool off.

Guy Paris
04-12-05, 09:33 PM
If the 337 is issued it's certification as a twin and all flight tests are done with both running, except for the obvious, could it be legal in the eyes of the FAA?
The Boeing 727 could be (2) engined ferried but not without several things to do. Think they were training department pilots only, secure the inop engine, (no rotation), insure as best as posibile the other (2) would continue to run. Others I am sure that I have forgotten. guy, the old 72 driver...