Twin Turbine, Single engine performance & safety

[Lychee]

I am very interested in hearing your opinions on the best or worst performing helicopters in the event of one engine failing.

[gulliBell]

B206, engine fails and you sure know about it real soon - need to apply rapid appropriate skill;
B212, engine fails in the cruise turns you into a B205 - no special skill required;
B412, engine fails at CDP and good chance you'll fly away (412EP that is) - a bit of skill is very helpful;
B214ST, engine fails anytime and you might notice, or might not - only abinitio pilot skill required;
S76A, engine fails and you know about it - need to apply prompt appropriate skill;
S76C above 23 deg C, same as for the A model;
S76C+, same as for 412 but love that 30 sec OEI power - a bit of skill is helpful;
S76C++, can't wait to give it a try!

[whatsarunway]

and the 76 b . . .turns into a 76a on two engines!

[MBJ]

Showing my age but Wessex 5!

Could absorb a maximum of 1550shp through the gearbox and each engine produced 1350shp. So you might notice one stop if it blew up but a rundown didn't show up on CWP and we only had one torque needle anyway.

Queen of the skies.

[BaronG]

Reminds of a saying from the FW world but may be fits helicopters too :

"In a twin, when an engine fails, the remaining engine will fly you to the scene of the accident" :E

BG.

[gulliBell]

Reminds of a saying from the FW world but may be fits helicopters too "In a twin, when an engine fails, the remaining engine will fly you to the scene of the accident"

Nope...in all the above helicopter types, an engine failure above Vbroc should result in continued flight and a safe landing. And as for that S76B, even at a hefty weight you can probably hover that beast on one engine! Love those PT6 engines.

[Blind Bob]

The BO 105 would manage about 80 kts single engine and then it was a 30 kt run on landing back at base. Throw out the crew and she would be able to hover taxi back to the hanger.
The EC 135 T2 has bags of power. MAUM at SL to +30 OEI. It still needs a smooth approach, but the FADEC won't allow you to overtorque or overtemp at the bottom, you just get Nr droop. So hover is no bover!:D

[Geoffersincornwall]

Bo 105 - was embarassing to teach single engine ops on this otherwise likeable beast. A little bit of turbulence and a demo of OEI (using training lmits of course but not at MOM) could become embarassing. It's a bit demoralising for the stude when his RoC suddenly disappears at the nearest bumpy piece of sky

S76B - beautiful, just beautiful. Talk to RS about his OEI landing back at the rig after fire warning light refused to go out.


AW139 - Now we are back in Wessie 5 territory. ......... Exceptional OEI performance. The difference is that whereas the Gnome might let you down the PT6 is rock-solid.


G

:ok:

[Camp Freddie]

I second Geoffers with the AW139,

no avoid curve at at all below 4000 density Alt.

and we had ours in the go around climbing at 1400 fpm OEI at MAUW.

Its awesome

regards

CF

[rotorspin]

Anybody with OEI experience of the 355NP?

Been told its a pretty outstanding machine

[SASless]

Chinooks on one engine are really a challenge.:E

[BlenderPilot]

B206, engine fails and you sure know about it real soon - need to apply rapid appropriate skill;
B212, engine fails in the cruise turns you into a B205 - no special skill required;
B412, engine fails at CDP and good chance you'll fly away (412EP that is) - a bit of skill is very helpful;
B214ST, engine fails anytime and you might notice, or might not - only abinitio pilot skill required;
S76A, engine fails and you know about it - need to apply prompt appropriate skill;
S76C above 23 deg C, same as for the A model;
S76C+, same as for 412 but love that 30 sec OEI power - a bit of skill is helpful;
S76C++, can't wait to give it a try!

Where I fly . . . . any of the above would end up kissing the ground eventually if the engine failed at anything other than optimum OEI speed.

Even the A109E Power has problems around here.

[perfrej]

I've only flown it once, but from what I understand an OEI condition is more or less a "continue flight", even at high gross weight... Someone with a rating may be able to fill in the details...

[Cron]

I have looked but are still baffled with a number of elementary and perhaps naive questions regarding twin engine helicopter performance. May I state them simply?

1. Can such a machine sustain its own weight on one?
2. Can it land and takeoff on one?
3. Can it land and take off one one plus some payload?
4. Can it land and take off on one at MAUW?

My reasoning is that if 1 to 4 are true then the second engine is purely in place for safety and more speed.

Put me right someone.

Thanks

Regards

Cron.

[800]

You will find that only a couple of aircraft in production are able to take off on one engine when they are configured for "normal" contractual operations.

The limiting factor for most helicopters is the rating of the transmission. A single engine (of the two) is usually not rated with enough power to operate the helicopter through all phases of flight by itself with MAUW.

[Nigel Osborn]

The Wessex 5 could. I remember picking up 9 soldiers from a clearing without any problem in Borneo.
Modern twins basically can't.

[bolkow]

chinooks are even worse with one rotor out!:}

[Arm out the window]

Cron, virtually all my helo experience is on singles, but I think I can answer some of your questions satisfactorily:

It may seem illogical, but the power required to fly a helicopter doesn't necessarily increase the faster you go. In fact, if you drew a graph of power required vs. speed, it would look a bit like the curve of a skate ramp - high at the start, low in the middle and high at the end. (This is because of various factors of airflow through the rotor disc and components of drag that we don't need to go into just now).

So, you need a lot of power to hover, a lot to go fast, and somewhat less in between.

If each of your two engines can produce a massive excess of power, then an engine failure at any time should be a doddle, because the good engine will just take up the slack, so to speak.

If the engines aren't up to the task of providing hover power on their own, but need to be working as a pair to do so, you won't be able to sustain the hover if one fails and will have to 'cushion' on to the ground with the other.

However, if you're flying above a certain speed (where power required becomes less), your remaining engine may be powerful enough to keep you flying. What about landing, you ask? Well, in that case, you'd need to do a 'running' landing (either on wheels, or scraping along on skids) so you could keep the speed up right to the ground.

Hope that helps.

[heliski22]

Geoff,

In one of our early Power Assurance Checks, we did it at low weight and had to restrict the Single Engine Tq to 90% to keep the thing on the ground. Even at that, it was starting to dance on the mains with the oleos well extended! I didn't try, of course, but it had all the feeling of being able to fly away!

If I recall your demos in the Sim correctly, 70% Tq gave us 140kts at 6400kgs. Bring one engine off-line and the Tq went to 140% on the remaining engine which, happily is the Max Continuous OEI!!! Similarly, 80% Tq gave us 160kts. Engine off-line and Tq went to 160% and so on.....

I do hope it never happens though, much cosier feeling with both of them churning away behind you!!!!

22

[What Limits]

Cron,

I can see your problem and I will try and keep it simple for all the others

Helicopter performance is affected by weight (mass), altitude and temperature, the WAT limits. Look in the flight manual and depending on manufacturer you could have several charts that give you the maximum weight for a given density altitude (pressure altitude versus temperature).

This weight would normally be the maximum for All Engines Operating (AEO), but there will be other charts for One Engine Inoperative (OEI) performance. This MAUW should give you enough performance to make an safe, comfortable approach and landing. Depending on aircraft this could be a running landing or an approach to a hover.

The second engine comes into its own during the critical phases of flight, take-off and landing. If you are at or below OEI MAUW and you are using the correct techniques, should a power unit fail you should be able to land safely.

For an explanation of proper techniques, Performance Class I, II and III and Category A operations have been done to death before (but is still confusing to those who have not done it, and to those that have!)

The answer to your questions, yes and no, yes and no, yes and no, yes and no. Clear?

[Heli-phile]

OK Its the old chesnut reborn. Last week I overheard a pilot waxing lyrical to some VIP passengers that Multi engine helicopters only had a 2% safety improvement over Single engine helicopters!! so for the extra cost the Multi engine was just a waste of another US$1.5 million

I would love to know the figures upon which this was based.
Are they comparing like with like?? only types I could think of would be AS350 v AS355 or A109 power v A119 Koala.

I fly both multi and single machines and I always feel happier in the twin, not just because of the extra donkey, but also the extra generator, hydraulic system Etc. Am I wrong to feel "safer" in the twin. what are the figures on this??

Please don't post the usual "well they still share the transmission/tail rotor/fuel source". that does not really prove one type over another rather its an event which is equally likely in either!!
just deal with the differences and statistics to support/refute multi engine safety advantage .

Thanks

[Phil77]

Heli-phile: In addition to this thread, where your post has been moved to, here is another (now locked) discussion about twin-engine/Cat A reliability and statistics you might find interesting:
http://www.pprune.org/rotorheads/154248-multi-engine-height-velocity-avoid-curve.html


Phil77

Thanks for the link: the thread is now unlocked.

Splot

[height is good]

Throw out the crew and she would be able to hover taxi back to the hangar


BB Me thinks you are having a dig about the weight of your crew:}