I think its a perfectly plausible question to ask a pilot in a flight review

Yeah we are drifting, but for the benefit of those who might not be familiar with twin engine helicopter limitations:

Let's say for example, in twin engine helicopter the 5 minute take-off power limit is 100% TQ. At that power setting the MGB sees 2x100%TQ = 200% TQ. The limiting factor is due to the MGB. Now let's say you are OEI in that same helicopter and you are operating at the OEI limit of 127% TQ. The MGB sees 127% TQ: remember, it can do 200% TQ for 5 minutes. Easy, no sweat at all for the MGB at any OEI limit. But that engine sure is getting hot pumping out 127% TQ for (up to) 5 minutes.

Which is why I said earlier, the 5 minute take-off limit is stop you ripping the guts out of the MGB/IGB/TGB and driveshafts, the OEI limits are to stop you ripping the guts out of the engines.

This is a simple explanation, the actual situation is slightly more complex because in some twins there are things such as 30 second OEI power limit, dual engine soft limiting, and dual engine blow away power, and combined TQ limits when one engine is better than the other, blah blah blah, but the principal is the same.

And finally, an angle to your question is why is the 2.5 min OEI limit half the 5 minute twin engine minute limit? It is only half in time, not half (actually double) in power. Why it is half in time I don't know, and I don't care, I'm only the pilot and I only know what is written in the RFM. And if that examiner asked me the same question that was raised at the opening of this topic, my answer would be "don't know, don't care". Sure, he is entitled to ask the question, but I don't need to know the answer.

gulli:
Good explanation. It almost mirrors one I received from a Pratt rep a number of years ago. We were going through a learning curve on our new EC135Ps where they were flying around OEI for various "reasons."

We changed the 1st two engines but on the 3rd they sent a rep and boroscope. A discussion started on the 2.5/30 min OEI which you can see in the downloads.

Long story short... if engine can hang for more than 2.5 why keep the limit. He like me think the 2.5 is a certification "check box" item. But cheaper than wringing out the rest of the system. He mentioned just look at the 139 if you get a little excited on the collective, you change half the M/R limit times.

When I asked the 3 engine divided by 5 mins question he offered to buy the 1st beer. Oh well, maybe next time. Thanks again. W1

And we now return you to your regularly scheduled program on the R44...

In which case I have nothing further to say as I know nothing about the R44.

Except, and this applies to R44. #1 Knowing the limitations of your aircraft is a good starting point. #2 Knowing why or how those limitations came into existence is not important, but if you care to find out, no harm in doing so. If you take a check-ride with me, expect questions on #1, there will be no questions from me on #2. I'll be perfectly happy if the student knows and applies all limitations.

Still on twins OEI, when only one engine is pumping power big-time into the MGB, it is getting it asymmetrically, instead of one from each side. This puts stresses on the gear teeth, bearings, MGB supports, and the little people inside the box putting coal onto the fire.

The exception of course is the B212 / 412 where the engines feed to a combining box before it goes to the MGB by a single shaft.

Regrettably a lot of that sort of thing used to go on, and still does...more and more newer design helicopters now have black boxes that record all sorts of numbers and any errant pilots can be identified by way of laptop computer and be retrained by way of Chief Pilot.

Yeah, but if you're burning up OEI engine limit time the engine gets changed, not the MGB. The MGB is designed for one engine pumping power big-time into it from one side. Just as the CBOX in 212/412 is designed for one engine pumping power big-time into it from one side.

Unfortunately, in my case #2 is all I have to go on when you pilot types show up with the odd light on and want to know why. But I digress again... W1

I hear you. Just be greatful those pilot types told you about the odd light. It's the pilot types that don't tell you about the odd light that are of concern.

Well, boys, I had a chance to talk to the tech guys at Robinson. Its the engine, not the MRG.


Proof: The gearbox in the Raven II is exactly the same as in the Raven I.


And that is why I believe it IS important to understand HOW the limits came to be as well as WHAT they are. If I'm in a crap situation, which I intend never to be as I do this just for fun (in fact I have a rule: at the first close call of any kind my Heli flying is done), I know I can pull takeoff power as long as I need it, and the result will be wearing the engine out sooner, NOT falling out of the sky due to a catastrophic failure of the gearbox.


If you want to go down the "just follow the rules" path, why bother to teach aerodynamics at all to student pilots? Just tell them "As you are slowing down, once it starts to shake around 20 kts you will need more power until you are really close to the ground"

OK then, I'd like to know how the Vne limit came to be in R44.

p.s. helicopter shaking as you slow down to 20 knots, that's a Robinson thing is it? The helicopters I fly don't shake, and if they do, it gets grounded for rectification.

I think he is loosely referring to the vibration you feel as you pass out of translational lift.

Many people know that the Vne in the R22 is due to forward cyclic position. At Vne (and most rear cg), the stick is forward enough that you are at the certification limits for controllability - the regs require you have enough push left to overcome a gust.

In the R44 I was somewhat convinced it might be retreating blade stall, but I talked with Pat Cox @ Robinson a couple years ago and he said no, that the Vne is just to control stress on the dynamic components and make sure components all reach their design lifetime.

Well, there you go, gulliBell. Now, if for some reason you need to exceed Vne in a R44, you'll know its possible. Something might break before its supposed to after you do it, but if you need to do it to save a life, go for it.


Can't really think of a legit life saving reason to exceed Vne though.....passenger in cardiac arrest, need to get on ground now? But, I guess in that case I'd just enter a vortex ring state, get down to about 50 ft agl, employ Vuichard, exit VRS at 10 ft, land, save the passenger.

Fantastic! We haven't had a good Vuichard punch up on here for at least a couple of weeks. Of course if Vuichard didn't work for the guy before you (smoking hole in ground etc) there's always autorotation as a gentler alternative...

Actually in an R44 you'd have to be trying pretty hard to exceed the sea level Vne, as in nose pointing distinctly downward. I don't think full 5-minute power would get you there in level flight.

The Vne for an S76 is 155kt, I managed to get it up to 180 briefly when I was travelling quickly with a 35kt tailwind and ran into a front with around 40kt headwind. No engineering problems or aspects to be looked at, the machine can cope with such things.

If you are not pulling its guts out with power, there should be no worries in an R44

Apart from this bit if you exceed Vne, you have reduced the life on components - you have no idea by how much or what damage you may have done.

It is simple - don't exceed the limits but of you do, inadvertently, have the aircraft checked throughly.

Do you want to be the one flying when a pitch change rod or control horn gives up early in its life due to overstress?

Well, you know Crab, that's the scary part.


"......aircraft checked thoroughly."


In the examples you chose, say a control horn has started a microscopic crack due to overstress in the Vne incident. Is the mechanic going to fluoroscope every part? Of course not. Could the crack propogate and fracture between 100 hrs? Hell yes.


Your advice is right on, but........its not actually that good to be an engineer and fly other peoples machines. I try not to think about it.

Gator - agreed but at least if it is reported, it might get checked more thoroughly each flight and possibly changed early just in case.:ok:

When we flew display manoeuvres in the Lynx, they were all documented and each cycle of each manoeuvre deducted a certain life from specific components.

Similarly on the Sea King, an overtorque, even if relatively minor, would reduce the life of the gearbox by a calculated amount dependent on the severity.

If you pretend these things don't happen and don't own up, you are building problems for yourself or others further down the line.

I own up. It's the other guys flying the thing I worry about, but can't do anything about, so I try not to think about.