Helicopter crashes at Lumberton airport

[Sir Korsky]

IIRC, our 206 daily preflight required pilots to hand pull the blades backwards to check the clutch operation. If you could hear/see the turbine rotating, you were good to go.

[[email protected]]

Having said that, slowing the rotor RPM a little during the descent would reduce ROD, no?

That depends on your RFM - if it says you can then it should specify a figure (on the Gazelle it was 330 Nr down from 380).

If it doesn't say you can do it then it is vital that you stay within your prescribed Nr limits. as you raise the lever, you move the 'driving' section outboard and increase rotor drag at the root (and possibly at the tip as well).

Overall you could see an increase in rotor drag which will need an increase in RoD to maintain the Nr.

Most people like to have as much Nr as they can for the EOL at the bottom and, even if you do droop the Nr for range you must recover it to normal for the EOL or run the risk of it being very exciting as there is not enough kinetic energy in the rotor to adequately cushion the touchdown.

As postulated earlier, it may well be what this chap did at Lumberton.

[nigelh]

Armyav8r ....I completely agree with all you say and this was my point when i got shot down by TC !
I believe the auto is near as damn it exactly the same with engine in idle ...but the EOL ( Landing ) bit will be different as you will notice that even at idle if you pull collective the machine will at least go light on the skids ....so the engine will be producing some lift right at the last bit when the rrpm droops .
Apparently you and i are wrong !!!!!!

[[email protected]]

To be fair Nige, I think it depends on where the idle sits in terms of holding Nr - what does it give you when you sit on the ground at idle?

It is, I suspect, way below useful Nr that would show a difference on an EOL - and that is based on my experience on a Gazelle.

[vaqueroaero]

As part of my previous life I was fortunate enough to spend 6 years as an instructor at a fairly well known OEM training facility in Fort Worth.
Whilst doing in house recurrency in a 206 with a colleague who had been working there for 35 or so years I was asked if I had ever suffered an engine failure.
Upon my negative response the throttle was retarded and then placed in the off position. We did ten or so engine off landings, and, as I recall there is no noticeable difference performance wise between having the engine off or at idle. The biggest difference was having the engine out horn going off and AW Brown telling me I'd better not screw it up as a go around was now no longer an option.
I have had a partial sprag clutch in a 206. We were trying to adjust the autorotational rpm and no matter what we did the rpm wouldn't change, but would drop to the bottom of the green with the Nr and Np needles staying stuck together. It wasn't until after about the third engine shutdown that we noticed that there was no needle split. You couldn't turn the blades backwards, but by putting your hand down the exhaust, with very little pressure you could hold the Np wheels in place while turning the blades. What would have happened to the Nr with an engine failure is an unknown, but it probably wouldn't have been pretty.

[nigelh]

Crab .. do you think that we all agree now , apart from your mate TC , that there is not a fundamental difference between the two states of autorotation?
IF there is one , it will show itself right at the end on the collective pull at low rrpm ? One would hope that is the case as I don't know many people who teach with shutting the engine down !!!!
I'm glad we finally got there .......

[Thomas coupling]

Crikey Nigelh, I've realy gotten under your skin haven't I
Sorry I haven't responded earlier, but I have a life to lead outside Pprune too you know.
In fact I'm spending less and less time on Pprune now having spent the last 17yrs since joining it repeating myself. So you can take succour in that perhaps.
I will say this once again but for the last time................
Many posts ago, there was another thread on engines off etc - someone asked how many EOL's I had done. I checked and rechecked my log books and can confirm I have done in excess of 2500 EOL's. From nose bleed height to downwind, to going backwards, to 100' and 120kts.
ALL of them were initiated with the throttle lever being retarded and locked in the off position, fuel to shut off, ignitions off. Engine(s) out. In fact I have the dubious honour of switching all the engines off in a Seaking -to a landing - before the RN decided that this was another silly practice we could all do without....and banned this.
I'm not sure where Crab did his EOL instructing but mine was in the FAA and the technique was an E.O. (as above) - with or without warnings.
[Not just to FI].
An auto on the other hand - is a descent to the hover/land/overshoot with the engine(s) running at FI until just prior to the final decision to hover / overshoot where the engine(s) is brought back online.
Flying along minding your own business and an engine(s) stopping without warning is a totally different ball game compared to entering an "auto" for obvious reasons.
The delay caused due to the jesus factor (in an EOL) will already have cost you an increase in RoD which will prove difficult to make up, as you sort out the entry. In an auto, friction in the drive train and the engine itself sitting at FI will all have a play in preventing you from drooping the Nr too low before a warning is sounded reminding you things aren't looking good. In an EO, there are no safety nets.
Friction, engine running and confidence knowing you have a safety net when you need it, all add up to a lower RoD in reality.
Whereas in an EO: no friction, no engine(s) lack of practice/manhandling and a rapid onset of RoD during the initial entry will exacerbate the RoD. If you are practiced/well rehearsed/lightning reflexes - you might be able to recover this RoD difference. In reality you simply have to manage it/live with it.
That's me done on this particular matter.
Don't respond Nigelh. I'm really tired of your wisecracks. Go find someone else to annoy. [I seem to recall asking you to stop stalking me a few times now - enough is enough]. Chill out FFS.

Any chance this thread could return to the subject matter?

[chopjock]

TC,

An auto on the other hand - is a descent to the hover/land/overshoot with the engine(s) running at FI

Sounds like you are saying it's impossible to auto rotate with the engine stopped?

[AAKEE]

Originally Posted by [email protected]

Out of interest, how many people actually check the functioning of their freewheels on a regular basis - we had specific checks for it on the Sea King but on a single you would need to enter auto and flare to split the Nr from the Nf (or equivalent).

Yep, in every damned shut down. Cause it was written in our flight manuals ( "Throttle to stop/off - needle split") on the three first types i flew. Its written in the spine, so eyes go there on later types that dont say this( except for when in the maintanence check flights, where there can be explicit checks for needle split, both in flight/auto or on ground run).

[nigelh]

Thanks TC .....you now say ..."Whereas in an EO: no friction,"
That's all I wanted you to admit 👍
Well done !

[wiedehopf]

disclaimer - not a helicopter pilot - but still technically adept

now on a piston helicopter if the rpm of the engine decreases there can't be any torque transmitted from the engine to the rotor system / gear box. also it does not matter which accessories are driven before or after the sprag clutch. it will be the same no matter how far the engine is below output shaft rpm if it is minus 100 or minus 10 percent.

on a turbine helicopter with a gas generator and a power turbine on the other hand... you can reduce the rpm of the gas generator only. that running at idle still makes air go through the power turbine which i believe will still turn at sufficient rpm although with relatively little torque to transmit some power to the rotor system.

so we see we have a difference regarding autorotation:
piston helicopter there is no difference between idle and engine off
turbine helicopter there a difference between idle and engine off

now add to that: i'm not so sure a turbine helicopter needs a sprag clutch. you could well say the power turbine as a load is not significant enough to warrant one. but i guess they put one in in case turbine or compressor blades decide to rampage through the engine .... which could be a significant load.

still idle power should impart enough torque onto the power turbine wheel so that it makes a noticeable difference compared to engine out even if there is a sprag clutch.

so nigel: tc might have said no friction but might have meant no residual torque being transmitted.
but i guess that's the different viewpoint piston vs turbine.

anyway there haven't been any concise explanations on the issue so maybe my train of thought will help with that maybe not. at least you got my brain working that's always nice so thanks for the discussion

[[email protected]]

wiedehopf - very useful, thank you, it concurs with what I was taught (on turbines) and shows that you can't automatically transfer knowledge broad brush to pistons

[[email protected]]

AAKEE - checking the freewheel on shutdown is all well and good but what about checking it before you go flying?

It is a simple check on a twin since you just retard each engine in turn to check for Nf split.

[212man]

on a turbine helicopter with a gas generator and a power turbine on the other hand... you can reduce the rpm of the gas generator only. that running at idle still makes air go through the power turbine which i believe will still turn at sufficient rpm although with relatively little torque to transmit some power to the rotor system.

so we see we have a difference regarding autorotation:
piston helicopter there is no difference between idle and engine off
turbine helicopter there a difference between idle and engine off

On a gas turbine engine there are two governing parameters - Ng and Nf/N2. With the throttle open the engine will match its Nf/N2 to the Nr and the needles will be joined and torque applied to the rotor system. With the throttle at idle the Ng will be governed only, there will be no regard for Nf/N2 and the needles will be split with no torque applied to the rotor system. Ergo, in autorotation the rotor will not benefit from the engine when the throttle is at idle.

[[email protected]]

No, but many of the accessories may.

[212man]

Originally Posted by [email protected]

No, but many of the accessories may.

I'm all ears....

[AAKEE]

Originally Posted by [email protected]

AAKEE - checking the freewheel on shutdown is all well and good but what about checking it before you go flying?

It is a simple check on a twin since you just retard each engine in turn to check for Nf split.

I think we might be closing up to OT.
Anyway, needlesplit check is to comfirm it is releasing now, to be checked safe for next flight. When running up you have one engage for the freewheel before taking of. If doing your check you still have one engage to do before flying. I do not think you add a lot of safety doing so.
The system with multiple engage-disengages during ground run after install of new freewheel unit ( and check for disengage during checkflight /autorotation on some types) and to check during each shutdown should be good enough, I think.

[wiedehopf]

it might well be that the gas generator on idle does not turn the power turbine fast enough to torque the rotor. then there is no difference to the piston case.

but i'd say it all depends on the gas generator and its idle setting doesn't it?

@crab
the accessories either get driven by the gas generator and never are an additional load during the autorotation regardless of idle or off. or they are driven from a point after the sprag clutch in which case if the power turbine does not torque the rotor it also does not torque the accessories and they are drag for the rotor.
so talking about the accessories really does not help. they should make no difference.

[[email protected]]

I'm clearly wrong then - not the first time and I'm sure won't be the last. I have looked back through various tech notes but can't see anything to support the idea, even the Sea King where the TR drives the MRGB accessories in auto.

Hey ho It's just the sort of rabbithole you can disappear down with a throwaway remark on PPrune - far too many knowledgeable people on here

[idle stop]

For what it’s worth, here’s my two pennyworth.
In 28 years of instructing I have probably done only hundreds of Simulated EOL (SEOL, to use EASA-speak) in piston helicopters, but thousands in turbine helicopters.
And just to clarify my own terminology a bit more, an autorotation is where the rate of descent airflow, rather than engine power, is driving the blades, and may end in a re-engagement of power, either at height, or in a flare recovery (ie the SEOL profile, but power used to recover to low speed IGE environment, no ground contact) or in a SEOL (engine running but disengaged and ground contact) or an EOL (engine not functioning, ground contact). OK, some purists may remind me that engine to MGB drive may have failed but the engine is left running to drive the TRGB…point taken!
My take is that in a piston, with the engine throttled back to idle, there would be a negligible difference, though I have never done a shut-down, thus actual, EOL in a piston.
In the turbines, it depends….
In the Gazelle, when you throttle back, the engine is completely disengaged because it has a mechanical centrifugal clutch. (BTW, 1000+ hrs as a Gaz military QHI)
In free-turbine installations there may be residual thrust at the flight idle (FI) throttle position. This is true for a couple of other light Airbus SET types, where there is a drop in NR at min collective pitch when the throttle is closed to the ground idle position. Less observable in the 206 variants, though I am open to contradiction: and only recently have I access to a 206 instrumented for flight data, so I may well have a scientific look when I next can. This may also be a function in the 206 of the high inertia rotor, so that a throttle closure to GI at committal height doesn’t manifest itself in NR drop before the flare commences.
In certification trials on a light twin with Allison engines, we found that in 4 minute OEI climbs, in identical WAT and geographical conditions, there was no discernible difference between a climb with one engine at GI and the same engine actually shut down. So conclusion: no residual thrust at GI. So with both engines at GI in autorotation in that type one might expect that to simulate faithfully the engines-out conditions.
I also had a little experience of autorotation and EOL trials in a 7-tonne class aircraft. Here with both engines shut down there was a 2-5% drop in NR at min collective pitch vs NR at GI…so definitely some residual thrust.
It is, perhaps, safest to assume that most SET may have some residual turbine thrust during practice autos, either at FI or GI throttle setting. But it's probable that for an unexpected engine failure the descent profile will result in potentially shorter range, due to the initial loss in NR before pilot intervention.
So, putting my FI(H) hat on again, two golden ‘Lessons Learned’ rules:
• In Autorotation, NR is King
• Never expect to go as far as you first think you might in auto; losing range is easy; stretching it is not!