"high side runaway" - other local terms?

[misunderstood]

Heard recently "high side runaway" in relation to turbine overfuel situation. I'm familiar with the term "turbine runaway" ('cause that's when the engineers are shouting "run away, run away"!), but not "high side runaway". In context it was clear enough, but made me think I should ask if you've got any local terms used to describe various engine startup/running/shutdown problems that may not be universal. I'm sure those who have operated internationally will have come across some.

[BlenderPilot]

Se volvio Loco el Motor

[Shawn Coyle]

Might be interesting to make up a lexicon of such terms. High side runaway might also be known as 'high side failure', 'runaway up',
'N2 shaft failure' (peculiar to one engine type), 'engine overspeed'.
Never heard it called an 'overfuel' though.
Any other suggestions?

[[email protected]]

I have always understood engine governor runaways as either going to max fuel flow or min fuel flow and called them 'runaway up' or 'runaway down'. The runaway up is likely to cause engine shutdown if overspeed protection is fitted whereas the runaway down will just leave the engine at flight idle - not much use unless you have a manual throttle to override the governor with.

[ppng]

If my memory is correct, the Puma flight manual talks about 'Governor Active Failures'. These can result in 'runaway up', 'runaway down', or 'Ng oscillations', depending on where the problem happens and where collective is at the time of failure.
When the weather is 'doggers' some people like to theorise about more esoteric governor malfunctions which are not covered by the flight manual. Whether these are realistic (and just 'glossed over' by the manufacturer) or are just the product of half-a-dozen bored minds is a moot point.
Nick? Shawn? Anyone?

[SASless]

Are not all mechanical failures just exactly like described in the approved flight manual?

Anyone have an N2 Overspeed Drive Shaft Failure on a CH-47A....it took the US Army and Boeing-Vertol years to invent that procedure....not that it was a "remarkable failure" or anything like that! Crikey....your life can pass before your eyes on that situation in a empty helicopter.....basically a "runaway up" that does not respond to either the normal trim or emergency trim systems....thus allowing one engine to go to Topping plus power...slightly over speeding the rotor system....and when you pull the Thrust lever (think collective) up under your armpit in order to control that wee excess Nr.....the other engine then approaches topping as well....which makes for the perfect occasion for a Chinese Parliment in the Cockpit. Normal Nr was 230 rpm....I saw the rotor tach as it was dropping back through 270 with full application of Thrust Lever (collective)....and it hung there for what seemed like years.....till we sorted out which Donk had stampeded for the barn. For a while, I thought I had prematurely bought Christmas presents....thinking they might more likely become going away gifts.

There are runaway ups, high side govenor failures, overspeeding governors.....but they are not all equal.:{

[Chopper Jog]

Fellow Chopper Boys,

Regardless the type (Sikorsky or Eurocopter) you fly, these terms relate to a malfunction of the FCU/Governor. My experience with these malfunctions is that a high side failure/Governor run-up or low side failure/Governor run-down relates to the fuel entering the FCU.

With a high side failure too much fuel is fed into the system via metering valves, causing the NR/T4, T5 to increase. And the reverse hold true with a low-side failure. If not controlled by the pilot, a possible engine shutdown may occur with condition 1 and a decrease in NR in the latter condition.

I have also seen (on the sim and line trg) some pilots take some time to analyse the situation to determine what engine is the faulty one. A great practice on the Black Hawk was to call NR/T5 straight away with any associated emergency light/audio.

If NR/T5 is high, that particular engine has experienced a high side failure and the pilot has to raise the collective or retard the engine lever. The other condition with decaying NR and low T5, indicates that a low side failure has occurred on that engine and the pilot has to lowered the collective or entering the emergency range.

In following this cockpit practice, the crew will be assured that the helicopter is in a safe flying condition and if necessary, adjust the power to control the NR - which in turn is what keep us alive.

Hope this explanation has cast some more light on this topic.

Over to the technical experts of the FCU, ECU and Governor components.

From downunder,

Chopper Jog

[helmet fire]

Shawn, I have also heard it commonly refered to as governor overspeed, and governor underspeed. But, like anything, I guess you tend to prefer what you learn first, even though there may be far more appropriate terminology, eg I am sure all will remember the continuing VRS and SWP threads! In this case I prefer High side failure (NR increasing), TQ Splits (NR 100%, but one engiine high, and one low), and Low side failure (NR decreasing) because they are descriptive of the problem and doesnt sound like too many other terms.

In remedy, I think there is one solution to all the aircraft I have flown (but I have not flown many!):
NR is life, fly NR first!!
Then:
A single engine is easy: react accordingly.
To diagnose a twin: always grab the high engine (based on temp, N1, N2, TQ or whatever you have left to indicate) and gently retard. For high sides, you have the bad engine in your hand if the NR returns to normal and other engine rises to compensate. If it is a low side, you know you have the good engine in your hand if the NR droops below the normal operating range and the other engine fails to compensate. In a TQ split, you have the good engine if NR droops, and the bad engine if NR remains at 100% and the other engine increases to compensate.

But again: I have not flown many types, and I have no three engine experience.

[belly tank]

About 3 years ago i was on the way back from a fire in a 206 and landed to refuel at fire control. everything appeared normal, frictions on etc, throttle back to idle as she spooled down after about 2-3 seconds after stabilising at idle the helo shut down, i looked at my crewmember and he said that was a quick shut down!!

I said i didnt do a thing!! she quit by herself.(i had plenty of fuel by the way!!)

turns out they had to replace the govenor

bt

[SASless]

I believe Nr is the key to analyzing engine malfunctions...as said by others....we live and die by Nr or the lack of it. If the Nr is higher than set previously...assuming no collective movement...right answer is a high side failure of some type....and a lower Nr would suggest a low side failure of some sort. No change in the Nr can be hard to define. The next step I would assume one would wish to take would be to determine what the N1/Ng and exhaust temps are doing....those at or very near topping and high Nr would point to a high side failure....and indications approaching flight idle indications and low Nr would suggest a low side failure. I would suggest the torque meters are of the least value in determing which engine is the culprit...and should be the last thing checked.

The cool aid acid test is simply to raise and lower the collective and see which engine correctly follows the collective movement.

[NickLappos]

Sassless has it right on, as usual.

For a "high side runaway" the bad engine fuel control/governor goes to max fuel flow, usually because the Nf/N2 reference is lost to the governor, which makes the control think the rotor is going down in rpm. The classic case is the Nf flex shaft failure in the previous generation of engines, where a flexible shaft runs from the power turbine section forward to the fuel control to provide the output speed reference. If that shaft breaks, the governor thinks the engine is winding down, so it tosses in more gas, and the engine runs away upward. FADEC engines have multiple references for N2/Nf so they are harder to fool.

The key in multi engine helos is to be SURE you have ID'd the bad engine, and that can be confusing.

For the pilot, these highside runaways are doubly bad in some cases because the pilot's triple tach uses the same reference as the control, so the pilot sees a big torque split and an N2/Nf needle going to zero. The natural tendency is to think the low N2 engine failed, and grab the lever of the healthy engine and do some foolish things. It is all too common for a healthy engine to be mistakenly shut down leaving the screwy one as the only power source. That usually means a trip home in a rubber boat, unless you are Sky King.

The absolute key is to do nothing when the torque split occurs. Nothing except stare at the rotor rpm. If the rotor is in the green, do not grab any levers, nor any fuels. Torque split, rotor in the green, hold altitude and airspeed and study the rotor rpm. If the rpm is dropping, of course, lower collective to get it to stop!

If the rpm is going up, power is too high, so blame the high torque engine. If rotor rpm is screaming upward, do not grab any levers, the odds are about 50% that you will grab the wrong one. The lever to grab is the collective pitch. Pull it up to absorb the extra power, you won't have to pull far. Once the rpm comes up about 3% the good engine will be at flight idle, loafing along, trying to be your friend, while the bad engine is at max torque, carrying the whole helicopter.

Some pilots will worry about that massive torque that they see, perhaps even a bit above the red line. Yes, you can worry, but don't grab any levers yet, at least until you and your stick buddy have discussed this thing. Rotor up means too much power, blame the up engine, confirm it, tell your cp to put his hand on it, confirm his hand is on the correct lever. Do this as carefully as the first time you bought a house, you will live with this decision about as long as a mortgage! Then pull gently back to get the torque to start creeping back. Don't wiggle the collective, it will confuse things if you are making the torque wiggle while he is trying to sort out if you two astronaughts have chosen the right lever! If you can control the torque by pulling back, just manually set the bad torque maybe 10% below the good engine, and the two of you coordinate the lever during the flight home.

In a single, the answer is easier. If the rpm is up, the engine is up in power, pull the collective to stop it from overspeeding, then coordinate a lever pull back with collective reduction to keep things in the green. Fly home gently, and make a shallow approach.

Some test pilots think the engine levers should be in the tail cone, inaccessible in flight, because of all the bad things that good people do with them. In an engine emergency, the engine lever should be the last thing you move, after the situation has been stabilized by collective pitch and brainpower. Airplane training where you have to jump on the controls and identify/feather fast is exactly backwards to a helicopter.

[Thomas coupling]

I like that description Nick - nice one.

I always remember:

Runaway Up - Nr UP
Runaway down - Nr DOWN

High speed shaft failure - Nr STEADY (or v.slight rise).

Look to the Nr gauge every time..........

[C4]

Eurocopter test pilot taught me "Didi's Law" for identifying the "bad engine"

"High rotor RPM, High engine bad engine,
Low rotor RPM, Low engine bad engine."

Very simple to remember, works every time..:ok:

[offshoreigor]

Hi All, long time!

I agree with Nick and SASless, but in my experience, to keep it simple, in most types (76, 61, 212), if the Nr is higher than your beep trim can go, its a High Side failure, if the N2/Nf has split off with Normal to lower than normal Nr, then its a Low Side.

Just my 2 cents worth.

Cheers, :eek: OffshoreIgor :eek:

[NickLappos]

offshoreigor,

It is a bit more complex than that. The up engine might not drag the rotor to the max beep value, since the good engine will probably unload within a few percent. If you look to max beep as the exact clue, you might be fooled. In fact, virtually any up Nr is the sign of a potential high side failure, along with the large torque split.

One clarifying thought. In some helos, there is another way to get the torque split and Nr change - a beeper failure. If the N2/Nf beeper fails upward, the engines might split in torque, but both are fully governed and fully servicable, just not together. This could also be a collective bias/Load demand spindle failure. The key thing with a beeper/bias failure is that the engine has no will to run to overspeed shutdown, it is normally governed, the governor is just set too high.

[helmet fire]

Good discussions.

I still like the simplicity of:
Fly the NR.
Grab the high engine - it will tell all.

I tried to wrack my brain of some of the more complex failures I have heard of, and there are a few in which assumptions of governor problems are not covered adequately by certain diagnostics. In one, a Black Hawk took off from a grass confined area. As they became comitted to forward flight, they got a low rotor warning, looked inside to find a healthy torque split as they lowered the lever to put NR into the green. NR recovered fine and beaut, TQ split substantially reduced to normal. Both engines followed the collective movement thereafter. As power was again applied to diagnose problem (on way back to base), it was noticed that one engine had a much higher TOT and NG. According to some of the diagnosis above, you would have assumed the low tq engine was bad, and taken it to governor lockout (ECU lockout) removing TOT limiting and overspeed protection.

This would have worsened the situation because it was actually caused by a massive clump of grass clogging the intake. As lots of power was applied, the bad engine reached TOT limiting at a low TQ setting, but the good engine continued to increase TQ to maintain NR until it too began limiting, and NR drooped. Grabbing the high engine after NR was under control solved the problem because it was then apparent to the pilots that the bad engine was limiting like it's supposed to, and NG speeds matched the TOT readings, but TQ was not being produced: ie ECU lockout is not going to solve that problem. IE taking the low engine govenor after low NR is not always the fix.

Just provoking discussion - not trying to preach! I have found the same technique useful with the twins I have flown so far (UH-60, B212, B412, BK117, AS355). I wonder how applicable it is to other twins?

[matador]

During my test pilot course for the CH 47 a wise guy told me this simple rule:
Do with your collective whatever the RPM's are doing:ok:
It works for every helicopter and for every system. It also makes things easier to find out what is going on, as said before if you have raising RPM you have a high side, the opposite also applies. It is really that simple.
Buen vuelo

[Steve76]

What Offshore means is that the NR will not drive out of the "green" range via a beep trim runaway.

A highside or lowside failure WILL drive outside the normal operating range.

Personally, I have never had trouble 'remembering' easy ways to identify a governor failure. Its just a matter of knowing your aircraft and understanding the system. I mean, if the NR is driving up, control it with the collective. Down, control it with the collective.
Easy peasy, lemon squeezy.... ;)

[helmet fire]

matador: the low NR does NOT always equal a low side governor - therefore I dont think that it "is really that simple".

Steve, you are right about the easy peasey bit - fly the NR, but once that is controlled, it is not always so lemon squeasy - especially with low sides and tq splits.

Another example of tricky low sides, and one that used to fool heaps (including me) in the UH-1H was a stuck anti ice inlet vane that would, in the worst case, result in a max of 25 psi torque available (typical training weight IGE hover required 30ish psi) before NR bleed. We all tried to go to emrgency governor, thus significantly worsening the situation when a bit of analysis would have gone a long way.

[offshoreigor]

Nick Lappos

You are the S76 Guru but, in my humble 14 year operational experience on the S76A, A+ and A++, ( in addition to my S61, BH212, BH205, BH206, HU500D) the writing is always on the wall. If the Governor fails to the high side on the Allison or the Ariel, the Nr will ALWAYS go above the ENGINE BEEP RANGE! That is to say, above 107 on an Allison and above 109 on an Ariel. This is a fact of life.

I believe this discussion was about a total High Side failure so I don't feel I am out of line here. I am not trying to simplify things, but I have always found that when giving recurrent training to my pilots it is always better to keep it simple and I find it much more useful to show people what they already know rather than what I know.

Cheers, :eek: OffshoreIgor :eek:

[Collective Bias]

This is not really about "High side failure", but regarding "diagnostic a failure" I just want to share this incident that happend to one of our aircrafts a few years ago.


Nightime (midnight), VFR conditions, two-pilot crew, HEMS flight.

MGB chip light about 3 min before destination (hospital heliport), emergency checklist consulted, and decided to land at heliport. Engineer called in to check aircraft. Verified some chips on MGB plug. Changed oil, cleaned filter and hover tested it for 45 min, before new MGB chip light came on. One small chip. Same procedure again with oil and filter, and decision was taken to relocate aircraft without medical crew to nearby airfield (aprx 2 min away) and continue investigation there in the morning.

Now it was about 4:00 AM, still pitch dark, tired crew, "just" going to make a 2 min flight and then off to bed, you get the scenario.

"Known" MGB problem!

During TO, just after CDP(wherelse) a loud bang followed by a large Tq split. #1 N1 and T5 skyhigh, #2 Tq maxed on tq gage.........and all the time the pilots had that known MGB problem in their mind.

They flew out of the situation, and after Vtoss they started to take action on the situation. Confirming that it was #2 engine that was "driving the rotor" and also responding on collective change.

They carefully retarded #1 engine to idle, left it on idle, and made a running landing at the airport.

They estimated that #1 engine was at max "values" for 30-45 seconds before being retarded.



Many things could be said about this event, but it was some good airmanship that saved this flight.

I think that "ignoring" the instrument readings, and first fly out of the situation coupled with a careful analys (slow?) of the situation, prevented the crew from doing a bad (quick) decision.


The failure, a rag sucked into the #1 engine compressor

:{

[Thomas coupling]

Helmet fire has a point...
Everyone's talking about sorting the immediate problem out which we all seem to agree on.....BUT as helmet stated; what then? You need to look objectively at what the instruments are telling you.

In the case with engine ingestion coming out of the LZ, you've now contained Nr. Which engine is to blame for the droopy (previous) Nr and low Tq settings...or is it high Tq settings on the other engine which is to blame??

It's 90% easy peezy...but that other 10% might spoil your day BIG TIME
:\

[NickLappos]

offshoreigor,

You are correct, on the 76 family where the operating rpm is at max, any extra power will lead to an up tick above the max beep. In some defense of my assertion otherwise, we are talking here in generic helicopter terms, so that is not a reliable cue for many helos that have some margin between normal operating rpm and max beep.

Thomas coupling,

The idea here is that added power will result in a torque split and a rise in rpm, while a power loss will result in a torque split and a droop in rpm. So, if rotor his higher, it is probably a high side faailure, if rotor is lower, it is probably a power failure on the low engine.

You are absolutely right, you must do a study of the gages, but when you do, look to the rpm first. If the rotor is up, power is up, blame the high torque engine. If the rotor is down, power is down, blame the low torque engine.

Then, use collective to keep the rotor in the green, and stabilize altutude with speed. Do not leap to slam engine levers anywhere, this is the important point. Doing nothing is better by far than doing anything bad, and there is no great rush.

Once you have a suspect, give him a fair trial before you hang him. Tell each other why it is a good idea. "The torque split and the high Nr makes me think number 1 has a high side failure, do you agree?" Actually think about what you are seeing, guys.

Take the proper engine lever in hand, confirm it, both of you, please! Then start it back gently, moving it back while spotting rotor rpm (and while not moving the collective, which will confuse you!). If the torque begins to drop, and the rotor rpm goes back to normal, you have it! Pull the bad engine about 10% below the good one, and fly home cordinating lever and collective to keep it that way.

For the interesting case where the engine was stuck at part power due to injestion (I have had this about 3 times due to internal engine problems or fuel control stickyness) the above works well. As the torque splits, the rpm will drop somewhat, so follow the rules.

One real issue in a low power situation is - Do you leave the bad engine running and fly home? If it is vastly unhealthy inside (split internal passeges that are leaking power) it could be very bad to let it run. If it is a simple sticky fuel control, no harm. But how do you know? My suggestion is to gain good oei flight, and decide the risk balance. If I am over golf courses, shut the bad guy down and go home. If over 100 miles of icy sea, leave the low one low and milk it home.

[Yabu]

A couple of years back, we had in the persian Gulf , a high side of sorts. It was refered to as Auto acceleration, caused by fuel leaking into the bearing on the N1 govenor shaft in the FCU side on a bell 412 EP. This had the effect of drying the bearing causing it to seize and the shaft broke. The result was very instant rise of N2 / NR and within seconds hit near 120%. The pilot reported that diagnosis was extremely difficult, due to the high airframe vibration, making the panel near impossible to read. In fact due to the shake could not ensure which engine went high. The result was engine meltdown, resulting in fire, which then gave the pilot an engine to work with. All this in a matter of less that 10 seconds. He managed to land the aircraft safely to an offshore location. The result was an engine destroyed, engine deck delamination due to vibration, drive train was damaged, and from what I was told the rotor blades measured 25mm longer than standard, so it sure wound up. As an aside the management and engineering of the company involved tryed to crucify the pilot, who was alone ( not 2 pilot) They said he must have placed the fuel govenor switch to manual to cause such overfuelling. He was later exonerated by Pratt and whitney who upon tearing the engine down found the root cause.
His reported way of dealing with the situation as it happened was to immediately pull collective to try an hold back the rapidly rising Nr. In the short time the engine was on fire so that he dealt with as per 412 manual. He said there was no time, and indeed great difficulty, to diagnose because of the high vibration he experienced. I suppose this was due to a severely out of balance roter system at the quickly rising Nr.
The guy was a very experienced 412 pilot, and was in his words lucky that the fire occured before the drive train or rotor system came adrift.

[NickLappos]

Yabu,

I'll bet that was a handful, and the pilot did very well, but I'll also bet it was worse than you described. The auto-accel is a term we use to describe how an engine runs away when a flammable liquid is poured into the engine (usually by a bad bearing seal or an oil leak in the engine inlet). In these cases, the oil that pours into the engine is a good fuel source, and the fuel control is of no help, since it an only cut back the legitimate fuel, while the "new" fuel source is uncontrolled and usually very much more than the engine needs. For example, if you dribble oil into the inlet at 2 ounces per second, the engine will speed up to give you takeoff power!

What your friend experienced was probably a bearing failure on the engine internal shaft, which allowed bearing lubricant oil to leak into the internal air path of the engine, driving it to well above max power. The vibration was that poor engine running with one internal shaft bearing gone, so that engine shaft was whipping like crazy. The fire is because the engine finally split open and the hot gases poured into the compartment and attempted to prove the rating of the fire zone.

Corrective action = pull collective to keep the rotor in the green, shut that engine down (but it won't really shut down until it runs out of oil!) and get ready for a shaft failure/engine fire or something like that. Your friend gets an A++ on that check ride!

[helmet fire]

Yabu,
I had heard that story - and apparently it happened again some three months after the first, again on a 412 in the middle east. I just hope that when something like that happens to me, that I can get out of it as well as your mate - what a great job. I heard that on the second one, that engine fire illuminated the T handle on the dash, and that was about all the pilot could see due to the vibrations, so he pulled it and solved the problem!

Nick,
I think we all agree on your methodology, and I note that you say that NR high is "probably" a high side, NR low is "probably" a low side. I believe that retarding the high engine will always give you a more accurate diagnosis with a high side, low side or torque split- and it is usually the diagnosis that gets you because most pilots can fly the NR successfully.

[Thomas coupling]

Nick, 2 probs with your theory:

1. Don't have another pilot to 'share the blame with'!
2. Don't have throttles!! Just switches from Flight to GI!!

Otherwise ....agree with you!

[NickLappos]

helmet fire,
I suggest that you not grab the high torque engine if its brother engine has quit. If you always grab the high engine, you will be wrong about 90% of the time, because if engine failures are rare, high side failures are much more so.

I strongly suggest doing nothing, because you really don't have to do anything for a while, and then grab the proper lever and test your theory slowly.

[helmet fire]

Nick,
thanks for that advice - I'll try not to grab the only live engine.....
We are ALL suggesting that slowly slowly approach, but I guess it never hurts to repeat that mantra.

It's interesting that 90% of governor failures are low side - I thought the vast majority were designed to fail high, and I must confess that the vast majority I have heard of have been high. None the less, grabbing the high engine is a diagnostic tool - not a solution. Of course to be used slowly.

[NickLappos]

helmet fire,

I didn't mean to imply that low side failures are more common. I think you are right, many governor failures are up, but all goveror failures are rare. The problem we see is that the common, garden variety engine failure creates a big torque split, and can be (and often is) mistaken for a high side failure of the opposite engine, with dsasterous results.

When I first checked out in twin helicopters, I found an entire school of pilots who were being taught to grab the engine levers at the outset, so that they were primed to do something with them. It is my earnest belief that the levers could be locked out for the first minute of any torque split emergency without creating a big safety change. Of course, other emergencies require their quick manipulation (fires, anti-torque failures, rapid shutdowns on the deck) so we must leave them in plane sight, but when one engine is acting strange, there is a disappointing percentage of time when the crew jumps too quickly and grabs the wrong one.

[Shawn Coyle]

If it wasn't an engine failure in a single engine helicopter, the best advice I was given was 'reach out and wind the clock'.
'Course, now that we have digital clocks....