Icing in Piston Engine Intake Ducts

[Dick Whittingham]

This is not a query about carburettor icing. The mechanism for this is well documented. However, most references say that icing in the intake duct cannot, or does not, occur with fuel injection sysytems

However, there is some potential for adiabatic cooling in the intake when the air valve is closed. Question. Is there any evidence at all out there of icing in the intake duct or manifold of fuel injected engines?

Dick

[clunckdriver]

Yes, it does take place, thats one of the reasons my 421 Golden Eagle has an alternate air flapper valve on the side of the cowl to let alt air in in the event of this happening, also have a manual selection we can use. Having said the preceeding its not seen very often, our main intake ice problem is just a side effect of normal airframe icing, of which there has been plenty this Winter.

[Dick Whittingham]

Thank you, Clunkdriver!

Could I just confirm that your Golden Eagle has fuel injection and not a "Stromberg Injection Carburettor" - whatever that is. In the references this one sometimes gets a mention as a sort of hybrid that does ice up occasionally.

Anyone else?

Dick

[SNS3Guppy]

The mechanism for this is well documented. However, most references say that icing in the intake duct cannot, or does not, occur with fuel injection sysytems

What sources say that induction ice doesn't occur? I've never seen one. Perhaps you're confused about what you're reading.

There is nothing about fuel injection that affects induction icing. In other words, in a discussion on induction ice, fuel injection is irrelevant; it has nothing to do with the price of tea in China.

Several kinds of induction ice can occur. Carburetor ice you've already mentioned. In aircraft equipped with carburetors, ice at the throttle plate and venturi area within the carburetor can experience icing which occludes airflow, or which impedes the fuel jet, starving the engine of fuel and airflow. Two types of carburetor icing occur; one involving use of the idle jet, usually with a closed or nearly closed throttle plate, and the second involving operations at high power settings or anything above idle, in which the main fuel jet is partially occluded or the entire venturi throat and becomes blocked. In practice, it takes fairly little ice to cause engine problems in the carburetor, and very little change in carburetor air temperature and mass airflow (throttle position and engine RPM) to affect a change in the rate of ice buildup or loss.

Two other types of induction ice occur, plus the hazard of ice passing through the induction and into the engine to cause blockages or damage.

Piston engines utilize an alternate air door for the engine inlieu of carburetor heat, as there's no carburetor to heat, and the mechanism of icing is different (or more appropriately, the manner in which icing affects the engine is different). Some engines use manual alternate air doors in the event induction air is blocked. Others use automatic doors, and some engine installations utilize "suck doors" that are typically held closed magnetically and drawn open if the induction is suddenly blocked. These latter doors have been responsible for a few engine failure when the door itself failed and was drawn into the induction, blocking airflow, and causing an engine failure. In fact, a fatality occurred at a corporate department where I worked due to this very thing.

Induction ice can form at the air inlet, in an injected piston engine installation. This often forms across an air filter. In most installations, there's no remedy for this, other than the use of alternate air.

Induction ice can rarely form inside the induction area, though most engine installations prevent this by using inlet filtering at some point in the induction (either actually at the inlet, or sometimes farther inside the airplane). Icing can form on the throttle plate, or in some cases can build at a bend or turn in the induction housing or tubing. This generally only occurs when a significant enough change in the induction diameter causes a change in airflow velocity and consequently pressure, and only when sufficient moisture is present, and even then only when ideal freezing conditions exist. This is unusual. Ice can form in the induction itself (rarely), or on the throttle plate.

Some installations use changes in the direction of the airflow to remove contaminants. This is more common in turbopropeller installations, especially PT6 installations, where the air induction is toward the back of the engine, rather than the front. Where a change in airflow direction occurs, it's known as a "mass particle separator," which allows ice, debris, birds, etc, or anything with mass to avoid being drawn into the engine directly. Various means such as "ice vanes" are used to increase this action in the presence of precipitation or contaminants (the King Air and Piaggio, for example). Most piston installations don't get that fancy.

I've never seen a reference that suggests induction ice doesn't occur in injected engines. In fact, I've never seen a reference that doesn't warn that induction ice can occur in injected engines, and that makes a provision for an alternate air source to accommodate that potential event.

[Tinstaafl]

I used to get induction icing regularly in the Islander, flying in winter around Shetland & down to Scotland. These were carburetted but it wouldn't have mattered a jot because it was snow & ice blocking the airfilter in the intake. Mind you, conventional carby ice was common too, but I'm tryign to address induction icing. I've never seen a design more prone to blockage than the BN2. Not even a drain hole (until one was added by the company :ok:) to let meltwater out of the housing. It's not fun at 0200 in gales digging my fingers around the filter to remove impacted snow so I could depart again.

[Dick Whittingham]

SNS3Guppy:

I think you are more confused than me. Look at the relevant UK AIP for a start. And one thing I did not want was a long irrelevant wander through the whole icing subject. And where did you get the idea that piston engines do not have carburretors? A typo, surely

Thank you tinstaafl

Dick

[SNS3Guppy]

I said nothing about piston engines not having carburetors. Where did you get the idea that I said any such thing?

What on earth is a "UK AIP?"

Your question was answered. You appear to have a comprehension problem.

You stated:
However, most references say that icing in the intake duct cannot, or does not, occur with fuel injection sysytems

Again I ask which reference states that icing in the "intake duct" cannot or does not occur in fuel injected systems?

I've never seen such a statement. In fact, induction systems using injection systems are required to have an alternate air source precisely because induction ice can occur.

Just as importantly, as already noted, injection has nothing to do with induction icing. One may not get carburetor icing, but other forms of induction ice exist, as previously outlined for you.

What part don't you understand?

[411A]

Guppy tends to run on-and-on-and-on...and never truly answers the question.:rolleyes:
Having flown large 4-engine piston airliners extensively, induction air icing has never been a particular problem (period) on both Douglas and Lockheed (and even twin engine Convair 440) designs...DC4, DC6, DC7, L1649...specifically.
Not saying it can't happen, just that I have never experienced same.

Cessna 401/402/411/421 types?
No, not those either....and I personally own one of the above.

Next question?:}

[SNS3Guppy]

You have a comprehension problem, too.

The original poster stated:
However, most references say that icing in the intake duct cannot, or does not, occur with fuel injection sysytems


He or she has been able to say which "most references" make such a statement. Have you ever seen such a statement? I certainly have not (except here).

The original poster questioned:
Question. Is there any evidence at all out there of icing in the intake duct or manifold of fuel injected engines?


The answer is yes, there is evidence of this, which can be seen first and foremost by the fact that an alternate air source is required for injected engines.

Icing can take place at several points in the system, depending on the system, operating conditions, etc. The details of this were previously outlined.

Apparently several paragraphs has exceeded both your attention span, and that of the original poster.

This does nothing to change the fact that induction icing can and does occur, and provisions are made for it in the design of air induction systems for piston aircraft. These provisions have also been outlined. Perhaps you missed that, too.

[Brian Abraham]

Dick, some real world experience from Bob Buck when he was with TWA, from “North Star Over My Shoulder”.

Later, so-called nonicing carburetors were developed, but they weren't; then came fuel injection, and that was supposed to be the ultimate cure, but it wasn't. An engine needs air, and that comes in from the outside, bringing along snow, rain, freezing rain, or whatever else is out there. The carburetor might not ice, but the induction system-all the passageways the inbound air flows through-will.

A new model Constellation came with fuel injection and consequently was announced as "nonicing," but we still had trouble because the induction system would collect snow that made the engines run erratically. What we needed was a way of shutting off the air coming from outside and taking it instead from under the cowling where it was warm and snow-free. The problem was that the airplanes didn't have this option, known as Alternate-A (A for air). We pilots urged the company to modify the airplanes, but they didn't want to-it cost money.

Every time we encountered snow as we flew across the ocean the engines acted up. The flight engineer played with different power settings, leaned or enriched the mixtures, anything. We flew on with the engines surging-running irregularly, but running. But it was damned disturbing to feel the rough-running engines, on instruments, with the cold, stormy North Atlantic 17,000 feet below. No one ever had a complete failure because of it, but there were no guarantees, and each time it occurred you squirmed, hoping this wasn't the one.

The solution was simple, and it came about on a flight I happened to be flying, New York to Shannon, Iceland, and Paris. One passenger was John Collings, our VP of operations, a thin-lipped old-time pilot who demanded perfection. International operations had recently been combined with domestic, which had been his bailiwick, so now he was boss of it all. Ocean flying was relatively new to him, and I think he may have had that uptight feeling people are apt to suffer when first heading out to sea.

We were about 100 miles off the coast of Nova Scotia, flying through light snow when I drifted back to the cabin for my smile-and-nod session with the passengers and to look in on John. I knew the snow would become heavier as we approached a low-pressure area. He was eating dinner.

He asked me, "How's it going?"

I kneeled down in the aisle and responded, It's going fine, right on flight plan, a little snow outside-we're 100 miles off the Nova Scotia coast."

Just then the engines decided they weren't happy with the snow and started to run rough.

'What's that?" John nervously asked.

"Oh, just snow making the engines act up---can't help it." I didn't feel comfortable and wanted to go back to the cockpit, but I decided to grind this into John a bit.

We chatted some more, as the engines' normal rhythm was disturbed by roughness.

"Don't you think you ought to be up front?"

It won't do any good-I've got a good flight engineer, he's doing his best; without that Alternate-A we can't do a hell of a lot to help." He was well aware of our demands for this modification.

The engines continued their slight surging-Collings looked nervous.

Bob, I want you to go back up front!" it was an order.

"Okay-see you later." And I sauntered back to the cockpit-glad to be there.

We finally flew out of the snow and the engines settled down to their smooth pounding. But not long after that trip the order went out for Alternate-A to be installed on the fleet.

[Dick Whittingham]

Thank you all. I should have given a tighter specification. I am looking for examples of intake duct icing in classic carb ice conditions, but without the carb, that is in fuel injected engines at OATs above zero and in high humidity.

The reason for this is that teaching in the schools seems to be edging from a literal acceptance of the statement that fuel injected engines do not suffer from carb icing to an assumption that intake icing is not a hazard in what would be carb icing conditions and I don't have much real world data about probabilities

Dick

[Zeffy]

The Piper Aztec (Lycoming IO-540 power) was quite prone to icing/clogging of the air filters, especially when flying in conditions of wet snow.

Normal air inflow was through forward-facing scoops beneath the nacelles, then upward through the air filter and into the induction manifold.

Snow/ice could accumulate on the outside of the filter and block the air supply.

The alternate air mechanism was a simple hinged door in the middle of the canister housing the air filter. Selection of alternate air would change the air source from the forward-facing scoop to the inside of the nacelle.

Pulling the alternate air knobs meant losing the ram effect of the intake scoops and a typical drop of a couple inches of MP. Not exactly helpful when the airframe was also accumulating ice -- but it was better than a complete loss of power.

Perhaps you'll have better luck than I in finding an Operating Manual for the old Aztec on line.

IIRC, the guidance spoke specifically to the conditions of heavy, wet snow.

[clunckdriver]

Dick, the engines in the 421B are Continetal GITSO {Geared, Injected.Turbo/Super charged/Opposed} Ours are 385 H/P, Ram conversion. I can only remember two cases of intake ice when in non airframe iceing conditions in about 3000hrs on type, in order to see the ice inside the intake one has to get ones head way forward and look back into the scoops. Both times I recall took place in about plus two degC air temp when the precip was very close to turning into freezing rain. Hope this helps and check your PMs.

[Piltdown Man]

Guppy, this is the UK AIP (http://www.nats-uk.ead-it.com/public/index.php.html). If you are studying for CPL/ATPL exams, this is one of the sources of knowledge. If it's in here, is HAS to be correct. But when studying for UK exams in the UK, it is important not to let real life and facts get in the way of "good" exam question. Fortunately, the better schools in Britain give you real answers as well at the exam answers - the two are often quite different.

PM

[SNS3Guppy]

Guppy, this is the UK AIP. If you are studying for CPL/ATPL exams, this is one of the sources of knowledge. If it's in here, is HAS to be correct.

Thanks for the reference. That's good to know, because I can't find anything in that reference that suggests what the original poster suggested. There is nothing which suggests or stipulates "that icing in the intake duct cannot, or does not, occur with fuel injection sysytems."

At that web site, the paper entitled "Induction System Icing on Piston Engines as Fitted to Aeroplanes, Helicopters and Airships" (http://www.nats-uk.ead-it.com/aip/current/aic/EG_Circ_2009_P_077_en.pdf) makes no mention of the notion that icing in the intake duct cannot or does not occur with fuel injection systems. The paper does cover the topics I previously described. Perhaps some will find it long-winded and useless too, though the original poster sees it as the center of the information universe. Seems he's hit a bit of an impasse.

Induction ice occurs in various forms.

[Old Fella]

As has been the case on previous occasions, SNS3Guppy has made an erroneous statement and then denied having done so. In his post (#4) Guppy says: Quote "Piston engines utilize an alternate air door for the engine in lieu of carburetor heat, as there's no carburetor to heat" Unquote. I am sure Guppy simply forgot to type the word "Injected" at the start of the statement. However, as is his want, Guppy denies having made the statement and instead tells Dick Whittingham who questioned him, and later 411A, that they each have a comprehension problem.

Back to the original question. Yes, induction icing can take place in either a fuel injected engine or a conventional carburetor equipped engine. Induction icing can occur whenever the the metal inlet duct surface is at or below freezing point and super cooled moisture droplets impact the duct. Ice forms immediately in this case. It can also occur when the OAT is near or below freezing and the aircraft enters visible moisture (cloud, sleet, rain) such as when on descent from above the freezing level.

[clunckdriver]

Dick, check your PMs!

[Brian Abraham]

Dick, there is probably no better description of intake icing than that given by the famed aviation author Ernest Gann in "Fate Is The Hunter". He describes his experience as a youthful copilot in a DC-2 flying over a mountain range.

We have merely nodded to fear. Now we must shake its filthy hand.

Both engines are cutting out --- first one and then the other. For one awful moment they both subside together. And there is a silence which is not really a silence but a chilling diminuendo of all sound.

This is the way you die.

At three minutes past two in the morning.

And as, suddenly the engines regain themselves. We can feel the surge forward, but we have lost five hundred precious feet! We are below that peak, wherever it may be.

Something must be done about the engines. Nothing else is of any importance whatsoever.

Hughen has yanked on fun rich mixture to the carburetors. He switches back to the main fuel tank and works the wobble pump. All the while he struggles to keep the ship in a semblance of straight and level flight.

Again the engines grow feeble. They are stricken with a mysterious disease. We might, argue with our flying senses, but we cannot argue with a manifold pressure gauge. Both instruments show a slow and steady loss of power. This cannot be due to carburetor ice. We are certain of that because other instruments tell us the heaters are working.

We lose another two hundred feet. Hughen anxiously jockeys the throttles. He checks the magnetos. Perfect. He steems more perplexed than frightened and I cannot think of a way to help him.

We are much too low. We can only be flying in a valley, but it is impossible to determine which valley since there are several shown on the chart. Nor can we know when this convenient valley will come to an abrupt end. Hughen begins a slow circle. Flying so, we will at least use up less territory.

"Maybe I'd better go back and start heaving things overboard." We must, absolutely. must, have altitude.

"Wait. I may need you."

Hughen has switched on the landing lights again and yanked open his side window so he can see the left engine clearly. After a moment he hauls the window shut. Then to my horror he cuts off the fuel mixture to his engine. Starved of fuel, it backfires angrily. At once Hughen pulls the fuel lever to full rich again. The response is like a cheer. His engine is putting out full power once more. He repeats the action with the right engine controls. Again, after a moments startled regurgitation, a welcome return of power.

We are able to climb at fifty feet a minute!

"Air scoops . . . icing over. Watch them. Soon as the power falls off, cut the mixture until they backfire. Then. slam them on again."

I must lean forward to see past the accumulation of ice along the fringes of my side window. It is swelling rapidly and is easily three inches thick in places. Beyond it, in the ghostly light, I can see the engine, now grizzle-bearded in ice.

The carburetor air scoop is an oval-shaped metal mouth on top of the engine. Through it must pass the air, which is as important to any combustion engine as fuel. Without air the engine dies as surely as a drowning human being. Normally, the mouth is approximately four inches wide. Now the reason for our loss of. power becomes all too obvious. Even as I watch, the ice accumulates around the lips of the mouth. It builds upon itself, decreasing the size of the opening like a closing iris until it is merely a black hole hardly more than the size of a dollar. The same thing is happening on Hughen’s side. Our engines are simply suffocating. Something must remove the ice before it closes their mouths entirely.

Hughen has had the nerve and courage to find a way. By backfiring the engines a tongue of flame spurts from the air scoops. It is not the flame but the force of air from the bowels of the engine which knocks away the closing ice. Two hefty belches seem to do the job nicely. Then there is a wait of three or four minutes until the danger point is reached again. It is a terrible abuse of the engines, but not so bad as asking them to fly through a mountain. Gradually, Hughen is able to nurse the ship to 4,300 feet. But even at full power, that is all. He holds the course reasonably steady on southwest, the direction from which we have come.

Between backfirings I am heavily engaged with the loop antenna. With luck, our position will again become important.

I am able to plot a series of bearing fixes. They show without doubt that retreat might be our salvation.

[mike-wsm]

Dick, I read your question as icing in non-carburettor engine air intake ducts. Have you been to RRHT (http://www.rolls-royce.com/about/heritage/branches/bristol.jsp) and looked at the history of icing in the Britannia intake ducts? Not a piston engine but a persistent sufferer from intake icing. There must be plenty of detailed reports available on those dusty shelves. Sorry if I've misunderstood and this is irrelevant.

PS - Lovely piece of Ernest K Gann, Brian.

[ChristiaanJ]

Brian,
Thanks for reminding me of "Fate is the Hunter". I lost my copy in the last move.... will have to buy another one.
CJ

[chris weston]

Brian,

Brilliant quotation.

Will get the book.

Does mirror finishing the induction tracts reduce icing - diminution of phase change nucleation etc?

Adds cost I know.

CW