One of the ongoing debates among King Air pilots has to do with the necessity to use the engine anti-ice system as the Pilot’s Operating Handbook (POH) directs: “Before visible moisture is encountered at +5°C and below, or at night when freedom from visible moisture is not assured at +5°C and below.” Depending upon your exact King Air model and the cowling modifications it may contain, power and airspeed always take a hit when the ice vanes are deployed. The speed loss may range from five knots to as much as 15 knots. It is this performance degradation that makes many pilots reluctant to pull the handles or flip the switches for ice protection.
The other factor that influences a pilot’s decision about ice vane usage is the experience and beliefs of other pilots with whom he or she associates. When the crusty old gray-beard that has been flying these airplanes for thousands of hours believes that ice vane usage is not very important, it is hard for the newbie to go by the book. In addition, when it is so frigidly cold outside that the water content in the air is already well-frozen, such that no ice adheres anywhere on the airframe, it is an easy leap of faith to conclude the engines are also not going to be harmed by ice.
I strongly disagree with the casual approach to ice vane usage and plead with you to indeed go by the POH’s directions. Let me explain why.
Depending on your piloting experience – specifically, how much time you have spent flying in clouds – I will wager that you have experienced a variety of icing events. Although the OAT may be the same from one event to another, the outcome can, and does, vary greatly. Whereas last week’s icing encounter really got your attention as the windshield heat barely kept up with the demand, this week the airframe came through without a trace. Go figure!
In support of those pilots who have a casual approach to engine anti-ice usage, perhaps they are the luckier ones and have had the great preponderance of their icing encounters be non-events. Hey, I can relate! Especially when we are up there in the high 20s or low 30s (thousands of feet) and the airframe is staying ice-free; it surely seems logical – but incorrect – that the engines will also be safe.
I am going to share two separate scenarios that happened to individuals that told their stories directly to me. My hope is to make you “scared straight” so that you will embrace the POH’s conservative approach to ice vane usage.
The first story involves an old friend of mine with whom I have conducted initial and recurrent King Air training since the 1970s. When I first met this fellow, he was flying a B90 and the various companies he advanced with moved up the King Air ladder so that he was checked out in just about the entire King Air lineup by the time he retired. Although he never argued forcefully with me about ice vane usage, being a kind southern gentleman, I know that he was reluctant to deploy the vanes unless the airframe was collecting significant ice. Nothing I taught could convince him that he was playing a dangerous game.
Then one evening while at home, I got a call from him. It went something like this: “Well, Tommy (He always called me that!), I guess I should have been listening better to you all these years when you preached about ice vane usage. Today, at FL280, we were in visible moisture that was so thin it could have been the contrails of a 747, 20 miles ahead! Of course, I didn’t activate the engine anti-ice. When I started the descent, and changed the power setting, I noticed that things weren’t matching up like they did before. This continued through the landing so I had the mechanics take a look. When they got the flashlights and mirrors to look at the first stage compressor blades, they reported bent blades on both engines. So now we are sending our engines out for repair and will install a couple of loaners in the meantime. I couldn’t believe it, but I saw it! You were right!”
The second story involved a B200 also flying in the upper 20s, but this time it was night over a dark expanse of the Australian Outback. The pilot noticed that the nav lights were giving a glow on the moisture they were in, so he extended the vanes. He was not sure how long he had unknowingly penetrated the tops of these smooth clouds, but doubted that it could have been for more than a few minutes. When he broke free of the clouds and retracted the vanes, he noticed a 400 ft-lbs, or so, torque split. In the descent, one engine started fluctuating and actually expelling some visible flames at times out of the exhaust stacks. That engine was found to have suffered first stage compressor damage – a bent blade.
For many years now I have always included a copy of a Pratt & Whitney Field Note in the section of my training manuals dealing with ice protection. I am sure those who have trained with me in the past or who have attended the King Air Academy recently have read this before, but I want to print it here for those who have not yet seen it:
In April 1982, a general correspondence was issued concerning the subject of Compressor Ice FOD (Foreign Object Damage). Winter is here again and after three incidents this month, it is time to reprint the original issue with a few new comments.
During this past winter, we have received several engines for first-stage compressor FOD. In each instance, a single blade has been bent with the damage being caused by a soft or dull object – in all probability, ice.
The PT6 nacelle intake system is the result of a very exhaustive and exacting research program. Many hours of development flying in icing conditions with such equipment as closed circuit television cameras in the intake, and fifty million flying hours have proven its effectiveness.
All flight manuals are very explicit when it comes to icing. “Deploy the ice vane prior to penetration.” The interpretation of icing, however, is sometimes a little more difficult. Depending on the OEM (Original Equipment Manufacturer), some will state that +5°C and visible moisture are the criteria. Others will only offer it as a rule-of-thumb. Meanwhile, pilots will, on occasion, wait until first appearance of ice on the windshield.
Night flying imposes an additional measure of difficulty. Here the criteria is sometimes only a check at regular intervals with the wing ice inspection lights. To properly understand when the ice vanes should be deployed, one must understand where the FOD comes from. First, it does not build-up on the intake, break off, and then go through the engine screen. The sheer mass of the ice will stop it from turning the corner and hitting the screen. Secondly, even if it were to get in the intake plenum, the low velocity air at the screen, along with the ¼-inch mesh, would preclude any damage. What actually happens if the vane is not deployed to perform the inertial separation of the moisture, is that this moisture will collect under the screen and freeze. Either when a piece breaks off, or when penetrating higher OATs and the ice separates due to melting, the engine sustains FOD.
The same will occur with snow. Although below the freezing point, if the deflectors are not deployed and the snow reaches the screen, there is sufficient radiant energy to melt and then refreeze under the screen.
Only if the flight crews understand this principle can they be convinced to properly manage the deicing vanes. One bent blade (which is typical of ice FOD) costs approximately 100 man-hours in shop labor, plus the blade cost and cost of the software kit for reassembly. In addition, when an engine gets disassembled, hot-section components often require premature replacement and some class “A” Service Bulletins require embodiment. This adds unexpected cost to the FOD encounter. I know the pilots will tell you that the ice vane deployment costs them a lot in aircraft performance, but when you consider our economic times, one bent blade can be much more expensive.
Since this was first printed, two areas have come to light as to why flight manual procedures are not being followed.
First is pilot education. Most pilots who have been involved with this FOD are not aware of the mechanism. Give them a copy of this field note. Last year, in the case of one operator, this is all that was necessary to resolve the problem.
The second item is block time, or sector time. The fact is simple: when you deploy the aircraft anti-ice system, the aircraft slows down – some more than others. On short legs this does not amount to much, but when you are flying sectors of greater than one hour, it can be significant.
I cannot overemphasize how important this item of ice FOD is. The issue has gone beyond the dollars and cents phase and is now affecting the reputation of the airframe and the engine.
Does that information – Right from the horse’s mouth, as it were! – give you a better understanding of the mechanism? An important take-away is that what occurs in the engine intake may have little or no similarity to what the airframe is experiencing.
Knowing how fickle ice can be – benign one flight, scary the next – always makes me think of the classic movie line spoken by Clint Eastwood in Dirty Harry: “Are you feeling lucky, punk?”
If you choose to continue to be casual in your deployment of ice vanes, you must be feeling very lucky! I hope your luck holds out. Because if it does not, then the airplane’s owner is going to be faced with a large, wasteful, maintenance expense. Flying a few knots slower will produce a lot less lost time than having the plane be grounded for a month or so for engine repairs!
I will leave you with a positive thought: Do you realize that specific range – the nautical miles you are traveling for each pound of fuel burned, calculated by dividing ground speed by fuel flow – almost always is improved due to ice vane deployment?! Amazing, but true. Unless you are flying at a very high altitude close to the certified ceiling, or fighting an extreme headwind, or you were using a reduced power setting closer to max range than max power, then this statement is true.
You see, since the Fuel Control Unit (FCU) is a governor for compressor speed (N1), the reduction in intake air density due to ice vane extension makes N1 want to increase due to less compressor air drag. But the FCU reacts by reducing fuel flow to keep N1 constant. The reduction in ground speed is proportionally less than the reduction in fuel flow, so the airplane actually becomes more, not less, fuel efficient. Write down your stabilized ground speed and fuel flow numbers next time, before and after ice vane deployment. Get your smartphone’s calculator and do the division. I’m right, aren’t I? Perhaps that will give you a little comfort when you observe the decrease in speed.
Bottom line? You’ve heard it before but I’ll state it again: “When in Doubt, Get ‘em Out!”
1 Comment
Great info thank you for sharing