Page 30 - June 2023

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-lb, 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 1⁄4-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 manhours 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.
28 • KING AIR MAGAZINE
JUNE 2023