Page 24 - Volume 11 Number 3
P. 24

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 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.
MARCH 2017

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