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Type II and the more common Type IV anti-ice fluids use non-Newtonian thickening agents so that they can stay put during ground operations, yet shear stresses on the fluid during the takeoff roll cause the fluid to come off the wings, leaving a clean wing behind that can provide proper lift at rotation. However, if the fluid itself fails to detach from the wing, it can degrade the aerodynamic performance of the wing just like any other contaminant.

Under what conditions of liftoff/rotation speed, aircraft design, temperature, and precipitation is the margin between "shear forces available to get the fluid off the wing" and "shear forces required to thin and remove the fluid" the least? You can assume that the fluid in use is a standard SAE Type IV fluid applied as per SAE ARP 4737.

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  • $\begingroup$ (BTW: if you wonder why I ask -- it's because I'm trying to ascertain what consequences there could be if you anti-iced a slow plane with Type IV) $\endgroup$ – UnrecognizedFallingObject Apr 14 '17 at 22:16
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Under the following conditions, we can't take off:

  • Temperature below -10 degrees C, and
  • Small Hail or ice pellets (i.e. METAR "PL" reported), and
  • Rotation speed below 115 knots

To operate with PL reported, we have to have Type IV anti-ice applied, and under those conditions of temperature + precip, they (yeah, "they") have determined that you need at least 115 knots to be sure the fluid adequately shears off. There are various ways to drive an increase to your VR speed; it takes a pretty light 737 to rotate at 115 knots. Other aircraft, of course, might normally rotate well below that speed.

I suspect that this limitation is probably applicable across the US airline industry, but the charts I have are specific to one carrier, so I won't generalize beyond that.

This (PL, -10C) is the only place in all the de-ice/anti-ice charts that we have this 115 knot limit. With all other forms of precipitation, we have a holdover time and the requirement to visually check the fluid for failure; if it has failed, then we don't take off but rather go back & get it washed off & re-applied. In the case of PL, there is neither a visual check nor a holdover time, but an "allowance time" after which you have to assume fluid failure & go back to start over. (With PL, you can't visually identify fluid failure -- so you have a conservative time, and after that you're just done.) Speculating, I'm assuming that the nature of the ice pellets is such that they can, in sufficiently cold temperatures, generate the conditions where the fluid needs a particular speed to shear off that some of our operations might not reach -- thus the 115 knot limit. Whatever can happen with the other precip, presumably can't keep the fluid from shearing off at whatever our lowest possible VR might be -- which I don't know offhand. Again, for aircraft with a slower VR, you might get those sorts of situations with combinations of precip & temperature that we wouldn't worry about.

Not a complete answer to your question, but hopefully this data point may be helpful.

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  • $\begingroup$ So "we" in this answer is "an US based airline", which makes "they" the FAA? $\endgroup$ – DeltaLima Apr 15 '17 at 8:17
  • $\begingroup$ @DeltaLima Maybe, or perhaps an industry group, or chemists at Dow or several of the Type IV fluid producers, or maybe just guys at our airlne who decided that we could ignore whatever cases required a VR above (values you'd never use in our operations anyway), or maybe some combination of these and perhaps others. But simply, "they" = the collective group of those who influence the rules we operate under. Which includes but can be broader than just the FAA. $\endgroup$ – Ralph J Apr 15 '17 at 14:09

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