Flintstone wrote:Home brewed de-ice fluid is a money saver but too much glycol will inhibit the shear speed. You don't want to get airborne with glycol covered wings. .
Harry Brown wrote:Inhibit Shear Speed? *You do ideally want to get airborne with glycol or more correctly low foaming propylene glycol covered wings, *that is the whole point of anti icing.
For a short time at the beginning of my career I too thought this (*
) but it simply wasn't explained to me properly the first time around.
De-icing as it is commonly called is often but not always a two-step process, first the de-icing (removal) followed by anti-icing (prevention). The purpose of anti-ice fluid is to prevent re-contamination on the ground between de-icing and getting airborne. Once the aircraft has passed a given speed during the take-off roll it (the fluid) is no longer needed as the airflow prevents precipitation from adhering. In fact were the fluid to remain it would have an adverse
effect on the wing's performance (for simplicity's sake this includes the tailplane).
Having fluid on the wing in flight is a bad idea in a similar way to having frost, ice and other contaminants there. It degrades laminar flow and raises stall speeds. Anti-ice fluid is therefore manufactured to a known viscosity so as to allow it to run off the wing somewhere between a 'safe' speed (where snow/ice/cold cack can't stick) and before rotation. From the NASA website:
Aerodynamic acceptance: to ensure the fluids do not compromise the aerodynamics of the aircraft at the point of take-off; ideally the fluids blow or shear off the wing & tail surfaces during the take-off roll. https://aircrafticing.grc.nasa.gov/2_3_2_1.html
(Apologies for the bright red text but this is a dull subject that I thought could do with some livening up.)
The leading edges and cowls are of course still at risk but they are protected by the anti-ice systems, essentially a piccolo tube within the leading edges and cowl openings through which flows bleed air tapped from the engines. But I digress...
For the benefit of readers unfamiliar with the sticky stuff here's a quick and dirty low-down on the four main types:Type I.
Red/orange, minimal holdover time (HOT**). No min rotation speed (so safe for use on all aircraft types). This is typically used to de
-ice an aircraft prior to applying anti
-ice fluid.Type II.
Clear/straw, HOT varies (see below) but typically 00:20-00:45 minutes. Minimum rotation speed 100 knots.Type III.
Yellow/green. HOT 00:10-00:20 minutes. Minimum rotation speed 60-100 knots.Type IV.
Emerald green. 00:35-1:15. Minimum rotation speed 100 knots.
** HOT will vary considerably dependant upon OAT, mixture ratio (100:0, 75:25 or 50:50 with water), temperature of mixture, brand of glycol and type of precipitation. The FAA, NASA, Transport Canada and other authorities update the holdover data tables annually.
Harry Brown wrote:On jet transport aircraft that require wing tactile tests during the walkround in icing conditions you can still feel the propylene glycol on the wing from the previous deicing.
Thanks. I know the procedure quite well and at this time of the year conduct it several times a day. What you describe as remaining from previous de/anti-icing is simply a 'leftover' due to there not yet being a fluid that will run off completely by airflow alone. In fact it must be removed (usually by blasting with hot Type I) before treating the aircraft again for another flight.
Harry Brown wrote:No fluid should go near the fuselage where it could flow over a windscreen on take off or near an APU which should be switched off for anti icing/deicing .
Quite so. In fact I think someone looking a lot like me and wearing my clothes wrote something about that at the bottom of the first page of this thread.
While you might think that I couldn't possibly comment.