Holdover time Type 4 100% vs 75%

[WhiteKnight]

I've noticed if you look at the HOT of generic type 4 fluids it gives you most of the times a better HOT with 75% mixture ratio than for 100%.
Does anyone have an explanation for that?

Thanks a lot

[Mad (Flt) Scientist]

I would suggest a combination of the scatter in the testing results for any given fluid, and the effect of combining multiple fluids into the generic tables.

Looking at the TCCA generic table (Table 20 in this year's HOT), abnd looking at the upper limit times, there are 13 cells where there are 100/0 and 75/25 data. Of those 13, 5 have the same upper limit, 5 have 75/25 higher and 3 have 100/0 higher. Of the 8 cases which are not the same, in two cases 75/25 is higher by 20 minutes, in 3 it's higher by 5 minutes, and the three where 100/0 is higher are by 10, 15 and 25 minutes. If you take the overall average of those 13 cases, the average is that 75/25 is better by 0.4 of a minute - effectively zero.

[captainsmiffy]

It is because the dilution alters the fluid viscocity....and the 75% can now absorb more contaminant before failure. Also, the fluid is diluted with hot water and this, in turn, imparts heat energy to the airframe. The diluted fluid has a better min HOTcompared to the undiluted fluid but the max holdover time generally is shorter.

There you go, whiteknight, wont have to mention this, now, in your 380 RTGS

[JammedStab]

I thought it was addition if warm water as well but after checking the other type IV charts, that theory doesn't seem to..........hold water. Read the first reply.

[Old Safewing]

Hi

Each manufacturer develops their own brand of fluid.
While glycol is the main constituent in the fluids the glycol ratio may vary by small amounts from brand to brand.
This ratio has no relationship to the 100% 75% or 50% mix ratio this is based on the total makeup of the fluid; ie wetting agents, corrosion inhibitors, vulgo rubber material in long chain polymers and glycol and if done so water.
The fluids manufacturers are naturally commercially sensitive to revealing their fluids make up.
Thus some fluids are better at managing snow over freezing drizzle or other forms of ice contamination.

What is important to remember that fluid integrity is governed by freezing, dilution and fluid layer thickness.

The glycol as a freezing point depressant converts ice to melt water. This melt water dilutes the fluid making it more runny /lowers viscosity. Lower viscosity fluids flow more readily thus on an uncontained surface like a wing the fluid drips/ flows off at an increasing rate over time of exposure. Over time the fluid layer will thin, as well as start to freeze as the freezing point depression diminishes towards ambient freezing temperature or aircraft skin temperature.

As many of you know water can be added to provide a level of economy. Additionally a battery of tests are required to qualify the fluid. One of these tests is on wing viscosity, ensuring that the fluid behaves in such a manner that it will stay on the wing long enough to protect but will eliminate during the latter stages of the take off roll to avoid it becoming a contaminant itself.

Type IV fluids are Non Newtonian fluids and behave in a somewhat counter intuitive fashion. While these fluids have pouring/spraying properties they can semi solidify if a force is applied to them like airflow giving them good staying ability on the aircraft structure. Occasionally some of this fluids are too resistant to elimination or their behavior does not conform to the strict testing criteria of fluids.

To redress these some fluid manufacturers may tweak their fluids constituents or declare that the fluid cannot be used at 100% concentration or in fact they might declare that 100% concentration contains water as part of the makeup of the fluid and sell it that way. Additionally LOUT or lowest operation use temperature may limit a fluids ability at lower ambient temperatures close to fluid safety buffers. As you can imagine when you’ve poured a lot of money into research, development and testing of fluids and they show promise the last thing you want is to fail.

So to that end I’m not sure you can compare like with like when comparing Type IV fluids. Some are better than others at certain jobs in certain weather conditions but may be less optimal in other weather phenomena. Most engineering departments will do an analysis on fluids based on the requirements for the weather conditions normally encountered for their airline operation, aircraft code, fluid type availability for the region and the old chestnut; cost.

I hope this helps.

[JammedStab]

Our de-icing manual touches on the subject but without detailed explanation of why....

"Longer Holdover Times for 75/25 Dilutions
For some brand-specific fluids, protection is increased in some cells when fluid concentration is reduced. The addition of certain quantities of water to some neat(undiluted) fluids can enhance their performance up to a certain point. Without knowing about this particular fluid mix phenomenon, an operator may think that the data presented in the tables are in error."

[megan]

While these fluids have pouring/spraying properties they can semi solidify if a force is applied to themThe other way round. When a force (spraying) is applied the fluid reduces its viscosity because of sheer stresses. When the force is removed viscosity increases. Thats why you have to hit the tomato sauce bottle to get the stuff out, the act of hitting reduces the sheer stresses thus lowering the viscosity - the fluid is said to have thixotropic properties.