David Slotnick

🧵regarding Lord of the Rings - related traumatic injuries, and whether access to modern Level 1 trauma centers could have decreased morbidity and mortality within the Fellowship. Here we will take a more evidence-based approach to some of the injuries in Middle Earth (1/ )

A short primer on “Holdover Times”, “FICON” & why the airlines just stop trying at some point: The wings generate lift, letting the jet fly. The shape of the wings & the way air flows over them make this possible. When the shape changes – due to snow & ice – the amount of lift does, too. How much? We don’t know…and if we don’t know, we don’t go. We don’t roll the dice. Deicing is practiced widely to enable flying in the weather, and these airports have good facilities. A part of deicing that doesn’t get talked about much is anti-icing. Deicing uses hot fluids (sometimes air if it’s just fluffy snow) to remove any snow or ice on the jet (“contamination”). But that’s not enough if there is still precipitation falling; the snow & ice will quickly re-accumulate. So after deicing, a layer of anti-ice fluid is applied. This fluid is slightly sticky & adheres to the wing. It’s colored so the folks applying it can see what they’ve missed. Snow & ice that fall on it melt, so the wing doesn’t get contaminated again. At around 100 knots on takeoff, the anti-ice fluid flows off the jet & airflow over the wings is normal…the jet will fly. At speed, the way the air flows over the wing prevents most icing buildup on the top & bottom of the wing, but it can form on the leading edge. That’s why the leading edge is heated using air from the engines along the silver strip you see on the wings (and around the engine inlets). It’s also why this weather is something you can fly through & land in, but not takeoff in: on the ground, the ice will form again. Anyway, the anti-ice fluid has a “holdover” time. It can only melt so much snow & ice before it’s washed off the wings & you are again unprotected. If you applied anti-ice, then came out with a fire truck & blasted it with water, the fluid will be washed off. In some conditions, the holdover times can be in excess of an hour. Say, very light snow. In others, though, the holdover time approaches zero…meaning that the moment you stop applying the fluid, it’s immediately degraded to no protection. Since it takes time to drive the trucks around & spray the whole jet, as soon as the truck moves to the next part of the jet the last part it sprayed is already unprotected. It’s a pointless exercise. These conditions are extreme, like freezing rain or very heavy, wet snow, but do occur occasionally. The other issue is the runway & how slippery it is. The FIeld CONdition (FICON) is a report of what the runways look like. For example, wet or 1/4” snow covering it. There are many variables to it…wet or dry snow, for example, or ice and they generally are specific to portions of the runway, divided into thirds. One end of the runway may be good, while the other end is not so good. If the runway is long enough, it’s possible to just land on the good parts & stop before you get to the bad parts. (To be clear, we don’t change our aim point or try to land longer down the runway under any conditions) The airports also measure the “braking action” of the runway using special equipment. The results are broken down into codes signifying how well you can expect the brakes to work. These run from 6 (dry, normal runway) to 0 (skating rink, brakes ineffective). They report the results as part of the FICON. For example: “Runway 18 FICON 5/5/5 WET” means the runway is wet, but the braking action on all thirds of the runway is pretty good. Airports can kind of “anti-ice” the runways, too. It’s not the same fluid, but it’s subject to the same principles of a holdover time & eventually stops working. As I said in a previous post, many think Thrust Reversers do most of the work in stopping the jet. That’s not true, it’s the brakes & the reversers alone will not stop you. So if the FICON goes to “1/1/0 ICE”…you’re going to have a bad time. In theory, you could takeoff on that. The problem is you are gambling you won’t need to abort the takeoff. If you did…you’re screwed. That’s all my space!