Saturday, March 12, 2011

Anti-Ice Without Electrical Heating

Yesterday's post was mostly about electrical heating elements that prevent ice from accreting on various parts of the airframe. Today is about anti-icing measures that do not involve electrical heating, except in as much as the flow of electricity always causes some heating.

The leading edge of the wing and tail are covered in what is essentially a network of bicycle wheel inner tubes stretched out straight, covered in a neoprene sheet and connected to a nozzle. At the pilot's command, an electrically operated regulated 18 p.s.i. bleed air inflates the tubes, called boots and pops off the ice. Whether the pilot commands it or not, at all times that the boots are not being inflated, suction pulls the boots back snugly against the wing. The suction also comes from the 18 p.s.i. regulated bleed air, through the same distributor as the positive pressure. I'll explain how it can both suck and blow later. This system is yet another controlled by a three position switch. The OFF position leaves the solenoids controlling the distributor de-energized, and the wing boots connected to vacuum and sucked against the wing and tail. The AUTO position opens the solenoid allowing 18 p.s.i. regulated bleed air into the wing boots, inflating them for six seconds. Then that pressure is dumped overboard and the wing boots gets sucked back against the wing while the tail boots get inflates for four seconds. After that the system rests for 170 seconds and starts over. The third position is MANUAL and all the boots will remain inflated for as long as the pilot holds the switch in that position. Electrical control of the solenoid can be assigned to either the left or right essential bus.

Inside the engine inlet is a probe called the P2T2 probe (or the R2D2 probe, depending on who is talking). Its job ought to be transmitting pressure and temperature readings for that station to the engine computer, but considering that it is kept ice free with P3 bleed air, it seems that it wouldn't be an accurate indicator of the temperature and pressure there. What am I missing? And does its output have to be translated by a C3PO probe before it can be used? This web page has condescending British people telling someone else who dared ask that anyone at the ATPL level who has to ask questions about a P2T2 probe is lazy and stupider than a toenail. Maybe so, but I know how it's anti-iced. Mostly I know. I'm not sure if it is automatically supplied with bleed air at all times or whether that's part of the inlet anti-ice.

I like the way the anti-ice for the oil cooler works. It simply runs the line taking hot oil from the engine around the lip of the air intake before routing it to the oil cooler. No electricity required.

The fuel anti-icing (i.e. to prevent ice crystals from forming in the fuel thus blocking the filters or lines) also uses heat from the oil, but not quite so simply. As the high-pressure engine fuel pumps draws fuel through the filter towards the fuel control unit, some fuel is routed off to the side, through a heat exchanger, heated with scavenged hot oil and then metered through a temperature-controlled anti-icing valve and sent right back to where it started before the filter. I don't know the exact mechanism; it probably involves a bimetallic controller, but the colder the fuel, the wider the anti-icing valve opens and the more fuel has to take the scenic route through the oil-fuel heater. An additional valve in the system, just before the heater, closes when the engine speed is below about 50 or 55% rpm, to prevent any fuel from being diverted during engine start, the only time such low speeds should occur.

Back to the very low tech, the manufacturer claims that the static ports and fuel vents on the airplane are anti-ice. They have no heating, it's just that the fuel vents are concave and out of the airflow, so there is nothing for ice to build up on, and the static ports are also very flat and at the rear of the aircraft. Just in case they were to ice up, there is an alternate static source located in the unpressurized forward baggage compartment.

There are two more anti-icing systems on the standard aircraft, but they are both irrelevant to this fleet, one because the system it belongs to has been removed, and the other because it was an option not installed.

Also, can someone please tell the person who writes the scripts for my bank's drones to read at me while I try to set up automatic payment on my new credit card that (a) a new credit card is not an occasion for "congratulations," and (b) "I want to inform you some points" is not English.

And in current news, when a huge tragedy strikes, and it's too much to take in all at once, people focus on the corners of it that are most relevant to them. I woke up to e-mail chatter about international flight schedules, plus this video of the tsunami associated with Japan's earthquake hitting an airport. Even people with no international connections have something in Japan they can relate to. I laughed to see the internet hasten to assure us that Maru the Cat and his owner are fine. But what is a mass tragedy if not thousands of individual tragedies, and, thanks to stringent building standards combined with preparation and training, millions of people who can say, "I'm okay, and so is my cat."


John Lennerton said...

Another question. Somewhere in the dark recesses of my memory, I seem to recall that one can leave the de-icing boots on the leading edge of the wing inflated too long, resulting in an ice coating that cannot be broken by the boot, since it has formed on the boot's shape at the point of maximum inflation. Is that true? The timing mechanisms seem to defeat that possibility.

The tragedy unfolding in Japan reminds me of when, in my youth, I traveled there to work at the Tokai 1 nuclear power station, some 100KM or so south of Fukushima, and maybe 150KM south of Sendai. The focus on the problems at Fuku 1 are one of those 'corners' of this tragedy. I'm hopeful it will amount to nothing more than a minor inconvenience to a neighborhood with much, much larger issues.

It is a beautiful countryside, with wonderful houses and farms and people. My heart goes out to those people. And their cats.

Wayne Conrad said...

I love these techno-whiz posts. I'm happy that write-to-remember and read-because-fascinated intersect so nicely.

Here's a topic, if you need one for later: What's the difference between deice and anti-ice? Is there any overlap between them? That is, can a system be de-ice when the icing is not so severe, but if the ice is forming quickly enough, it's only anti-ice and can't be counted on to remove already formed ice? Is there a regulatory difference between deice and anti-ice for flight into known icing?

majroj said...

click here
No, the deicing boot doesn't fend off big bird strikes.

I saw similar photo from USAF safety journal around 1980. Bald eagle struck at the edge of a boot so there was one talon and a wingtip sticking up from between the nearly intact boot and the duralumin wing edge.