Friday, March 11, 2011

Heating Elements: Making Coffee, Shedding Ice

You know you've been reading too many airplane manuals when you can't turn on the coffee maker without thinking, "horizontally-mounted unlabelled three-position rocker switch spring loaded to the centre position, with an adjacent amber indicator light illuminating when the heat cycle has been activated by momentary left pressure on the rocker switch," and then wondering about the circuitry of the heating cycle and of the timer that deactivates it if you walk away and leave it on.

The Screaming Whippet has electrical heating elements, but not for making coffee. Both left and right pitot tubes are electrically heated, from their respective DC power buses, and there is a switchable loadmeter in the cockpit for checking their operation. The pitot heats are operated by a vertically mounted three-position rocker switch, but the upper, pitot heat position is rarely used, in favour of the lower position which heats both the stall indication vane and the pitot tubes. The windshields are also electrically heated, through the largest (60A & 50A) circuit breakers on the airplane, via the left and right essential DC buses. With windshield heat selected to LOW, the panes in front of both the pilot and the copilot are heated together, with single thermostat, such that if the sun is to one side or the other of the airplane and one windshield reaches the 90-100 degree Fahrenheit temperature, it will cycle off despite the other one not being hot yet. With the windshield heat switch set to HIGH the windshields cycle separately, with twice as much power as before going to the captain's side. I'll have to check, but I think the FO may not get any more heat on HIGH.

Let's see, what else is electrically heated? The propellers are. On the leading edge of each blade there is a little grid of wires, kind of like the ones on your car's rear window defogger, except that these are under a rubber cover. There is an inner and outer element, although it looks like just one. And there's a really really long skinny cord so that it can wrap round and round and round the propeller without coming unplugged. No, I'm kidding about that. It's tricky to have an electrical cord on a propeller. There is instead a slip ring assembly with a brush block so that electrical contact can be made without a hardwired connection as the propeller goes round and round about two thousand times a minute. Each propeller heating element draws power through a 25A CB on its same-side 28V essential DC bus. The system operates on a timer, switching on all inner and then all outer elements on either propeller, and it's not crystal clear from the description whether that means a four stage cycle with only one propeller at a time or a two stage cycle with the inner for both props together and then the outer for both props. There's an ammeter that shows the current being supplied to the prop anti-ice; the manual says it should be about 14-18 A per propeller, which suggests to me that they are both on at once. You can switch the ammeter back and forth to show the left propeller or right propeller draw. But then it also says there is a separate timer for each propeller, so I'm not quite sure. When you turn it on, heating starts wherever it left off last, not at any predetermined position.

Propeller heat is on the same switch as nacelle inlet anti-ice, but in the case of a failure of one essential DC bus, only the inlet anti-ice can be switched to the other. Power can not be supplied to the propeller elements on the side with the failure.

I didn't mentioned the inlet anti-ice before the above paragraph, because it doesn't include an electrically heated element, but seeing as it's on the same switch as the propeller heat, I'll put it here. Inlet anti-ice refers to the engine inlet, the part at the front of the engine where air comes in to make the engine work. It is important that this area not become clogged with snow or ice. Instead of using electricity to heat this area, really hot air is blasted into it. This works well and being that the engine inlet is really close to the engine, it has a ready supply of really hot air. It's not exhaust: that's too hot and is at the other end of the engine. (A number of readers will now being going clue! because believe it or not "the exhaust comes out the back of the engine" is not a universal truth with airplanes). This is bleed air that has been heated by compression in the front part of the engine.

I will now quote some facts about the engine intake air from the training manual. Keep in mind this is a training manual, a collection of useful information felt essential for new pilots to learn before flying this airplane. It is not a maintenance and repair manual or a set of blueprints. The first paragraph of the description tells me that bleed air is obtained for this purpose from two different ports, one being "the main anti-icing bleed port" and the other the "cabin pressurization bleed port." It doesn't tell me the relative location of these ports in the engine or how the two sources are different, so I'm not sure why I should care, but in the second paragraph they want to be sure that I know that "the flow from the main anti-icing bleed port runs through an anti-ice valve to a tee: the tee has an orifice of 0.234 inch diameter." Sadly, I will now remember this, and possibly I will also remember that "the line supplying the left-hand side of the distribution duct has a 0.204 inch diameter orifice." Why? Why am I burdened with this information? I understand that the brain does not actually fill up with facts, so that having this one thrust upon me does not decrease the likelihood of my remembering the essential items to complete in case of an engine fire (stop it, close the firewall fuel shutoff, close the hydraulic shutoff, discharge the fire bottle), but still, it's distracting.

Manual-provided information of which I do approve includes more about the switches and annunciator lights. The switches (one for each side) each have three positions: ON, OFF and TEST. In the ON position, the electrically operated valve that allows bleed air to flow through those very specifically sized orifices to do its deicing job is commanded to open, and if the circuitry can verify that the valve did indeed open, the appropriate INTAKE HEAT ON annunciator is illuminated. In the OFF position, the valve should close. Assigning bleed air to this task comes at a cost: a drop in engine torque and a rise in exhaust gas temperature. That is to be expected because the compressed air would otherwise have been used to cool and contain the combustion area and to drive the power turbines. Because of the performance hit we would take if the valves did not motor to the off position when so commanded, electrical power to those valves can be taken from either essential DC bus. There is also a means of verifying that they actually did close: when the switch is depressed to the test position, if the circuitry can verify that the valve is closed, the INTAKE HEAT ON annunciator illuminates. That's kind of backwards, but pretty clever, really. And far more interesting than the diameter of the bleed air orifices. Hey, I just figured out why they included the diameter of the bleed air orifices in the manual. It's to ensure that the test function of the inlet anti-icing valves could be far more interesting than something.

There are more anti-icing measures on the Screaming Whippet, but there are several more anti-icing systems on the airplane, most of which have an electrical component, but I'll put them in a different post, because this one is mainly about heating elements.

Also I've been reading over the details of the job offer. I think this is a real job. Like I get paid extra for working on statutory holidays. Just to keep the Canadian content up, I'll tell you what they are: New Year’s Day, Good Friday, Victoria Day, Canada Day, Labour Day, Thanksgiving Day, Remembrance Day, Christmas Day, and Boxing Day. It seems that for International Ice Cream for Breakfast Day (first Saturday in February) I'll be on my own.


John Lennerton said...

You may be paid extra for working holidays, but if Captain Dave is any indication, you'll still be working those holidays many years from now. I can only hope you're driving the equivalent of Fi-Fi at that time. In the left seat.

You didn't tell us which bleed air is used for nacelle inlet anti-ice. Is it the same as is used for cabin pressurization? If so, does cabin pressurization reduce power?

Anoynmous said...

The only thing I think when I turn on a coffee maker is "selected on and indicating". This is despite the fact that only coffee maker I ever use doesn't actually have an indicator light.

You have given lots of clues now. Nobody should have any problem zeroing in on the bird you're describing.

And you need a companion for that last holiday. Nobody should eat ice cream alone.

Aviatrix said...

Don't say that, Anoynmous. I myself wouldn't have recognized the aircraft just from that a month ago. And believe it or not, I have already accepted an invitation for IICFBD 2012. If you're in town I could probably invite you along, though.

John: The manual itself is oddly inspecific describing the source of the inlet anti-ice. It is prior to the bleed air switches, but described as a blend of two sources, names but not made clear exactly where they come from. I think they are both P2.5/P3.

Dafydd said...

Aviatrix - Thank you for the heads up on IICFBD , I never heard that before .

Feb 4 2012 - it's in the Calendar

Jez said...

Don't worry, Aviatrix, "more interesting than the test function of the inlet anti-icing valves" definitely describes this blog. Keep up the great posts!

D.B. said...

I'm fairly sure I know what the "Screaming Whippet" is - but as I write this I can now think of 2 different aircraft that would fit all the clues. High wing or low?

Anonymous said...

You are a really good wordsmith! I can actually read this tech stuff without getting sleepy. Very good!

Sarah said...

"... the tee has an orifice of 0.234 inch diameter." Sadly, I will now remember this, and possibly I will also remember that "the line supplying the left-hand side of the distribution duct has a 0.204 inch diameter orifice." Why? Why am I burdened with this information?

Having read tech and training manuals most of my life, the answer is probably both

1) Whoever wrote it could not distinguish between useful information and all information, and

2) There is a typo and only one of 0.204 and 0.234 is correct. :)

Devil In The Drain said...

And there's a really really long skinny cord so that it can wrap round and round and round the propeller without coming unplugged.

I love this stuff!

When (if) you get tired of flying, could you please get the job writing the manuals?

kschendel said...

D.B, re high wing or low: an earlier post had the answer to that one, albeit indirectly.

Anoynmous said...

I myself wouldn't have recognized the aircraft just from that a month ago.

I would probably recognize only one or two specific models of airplane from my own experience. But Google does a great job of narrowing things down from just a few specific numbers. Someone has posted flash cards online that match your descriptions perfectly, and more than one person has described the experience of flying one using language that confirms the SW name.

Michael5000 said...

Hold out for National Flag of Canada Day.