Monday, March 14, 2011

The Short Bus

When I was working on my commercial licence I met a guy who flew the Screaming Whippet and one of the things I remember him saying about the aircraft was that it had a very complicated electrical system. I was looking forward to meeting this beast, but we were warned in ground school not to pre-read about it in the manual, because there were many options and modifications and that we'd get confused if we did. So it was unveiled on I believe the morning of the second day of class.

It's not so bad. Yeah, it has DC power and three different flavours of AC power, but I can understand the circuitry. There are two 24V lead acid batteries, two 28V starter-generators, and capacity to accept ground power (max 1000A), which can power electrical systems and/or charge the batteries. I've done this enough time that those are pretty standard numbers. The batteries each have their own relay connecting them to the system, then an additional relay connecting the combination, plus the GPU if connected to the battery bus. The battery bus is power distribution central on this airplane. It's located in a junction box behind the left pilot seat, and everything connects to it. Each starter-generator connects to it through a generator relay and a 325A current limiter. The DC battery bus provides power to the left and right essential buses, through 225A current limiters and also to a non-essential bus, through a 150A circuit breaker. The current limiters and circuit breaker are all located in the junction box. Each of those three DC buses described also has a bus tie switch. They are on the circuit breaker panels, under the windows on the left and right sides of the cockpit.

Most of the services are quite predictable, with the left side of the airplane being on the left bus and the right side on the right bus. There are also nine or ten (the tenth is optional) transferable items that can be switched from the left to the right DC bus in the event of a left DC bus failure. They are the turn and bank indicator, the fuel crossflow valve, the valves controlling wing and tail deicing, the windshield heat, the valve controlling left inlet anti-ice, the valve controlling right inlet anti-ice, the flap and gear position indicators, the valves controlling landing gear operation, the solenoid that applies vaccum to open the dump valve and (optionally) the pilot-side DC cockpit instruments. Flaps, nosewheel steering and all exterior lights, including landing lights are on the non-essential bus.

Each essential DC bus has one inverter, and each inverter is connected to 26V AC bus and a 115V AC bus, so four AC buses in total. The L and R 115V buses are connected together through a circuit breaker, as are the two 26V buses, so that with one inverter online, all four buses are powered. Only one inverter can be online at a time. The fuel pressure, oil pressure and some cockpit instruments are on the 26V AC bus. The fuel quantity gauge takes 115V AC. The cockpit edge lighting is powered from the 115V bus, but stepped down to 6V. Other instruments may be on one or the other depending on the airplane, you need to check the CB panels to see.

My brain is full. Remember this two weeks worth of system spam you've been receiving was a three day blitz for me. And I have SOPs and limitations to learn on top of this. I think I'll go jump on the treadmill for an hour. There's too much snow outside to go running, but it's too slushy to go skiing.

Today is Pi Day. You should celebrate by eating pie. You may heat it in an electric, gas or wood-burning, dung-burning or solar powered oven. Or another sort if you have one I didn't think of.

P.S. I got on the scale after jumping off the treadmill. Target weight minus five hundred grams. Booyah! No sugar, no alcohol and get some damned exercise. It works, people. Girl's got legs and she knows how to use 'em. Now to keep it off.


Peter said...

Another interesting post. Speaking of pi day, have you seen this musical interpretation of pi?

Jez said...

I had a pie for pi day, but no one here in Australia gets it, since it's 14/3 here rather than 3/14 !

Anoynmous said...

March 14 is *National* Pi Day in the US. I always thought the *International* Pi Day should be 22 July.

Ed said...

For somebody who's worrying about her weight, celebrating pi day is irrational.

cockney.steve said...

you said," the fuel crossfeed valve,"

Am i not paying sufficient attention,here at the back of the short bus, or did you say there was no crossfeed, but transfer was accomplished by wing-down or sideslip ?

all interesting stuff, and whilst I haven't bothered to try and identify the aircraft, the technology and systems is a source of fascination to me and no doubt, a large percentage of your really sounds like you're progressing from ox-cart to thoroughbred carriage and pair.
verification = terable.....describes the PI pun?

Sarah said...

I thought the short bus was the A319 ...

Thanks for the light post after a grim weekend of tsunami news coverage.

Happy Pi Day!

Aviatrix said...

Ed: I'm not worrying about my weight. Worrying is for things you are powerless over. I've simply done something about it. Pie can form a healthy part of a balanced 1240 calorie per day diet. I will be having tofu pot pie for lunch, for example.

Steve: Correct. There is no crossfeed, only crossflow, and look at that I typed "crossfeed." The electrical power is simply to open the valve. I'll correct that.

Tim G in MN said...

Pecan Pie warmed in the microwave with a scoop of ice cream... Did you know that hat sizes are drived by measuring the circumfrance of your head in inches and dividing by Pi? Best of luck with the training!

coreydotcom said...

I read something about it actually being half-tau day... pi is actually wrong according to the tau manifesto. Look it up - there's a cool little website about it (if you care!).

Also...1240 seems pretty restrictive (maybe it isn't for a girl...). When I was still playing football and hockey at a high level we were encouraged to eat 4000 a day but I had trouble getting that high (in the calories lol). Now, I exercise regularly, so I try and keep it under 2500 to maintain weight and when I want to lose weight before a race or something I keep it to 1800 - but 1800 is not much so I can only imagine 1240!

Bonne journée!

Sarah said...

Hey coreydotcom, thanks for mentioning tau-day. I thought of it but what's a few extra factors of 2 really matter after all.

It does make "the most beautiful equation in mathematics" a little more pretty: e^iτ = 1

cosmic, eh?

Aviatrix said...

It does seem low, seeing as "2000" is the typical number for a human, but the daily allotment was calculated for me by a most excellent iPod app called myfitnesspal. It's free, and makes it so incredibly easy to track what I eat and expend that it's not even the tiniest bit annoying to use. It also looks at daily nutrition, making me realize I'm getting too much vitamin A and not enough iron in my diet. The 1240 is actually a baseline. It tells me how many calories I have left in the day, and the number increases when I report exercise and decreases when I report food. My actual consumption is more like 1800 a day, but if I sat on the couch all day the max would be 1240.

And yeah, girl.

Sarah said...

Thanks for the pointer to "myfitnesspal". Installed.

I've been carrying around 10 extra kilos for too long, maybe this will help. Apparently exercise is not enough.

Traveller said...

I've enjoyed the systems posts.

Congrats on the success in achieving your target.