I'm not sure if this is a cohesive description of the engine start sequence or an unpublished chapter of Joyce's Ulysses in which we follow Aviatrix's stream of consciousness as she slides into aircraft manual-induced, chocolate-deprived madness. Either way, come for the engines, stay for the Star Trek references and sexual innuendo.
When I start my car, I put my foot on the brake, the key in the ignition, turn and hold the key until the engine sounds just right, then release the key and the engine continues. That's a little bit complex. You have to get a feel for when to release the key. When I first learned to drive there was another complication, in that I had to put my foot on the gas as well and give it just enough gas to start. But many of you start old manual transmission cars every day without even thinking about the process. And some of you start up an electric car by, I understand, pushing a button. I think they just push a button to start the starship Enterprise too, so this is clearly the way of the future.
Push button airplanes exist, and starting mine does begin with pressing a button, but it's a little more complicated than that, and I have to know and understand the whole sequence, including the stuff that does happen automatically. I'm sure I'll be quizzed on this all during the test, and if I'm going to feel helpless someday when I can't get the engine started, I'd best know exactly what is failing to happen.
I need to be sure that the inlet and exhaust are free of debris, that the first stage compressor has no visible damage, that the P2T2 probe is clear, the propeller is on the blade angle locks, and that it moves freely. I should check the oil, make sure the speed levers and power levers move freely and are set in the low and just ahead of ground idle positions, respectively. If I'm using a GPU I should make sure it's supplying 24 V and 1000 A, and if I'm using batteries I should check that they have a good charge.
0 - 10% RPM
After the prestart checks are complete, I press and hold the start button. As a result:
- the #1 start control relay closes
- the starter relay closes
- the oil vent valve opens
- the anti-ice lockout valve closes
- the starter engages to rotate the engine
And let's see what that all means.
The #1 start control relay "is used to control the starter relay and the oil vent valve during ground starts, and the propeller unfeather pump during air starts." It "also opens the voltage regulator 'B' to starter-generator 'A' lead to prevent field feedback during starts." The first part I get: The #1 start control relay is the electrical string you pull on to get the next two things on the list to happen. If the second part of the description mentioned some kind of particle or anti-matter, I would swear that was from one of the episodes of Star Trek: The Next Generation where Wesley saved the ship again. I can surmise that for some reason the starter-generator has an 'A' lead, that the voltage regulator has a 'B' lead, and that these two are normally connected, but that that connection can lead to undesirable feedback during starts. I can further surmise that the voltage regulator is there to regulate generator output, but seeing as the generator isn't put on line until after it has finished being a starter, that there's no need for it to be connected during start.
The starter relay "energizes the starter generator, during ground starts only." That makes sense, because during air starts the starter is not needed: rotation of the engine from the propeller is sufficient.
The oil vent valve is a clever little kludge to prevent the starter having to work against cold sluggish oil pressure as it rotates the engine. It opens up the oil system to allow it to ingest air during ground starts.
The fuel anti-ice lockout valve closes off the line through which the fuel system cycles cold fuel to a heat exchanger with the oil to prevent ice crystals forming in the fuel. At start the oil isn't warm anyway, so the valve is closed to keep the fuel pressure high.
So that's two things that happen when the start button is pressed, two more that happen because of one of those (or three if we count the A-B lead thing) and one one that happens because of the second wave. And that's all before anything even catches on fire.
10 - 55% RPM
When engine RPM reaches 10% the fire is supposed to start. Specifically:
- the 10% speed switch closes
- the #2 start control relay closes
- the series/parallel relay connects the batteries in series, if selected
- the SPR valve is energized
- the fuel solenoid valve opens
- the 'primaries only' fuel solenoid opens
- ignition turns on
And I know what most of that means.
The speed switches contain
reverse tachyon tribarium warp core interfaces a DC regulating system, a signal conditioning amplifier, timing pulse generators and voltage discriminating circuits (it only allows straight voltages to go through, and the ones with suntans don't get to use the good wires). The speed switches contain STUFF, okay, including three relays. The 10% relay (which the voltage discriminating circuits probably think is an overestimate, or maybe a lifestyle choice) is normally open, the 55% relay is normally closed and the 90% relay is normally open. They receive electrical signals from the "tach gen" which for the purpose of my own amusement I will imagine stands for tachyon generator. Each relay changes its respective state when engine rotation reaches the corresponding speed. The 90% one isn't important to engine starting, but I didn't want to leave it out, as it's probably already getting enough hassle from the voltage discriminating circuits. Best I can tell from the ten pages of starter circuit diagrams, when the 10% speed switch closes, it energizes the #2 start control relay, and the #2 start control relay does the four other things on the list: ignition on, batteries to series, SPR energized and fuel solenoid on.
Hey who thinks I should be allowed chocolate now?
The series-parallel switch sounds like something Scotty would do at the last minute to keep the Enterprise from falling into a black hole.
"It's no use Captain. We haven't got enough juice to get the warp engines online."
"You've got to get them online Scotty, or we're all dead!"
"Well I suppose if I cross connected the dilithium crystals from the other nacelle, it could possibly give enough of a boost to get the warp core online. It's never been done before, laddie."
And then he does it, and it works and the he publishes a paper on the subject, and the manufacturer comes out with a bulletin saying never to do it because it voids the warrantee. It's just like that. In fact it's hard to believe that some desperate pilot didn't invent it while stranded on a reserve somewhere, except that this isn't a bush plane. The manufacturer has set it up so you can flick a switch and the batteries, usually connected in parallel, one to each starter-generator, automatically go into series when the power is needed most. My company, however, says that the power is not needed most at 10% but rather at about 18-28% where there are bad vibrations (seriously, that's the reason), so what I'm supposed to do is start the engine with the series-parallel switch in parallel, but if the engine rpm is at or above 18% and not increasing steadily by 1% per second through 25%, I should flick the switch into series then. I was so impressed by this in groundschool I immediately asked why I wouldn't do it every time. It's because it's very hard on the batteries and if you drain both of them together, you only have one shot, so you have to hit it with your best shot. Also if you do it at night you need to have a flashlight handy, because it will dim the lights and you won;t be able to see the engine instruments.
The SPR valve is another thing they didn't get quite right, according to company procedures. It sounds all cool and modern with the TLA and all, and it stands for Start Pressure Regulator which brings to mind the idea that it might be some sort of computer. Nope. It's the primer. Yeah, the primer. I have to prime this puppy. Primer is available between ten and fifty-five percent rpm, but I should prime it between zero and ten percent, then not prime it between ten and twenty-five, then go back to priming until 55% when the 55% speed switch opens and de-energizes it. The SPR bypasses the fuel control unit and fuel flow transmitter, going straight to the primary fuel nozzles.
The fuel solenoid valve is the thing that lets the fuel into the engine so the igniters have something to ignite. I'm sure that makes them very happy. You can close these valves mechanically with an emergency stop control, but then you can't open them again. Ever. Okay probably not ever. I assume maintenance can fix you up again. The primaries only solenoid allows fuel through only the primary starter nozzles, not the full manifold.
The igniters are not something that I have a lot of information on, but the ignition exciters supply "a nominal high voltage of 18,000' volts to the igniters" so the exciters seem to be serving as fluffers to the igniters. The igniters are some kind of high tech spark plug that lights the fuel. I know they are different than glow plugs, because I flew an airplane once on which some models had glow plugs and some models had igniters and this was apparently different enough that I was supposed to care. (Can you tell I didn't?) I pretended to at the time, and phasers on kill, I swear I will pretend to care about this, too. The igniters light my fire. Can't start a fire without a spark. What is wrong with me? I almost never quote song lyrics. Because I don't know any.
A light comes on when the igniters are working, and the EGT should rise, indicating that something is actually on fire in there, and that the hot exhaust is coming out the right end. When it happens, the pilot can release the start button. If it doesn't happen within ten seconds of reaching 10% rpm, or before reaching 20% rpm, the pilot should shut down the engine. She does this by pressing the stop button, pulling the stop and feather control and then continuing to motor the engine for ten seconds with the starter test switch.
When the 55% switch (which the book calls a 50% switch, but the book is for an older model of engine, so I have to assume that changed) opens, everything that the #1 and #2 start control relays did gets undone, except for the opening of the fuel solenoid:
- the starter relay closes
- the starter turns off
- the anti-ice lockout valve opens
- the oil vent valve closes
- the series/parallel relay goes back to parallel, if it was used
- the ignition turns off
- SPR becomes unavailable
- primaries only fuel solenoid closes
Whew, that feels good. It was actually worth it. It took too long, though. I spent a couple of hours on that, between course notes, two manuals and the engine supplement. And stalking around hoping chocolate would spontaneously appear.
Is there some kind of vitamin, found only in chocolate, from which deprivation makes it difficult to study without degenerating into lame Star Trek references, old song lyrics and crude jokes?
I'm also getting antsy, waiting for my training to be scheduled. I have a job offer already, but until I actually go online as a pilot, I don't really have a job. They scheduled the first two while I was there, and I know the second one had a PPC ride today. But it could have been delayed by aircraft or training pilot availability. That happens. Or they decided they hated me and are just going to ignore me. It's not paranoia when they really are out to get you. I e-mailed the chief pilot with a question about where I should send my paperwork, kind of the way I ask ATC for an altimeter setting when I haven't talked to them in a while.