Wednesday, March 16, 2011

Engine Startrekinsey Sequence

I'm not sure if this is a cohesive description of the engine start sequence or an unpublished chapter of Joyce's Ulysses in which we follow Aviatrix's stream of consciousness as she slides into aircraft manual-induced, chocolate-deprived madness.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.

Before Start

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.

55% RPM

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.


TgardnerH said...

My job has a recurring lucrative summer gig that they hire in-company for, and I did it last year, and things went well, and I was expecting to find the in-company application in my email around the same time. The same time came and went. I emailed people, no reply. A month came and went. I called people. I got paranoid, and thought I'd annoyed the wrong person and was getting ignored. Application showed up yesterday.

(Point being, I feel your pain, and a little paranoia is a good thing)

grant said...

if an aviatrix from say the 1930s were to read your description, it would be about as intelligible to her as the Star Trek lingo. My how things change...

Switches off? Switches off...
Contact? Contact! Clear to swing prop....

Ward said...

I wonder: how many men did you lose after the mention of fluffers?

(Verification word seems appropriate: monicka )

Anonymous said...

At least it's not as bad as the (somewhat unofficial) start procedure I've heard of for some electric trains. The problem is that these trains are powered by electricity from the overhead wire, and the thing that actually connects to the overhead wire is kept raised by air pressure, which is provided from a compressor and stored in an air tank. The compressor is of course electrically powered, and since it's a fairly big thing, it won't run on batteries, only on power from the overhead... which is a bit of a problem if the current collecting device is down, and the air in the tanks has run out. At that point, one option to get things going is to close the circuit breaker for the compressor, climb up to the roof, stand on the frame of the current collector (which sits on insulators) and raise the thing up yourself and hold it against the 3000 volt wire until the compressor has run long enough to build up some pressure to do things the normal way.

Jez said...

Anonymous, electric trains nowaways always have batteries to power the compressor to refill the air reserve in those situations. If the battery were to run dead for some reason, you'd send out a maintenance truck with a new one, not stand on top of the train!

Aviatrix, I'm a little confused by the series/parallel switch. You mention that the normal situation is having the batteries in parallel, which means the DC voltage is say, V volts (24, right?), with "I" amps of current available. If the batteries were changed to series, that would increase the voltage to 2V and decrease the current to 0.5I. But a higher voltage which would tend to make light bulbs brighter, not dim them. That's my first conundrum.

The second is that regardless of the voltage, the available power from the batteries would remain the same at P=IV. There must be more to this somehow? Do the starter motors have a design where higher voltage but lower current results in more torque at certain RPMs?

(My electricity knowledge running a little rusty... especially the bits about motors!)

nec Timide said...


Your comments about electricity are correct as far as they go. But you are talking about the volage and current the batteries are able to deliver in each configuration. For starting as important is the current the starter motor is able to consume for each battery configuration.

Assuming the internal resistance R of the motors stays the same when the batteries switch from parallel to serries (a bad assumption but close enough for illustrative purposes), the doubling of the voltage would cause an increase in current through the motor, from V/R up to 2V/R or 0.5I which ever is less. This likely increases the amount of power the starter motor is dissipating (or there is no reason to do it) which would increase the torque and the heat generated by the motor.

Ed said...

As nec Timid says, increasing the voltage by putting the batteries in series will increase the current (that's the point of doing it). Because of the increased current the voltage across any one of the batteries (which the DC buses are presumably still connected to - rather than to the series combination) will drop.

A normally open relay is one in which the contacts (output) are open when the coil (input) is not energized. If the coil is normally energized then the normally open relay will normally be closed. Ok?

Aviatrix said...

I think you guys have managed to answer all the questions yourselves.

Normally the batteries are independent, so that while one is starting an engine, the other one is powering the cockpit lights. But if they are selected to series for a start, at the point where the relay closes to connect them then the starter draws current from both batteries.

The instructor did not think this would double the voltage, and corrected a slide that suggested it would, so you can argue that point with him and the slide.

Sarah said...

Very good! A long technical post, with humor well used.

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?

All of these are good mneumonic devices for a tired brain.

But if you don't use caffeine, which would overpower chocolate, the effect of cocoa could well be missed by your brain....

mattheww50 said...

The reason for the switch from parallel to series on the batteries is the starter is a DC motor. Peak Torque on a DC motor is at zero RPM. As the motor spins faster, it develops what is called Back EMF, or reverse voltage, effectively reducing the voltage to spin the motor. (Most DC motors have very low resistance, so the initial start current tends to be enormous. If the current remained that high for any length of time, the motor would burn up from the heat generated). The Back EMF causes the current to the motor to be reduced as the starter motor RPM increases.

The faster a DC motor spins, the lower the torque (and power output). Unfortunately the faster the turbines spins during the start sequence (until the fuel actually ignites), the more energy you need from the starter. So at some point the only way to spin the turbine faster is to find a way to increase the power available from the starter/generator, hence the need to switch the battery configuration from parallel to series.

The switch from parallel to series doubles the voltage available to the motor, However the back EMF is directly proportional to starter motor's RPM. The effect of the switch is to substantially raise the available power output from the starter (far more than the doubling of the applied voltage suggests), which then spins the turbine faster, and improves the conditions inside the engine to actually start burning the fuel.

The good news is that once the engine has started, the starter motor turns into a hefty generator. IIRC, it is rated 300 amps. So if you can get one engine started, it can recharge the batteries very quickly, and provide most of the power to turn over the other engine.

The analysis about the switch from parallel to series doubling the current however is incorrect.
Because the actual current being drawn is based on the effective voltage applied, which is the voltage from the battery MINUS the back EMF generated by the motor itself. So if the Back EMF is say 12 volts, doubling the applied voltage from 24 to 48 volts raises the effective voltage on the starter motor from 12 Volts to 36 volts, and if the battery could supply it, the current would actually triple.

The motor's DC resistance is irrelevant to current draw of the motor except at zero RPM. The starter motor really can draw about 700 amperes at zero RPM, but the current draw falls dramatically as the engine 'spins up'.

cockney.steve said...

^^^^ Ah! and some of us ancients remember how jump-leads and a series connection could usually start the most obstinate car...but on-board electronics would not tolerate that, wiich is why, i suspect, Aviatrix's new ride has such a complex switching arrangement.

Anonymous said...

I love Aviatrix' style describing the engine start sequence!
One really might imagine the plane is a Star Trek ship beeing 'modified' in the desert like the plane in 'The flight of the Phoenix' and firing it up is going to happen only once or never ;-)
Great writing!!

Regarding the parallel/series switch for the batteries:
From the reading, 'parallel' in this case means 'independant', so one battery is powering only one starter motor.

Once you put both in parallel/seriell to supply the starter motor(s), this/these will be able to draw a lot more power (I or U) from the battery pack and thus both be seduced real quick ;-)

GPS_Direct said...

So, I'll assume you can see the 1st tage compressor vanes through the intake? The non-reversed flow style of engine would indicate that. And, do you swing the prop to check the vanes (assuming you can't see the entire compressor face at once)? Lacking a free-turbine would make that easier, unless there are prop brakes.

And, this post gets a "sex" tag for "fluffer," but the pneumatic post doesn't for "fellatio"? Chocolate withdrawal must be a multi-system thing...

A Squared said...

The instructor did not think this would double the voltage, and corrected a slide that suggested it would, so you can argue that point with him and the slide.

I'm not going to pretend to know what is really going on in the whippet's electrical system, but, if the batteries are in fact connected in series, in the sense of "series" as it is universally meant in the electrical world, that absolutely will double the voltage.

There are only two possibilities:

The instructor is wrong.

or is he is correct, and the voltage to the starter is not doubled when you move the switch o series, then the switch is doing something other than connecting the batteries in "series" as the term is commonly used.

Not knowing what that switch really does, I have no idea which is the case, but I guarantee you that if you connect two batteries in series, you will get a voltage equal to the sum of the individual batteries' voltages

majroj said...

1. Chocolate = theobromine.
2. Copy that fluffer thing. Loose association and flight of ideas there. That's sort of the visual equivalent of pitch correction in a pop recording vocal, no?
3. Man, no wonder they stopped trrying to use small turbines in cars...although Granatelli's tore UP Indianapolis.