Feb 26, 2007

A Day in the Life (11) - pushback and engine start...

A description of a typical day's flight from Montreal to Vancouver and back - as a backdrop for a detailed, non-technical description of what an airline pilot does.

Log Entry 2003 - A320 Capt - CYUL - Main Ramp

Departure Time: plus one and counting...

A lot of effort went into the A320 flight deck design. They got it mostly right. But one thing they got wrong in my opinion is they put the lever to set the parking brakes and the knob to initiate engine starts in the same general location and both require a clockwise twist to operate. So the physical action of starting the engines during pushback is very similar to that of setting the parking brake at the end of pushback. That’s an accident waiting to happen. I was the first officer one night a few years back when I saw it almost materialize.

We were on the last leg home after a long day, and of course there were several distractions during the pushback. I noticed the captain reach down to start the engines but actually grab the parking brake switch and begin turning it. I called a warning at the same instant he realized his mistake and corrected it. A loud jolt shuddered through the aircraft as the brakes pulsed on and off. Our tractor driver, feeling the sudden impact, lurched us to a stop and called on the intercom, alarm in his voice. The captain acknowledged his mistake and apologized. We waited tensely while a mechanic was called out to inspect the nose gear.

Thankfully no damage had been done. Tow bars do have a shear pin designed to break before serious damage occurs but that's a last resort and they don't always function perfectly. Nose gears sometimes get damaged as do pushback tractors and worst of all people when they come up against unyielding heavy metal. Ramp staff are often hurt when pushbacks suddenly go wrong. It's a dangerous phase of the operation for them.

Since upgrading to captain I've been very methodical about reaching for that particular control. More than once I've caught myself as my fingers threatened to get ahead of my thinking. A danger in any airplane.

Today the push starts smoothly. We hear the roar of the pushback tractor slowly overcoming inertia then we are moving. FO Paula stabs her audio panel selector to the VHF2 radio and tells the ‘iceman’ we’re on our way. The aircraft clears the loading bridge then the tractor driver calls; "Clear to start engines…"

“Starting engines - two and one,” I reply. Carefully I clutch the correct button and initiate the engine start procedure:

Eng Mode Selector................IGN/START

I rotate the knob then pause for just a second as the engine parameters appear on our display screens.

Engine Master 2 switch…….ON

I announce “Starting engine two.” I lift the engine master two over its gate and let if fall into the ON detent. I'll hold onto it until the engine has stabilized. Despite the ‘automatic’ start cycle there are several situations where I may need to take action. For example if we prematurely lose electrical power I'll terminate the start and apply a procedure to purge any accumulated fuel from the engine. Our manuals list several other cases where the Airbus auto-matrons won't help me and besides, they aren't the ones who'd be fired if a five million dollar engine gets cooked during startup. So I pay close attention and wonder again at this entire philosophy of automation.

Turbofan engine primer:

Modern jet engines are comprised of two sets of compressor blades driven by their own exhaust turbines attached through concentric shafts. They vaguely resemble empty thread spools and are named N1 and N2. Both spools spin at such high revolutions that it is frightening for pilots to think about while suspended over inhospitable environments, hours from the nearest airport, hoping nothing goes wrong. So the tachometers are displayed as a percentage of maximum rpm. Much easier on our nerves.

Turbofan basics are covered at wikipedia http://en.wikipedia.org/wiki/Turbofan
For those more technically inclined a detailed description is available at: http://www.b737.org.uk/powerplant.htm

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On previous aircraft models I had key startup parameters I was required to note and emphasize with a verbal callout. On the Airbus we are still responsible to monitor the sequence, but we no longer make these calls. This inspired someone to quip that these engines are so smart they actually start without anyone talking to them. But decades-long-habits don’t just go away. The eight-track in my head still plays the callouts “silently” as I confirm each step:

Engine Master 2 switch…….ON
The start valve opens, the bleed pressure remains ‘green’ and the N2 tachometer increases. Within a few seconds the oil press increases. When I see 16% N2 I also see a message telling me which set of ignition “spark plugs” kicks in. We have two, named A and B. Usually they alternate between starts.

At 22% N2 the fuel metering valve opens and we check that the fuel flow reading increases. Within fifteen seconds of that we need to see the EGT (Exhaust Gas Temperature) begin increasing. If the fuel flow behaves abnormally it can be a sign of a couple of different start malfunctions. This morning it’s just right.

By this time we should see some increase in the N1 tachometer. This confirms that the fan section of the engine isn’t jammed. I’ve read at least one report where pooled rain or snow inside an engine literally froze the N1. I've also read of someone in another company attempting to start an engine with a plug still in place. Now, an engine plug or cover looks a lot like a kid’s round plastic wading pool. It's usually colored bright yellow and is inserted in the front engine opening to keep out debris. I can only assume that someone slept through their exterior inspection. But our fans spin up freely this morning just like usual.

At 50% N2 we see the start valve close and the igniter switch off as the engine becomes self-sufficient. It should now continue to accelerate to a normal idle. If it hangs up though, we are responsible to notice and do something about that too.

At about 52% N2 we feel a thump in the airframe as the engine two generator comes online and takes over from the APU.

FO Paula looks for the stable engine parameters. In ground school my instructor drove these into me by chanting after every engine start; “twenty, forty, sixty, thirty." Just how much can that old eight-track of my mind hold I wonder as I look for approximately: N1 20% ; EGT 400°C ; N2 60%; Fuel flow 300 kg/h.

Our airplane jerks softly to a full stop against the towbar. The outside ramp noises in my headset warn me that someone below is keying his microphone:

“Pushback complete. Set brakes.”

Departure Time: plus three and counting…

2 comments:

Steve Lin said...

Dear Captain ALUWING:
Inadvertently find your blog when I searching a Towbar on the web.
The "parking brake" things you mentioned actually happen to one of my colleague in 2005.
He was been wakes up in 5:00 am for a non schedule flight. when He cam on board, there were some previous fault record about the door open message on the log, during the push back and Eng start, his FO keep talking about how to dealing with those condition, what's happened next was almost the same as your story, but result was totally different.
The tow bar do break away as designed, but it goes up ward into the nose gear strut, cause serious damage and whole nose gear assembly have to be replace and he lost his Captain ranking for a year.
So these things do happened and not only once!
A330 Captain Steve Lin

Aluwings said...

Capt. Lin,

Thanks for adding this story. It sure reveals how much distraction we face on the flight deck and how this can lead to near-disasters.

Too bad about your colleague's misfortune. I'm relieved that no one on the ground was injured.