Jun 2, 2007

A Day in the Life (21) Vertically Challenged

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. (check left hand column for series index).

Log Entry 2003 - A320 Capt - Cruise

Time: CYVR Arrival minus 4H50M

Earth’s highest point is Mount Everest at 8846 meters (29,028 feet). The lowest point on earth is the Pacific Ocean's Marianas Trench, near Guam at 10,924 m (35,840) feet deep. So the total vertical relief of the earth’s surface is 19, 770 meters - just under 20 kilometers. That seems like a pretty craggy surface at first glance.

But consider that the circumference of the earth is about 40,000 kilometers. If we were to shrink the globe down to the equivalent size, apparently it would be smoother than a billiard ball.

Then, moving upward from this surprisingly smooth surface, I know that at just 18,000 feet we are already above half of the earth’s atmosphere, by mass. Of course it continues to stretch upwards to one-hundred thousand feet and more, but by then it’s so thin it’s not a place we can survive or fly conventional airplanes. On our billiard ball model, our entire “useful” atmosphere could be represented by a film of baby oil. That’s all that stands between us and the oblivion of deep space.

You could say we earthlings are vertically challenged. Our world is a very compact place when we start measuring ups and downs. And a lot can happen in a short distance. So I’m not overly surprised when the turbulence stops as suddenly as it began once we've descended a mere 500 feet.

FO Paula glances over and asks, “Do you want to ask for flight level two seven zero, wrong-way, for now?” Normally the odd-numbered flight levels are for eastbound flights only.

“Good idea,” I reply and key the mic and make the request.

“I’ll do even better for you,” the controller replies. “Goose Air one-eleven, maintain block altitude two-six zero through two eight zero.” I read this back and thank him. We now have some freedom to climb and descend a little to track this smooth river of air.

FO Paula locks us onto flight level two seven zero, at least for now. I wait a few minutes to see if the smooth air will hold. It does. I switch off the seat belt sign.

“Let’s try again.”

A growling stomach soon reminds me that my early morning coffee and snack are a long way down. When Connie returns with my breakfast tray I dig right in. Our Standard Operating Procedures suggest that the two pilots should not eat at the same time. But they also say that hypoglycemia is not a good thing. As further insurance I note we have good alternate airports along the route -- safe harbour should the green eggs and ham turn us both green.

As we dine, it's time to catch up on our mutual continuing sagas of life, the universe and everything. I happen to notice our flight management computer now says we’ll be arriving in Vancouver with nearly empty fuel tanks. The old computer adage, garbage in - garbage out applies here. The dire prediction is triggered by the unplanned altitude changes we've made. Yet, it’s disconcerting to see negative numbers in the “extra fuel” display window, so I add the step climb predictions at a couple of places along the way.

As I do so, the fuel prediction gradually adjusts towards the original flight planned amount. But still I am acutely aware that as long as we are trapped at this relatively low level, our reserve fuel is steadily being consumed.

The clock is ticking. This clock is not marked with one’s and two’s and sequential numbers up to twelve. Rather it sports a big ‘E-for-empty.’ Tic, tic, tic…

Time: CYVR Arrival minus 4H30M

2 comments:

Anonymous said...

How big of a difference is there in fuel flow numbers for the 'bus between FL 270 and your original planned altitude?

_____________________________ said...

For round numbers the difference in fuel flow would be about 400 to 500 KG per hour.

Typically, our minimum fuel that must by law remain in the tanks at touchdown is about 1,200 KG. (This provides 30 minutes of 'circling')

Small increments are included for the approach and landing, and if we have fuel for an alternate airport (we don't always), then another small increment is added to account for the go-around. Any known delays that we expect to encounter will require extra fuel.

If we are warned ahead of time that turbulence will prevent us from attaining our ideal altitude, we'll carry extra fuel for that. But we don't always get that information before departing.

For contingencies we carry about 400 KG more. Obviously that will get used up pretty fast if we are stuck too long at FL270, for example, or if we need to detour around thunderstorms.

Of course the winds will be significantly different at FL 270. Due to the way the computer-planning system finds the best possible route and altitude profile to destination, any deviations tend to make the fuel situation worse.