Jul 18, 2007

A Day in the Life (27) Ready…

A description of a typical day's flight from Montreal to Vancouver and return - 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 - nearing TOD (Top of Descent)

Time: CYVR Arrival minus 0H45M

With Vancouver in view on our navigation screen, FO Paula decides it’s time for her official briefing. Once she sees I have my charts out and I am ready to follow along, she launches into the prescribed litany, telling me all about the descent, the crossing restrictions, the landing configuration she’s planning and so on.

I dutifully check that the trolls have all been assigned their appropriate parts to play, poking Macdoo with any needed numbers as I go. When all is entered to my satisfaction, I let her know and she finishes up the briefing as she double-checks my work.

Some of the considerations that go into our descent planning:

Wind— we start the descent later against a headwind and earlier with a tailwind. The part that’s hard to know is whether the wind will persist all the way down or will change significantly as soon as we leave cruise altitude.

Landing weight — we start the descent further away when we are heavy and closer when we are light.

Crossing restrictions - most busy airports have defined arrival paths that require us to hit certain navigation points at specific altitudes and speeds. These usually force us down earlier than we’d like, then require us to add power and ‘drag’ the airplane along level again until the next descent stage. This is necessary to interweave us with traffic outbound or landing at nearby airports.

Anti-icing requirements - if we need to keep the engines and maybe even the wings heated in descent to prevent ice forming, we need to start the descent earlier. The engines provide the hot compressed 'bleed air' for this job, so we have to run them at higher than idle power - effectively lengthening our descent distance.

Descent Speed - If we are running late and plan to use a higher descent speed to gain time, we will begin the descent closer to the destination. Higher airspeed means more aerodynamic drag which gives a steeper descent. But with the price of fuel the way it is, we don’t do this as much as we used to. It’s more economical to start the descent earlier and glide down at a lower speed which trades more forward distance for altitude.

In an ideal setting we close our engines to idle power at the top of the descent, and keep them like that all the way to the final approach alignment with the runway. If our calculations prove to be off a little we can compensate by adjusting our airspeed. Dive faster to get down sooner, or reduce our airspeed to glide further. If this isn’t enough, then we extend the speed brakes to add more drag and descend more steeply, or in the other case, we open the throttles to add engine power.

It’s a sweet feeling to glide all the way down from say thirty or forty thousand feet slowing down at the appropriate times, extending the flaps and landing gear at just the right times, and then applying engine power for only the last few minutes to lock in our final approach speed to the runway.

Every pilot has their own “special” formula for working out all these variables to come up with the most accurate descent point. But a quick and simple rule of thumb is to take three times the cruise altitude in thousands of feet and use that as the miles to fly to landing.

So if we are landing at Vancouver (sea level) and we’re at 39,000 feet like today, three times thirty nine gives us 117 flight path miles to touchdown. To this we add and subtract adjustments for the factors I just mentioned. Before the use of flight management computers which include vertical navigation guidance, it was a point of pride for pilots to come up with the best possible estimate.

With the Auto-Bus we give Macdoo the forecast winds for the descent, along with any required crossing restrictions, and then sit back and watch the Genies come up with a descent point. But again the old computer adage “garbage in, garbage out” applies. If the guidance system has an inaccurate lateral path in the program, then of course the vertical path will be wrong.

Also, the automation needs careful watching during descent. Especially if the winds are different than forecast. I’ve seen it happen many times where the navigation Spirits will show us a GREEN number at a key crossing restriction which means “I’m gonna make it…”, only to have it turn AMBER at the last moment which is the computer equivalent of throwing hands in the air shouting, “I'm not gonna make it! I'm not gonna make it! Do something!”

Today, FO Paula finishes up her briefing by reviewing the crossing restrictions and other data I’ve entered. When she gets to the descent speed item she pauses to ask if I’m interested in a higher-than-normal descent speed to try to improve our gate arrival time. I am. She taps in a number just ten knots below the redline limit. She then finishes the briefing and I ensure she knows how and when we’ll transfer control after landing.

Finally she calls for the “pre-descent checklist” and I pull out the metal card. Following the script again, I perform the final confirmation:

“Approach Briefing; Completed.
Landing Data; Set.
Pressurization; Checked.
ECAM Status; Checked.
Nav Accuracy; Checked.
Pre-descent checklist complete.”

The entire procedure flows smoothly and includes important cross-checks of crucial data, and it took us many hours of hard work to make it look this easy.

But that's it. We’re set.

Time: CYVR Arrival minus 0H30

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