Motion Lotion: What’s the Commotion?
“The only time you can have too much fuel is when you’re on fire.” –Anonymous Pilot
Those are words to live by, in the flying business–but jet fuel is expensive. In fact, it’s just about the largest expense in the operation of the airline, which is why it makes sense to use fuel as sparingly but sensibly as possible. But as a passenger, what’s it to you?
Well, for starters, this:
Do we go around it? Above it? Through? You won’t like the last option, but fuel is the double-edged sword in this fight: more means we’re heavier, which limits our climb. Plus, going around the weather will burn more fuel, limiting our options at our destination:
We’re at 36,000 feet now, which is just about the optimum altitude. “Optimum” is a moving target: as you burn off fuel enroute, the jet gets lighter and the wing can handle a higher altitude, which means the engines can operate at a lower thrust setting, thus saving fuel. We’re within 200 feet of the max if we climb to 38,000 feet to top the weather. We can wait till the “max” readout shows “380,” or really, from experience, we know that in the time it takes to request and receive the clearance, plus what we’ll burn in the climb, we’ll be at the correct weight. But, there’s always a catch.
The airspeed tape on the left shows us a very narrow operating range at the top end of our altitude capability. That is, your range of acceptable airspeed is from about 212 to about 245. The “chain” above that shows the area of high speed buffet, meaning parts of the aircraft, above that speed, will begin to go supersonic. More importantly, though, in my mind, is Mach tuck: swept-wing jets tend toward a pitch down near the high speed limit, and guess what a pitch down does: your high speed becomes even higher. In a jet, particularly a passenger jet, if you don’t recover aggressively and immediately, you will not be able to stop what will become a dive.
On the bottom of the tape is the yellow line we call “the hook,” which is the slow speed stall. If you go below that speed, your airfoil will stall, and you will fall.
So, at 38,000 feet, we have very little margin between the high and low speed buffet, requiring extreme vigilance on our part: turbulence, mountain wave action, or a drastic updraft of any kind can push us beyond either speed limit. Which is also part of the balancing act the captain must perform:
I insert a slower Mach number in order to cruise more toward the middle of the range between the high and low speed limits. That, too, though, will affect our arrival time, won’t it? But that’s a balance I feel can be maintained, knowing that we’ve picked up some direct routing already. I’d rather sacrifice some time (and really, fuel) to gain a better pad between any adverse effects (mountain wave, thunderstorm up drafts, windshear, clear air turbulence) that could push us into either boundary.
And, I’ve already checked: the winds at the higher altitude are more favorable. To be even more accurate, I’ve requested a data-linked update to our flight management system, updating the projected winds the computer is using to calculate the times, distances and fuel burn it displays because what we data-linked into the system on preflight hours ago may not still be accurate:
The photo makes it hard to see, but the new, uplinked wind speeds are highlighted, all I need to do is push the “EXC” (execute) button and the entire nav calculation will be updated in a matter of seconds.
Climbing early has taken us out of more headwind earlier, so I believe the ETA will be largely unaffected. This hunch is borne out as we progress in our flight:
We cross Pocatello, Idaho (PIH) six minutes ahead of schedule and up 700 pounds on fuel. If, however, the higher altitude winds were less favorable, we’d end up with the same result by going around the weather (more miles at regular cruise Mach) as by climbing above the weather (less miles at a slower speed). The latter option is better, fuel-wise, as you can see from the fuel log above. But we’ll do whatever is safest and most optimum first, and worry about timing later. Plus, if we don’t have what I consider a comfortable high speed-low speed margin at the higher altitude–we’re not climbing, we’ll just have to fly the additional miles (and minutes) around the storm.
It’s not just air miles between us and Seattle–it’s a constant balancing act of time, fuel, altitude and route. It all goes on steadily, quietly but relentlessly in the cockpit, but we all share the payoff in the end.