By Thomas A. Horne
Today’s business jets and airliners use engines that have their design roots in World War II technology. But they’ve come a long, long way in the intervening years.
Britain’s Gloster Meteor was one of the first operational jet fighters, and was equipped with two Whittle W.2B engines of 1,700 pounds of thrust, giving it top speeds of 362 knots. Germany’s Messerschmitt 262 had two Junkers Jumo engines of 1,300 pounds of thrust, and could reach 485 knots. These early designs were pure turbojets, meaning that all the air flowing into them was directed through a single compressor and power turbine that share a common shaft. In between those components is a combustion chamber. Air enters the engine, gets compressed by the compressor, is mixed with fuel and ignited in the combustor, then sent through the power turbine to generate thrust. These early turbojets could make an airplane fly fast, but engine temperatures ran very hot, they left trails of exhaust smoke from incomplete combustion of fuel, had high noise levels, and terrible reliability (word has it that the Jumo 004 engines would last only 25 hours).
Better turbojets persisted up to the 1960s and were used in early business jets, such as the Rolls-Royce Viper engines used in the Hawker-Siddeley H.S. 125. But the need for quieter, more fuel efficient jet engines was met by the development of turbofan engines.
Turbofans, unlike turbojets, send most of the incoming air around the core of the engine, in effect bypassing it. The fan section located at the very front of the engine accelerates this air and sends it out the back of the engine. At the same time, additional thrust is produced by combustion in the core.
The result is an engine with a high bypass ratio. The bypass ratio is the ratio between the mass of air flowing around the engine core and the mass flow rate going through the engine core.
Today’s modern business jets typically have bypass ratios in the neighborhood of 3:1 or more. Cessna’s Citation X, for example, has Rolls-Royce AE3007C1 engines of 6,764 pounds of thrust and bypass ratios of 5:1. The early-1980s Citation IIs have 2,500-pound-thrust Pratt and Whitney JT15D engines with 2.5:1 bypass ratios.
High bypass ratios are desirable because they yield lower exhaust speeds, and thus have better fuel specifics—meaning they burn less fuel per mile. This is mainly because the thrust generated by the bypassing air isn’t created by burning fuel. In effect, it’s “free” thrust. Another good reason for turbofans is their lower noise. That’s because the bypassing air mixes with, and muffles, the air coming from the noisy core.
Jet engines with low—or no—bypass ratios are great for fighters and other fast airplanes (the Concorde’s Olympus engines had 0:1 bypass ratios). Low-bypass engines produce more thrust because of their higher exhaust speeds, but in the civil-aircraft market, high-bypass engines win hands-down.
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Do you remember the computer, “HAL,” attempting to take over the spacecraft in the 1968 classic 2001: A Space Odyssey? (“Just what do you think you’re doing, Dave? Dave?”) That movie gave voice to our fear that we would one day become slaves to our technology. In many cockpits, that has certainly become true.
Back in 1983, when Ronald Reagan was in the White House, I was hired to fly my first “all glass” airplane, a new Canadair Challenger 600. It was a marvelous display of technology, with its too-cool TV screens, blinking lights, magic buttons, and an annoying, disembodied voice we called “Bitchin’ Betty.” At first, we wondered what we needed all that technology for. Turns out, that question was quite prescient.
While we integrated all this computer magic into our everyday flying, attempting to rise to the challenge of operating it in spite of its quirks, “operating logic,” limitations, and engineering misses, the technology became more common, refined, capable, and helpful; we began to take it for granted, yet few paused to question its role. “Who is the master here? Who is the slave?” That failure led to a whole new category of pilot error. Here’s what often happens when we get the master/slave relationship confused.
Altitude busts. Pilots are often busy typing on the computers instead of ensuring that the autopilot will make the crossing restriction or level off at the proper altitude.
Missed ATC calls. “Was that call for us?” has become even more common in our cockpits as we are distracted by the magic in front of our eyes. This can lead to clearance errors and loss of separation from other aircraft.
Near collisions. Let’s face it: Given a choice between staring at a “blank” sky and an ever-changing TV screen, pilots will choose the TV. Every time. But not all traffic appears on the screen.
Taxi errors. When you are moving about the taxiways with your head down, it’s quite easy to miss your turn and pull into the path of a departing or landing aircraft.
Establishing who is boss in any technology relationship is a crucial first step, one that’s often missed in the rush to play with the new toys. We’d rather play with the buttons and knobs than decide how (and if) we want them to serve us. That’s understandable, because technology is compelling, exciting, amazing, and endlessly fascinating.
But when we try to serve this “slave” we discover that it has no brain; cannot plan; does not have the big picture; does not “think” or reason with the complexity of a human pilot; does not know how to prioritize; cannot hear ATC calls; does not know the danger of non-TCAS airplanes that get too close; has no concept of time management, or even why time is important; has no consciousness; may not comprehend spatial relationships with other aircraft, terrain, or weather; and has no experience to draw from.
It does not ask questions; does not learn from its mistakes; has no awareness of itself or things outside; knows only what someone (mostly programmers, not pilots) taught it; does not know the limitations of the machine to which it’s bolted; does not know the FARs, the AIM, or the POH; does not know how to fly; only knows one way to do things—its way; and, perhaps worst of all, constantly solicits and demands your complete attention. Still want to serve that “slave”?
In the headlong rush to learn and conquer our electronic flying world, we somehow lost sight of the reason that this equipment was added to our cockpits: to make flying easier by giving us a better way to display and manage information. All cockpit protocol should boil down to this: If the magic serves us, use it. If it doesn’t, minimize, ignore it, or turn it off.
Next time you go heads-down to type on the keyboard, twist the knobs, or just stare at the display, ask yourself: Is what I’m doing really necessary? Am I doing this because it makes the display pretty, or because it’s satisfying to prove I know how to do it? Do my actions serve me or am I serving the slave? More simply: “Do I need to do this now?” Often, the answer is no. Are you listening, HAL? HAL?
Michael Maya Charles is an aviation writer and commercial airline pilot.