Calling on experience and instinct
"Measure it with a micrometer, mark it with chalk, then cut it with an ax." Whoever came up with that one was thinking about flying jets. The engineers who design jets like nothing better than an absolute, no-wiggle room performance chart. The pilots who fly jets prefer easy-to-use, practical rules to live by. Something had to give. Somewhere between the drawing Hoard and the runway a valuable reality check evolved: rules of thumb.
Rules of thumb are gems of common-sense wisdom, designed to let you see the forest, despite all the trees in the way. They're the little voice that tells you something makes sense (or doesn't) despite whatever else you're being told.
Don't get me wrong, I like being precise when I fly. I mean, who wouldn't? It's like clean air and water; no one could be against it. But after years of flying jets for a living, I can say that sometimes all that precision just doesn't matter. Sure, I want to know to the last pound what we weigh for takeoff and how much fuel we'll burn en route. But I also realize it isn't always possible to know all the answers with complete precision. Nor is it truly necessary. That's when rules of thumb are handy to have about.
A recent transcontinental flight in a Boeing 737-800 from New Jersey's Newark Liberty International Airport to San Francisco International Airport is a case in point. The first chance to run the "smell test" of reasonability comes in the weather room with my check of the paperwork — the dispatch release, flight plan, and weather package.
I normally start with the dispatch release. It shows our estimated payload is 30,600 pounds, which tells me we should be full of passengers. The airplane has a total of 156 passenger seats. At the FAA approved average winter weight of 185 pounds per adult (180 pounds in the summer without the overcoats and galoshes) and 80 pounds per child between the ages of 2 and 12, the multiplication gets us close to our estimated payload. Throw in some cargo (not passenger bags, which are already included in passenger weights) and the 30,600 pounds makes sense to me. We'll have a better idea just before push time. That's when we'll receive the actual passenger count from the lead flight attendant and compare it with the weight and balance sheet that prints out in the cockpit. Everything needs to add up to 174,700 pounds or less, our maximum allowable taxi weight.
Of course, the chance the average passenger weight for this flight is really 185 pounds is slim. Maybe it's 180, or 193. But does it truly matter? If somehow our real takeoff weight is 1,000 or even 2,000 pounds heavier than we think, the effect will be slight. Our V 2, or takeoff safety speed, could be off, but only by a single knot. We'll use up a tad more runway for takeoff, but not enough so that anyone will notice. If truth be told, it's hard to fly within one knot of accuracy anyway. Yes, the FAA would look sternly upon us for knowingly departing even a pound over our 174,200 maximum takeoff weight. But the agency gives a wink and a nod to the "average" passenger weight theory, understanding that it probably varies a bit from flight to flight.
The dispatch release notes that the flight is planned at low altitude and high speed for an on-time arrival. Also, moderate turbulence is reported at all altitudes above Flight Level 290 during the first half of the flight. Sure enough, the flight plan shows we'll be at FL260 until passing Denver, at which point we'll climb to FL350 for the remainder of cruise. Reading further I can see why. At FL350 we would have a strong winter jet stream howling at 160 knots on the nose. Down at FL260 we can expect a more reasonable 70-knot headwind, and by Denver we'll be out of the jet stream at FL350.
So far things make sense. With everything being equal, a good rule of thumb for fuel efficiency in a jet is to fly as high as you can for the aircraft weight and to operate at long-range cruise speed. If you want to go fast, choose a middle flight level, something close to FL270, where true airspeed for a given Mach number is highest. And be prepared to burn more fuel. Today, winds and turbulence dictate that FL260 initially makes the most sense for on-time and passenger-comfort reasons.
Conditions on the West Coast are forecast to be good at our arrival time of 5:37 this afternoon. The dispatcher has chosen to list San Jose as an alternate, even though we don't legally need one. Good. He and I both know that San Francisco is notorious for rapid changes in forecast conditions. Late-afternoon fog and low ceilings seem to show up sometimes without the weather forecaster's blessing. A good rule to live by in any airplane is to temper weather forecasts with past experience.
There's also a sigmet for a chance of severe turbulence below 12,000 feet in much of the Northeast. This only pertains to the first five minutes or so of our flight. I make a mental note to brief the flight attendants to remain seated until they hear from us that it's safe to get up. But the little voice in my head tells me we're unlikely to experience severe turbulence. Over the years I've seen hundreds of similar sigmets, and almost never has the turbulence been severe. Short of flying into a thunderstorm, the possibility that "the aircraft may be momentarily out of control" or that "unsecured objects are tossed about" (Aeronautical Information Manual definitions) is slight. In fact, I would estimate that less than two or three hours of my entire career have been spent in anything resembling severe turbulence. If I was flying a Mooney or Cessna today I'd be more concerned.
Once settled in the cockpit, my first officer and I discuss three airworthiness directives that apply to this particular airplane. We're prohibited from using speed brakes above 300 knots (or serious structural damage to the tail might occur). We need to use the number-two com radio for all communications while on the ground (because of interference problems on the number-one com). Last but not least, we need to remember to turn off the center fuel pumps when the center tank quantity is down to 1,000 pounds (or risk a fuel tank explosion).
None of these issues was discovered in the certification process for the -800 series. They all cropped up after the airplane was flying for some time. Having flown seven of the nine 737-series models from the -100 through the -900, I think it's time for a new rule of thumb. When an airplane starts displaying these kinds of growing pains, it's time to stop stretching it. (Memo to Phil Condit at Boeing: Enough already.)
Minutes before pushback the weight and balance spits off the printer. Our takeoff weight is 173,941 pounds, a bit under maximum. Our stabilizer trim setting is exactly 5.4 degrees. Since this falls comfortably within the green arc, it passes my basic sensibility test. It could be off by a degree or two and I probably wouldn't notice the difference at rotation. But if it was set well outside the green arc, there could be a control problem.
We take off and climb to FL250 without incident — no turbulence of any kind. Cleveland Center soon gives us good news — we're cleared direct to FMG, the Mustang VOR. This represents a single navigation segment more than 1,600 miles long. Naturally we hope this shaves off some time. But the reality is clear to us seconds later as the flight management computer (FMC) recalculates our estimated time of arrival. The shortcut has shaved only one minute from the planned five-hour and 53-minute flight time. And it saves a scant 100 pounds of fuel.
This just proves another rule of thumb I've come to appreciate. Shortcuts usually don't help as much as pilots like to think. If the new clearance results in a greater net headwind, it can even increase the flight time. Fortunately there's a corollary. Deviations around thunderstorms don't seem to add a lot to total flight time either.
We're well past Denver in smooth air at FL350 when we get the first call from a flight attendant to "warm it up in first class." I've never figured this one out. Sometimes we cross the country without a single temperature complaint. Other times the interphone never stops ringing. Most baffling of all, the passenger cabin temperature setting always seems about the same. My rule of thumb for this one is simple. Don't touch the temperature control unless asked to do so. The number of complaints rises drastically with the number of times the temperature controllers are fiddled with.
We fly through a cirrus layer. At minus 58 degrees Celsius, icing isn't a problem. In fact, at any temperature colder than minus 40 degrees C we don't even need to turn on the anti-ice systems. The rule of thumb here is that you're most likely to encounter serious icing in a jet at lower altitudes and slower speeds, not at cruise in the upper flight levels. And as with turbulence, we seldom experience "severe" icing.
It's nearly time to start down. The FMC indicates we have 37 miles to go until reaching top of descent. It plans on our crossing a certain navigation fix at 10,000 feet and 250 knots. A useful rule of thumb for descent in the 737 is to use a 3-1 ratio. Multiply the number of thousands of feet to lose by three, and that is the approximate number of miles needed for an idle power descent. With a strong headwind I then subtract 15 miles, with a strong tailwind I add 15.
There's only a slight headwind by now, so 75 miles to lose 25,000 feet seems about right. I add an extra five miles in order to slow to 250 knots at 10,000 feet. The FMC is planning to do all this in 67 miles. Since (by experience) I don't trust the VNAV feature to always make altitude restrictions, I start down 80 miles out. But even with this, an "Add Drag" message soon appears on the computer screen. The FMC is recalculating and now doesn't think we'll make the restriction. But a quick glance at the FMC descent page confirms what I suspect. We're really not high at all. We're just 50 feet above the planned descent path. For some reason the engineers who designed the FMC figured this is important for pilots to know. Most pilots just find such nuisance messages annoying.
Not long afterward we land at a weight (I think) of 139,700 pounds, although we might have weighed somewhat more (or less) than this. Our target speed for touchdown with 30-degrees of flaps is 151 knots, but maybe it ought to have been 150 or 152. Fact is, anything in that range would be a safe airspeed.
Parked on the gate, I'm sure of just one thing. It feels good to stand up and stretch.
Vincent Czaplyski holds ATP and CFI certificates. He is a Boeing 737 captain for a major U.S. airline.
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