Questions from the Cockpit: Weighty words

Noah, a student pilot in Arizona, writes: I’m trying to get my head around weight and balance, especially the terms: Moment, arm, station, datum… None of them seem particularly descriptive. Can you help me out?

Happy to help out with your weighty problem!

OK, I won’t quit my day job and try to become a comic. But I’m glad you wrote in.

For an industry that relies overly much on mnemonic devices and other clever tricks to help commit material to memory, there is little help out there for the student pilot to master weight and balance terms.

I have a few tricks up my flight suit’s sleeves for some of the terms, but others you’ll just have to memorize old school: By writing them on a chalkboard 100 times after ground school.

Now, for my non-pilot readers, weight and balance is a somewhat complex series of mathematical problems pilots must solve before every flight.

Weight, just like it does for people, affects the performance of airplanes. How much an airplane weighs is variable and depends on how many people are in it, what their cargo is, and how much fuel is in the tanks. That’s not so difficult, as math goes. You just metaphorically put everything on a scale.

But with airplanes, where the weight goes is a second, closely related, and more complex problem.

You can imagine what would happen if you put 152 gold bars in the aft cargo compartment of a Cessna 152. Right. Other than quickly becoming the most valuable 152 on the planet, it would be sitting on its tail. This is the balance part of weight and balance, often just called W&B in the biz.

And learning W&B requires both math and English skills — an automatic barrier to learning, as most people are only good at one or the other, but rarely both. As you wrote about terms, I’ll assume you are a math wiz.

Myself, I’m better at English, so hopefully I can help you out with ways to internalize the most common terms, such as station, datum, arm, moment, and the assorted weights.

Let’s start with station. Station is a location in the airplane of something that you are weighing. Specifically, it is a pre-identified position, say the back passenger seat. So it’s a place.

And that’s how I remember it: Just like a train station or a bus station is a place.

It’s also a number that indicates how far away the place is from something called the datum.

Datum drives student pilots — and a lot of fully certificated pilots — nuts. Also known as the reference datum, it is the point from which, if you were using a metaphorical measuring tape, you’d measure the location of the various stations in the airplane.

Not only is the word datum meaningless to most people, it’s not even a real thing in the first place. The FAA defines datum as “an imaginary vertical plane” from which all measurements for W&B are taken.

Imagine that. An imaginary plane.

Oh, not an airplane, but a geometric plane. But that’s not the crazy-making part.

The crazy-making part is that the imaginary plane can be anywhere. In many airplanes, it’s the firewall. In some, it’s at the wing root. In others, it’s the tip of the spinner. In yet others still, a place floating in space in front of the airplane.

Here’s the deal: It’s just a set location from which to measure the location of the contents of the airplane. It’s chosen by the manufacturer. It doesn’t matter where it is, because it simply doesn’t matter. A datum can be anywhere and the math will work, so long as all the stations are measured from the same datum.

As to the funky term for the locationless, imaginary, geometric plane, here’s my tip for remembering the meaning of the word (even though it’s a bit of a stretch). The word data is plural — in other words, lots of pieces of information. Datum is singular, or one piece of information. So like Tolkien’s One Ring, the datum is the One Location from which all W&B measurements are taken from.

Now, the distance between the station and the datum is called the arm. Too bad they didn’t call it the leg instead, I think we’d all grasp that more quickly. We are used to thinking of measurements between points as being legs.

Personally, I just think about aviation costing me an arm and a leg, and that leads me to the leg of a cross-country flight, which is a measurable distance, and then that brings me full circle to arm again, and voilà! Arm = a distance.

But moment is not so easy, so we’ll need to spend a few moments on it. It has nothing to do with time, as we are used to using the term in common speech, but instead, with force.

In W&B, and in physics, it’s a mathematical expression of the relationship between weight and distance. If you hold your heaviest aviation textbook close to your body, it doesn’t seem to weigh much, but at arm’s length, it feels heavier. That’s moment in action.

The greater the arm, the greater the force from a given weight. Your fully extended arm has more, well, arm in the weight and balance sense. How cool is that?

Moment, I grant you, is not only a bit of a strange concept to grasp — the same weight being able to exert variable force — but is also a funny word.

That said, if you follow moment’s etymological journey, I think you’ll find it makes perfect sense.

The term comes to us from the Latin word momentum, which we tend to use as the reduction in inertia when something starts to move, such as overcoming momentum or gaining momentum.

But the original meaning of the word was simply “movement” or, critical for us, “movement power.”

Ah ha! Moment is movement power. The longer the arm, the greater the moment. Or said another way, the longer the arm, the greater the movement power a given weight has.

Now speaking of weight, we come full circle in your weighty problem. There are a lot of weights in W&B. Most, thankfully, are self-evident, like maximum landing weight, maximum ramp weight, maximum takeoff weight, maximum zero fuel weight, and just plain old maximum weight.

But then we have the chalkboard weights. The ones you just pretty much have to memorize. Those are licensed empty weight, standard empty weight, and basic empty weight.

Licensed is the plane as built, with fuel and oil drained out of it. As that is a status in which an airplane is unfit for flight, you generally won’t be using it as a pilot (although you may see it used as a term in vintage airplanes W&B guidance).

Standard empty weight is the airplane, permanently installed equipment, full engine oil, and unusable fuel only.

Basic empty weight — the one most commonly used by pilots as the starting point for W&B — is that standard weight above, but adding in optional equipment. Basically, a ready-for-flight airplane with empty gas tanks. So, I guess, not really ready for flight, but it still makes for a great starting point for our W&B mathematics.

So hopefully, you are now armed with datum knowledge that will last you for more than a moment.