Why Do Planes Fly So Slow?

I'm in New York now, momentarily!

1. Why Am I Writing About Planes?

This blog usually has a very focused focus. I write about philosophy and politics and headaches and college admissions and Jesus and Torah and AI and nothing else!

This is what you’ve come to expect from me! Focused focus.

And yet, here I am, with a post about planes, from totally out of the blue. Why?

Because I’m going on a trip! And, much like a four-year-old, I can’t really think about things that aren’t right in front of me, and this is what’s right in front of me:

Veeeeeeooommmm!

That’s an Airbus A321. My flight on it took me from Detroit—DTW, in fancy airplane talk—to New York City’s LaGuardia Airport—LGA. DTW is a really nice airport—one of the nicest I’ve ever seen. We have this cool fountain thingy:

Sploosh!

And we have this wacky tunnel thingy too:

Wow so blurry! Unfortunately, I am too lazy to walk to where it is (it’s really very out of the way), so the internet’s version must suffice. (source)

But as cool as the airport is, Michigan itself has a certain… anti-charm to it this time of year:

“Springtime gloom,” I call it

So I’m getting out! I’m spending a couple days in New York, and I’m visiting a couple colleges that let me in (Columbia today and Fordham tomorrow).

But, of course, to get to New York, I had to fly on an Airbus A321. Which took a little more than an hour and a half in the air. Not so long! But not so quick either…

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2. How Long Should It Take?

The Airbus A321 has a maximum speed of 544 mph (that’s 879 kph for my international [🤮] readers). And it’s 501 miles (806 km) from DTW to LGA.

Here comes some math, please bear with me:

\(\frac{501\ mi}{546\ mi/hr} = 0.92\ hr = 55\ min\)

55 minutes! A good bit less than my flight time! So what gives?

First of all, planes usually can’t fly straight from point A to point B. Wind does funky things, and you need a certain angle of approach, and you’re not supposed to fly over the secret military bases where they’re hiding the Epstein papers, so my flight path actually looks like this:

Ew. (source)

That jaggedy mess means I’m actually flying a disgustingly-inflated 539 miles (867 km). So we must redo our math!

\(\frac{539\ mi}{546\ mi/hr} = 0.99\ hr = 59\ min\)

Well, that didn’t change much! Where’s the rest of the delay coming from?

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3. Planes Fly Slow

The Airbus A321 has a maximum speed of 546 mph (879 kph), sure. But it has a cruising speed of only 518 mph (834 kph). Planes only fly something like 95% as fast as they can.

What the hell is going on?

As Jerry Seinfeld points out, airplanes absolutely can move at their maximum speed, and they sometimes do!

So I'm on the plane, we left late. Pilot says we're going to be making up some time in the air. I thought, well isn't that interesting? We'll just make up time. That's why you have to reset your watch when you land… Of course, when they say they're making up time, obviously they're increasing the speed of the aircraft. Now, my question is if you can go faster, why don't you just go as fast as you can all the time? C'mon, there's no cops up here. Nail it. Give it some gas! We're flying.

Why don’t they just give it some gas?

Drag!

See, airplanes have, basically, four different forces acting on them at all times:

We can (mostly) ignore lift and weight, because we’re worried about speed, and speed is (mostly) a function of thrust and drag. How hard your engines are pushing forward, and how hard the air is pushing back.

The magnitude of the drag force—how hard the air is pushing back—follows an interesting law:

\(F_{D}=\frac{1}{2} \rho v^{2} C_{D} A\)

You can ignore just about everything beyond that v², they’re all constants. But the v² is very important! It tells you that as you get going faster, the air resists you quadratically. A doubling in your speed means a quadrupling in drag. A tripling means a nontupling.

So when an Airbus flies at 546 mph instead of 518 mph, it faces a drag force about 11% stronger. That might not seem like a lot, but it could add up over the course of a flight! To fight an 11% stronger drag force, you have to increase your thrust force by 11%, which means burning 11% more fuel, which is, for an A321, another 200ish kg every hour. Which costs the airline a grand total of… $150/hr.¹

Not a ton of money! In fact, that’s less than $1 for every passenger on the plane. How much time could that $1 save?

At cruising speed, the flight takes:

\(\frac{539\ mi}{518\ mi/hr} = 1.04\ hr = 62\ min\)

Only three minutes longer!

Would you pay $1 to save three minutes? That’s a judgment of your time to be worth at least $20/hr, which, hopefully, it is! But also, it’s three minutes. No one cares! Seinfeld was wrong—flying slightly slower doesn’t explain why my flight is more than an hour and a half long.

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4. Oh, Bureaucracy

Planes spend a lot of time not flying, even when they have passengers in them. They sit on the runway to get last-minute fixes, they taxi in circles waiting for their turn to take off, and then they fly in circles waiting for their turn to land.

Different airports make planes spend different amounts of time taxiing.² At the worst airports—JFK in New York and DEN in Denver—departing planes spend, on average, 27 and 24 minutes driving around. At DTW, the average is a more-reasonable-but-still-very-annoying 19 minutes.

Planes have to taxi around when they land too. Again, JFK is a terrible place, and arriving flights have to spend 14 minutes driving around before they reach their gate. I can’t find data for LaGuardia, but the similar Newark (EWR) makes arriving flights taxi for only 10 minutes.

So how long should my flight take?

\(62 + 19 + 10 = 91\ min = 1.5\ hr\)

There we go! A flight time of an hour and a half.

Just waiting for other planes to get out of our way makes up 50% of the flight. And I haven’t even mentioned driving to the airport, parking, going through security, boarding, deplaning, and all the other terribly frustrating, time-consuming, and soul-devouring parts of modern travel.

But, when all is said and done, I’ve made it from the wilderness of the frontier to the lights of the big city in less than six months, and with absolutely no dysentery.

So I, for one, welcome our 95%-speed, endlessly taxiing, slowly boarded aircraft overlords.

1. Recent jet fuel prices, I’m using North America numbers.

2. This part is based on data collected and prepared by some weird org called EUROCONTROL for the winter of 2019-2020 (pre-covid!).