The Big Push

 

February 21, 2019

THE MEDIA THIS WEEK has been all aflutter over winds. An unusually powerful jet stream has settled in above the United States and North Atlantic, allowing eastbound commercial flights to take advantage of super-strong tailwinds. Planes are traveling coast to coast in under four hours, to Europe in five.

Jet streams are high-altitude, fast-moving rivers of air. In this case very fast-moving. They run west-to-east, and are strongest in the winter. To the extent that it’s practical, eastbound aircraft will be flight-planned into the heart of a jet stream to take best advantage — so long as turbulence isn’t an issue — while westbound planes will be routed away.

Beyond that, there’s really no issue here for pilots, other than the fun that comes with watching your groundspeed reading click into the 700-knot realm.

But wait a minute, you’re thinking. How can a regular old commercial jet travel almost supersonically without breaking apart? Don’t planes have maximum certified speeds, and aren’t those limits being exceeded? Isn’t that unsafe? There’s a lot of confusion on this topic, and the gist of it revolves around people’s misunderstanding of the differences between groundspeed and airspeed.

Yes, there are two kinds of speed: there’s speed relative to the ground, and speed relative to the air. Let’s look at both.

First is airspeed. This is the plane’s velocity relative to the mass of air around it. Or, if it helps you, think of it as the speed of the air moving across the wings. This is the speed that the airplane, and by extension the pilot, is most concerned with. The airplane recognizes only this speed. Imagine you’re in a plane sitting at the end of the runway, not moving with the brakes firmly set, and there’s a 50-knot headwind roaring down that runway directly at you. What does your airspeed show on the gauge? Fifty knots. Your motion over the ground, of course, is zero, which brings us to the second type of speed…

That would be groundspeed. It’s self-explanatory: it’s how fast the plane is moving across the ground. Without bringing wind into the picture, airspeed and groundspeed are the same. If the air is perfectly calm, your speed through the air is the same as your speed over the ground. A headwind will cause groundspeed to decrease. A tailwind will cause it to increase. Neither of these, however, has any effect on airspeed. The speed sensed by the airplane — the air moving over the wings — has no bearing on how fast it is moving relative to the ground.

Make sense?

Thus, there are no concerns about breaking the sound barrier, exceeding design limitations, or any of that. We have cockpit readouts of both groundspeed and airspeed, but only airspeed matters from a safety perspective. The plane, you could say, has no idea how fast it is traveling across the ground, and could not care less. The concern, instead, is what happens when three dozen planes arrive two hours early and there’s no gate space!

Primary flight display on a modern jet. Airspeed is shown in the left vertical column.

Now let’s get really geeky for a minute. This is advanced level stuff, and most of you will probably click away. For the rest of you, here goes:

If a headwind were to suddenly cease, you lose that air going over the wings, and airspeed decreases accordingly. To compensate, you’d need to increase power or lower the nose. Alternately, if a headwind were to suddenly increase, now you have more air rushing over the wings. Assuming you maintain the same power setting, airspeed now increases by that many knots. In practice these wind-induced changes occur subtly, and pilots (or autopilots) adjust power or pitch to compensate. On those rare occasions when they occur suddenly and more radically, that’s windshear.

Still with me? Okay, now I’ll really make your brain hurt…

One problem is that as the air thins out at higher altitudes, the airspeed indicator no longer gives an accurate reading of what your airspeed actually is. It’s giving you an indication only of how much resistance and friction there is — i.e. what the plane “feels.”  For obvious reasons this reading is important, as stall speeds and other limitations always reference this “felt” speed. But, it’s not a true reading of how fast the plane is moving through the air. Notice the word true. How fast are you really, actually, truly going?  Well, the rule of thumb, to account for that thinning air, is that for every thousand feet of altitude, you’re traveling about two percent faster than what your indicator shows.

And this brings us to a third type of speed, which we call true airspeed. People are often surprised to learn that at cruising altitude, a jetliner’s airspeed indicator will sit somewhere in the range of 250-290 knots, give or take. But is this how fast you’re actually flying? No. You’re actually, really, truly going 60 percent faster than that. Your true airspeed, in other words, is somewhere around 500 knots. The plane, though, is stupid, and it only feels like it’s moving at 260 or 280 knots. And that’s fine with you, because the plane will stall, or overspeed, or do any of the other wacky things it might do, based on what it thinks it’s doing, what it feels it’s doing — not what it’s truly doing.

This is one of the reasons we rely on Mach number, rather than indicated airspeed, during high-altitude cruise. Mach (Ernst Mach, his name was) is a percentage of the speed of sound, expressed with a decimal. For instance, Mach .80 (a pilot would say “Mach eight-zero”) is 80 percent of the speed of sound. Jets normally cruise at anywhere from .78 to .86 Mach, depending on altitude, aircraft type, and fuel considerations. Above 25,000 feet or so, air traffic control spacing is always based on Mach.

So, yeah, we have gauges that show Mach number, indicated airspeed (IAS), true airspeed (TAS), and groundspeed (GS). Seldom are they the same, unless you’re at or near sea level with calm winds. Cruising over the ocean with a roaring tailwind, they might read, for example, .79, 280, 450, and 550 respectively.

There’s also something called calibrated airspeed (CAS), which… never mind; there’s no need to go there.

That’s really all you need to know. If you think you’ve got it, try this quiz…

1. You’re at 4,000 feet with a 15-knot tailwind.  IAS shows 180.

What’s your true airspeed?
What’s your groundspeed?

2. You’re at 15,000 feet, no wind.  IAS is 250.

What’s your true airspeed?
What’s your groundspeed?

3. You’re at 20,000 feet with a 40-knot headwind, IAS shows 300.

What’s your true airspeed?
What’s your groundspeed?

4. You’re at 35,000 feet.  TAS shows 430 knots, and there’s a 50-knot tailwind.

What does your IAS show?
What’s your groundspeed?

5. IAS shows 200 knots at 10,000 feet, with a 10-knot tailwind. Air traffic control tells you to climb to 20,000 feet and increase to 300 knots. As you climb, you encounter a 50-knot headwind.

Will your groundspeed decrease or increase in this climb? By how much?

6. You’re landing at Mexico City, elevation 7,400 feet (you can round up to 8000). Calculated Vref (landing) speed over the runway is 140 knots. There’s a 10-knot tailwind.

What’s your true airspeed as you touch down?
What’s your groundspeed?

7. You’re taking off from Mexico City. Your calculated rotation speed is 155 knots. There’s a 10-knot tailwind.

What’s your true airspeed when you begin to lift from the runway?
What’s your groundspeed?

Those last two are a good illustration of why operations at high-altitude airports can be tricky. High TAS and GS mean that takeoff and landing distances can be unusually long.

Still there?

Very good. Here is a picture of some cheese balls:

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32 Responses to “The Big Push”
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  1. Mari says:

    Such great content! So much of this is unknown or misunderstood. Very interesting. Thanks for sharing!

  2. Tod says:

    On a light note, how does an airline fly not only to the wrong airport but the wrong country?

  3. Avery Greynold says:

    Re: “If the air is perfectly calm, your speed through the air is the same as your speed over the ground.” Only true when your altitude is zero or the earth isn’t round. Yes, I’m a bad person. Please delete this comment.

  4. Rod says:

    Jim M, Don Suter and I are still waiting for that shipment of cheese balls as consolation prize for doing so badly on the test.

  5. ChrisW says:

    Have you ever written about the “coffin corner”?

  6. jon says:

    excellent!!

  7. Ian says:

    In the cockpit, groundspeed and airspeed are both shown in knots. Knots and miles per hour are essentially the same thing. Except, a knot is a NAUTICAL mile-per-hour rather than a statute one. Nautical miles are a little bit longer than statute miles, so 800 knots is a little bit faster than 800 m.p.h. (There’s more about this in my book if you’re interested.)

    You’re right though, 800 knots is extreme. I’ve changed the text to read 700 knots. That’d mean 200-knot tailwind, give or take, which is unusual but not impossible.

  8. Colm says:

    “Jet streams run west-to-east”
    “eastbound aircraft will be flight-planned into the teeth of a jet stream to take best advantage”

    Is this correct? or am I misinterpreting it,

  9. MW says:

    Long ago, somewhere on the internet, I saw a claimed-to-be-true story. The teller was a light plane pilot, and one day there was a strong and steady wind through their town, so strong that it exceeded the minimum flight speed of their plane. So they flew into the wind through the town at an altitude of a few meters, adjusting speed by thrust, and ‘stopping’ (ground speed zero) at the stop signs as they went.

    • Rod says:

      Apocryphal, I’m sure, as so many good stories (and “quotations”) are.
      The problem with a strong and steady wind in a town is that such a phenomenon is basically impossible from a pilot’s viewpoint, because it can’t blow Through hills, buildings, trees, houses, etc. It has to go over and around them, and this will cause it to tumble and roil. (Think of a rock in a shallow, fast stream — what happens around and downstream of the rock? Air is a fluid like water.) So — even with improbably wide and amazingly empty, copless streets pointed straight into that wind — at a height of a few metres they’d be battered by wind shear. And the slower they flew, the more compromised their position aerodynamically.
      If they were lucky they’d merely get wrapped around a telephone pole.

    • Patrick says:

      See Rod’s comment, above. Theoretically you could do this, if the the wind was EXTRAORDINARILY consistent. In real life, however, the gusts and whatnot would make such a feat pretty impossible.

      Flying backwards, though… that one is comparatively easy.

  10. Roger Lemberg says:

    Apropos ground speed: go to YouTube and search on “sr71 speed check”. There’s nothing like a great story teller telling a great story which Major Brian Shul is and does.

    As much as we admire those graceful airliners, we’ll all admit that the Blackbird is in a class by her self.

  11. Eric Ressner says:

    “f a headwind were to suddenly cease, you lose that air going over the wings, and airspeed decreases accordingly. To compensate, you’d need to increase power or lower the nose.”

    Wait, no, that can’t be (or if it can, my head really does hurt!)

    The engine thrust is what creates air speed, right? Because the only thing the plane is thrusting against is the surrounding air?

    So, yes, if the headwind ceases instantaneously, you lose air speed. But if the engine thrust is left alone, wouldn’t it gradually return to the same air speed as before?

    Or maybe I’m reading you wrong and you DO want to increase power — but only until the desired air speed is re-established, then back to the original setting.

    NOTE: not a pilot here! Be gentle.

    • Rod says:

      I should leave this to Patrick but can’t resist. I flew light planes for 20 years (never broke one), an activity I think Patrick sort of dismisses, but everybody at least starts that way. (And it’s not a bad way to get yourself killed if you don’t know what you’re doing.)

      My instructor had a sort of genius for thought experiments (like Patrick’s plane on a runway facing a 50-knot headwind). He came up with precisely this one: Imagine you’re flying at 100 knots into a steady 100-kt headwind. You would be hovering above the ground. Now imagine that this wind magically ceases in a split second. What would happen? The key word in your question, I think, is “gradually”. Because if you failed to take other action in the meantime, other unlovable things would occur, such as aerodynamic stall and possibly a spin.

      In this thought experiment, you’d see your airspeed abruptly fall to 0. Trusting your airspseed indicator, you would lower your nose (gravity would then cause your airspeed to rise) and add power (helping it rise still further). Both keep you away from a stall, which ain’t necessarily curtains but Can be, especially at low altitude.

      All this is supposed to be second nature before you solo (it had better be). Yet the Colgan Air crash almost exactly 10 years ago in Buffalo and the Air France Rio-Paris crash a few months later had highly experienced pilots doing exactly the wrong thing. Which beggars belief, except that we’re all basically cavemen with ties.

      • Rod says:

        Sorry, ran out of space.
        The thought experiment I described is Extreme (cooked up to make the point that you’re not going to leap instantaneously from 0 to flying speed). Stall, though, is a question of the angle at which your wing meets the oncoming air. “If a headwind were to suddenly cease”, I think a properly trimmed aircraft would automatically lower its own nose. But to be safe you’d have to help things along. Maybe large airliners are different — I just don’t know.

        By the way, the Air France crew shouldn’t be judged too harshly in that they were being barraged with all manner of loud warnings and a degree of false information. (Apparently, and perversely, every time they started doing the right thing, the stall warning started up again, so they backed off and reverted to the wrong thing.) The confusion must have been ghastly.

      • Alan Dahl says:

        I have an entirely different theory on the Colgan Air crash. That series of aircraft (the Q400) was known to have a seat adjuster problem where the seat would appear to latch but then suddenly slide back. The PIC was on the short side so my theory is that the seat suddenly slid back, the pilot grabbed the wheel to instinctively catch himself and bang they were in a stall. Certainly they still did everything wrong from that point but it would explain why they got into the situation in the first place without the pilot forgetting all his training as to what the control inputs do. There is of course no way to prove this theory.

        • Rod says:

          Except that this would presumably have been audible on the CVR with an accompanying “WTF?!” from the captain, and the seat would presumably have been ascertained by the investigators to be not properly latched. I mean, maybe that’s impossible, but it’s incredible some of the things they can determine despite the massive destruction in cases like this.
          And as you say, they certainly screwed up the rest, and had, I believe, been having a jolly old chin-wag rather than concentrating on the approach.
          Finally, if plausible, Colgan’s lawyers would have clung to that theory like a leech on amphetamines. Is it out of the courts yet?

        • Patrick says:

          In addition to Rod’s points… I think the FDR records the RATE of any control surface deflection. In this scenario, the rate (and the extent) of elevator deflection would be pretty extreme. Does that jibe with what we know?

  12. Richard Taylor says:

    This couldn’t have come at a better time. Just got back from a transcontinental flight from LA. All the questions I had while looking at the screen in front of me and seeing speed well over 600 mph were
    answered in this post. Many thanks.

  13. Jim Paboucek says:

    Have you noticed that Cheese Balls are a lot bigger than they were twenty or thirty years ago? What’s up with that? It’s not as easy to eat 3,000 of them in one sitting, but I still do it.

  14. Rod says:

    Well I can’t leave Jim M to face the dread possibility of excruciating public humiliation all alone, now can I? (with my luck, his answers will be bang on, mine pathetically off)

    I did this with a pencil and paper and a temp of 38.5°C. Hey, how’s that for a butt-covering excuse? Challenge open: Can anyone beat it?

    1) true airspeed: 187 kt / groundspeed: 202 kt
    2) true airspeed: 286 kt / groundspeed: 286 kt
    3) true airspeed: 360 kt / groundspeed: 320 kt
    4) IAS: 280 kt / groundspeed: 480 kt
    5) Groundspeed will increase (from 230 to 310 kt).
    6) TAS: 151 kt / Groundspeed: 161 kt
    7) true airspeed: 167 kt / groundspeed: 177 kt

    • Don Suter says:

      Air speed test!!!
      1) TAS 194.4 Ground speed 209.4
      2) TAS 325 Ground speed 325
      3) TAS 420 Ground speed 380
      4) IAS 253 Ground speed 380
      5) Speed increase by 120
      6) TSA 170.72 Ground speed 180
      7) TSA 177.94 Ground speed 187.94

      If this is not right I blame it on Old Age….

  15. Simon says:

    Nice article, Patrick. This physicist likes your nerdy pieces too.

    In fact, how about an article explaining what we see on that PFD up there? Considering that ‘s The Pilot’s view from the office for so much of his time, how about you explain to the rest of us what all the things are you see on there.

  16. Kingfish says:

    Is hitting the jet stream at all perceptible in the cabin? Or do the increased winds come on more gradually as you ascend?

  17. Craig B says:

    Always makes landing at La Paz airport and the velocity relative to the mountains towering over either side of the valley an interesting experience compared to landing at more common elevations.

  18. Jim M says:

    Continuation:

    6. You’re landing at Mexico City, elevation 7,400 feet (you can round up to 8000). Calculated Vref (landing) speed over the runway is 140 knots. There’s a 10-knot tailwind.

    What’s your true airspeed as you touch down? 156 KTAS
    What’s your groundspeed? 166 KN/191 MPH/307 KPH

    7. You’re taking off from Mexico City. Your calculated rotation speed is 155 knots. There’s a 10-knot tailwind.

    What’s your true airspeed when you begin to lift from the runway? 173 KTAS
    What’s your groundspeed? 183 KN/211 MPH/340 KPH

  19. Jim M says:

    Ok, I’ll take a crack and risk public humiliation. Spoilers below if you care about such things.
    As with everything else these days, there’s an app for that: Airspeed Converter for iOS. It also takes temperature into account, all calculations done for a standard day. There are also websites that will calculate TAS from IAS:
    http://indoavis.co.id/main/tas.html

    1. You’re at 4,000 feet with a 15-knot tailwind. IAS shows 180.

    What’s your true airspeed? 191 KTAS
    What’s your groundspeed? 206 KN/237 MPH/381 KPH

    2. You’re at 15,000 feet, no wind. IAS is 250.

    What’s your true airspeed? 225 KTAS
    What’s your groundspeed? 225 KN/259 MPH/417 KPH

    3. You’re at 20,000 feet with a 40-knot headwind, IAS shows 300.

    What’s your true airspeed? 400 KTAS
    What’s your groundspeed? 360 KN/414 MPH/666 KPH

    4. You’re at 35,000 feet. TAS shows 430 knots, and there’s a 50-knot tailwind.

    What does your IAS show? 252 KIAS
    What’s your groundspeed? 480 KN/552 MPH/888 KPH

    5. IAS shows 200 knots at 10,000 feet, with a 10-knot tailwind. Air traffic control tells you to climb to 20,000 feet and increase to 300 knots. As you climb, you encounter a 50-knot headwind.

    Will your groundspeed decrease or increase in this climb? By how much?
    200 KIAS @ 10K ft. = 232 KTAS + 10 for tailwind = 242 KN groundspeed
    300 KIAS @ 20k ft. = 400 KTAS -50 for headwind = 350 KN groundspeed, groundspeed increases by 108 knots, assuming that ATC issues speeds in KIAS.

  20. Susan says:

    Thank you for the cheese balls. I was hungry 🙂 But seriously, this was a little mind bending and a fun read. Thank you.

  21. Peter O'Leary says:

    Great article. The best part was the cheeseballs, it made the science lesson enjoyable.

  22. I read it all – skipped the quiz 😉
    Funny – just a couple of hours ago my Dad mentioned he saw a speed on FlightRadar 24 of about 800 knots…incredible…I have conversely been on the flights from BOS to SAN that took 6.5+ hours and never broke 400.
    Good article – learned a lot!

  23. Gene says:

    What, no CAS?

    🙂