Tunnel-in-the-Sky Displays

25 Feb 2020 by Christopher Noessel

“Tunnel in the Sky” is the name of a 1955 Robert Heinlein novel that has nothing to do with this post. It is also the title of the following illustration by Muscovite digital artist Vladimir Manyukhin, which also has nothing to do with this post, but is gorgeous and evocative, and included here solely for visual interest.

See more of Vladimir’s work here https://www.artstation.com/mvn78.

Instead, this post is about the piloting display of the same name, and written specifically to sci-fi interface designers.

Last week in reviewing the spinners in Blade Runner, I included mention and a passing critique of the tunnel-in-the-sky display that sits in front of the pilot. While publishing, I realized that I’d seen this a handful of other times in sci-fi, and so I decided to do more focused (read: Internet) research about it. Turns out it’s a real thing, and it’s been studied and refined a lot over the past 60 years, and there are some important details to getting one right.

Though I looked at a lot of sources for this article, I must give a shout-out to Max Mulder of TU Delft. (Hallo, TU Delft!) Mulder’s PhD thesis paper from 1999 on the subject is truly a marvel of research and analysis, and it pulls in one of my favorite nerd topics: Cybernetics. Throughout this post I rely heavily on his paper, and you could go down many worse rabbit holes than cybernetics. n.b., it is not about cyborgs. Per se. Thank you, Max.

I’m going to breeze through the history, issues, and elements from the perspective of sci-fi interfaces, and then return to the three examples in the survey. If you want to go really in depth on the topic (and encounter awesome words like “psychophysics” and “egomotion” in their natural habitat), Mulder’s paper is available online for free from researchgate.net: “Cybernetics of Tunnel-in-the-Sky Displays.”

What the heck is it?

A tunnel-in-the-sky display assists pilots, helping them know where their aircraft is in relation to an ideal flight path. It consists of a set of similar shapes projected out into 3D space, circumscribing the ideal path. The pilot monitors their aircraft’s trajectory through this tunnel, and makes course corrections as they fly to keep themselves near its center.

This example comes from Michael P. Snow, as part of his “Flight Display Integration” paper, also on researchgate.net.

Please note that throughout this post, I will spell out the lengthy phrase “tunnel-in-the-sky” because the acronym is pointlessly distracting.

Quick History

In 1973, Volkmar Wilckens was a research engineer and experimental test pilot for the German Research and Testing Institute for Aerospace (now called the German Aerospace Center). He was doing a lot of thinking about flight safety in all-weather conditions, and came up with an idea. In his paper “Improvements In Pilot/Aircraft-Integration by Advanced Contact Analog Displays,” he sort of says, “Hey, it’s hard to put all the information from all the instruments together in your head and use that to fly, especially when you’re stressed out and flying conditions are crap. What if we took that data and rolled it up into a single easy-to-use display?” Figure 6 is his comp of just such a system. It was tested thoroughly in simulators and shown to improve pilot performance by making the key information (attitude, flight-path and position) perceivable rather than readable. It also enabled the pilot greater agency, by not having them just follow rules after instrument readings, but empowering them to navigate multiple variables within parameters to stay on target.

In Wilckens’ Fig. 6, above, you can see the basics of what would wind up on sci-fi screens decades later: shapes repeated into 3D space ahead of the aircraft to give the pilot a sense of an ideal path through the air. Stay in the tunnel and keep the plane safe.

Mulder notes that the next landmark developments come from the work of Arthur Grunwald & S. J. Merhav between 1976–1978. Their research illustrates the importance of augmenting the display and of including a preview of the aircraft in the display. They called this preview the Flight Path Predictor, or FPS. I’ve also seen it called the birdie in more modern papers, which is a lot more charming. It’s that plus symbol in the Grunwald illustration, below. Later in 1984, Grunwald also showed that a heads-up-display increased precision adhering to a curved path. So, HUDs good.

n.b. This is Mulder’s representation of Grunwald’s display format.

I have also seen lots of examples of—but cannot find the research provenance for—tools for helping the pilot stay centered, such as a “ghost” reticle at the center of each frame, or alternately brackets around the FPP, called the Flight Director Box, that the pilot can align to the corners of the frames. (I’ll just reference the brackets. Gestalt be damned!) The value of the birdie combined with the brackets seems very great, so though I can’t cite their inventor, and it wasn’t in Mulder’s thesis, I’ll include them as canon.

The takeaway from the history is really that these displays have a rich and studied history. The pattern has a high confidence.

Elements of an archetypical tunnel-in-the-sky display

There are lots of nuances that have been studied for these displays. Take for example the effect that angling the frames have on pilot banking, and the perfect time offset to nudge pilot behavior closer to ideal banking. For the purposes of sci-fi interfaces, however, we can reduce the critical components of the real world pattern down to four.

Square shapes (called frames) extending into the distance that describe an ideal path through space
1. The frame should be about five times the width of the craft. (The birdie you see below is not proportional and I don’t think it’s standard that they are.)
2. The distances between frames will change with speed, but be set such that the pilot encounters a new one every three seconds.
3. The frames should adopt perspective as if they were in the world, being perpendicular to the flight path. They should not face the display.
4. The frames should tilt, or bank, on curves.
5. The tunnel only needs to extend so far, about 20 seconds ahead in the flight path. This makes for about 6 frames visible at a time.
An aircraft reference symbol or Flight Path Predictor Symbol (FPS, or “birdie”) that predicts where the plane will be when it meets the position of the nearest frame. It can appear off-facing in relation to the cockpit.
1. These are often rendered as two L shapes turned base-to-base with some space between them. (See one such symbol in the Snow example above.)
2. Sometimes (and more intuitively, imho) as a circle with short lines extending out the sides and the top. Like a cartoon butt of a plane. (See below.)
Contour lines connect matching corners across frames
A horizon line

This comp illustrates those critical features.

There are of course lots of other bits of information that a pilot needs. Altitude and speed, for example. If you’re feeling ambitious, and want more than those four, there are other details directly related to steering that may help a pilot.

Degree-of-vertical-deviation indicator at a side edge
Degree-of-horizontal-deviation indicator at the top edge
Center-of-frame indicator, such as a reticle, appearing in the upcoming frame
A path predictor
Some sense of objects in the environment: If the display is a heads-up display, this can be a live view. If it is a separate screen, some stylized representation what the pilot would see if the display was superimposed onto their view.
What the risk is when off path: Just fuel? Passenger comfort? This is most important if that risk is imminent (collision with another craft, mountain) but then we’re starting to get agentive and I said we wouldn’t go there, so *crumbles up paper, tosses it*.

I haven’t seen a study showing efficacy of color and shading and line scale to provide additional cues, but look closely at that comp and you’ll see…

The background has been level-adjusted to increase contrast with the heads-up display
A dark outline around the white birdie and brackets to help visually distinguish them from the green lines and the clouds
A shadow under the birdie and brackets onto the frames and contours as an additional signal of 3D position
Contour lines diminishing in size as they extend into the distance, adding an additional perspective cue and limiting the amount of contour to the 20 second extents.

What can you play with when designing one in sci-fi?

Everything, of course. Signaling future-ness means extending known patterns, and sci-fi doesn’t answer to usability. Extend for story, extend for spectacle, extend for overwhelmedness. You know your job better than me. But if you want to keep a foot in believability, you should understand the point of each thing as you modify it and try not to lose that.

Each frame serves as a mini-game, challenging the pilot to meet its center. Once that frame passes, that game is done and the next one is the new goal. Frames describe the near term. Having corners to the frame shape helps convey banking better. Circles would hide banking.
Contour lines, if well designed, help describe the overall path and disambiguate the stack of frames. (As does lighting and shading and careful visual design, see above.) Contour lines convey the shape of the overall path and help guide steering between frames. Kind of like how you’d need to see the whole curve before drifitng your car through one, the contour lines help the pilot plan for the near future.
The birdie and brackets are what a pilot uses to know how close to the center they are. The birdie needs a center point. The brackets need to match the corners of the frame. Without these, it’s easier to drift off center.
A horizon line provides feedback for when the plane is banked.

THIS BAD: You can kill the *sense* of the display by altering (or in this case, omitting) too much.

Since I mentioned that each frame acts as a mini-game, a word of caution: Just as you should be skeptical when looking to sci-fi, you should be skeptical when looking to games for their interfaces. The simulator which is most known for accuracy (Microsoft Flight Simulator) doesn’t appear to have a tunnel-in-the-sky display, and other categories of games may not be optimizing for usability as much as just plain fun, with the risk of crashing your virtual craft just being part of the risk. That’s not an acceptable outcome in real-world piloting. So, be cautious considering game interfaces as models for this, either.

This clip of stall-testing in the forthcoming MSFS2020 still doesn’t appear to show one.

So now let’s look at the three examples of sci-fi tunnel-in-the-sky displays in chronological order of release, and see how they fare.

Three examples from sci-fi

So with those ideal components in mind, let’s look back at those three examples in the survey.

Quick aside on the Blade Runner interface: The spike at the top and the bottom of the frame help in straight tunnels to serve as a horizontal degree-of-deviation indicator. It would not help as much in curved tunnels, and is missing a matching vertical degree-of-deviation indicator. Unless that’s handled automatically, like a car on a road, its absence is notable.

*Starship Troopers* (1986) We only get 15 frames of this interface in *Starship Troopers*, as Ibanez pilots the escape shuttle to the surface of Planet P. It is *very* jarring to see as a repeating gif, so accept this still image instead.

Some obvious things we see missing from all of them are the birdie, the box, and the contour lines. Why is this? My guess is that the computational power in the 1976 was not enough to manage those extra lines, and Ridley Scott just went with the frames. Then, once the trope had been established in a blockbuster, designers just kept repeating the trope rather than looking to see how it worked in the real world, or having the time to work through the interaction logic. So let me say:

Without the birdie and box, the pilot has far too much leeway to make mistakes. And in sci-fi contexts, where the tunnel-in-the-sky display is shown mostly during critical ship maneuvers, their absence is glaring.
Also the lack of contour lines might not seem as important, since the screens typically aren’t shown for very long, but when they twist in crazy ways they should help signal the difficulty of the task ahead of the pilot very quickly.

Note that sci-fi will almost certainly encounter problems that real-world researchers will not have needed to consider, and so there’s plenty of room for imagination and additional design. Imagine helping a pilot…

Navigating the weird spacetime around a singularity
Bouncing close to a supernova while in hyperspace
Dodging chunks of spaceship, the bodies of your fallen comrades, and rising plasma bombs as you pilot shuttlecraft to safety on the planet below
AI on the ships that can predict complex flight paths and even modify them in real time, and even assist with it all
Needing to have the tunnel be occluded by objects visible in a heads up display, such as when a pilot is maneuvering amongst an impossibly-dense asteroid field.

…to name a few off my head. These things don’t happen in the real world, so would be novel design challenges for the sci-fi interface designer.

So, now we have a deeper basis for discussing, critiquing, and designing sci-fi tunnel-in-the-sky displays. If you are an aeronautic engineer, and have some more detail, let me hear it! I’d love for this to be a good general reference for sci-fi interface designers.

If you are a fan, and can provide other examples in the comments, it would be great to see other ones to compare.

Happy flying, and see you back in Blade Runner in the next post.

Ship Console

20 Dec 2016 by Christopher Noessel

The only flight controls we see are an array of stay-state toggle switches (see the lower right hand of the image above) and banks of lights. It’s a terrifying thought that anyone would have to fly a spaceship with binary controls, but we have some evidence that there’s analog controls, when Luke moves his arms after the Falcon fires shots across his bow.

Unfortunately we never get a clear view of the full breadth of the cockpit, so it’s really hard to do a proper analysis. Ships in the Holiday Special appear to be based on scenes from A New Hope, but we don’t see the inside of a Y-Wing in that movie. It seems to be inspired by the Falcon. Take a look at the upper right hand corner of the image below.

Piloting Controls

25 Apr 2014 by Christopher Noessel

Pilot’s controls (in a spaceship) are one of the categories of “things” that remained on the editing room floor of Make It So when we realized we had about 50% too much material before publishing. I’m about to discuss such pilot’s controls as part of the review of Starship Troopers, and I realized that I’ll first need to to establish the core issues in a way that will be useful for discussions of pilot’s controls from other movies and TV shows. So in this post I’ll describe the key issues independent of any particular movie.

A big shout out to commenters Phil (no last name given) and Clayton Beese for helping point me towards some great resources and doing some great thinking around this topic originally with the Mondoshawan spaceship in The Fifth Element review.

So let’s dive in. What’s at issue when designing controls for piloting a spaceship?

First: Spaceships are not (cars|planes|submarines|helicopters|Big Wheels…)

One thing to be careful about is mistaking a spacecraft for similar-but-not-the-same Terran vehicles. Most of us have driven a car, and so have these mental models with us. But a car moves across 2(.1?) dimensions. The well-matured controls for piloting roadcraft have optimized for those dimensions. You basically get a steering wheel for your hands to specify change-of-direction on the driving plane, and controls for speed.

Planes or even helicopters seem like they might be a closer fit, moving as they do more fully across a third dimension, but they’re not right either. For one thing, those vehicles are constantly dealing with air resistance and gravity. They also rely on constant thrust to stay aloft. Those facts alone distinguish them from spacecraft.

These familiar models (cars and planes) are made worse since so many sci-fi piloting interfaces are based on them, putting yokes in the hands of the pilots, and they only fit for plane-like tasks. A spaceship is a different thing, piloted in a different environment with different rules, making it a different task.

Maneuvering in space

Space is upless and downless, except as a point relates to other things, like other spacecraft, ecliptic planes, or planets. That means that a spacecraft may need to be angled in fully 3-dimensional ways in order to orient it to the needs of the moment. (Note that you can learn more about flight dynamics and attitude control on Wikipedia, but it is sorely lacking in details about the interfaces.)

Orientation

By convention, rotation is broken out along the cartesian coordinates.

X: Tipping the nose of the craft up or down is called pitch.
Y: Moving the nose left or right around a vertical axis, like turning your head left and right, is called yaw.
Z: Tilting the left or right around an axis that runs from the front of the plane to the back is called roll.

In addition to angle, since you’re not relying on thrust to stay aloft, and you’ve already got thrusters everywhere for arbitrary rotation, the ship can move (or translate, to use the language of geometry) in any direction without changing orientation.

Translation

Translation is also broken out along cartesian coordinates.

X: Moving to the left or right, like strafing in the FPS sense. In Cartesian systems, this axis is called the abscissa.
Y: Moving up or down. This axis is called the ordinate.
Z: Moving forward or backward. This axis is less frequently named, but is called the applicate.

Thrust

I’ll make a nod to the fact that thrust also works differently in space when traveling over long distances between planets. Spacecraft don’t need continuous thrust to keep moving along the same vector, so it makes sense that the “gas pedal” would be different in these kinds of situations. But then, looking into it, you run into a theory of constant-thrust or constant–acceleration travel, and bam, suddenly you’re into astrodynamics and equations peppered with sigmas, and you’re in way over my head. It’s probably best to presume that the thrust controls are set-point rather than throttle, meaning the pilot is specifying a desired speed rather than the amount of thrust, and some smart algorithm is handling all the rest.

Given these tasks of rotation, translation, and thrust, when evaluating pilot’s controls, we first have to ask how it is the pilot goes about specifying these things. But even that answer isn’t simple. Because you need to determine with what kind of interface agency it is built.

Max was a fully sentient AI who helped David pilot.

Interface Agency

If you’re not familiar with my categories of agency in technology, I’ll cover them briefly here. I’ll be publishing them in an upcoming book with Rosenfeld Media, which you can read there if you want to know more. In short, you can think of interfaces as having four categories of agency.

Manual: In which the technology shapes the (strictly) physical forces the user applies to it, like a pencil. Such interfaces optimize for good ergonomics.
Powered: In which the user is manipulating a powered system to do work, like a typewriter. Such interfaces optimize for good feedback.
Assistive: In which the system can offer low-level feedback, like a spell checker. Such interfaces optimize for good flow, in the Csikszentmihalyi sense.
Agentive: In which the system can pursue primitive goals on behalf of the user, like software that could help you construct a letter. This would be categorized as “weak” artificial intelligence, and specifically not the sentience of “strong” AI. Such interfaces optimize for good conversation.

So what would these categories mean for piloting controls? Manual controls might not really exist since humans can’t travel in space without powered systems. Powered controls would be much like early real-world spacecraft. Assistive controls would be might provide collision warnings or basic help with plotting a course. Agentive controls would allow a pilot to specify the destination and timing, and it would handle things until it encountered a situation that it couldn’t handle. Of course this being sci-fi, these interfaces can pass beyond the singularity to full, sentient artificial intelligence, like HAL.

Understanding the agency helps contextualize the rest of the interface.

Inputs

How does the pilot provide input, how does she control the spaceship? With her hands? Partially with her feet? Via a yoke, buttons on a panel, gestural control of a volumetric projection, or talking to a computer?

If manual, we’ll want to look at the ergonomics, affordances, and mappings.

Even agentive controls need to gracefully degrade to assistive and powered interfaces for dire circumstances, so we’d expect to see physical controls of some sorts. But these interfaces would additionally need some way to specify more abstract variables like goals, preferences, and constraints.

Consolidation

Because of the predominance of the yoke interface trope, a major consideration is how consolidated the controls are. Is there a single control that the pilot uses? Or multiple? What variables does each control? If the apparent interface can’t seem to handle all of orientation, translation, and thrust, how does the pilot control those? Are there separate controls for precision maneuvering and speed maneuvering (for, say, evasive maneuvers, dog fights, or dodging asteroids)?

The yoke is popular since it’s familiar to audiences. They see it and instantly know that that’s the pilot’s seat. But as a control for that pilot to do their job, it’s pretty poor. Note that it provides only two variables. In a plane, this means the following: Turn it clockwise or counterclockwise to indicate roll, and push it forward or pull it back for pitch. You’ll also notice that while roll is mapped really well to the input (you roll the yoke), the pitch is less so (you don’t pitch the yoke).

So when we see a yoke for piloting a spaceship, we must acknowledge that a) it’s missing an axis of rotation that spacecraft need, i.e. yaw. b) it’s presuming only one type of translation, which is forward. That leaves us looking about the cockpit for clues about how the pilot might accomplish these other kinds of maneuvers.

Output

How does the pilot know that her inputs have registered with the ship? How can she see the effects or the consequences of her choices? How does an assistive interface help her identify problems and opportunities? How does as agentive or even AI interface engage the pilot asking for goals, constraints, and exceptions? I have the sense that Human perception is optimized for a mostly-two-dimensional plane with a predator’s eyes-forward gaze. How does the interface help the pilot expand her perception fully to 360° and three dimensions, to the distances relevant for space, and to see the invisible landscape of gravity, radiation, and interstellar material?

Narrative POV

An additional issue is that of narrative POV. (Readers of the book will recall this concept is came up in the Gestural Interfaces chapter.) All real-world vehicles work from a first-person perspective. That is, the pilot faces the direction of travel and steers the vehicle almost as if it was their own body.

But if you’ve ever played a racing game, you’ll recognize that there’s another possible perspective. It’s called the third-person perspective, and it’s where the camera sits up above the vehicle, slightly back. It’s less immediate than first person, but provides greater context. It’s quite popular with gamers in racing games, being rated twice as popular in one informal poll from escapist magazine. What POV is the pilot’s display? Which one would be of greater use?

The consequent criteria

I think these are all the issues. This is new thinking for me, so I’ll leave it up a bit for others to comment or correct. If I’ve nailed them, then for any future piloting controls in the future, these are the lenses through which we’ll look and begin our evaluation:

Agency [ manual | powered | assistive | agentive | AI ]
Inputs
- Affordance
- Ergonomics
- Mappings
  - orientation
  - translation
  - thrust
- consolidations
Outputs (especially Narrative POV)

This checklist won’t magically give us insight into the piloting interface, but will be a great place to start, and a way to compare apples to apples between these interfaces.

Sci-fi interfaces

Stop watching sci-fi. Start using it.

Category Archives: ~ Piloting