HUD

Many years ago I failed at becoming a CFI. On the first lesson the instructor told me to put the airplane in the climb attitude. He said, “Look out at the horizon, and see the proper angle between the nose, and wing, and the horizon. That’s the climb angle.”

I said, “No, just look in here. See where the little airplane is relative to the horizon on the attitude indicator. And monitor the airspeed indicator and vertical speed indicator. That’s how you determine the proper climb angle.”

No hope for me. To become a CFI I would need to reverse everything I practiced in real life flying and look out the windshield for imprecise attitude and flight path information.

The solution was to skip over becoming a CFI and go straight to CFI-I, where my mission was to tell pilots new to IFR flying to stop looking outside and scan the instruments.

HUD

HUDs were once a novelty, but have started to show up in more general aviation airplanes.

Well, it turns out I may have been about 50 years ahead—or behind, depending on your view—of how we will all be able to fly before long. With development of conformal flight displays we will all one day be able to fly as though we are looking out the window on a clear day, no matter the clouds and visibility around us.

The real technology breakthroughs to make perpetual VFR possible came with development of head-up displays (HUD). A HUD displays all essential flight data on a combiner glass in front of the pilot’s eyes. A pilot can look through the glass and see an unobstructed view through the windshield, but thanks to magical optics, also read airspeed, altitude, and most importantly flight path data without changing his eye focus from infinity.

The military was the first adopter of HUD because, for obvious reasons, a fighter pilot needs to have the best possible vision of the threat environment and weapons delivery situation around him. By looking through the HUD glass the pilot can see the target visually, plus all the information needed to properly target the weapons.

Civilians soon realized that a HUD would be extremely useful in low approach conditions because the pilot could see all necessary flight and approach guidance information without taking his focus off the runway that is about to appear out of the murk ahead.

HUD guidance with its flight path marker (PFM) is so precise and so reliable that the FAA certified Category III ILS approaches to be hand flown down to visibilities of 600 feet. Without the HUD, only multi-channel autopilots are certified to fly approaches in such low conditions.

But HUD technology did not spread beyond a minority of transport airplanes and the largest business jets for many years because of the cost and physical size of the equipment.

HUD guidance is so precise because it relies on inertial sensor inputs. Until 20 years ago, or so, inertial guidance systems were large and extremely expensive. They relied on laser ring gyros—or in the early years on a platform of spinning rotary gyros—to measure tiny accelerations and thus calculate the airplane’s precise flight path. And it’s flight path, not attitude, or airspeed, or vertical speed that shows us where the airplane is going instead of where it’s pointed.

In a development few of us saw coming, creation of micro electronic mechanical sensors made it possible to build an inertial sensor so low cost and so compact that we can now see our flight path marker and synthetic vision in airplanes ranging from the largest jets to the smallest homebuilts.

The other size and cost issue with HUD has been the projector and combining lens that makes it possible to see both the view ahead and the displayed data clearly under all light conditions. The system uses a projector that is very bright to shine the data display onto the HUD glass. Miniaturizing the projector has physical limits so cockpit space is an issue. And the electronic and illumination technology required is costly.

However, advances are making it possible to display data on a HUD glass internally without need of an image projector. That can allow for a compact display and will bring cost down exactly as the non-moving gyro development made synthetic vision and flight path display affordable and available.

As HUD technology proliferated at the upper end, it quickly became apparent that the primary flight display (PFD) in the panel needed to change. It is distracting and takes at least a small amount of time to look up and down from a HUD with its image oriented to the real world outside to a conventional PFD with its display centered on the attitude and course data. Thus the conformal display was born. To maximize the benefit of seeing the real world outside through a HUD, the PFD in the panel needs to conform to that view.

G1000 PFD with flight path marker

The key to a PFD is the flight path marker, which shows not where you are pointed but where you are going.

In a conformal PFD the little airplane symbol that has been the center of attitude indicators since the gyro horizon was invented is gone. In its place is the flight path maker (FPM)—a small circle with small horizontal lines to represent the wings—that shows where the airplane is going. Across the center of the conformal display is the zero reference line. It’s not the “artificial horizon” but the point that indicates your flight path is going up or down. If the FPM is above the zero line you’re climbing. Below, you’re descending. Actual pitch attitude information is pushed out to the perimeter of the display. Same for bank angle indicator. Remember, the FPM shows where you’re going, not your attitude.

And the conformal PFD has full screen synthetic vision. So you see the terrain under or around the airplane, and the airport environment just as you would looking out the windshield, or looking through the HUD.

But how do you know the airplane attitude with a conformal display? Exactly as you do looking out the window visual conditions. The entire display background is a synthetic view out the windshield on a clear day. You see the horizon, terrain, obstructions and, if nearby, the airport environment and runways. No interpretation is needed, at least not more than visual flying on a clear day.

Conformal displays are still developing and there is no “standard” format for all symbology and data presentation as we see with conventional flight displays. Falcon’s conformal display looks a little different than Gulfstream’s, and Boeing’s has some differences, too. Everyone has reasons why their format is optimal, but in time the best and most useful features will become common.

The key is that with a full conformal display cockpit we are flying as though we are looking out the window in clear skies while still seeing the most precise flight guidance available. Are you flying instruments, visual, or both? Really both. And that’s the best of all worlds.

I can’t predict when we will see a truly conformal cockpit in piston airplanes, or even the light end turbines. It takes a lot of computer power to refresh the flight display so that it maintains a true image of the world around and under the airplane. But then I never expected to live to see precise flight path marker, synthetic vision, and even rudimentary HUDs in light airplanes. So who knows?

Maybe when the conformal display cockpit arrives in light airplanes I’ll go back and get that CFI because I can look outside and see the instrument data at the same time.

15 replies
  1. Dale Hill
    Dale Hill says:

    Mac, Have you read my earlier article about when the ‘Magic’ dies?

    https://airfactsjournal.com/2021/11/when-the-magic-dies-flying-with-and-without-huds/

    As they say, ‘It happens’, even with the most sophisticated systems. Your INS can wander off to another continent, your GPS can phone it in, or the circuit breaker on your HUD can ‘pop’ for some unknown reason. They are, after all, only machines and machines wear out, short-circuit, or go stupid. When that happens, one has to be ready to fly the airplane in those conditions in which you find yourself, not wishing for daylight, CAVU, and light winds.

    Reply
    • Chris Barker
      Chris Barker says:

      I agree with your reservations, Dale. In addition, the HUD is really useful but a) it has to be scanned – all the information doesn’t just jump into your brain, however close together all the data points might lie; and b) although the HUD is really useful on an instrument approach, particularly since the aircraft symbol provides actual flight path for when you can see touchdown point, you can’t focus on both HUD and runway at the same time.

      Reply
  2. OttawaCanuck
    OttawaCanuck says:

    “Look out at the horizon, and see the proper angle between the nose, and wing, and the horizon. That’s the climb angle.”

    Good for you for speaking out against that dangerous misinformation all those years ago. As you know far better than I do, using the same memorised pitch angle for every flight— rather than finding the angle that gives you the correct airspeed — will kill a new pilot the first time they load up the family and take off at max gross on a hot summer day, or depart a high-density-altitude airport.

    Of course, the other stuff your younger self mentioned, like the attitude indicator, was overkill — once you’ve used the ASI to establish the best pitch angle for that flight and verified positive rate of climb, you want to have your eyes mostly outside when you’re VFR — but even a J3 Cub has an airspeed indicator.

    Reply
  3. RichR
    RichR says:

    No argument on the inherent goodness of comprehensive information in an easily absorbed format. Despite all the technology, one still needs to develop a “feel” for any airplane you fly. Worst case may be the Air France stall to the ocean scenarios when conflicting information overload requires “mentally decluttering” all the high tech conflicting inputs, and relying on the basics to rebuild your SA. Feel also enables those hairs on the back of your neck…not to act on without further analysis, but to prompt you to cross check that all is well, and if not, prepare your mind for some dissonance to better absorb the new state. Examples would be nose (and potentially, AOA) higher than expected, mushy roll control, lower airspeed than pitch/power should yield, loaded up controls, etc…all of this running in your mental/tactile background will pop a flag when you’re busy or distracted to refocus on “aviate”.

    Reply
  4. TomB
    TomB says:

    Using instruments-PFD-whatever. With all the new golly gee whiz equipment we have available it’s very easy to get over reliant on the electronics so that we are “eyes in” the cockpit too much which is an invitation to mid airs. I have been a CFI-I since 1968. I think it is essential to teach students with their eyes outside as much as possible. Even with a HUD it is easy to focus on all the symbology instead of scanning for traffic.
    I fear a new generation of pilots who are focused on the technology and fly into something/someone.

    Reply
    • Mac McClellan
      Mac McClellan says:

      Hi Tom B,
      The beauty of HUD and conformal display is that you are looking at the “something “ else no matter whether you are looking inside or out. The best of all worlds.
      Mac Mc

      Reply
  5. RC
    RC says:

    After flying HUDS on a Boeing, I have to say they are amazing tools during approach. I’ve flown, and monitored, the manual CatIII
    approaches and am still shocked at how easy the HUD makes it.
    Other phases of flight, however, didn’t impress me as much. Works fine, just seemed too sensitive, if that
    makes sense, to suit me.
    Be curious to fly behind one of the ‘portable’ systems I see for light GA aircraft.

    Reply
  6. Gerry M
    Gerry M says:

    In 1974, I was forced to land “blind” in fog at KELP. Luckily I’m still here to talk about it. (I made a localizer approach, kept the autopilot coupled, and saw the runway edge lights just before I heard the tires chirp.)Fast forward to 2019 and have used Foreflight with synthetic vision to successfully make multiple practice approaches to within 3-4 feet of the runway centerline. With HUD and conformal integration, it can only get better and safer. Thanks for a great article.

    Reply
  7. Stephen Phoenix
    Stephen Phoenix says:

    Brings me back to the idea: If we had such tools, with traffic also shown, would we need so much ATC and procedures? Seems like it would be possible to fly in clouds just like VFR and self maintain traffic and terrain separation. Only busy airports would require a traffic cop or automated flow control.

    Reply
    • Mac MCCLELLAN
      Mac MCCLELLAN says:

      Hi Stephen,
      About 15 years ago the very concept you mention was much discussed. It was generally called “free flight” with pilots using technology to separate themselves instead of relying on controllers.
      The technology wasn’t quite there yet, but it’s getting close. In fact, a type of free flight has been implemented on the North Atlantic tracks where pilots can use TCAS separation to allow a faster airplane to overtake a slower one ahead. But that all happens with many, many miles of separation, not the few miles in trail we need to fully utilize a runway.
      There are many unanswered questions about Free Flight, more of them procedural than technical. For me the biggest question is how to we keep pilots from drag racing to the final approach fix?
      Having spent much of my life racing sailboats I know that sailors–and certainly pilots–will use rules, whatever they may be, as a weapon to get ahead and stay ahead.
      Mac Mc

      Reply
      • RichR
        RichR says:

        ATC assigns the sequencing, pilots are responsible for the separation…and if pilots choose to bust sequence/separation, I’m sure ADS-B and updated registrations make it easier than we’d like to automatically send the violation in the mail!

        Reply
  8. Mac MCCLELLAN
    Mac MCCLELLAN says:

    Hi Rich R,
    You hit upon my problem with Free Flight–minimum separation standards.
    Proponents of Free Flight say traffic flow would increase because we pilots are willing to fly closer together just as we do when flying visual approaches.
    But on a visual approach there is no strict minimum separation standard. Under Free Flight there would need to be.
    So, once there is a standard, as you point out, somebody takes the hit for violating it. ATC radar, now including ADS-B, is recorded constantly and software monitors for any loss of minimum separation. If that happens, an investigation determines if it was the controller’s or the pilot’s fault.
    What does the wise and successful controller do? Build in margin. If the minimum separation is three miles at the same altitude, he keeps airplanes four, five or six miles apart. What would the wise and successful pilot do if minimum separation standards transferred to us? Build in margin.
    I have no idea what the guy in front of me, or off my wing, is going to do. If he slows or turns unexpectedly–none of us have ever seen that before–and I’m flying right on the minimum separation number who’s at fault? I don’t know, and I don’t want that letter you mention.
    Like any pilot I love the idea of Free Flight. Taking care of our own business. Then I fly into an uncontrolled airport with a mix of traffic from J3s to Gulfstreams and wonder what the pilots around me are going to do next.
    Mac Mc

    Reply
    • RichR
      RichR says:

      since no one else is paying the bill anymore, my acft is strictly VFR…”free flight”, and like you, wondering what the folks around me are going to do next…but then I’ve always wondered that! Cheers.

      Reply

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