An Aircraft HUD for the Rest of Us

 - August 24, 2018, 11:59 AM
MyGoFlight's SkyDisplay head-up display (HUD) is targeting the high-performance piston single, turboprop, and light jet markets.

It’s a little hard to believe, but I’m flying a Cirrus SR22T equipped with a real head-up display (HUD), and despite the fact that it’s going to sell for about one-tenth to one-twentieth the cost of a traditional HUD, the SkyDisplay HUD, designed by MyGoFlight, really works.

The flight took place on a sunny August day out of Centennial Airport in Denver and included two IFR approaches into Colorado Springs and another approach back to our origin.

One important note: the SkyDisplay HUD is non-conformal, which means that the symbology doesn’t exactly match the terrain in the outside world, although the HUD’s flight path marker (FPM) and flight director cue do mirror navigation and attitude information and thus can be used to fly instrument approaches with great precision. Non-conformity means that you can’t use the FPM to determine whether you’ll clear a bank of clouds, nor can you point the FPM at the runway touchdown zone as you can with a conformal HUD and expect the airplane to fly right to that spot.

The SkyDisplay HUD is the brainchild of company founder and CEO Charles Schneider, who was curious why he could have a HUD in his BMW automobile but not in his SR22. The typical platitudes that aviation is different, lower volume means higher prices, avionics certification is expensive, etc., did not faze Schneider, and he continued researching HUDs for light airplanes.

The first step was to figure out how the BMW HUD worked, so he bought a used one on eBay and took it apart. His early attempts to make an airplane HUD fizzled but helped keep the dream alive.

He quickly figured out that to be useful, an airplane HUD must replicate most of the features of larger HUDs on business jets and airliners, but to achieve a lower price, the HUD couldn’t be conformal. That means it must show primary flight display symbology (airspeed, altitude, rate-of-climb, navigation information, and the FPM-based flight director) on a combiner display positioned in the pilot’s field of view when looking through the airplane windshield. The pilot must be able to focus on infinity (to see the outside view) and also see the HUD symbology at the same time. And the HUD symbology had to be driven by rock-solid air data, attitude and heading reference systems (ADAHRS), which are available from modern avionics.

There are advantages and disadvantages to a non-conformal HUD. Obviously, the low price is a big plus, but another bonus is that it’s not necessary for the pilot’s head to be positioned at a specific point (in the eyebox) to ensure proper viewing and conformality. For this reason, the SkyDisplay HUD combiner can easily be adjusted for wherever the pilot sits, instead of adjusting the pilot to match a fixed-position combiner.

As mentioned, a non-conformal HUD doesn’t give the pilot the ability to use the FPM as a flight-path vector, and the HUD symbology isn’t overlaid precisely over the ground features as would be the case with a conformal HUD. In Schneider’s opinion, that is well worth the lower cost. “The tradeoffs had to be made,” he said.

The FPM is also caged, which means that while it moves vertically, it is locked horizontally in the center of the HUD. The HUD does display a small “T” symbol on the zero-pitch line to show which way the airplane is tracking (left or right) so the pilot knows where to look for the runway when flying an approach in a crosswind.

The MyGoFlight SR22T is equipped with Avidyne displays, dual Garmin 430 navigators, and an Avidyne R9 autopilot. The SkyDisplay HUD is permanently installed on the ceiling above the pilot’s seat and weighs less than two pounds. This includes the projector and combiner, plus an aircraft interface device mounted remotely to deliver ADAHRS data to the HUD graphics computer from the avionics. In some installations, Aspen Avionics displays, for example, the interface won’t be needed as the Aspen units output correctly formatted data.

There is a notable difference between the SkyDisplay HUD and traditional monochrome business jet and airliner HUDs, and that is the MyGoFlight HUD displays two colors on the combiner. The SkyDisplay HUD can output full color, but “only two colors can safely be used in all light and background color situations,” according to the company, and these are green and magenta.

The SkyDisplay HUD employs magenta for the flight director cue and other targets, armed modes, and alerts.

Targets include airspeed, altitude, and heading bugs, and the tracking “T” as well as lateral and vertical deviations and the course deviation indicator (CDI).

Armed modes are primarily "altitude selected but not yet captured" or "glideslope armed and not captured."

Alerts can include messages, for example, vertical speed direction or messages about engine parameters that are out-of-range (although this feature wasn’t available in the version flown for this article).

Putting these items in magenta is doubly effective: it calls attention to the item and it makes it easier for the pilot to understand that the flight director cue is a target that the pilot must capture with the FPM in order to follow the flight director commands. Some pilots call this “putting the thing inside the thing,” but it is just another way of depicting the flight director command bars as a smaller circular cue (magenta in this case) and the airplane as the FPM that the pilot or autopilot can manipulate by placing the larger FPM circle directly over the smaller flight director cue circle. It’s easy to verify that this is accurate by looking down at the PFD and seeing that the flight director command bars are lined up with the aircraft symbol.

Flying the SkyDisplay HUD

I flew in the left seat with Bob Stedman, president of Independence Aviation, a flight school and management company based at Centennial Airport. Stedman has been flying the demo flights for MyGoFlight.

Weather conditions were perfect VMC in Colorado for our flight, unfortunately, as I didn’t get to experience flying the HUD in poor visibility.

SkyDisplay HUD
Looking outside while viewing flight symbology is easy using MyGoFlight's SkyDisplay HUD. (Photo: Matt Thurber)

The SkyDisplay HUD has simple controls and it switches on when avionics are powered up. MyGoFlight warns pilots that the SkyDisplay HUD is to be used “as a supplement only to primary navigation instruments to enhance situational awareness” and that “The device is NOT designed to be used as a primary flight instrument and should NOT be relied upon as such. Pilot according to the POH for this aircraft and FAR/AIM.” The pilot must click the set button on the HUD controls to accept these two messages.

The only remaining step is to set the HUD brightness with the up and down arrow buttons, or the pilot can leave it in auto brightness mode. A button is available to put the HUD into standby mode, which blanks the display, but it would be just as easy to flip the combiner horizontal to remove it from the pilot’s view.

During takeoff, the pilot can use the HUD to watch airspeed and more precisely lift the nose at the correct rotation speed while also looking ahead of the airplane and keeping it on centerline, according to Schneider.

After taking off from Centennial’s Runway 17L using the HUD, which did help with the rotation, I headed south toward Colorado Springs.

I wanted to see how well the HUD FPM works in attitudes not normally flown and chose to do some steep turns.

Keeping the FPM on the HUD’s zero-pitch line (which displays the local horizon) makes the airplane stay level, another handy feature of the FPM. I tried to keep the FPM on the zero-pitch line during the turns to avoid climbing and descending. The FPM helped me maintain the selected altitude and also showed me when I was too high or low, but it also allowed me to keep my head outside looking for traffic while using the FPM to try to stay level in the turn.

With the autopilot switched on, Stedman set us up for the ILS to Runway 17L at Colorado Springs. All I had to do was keep the green FPM overlaid on the magenta flight director cue and manage speed with the throttle, and the Cirrus maintained a precise path on the approach.

I have noticed when flying airplanes with HUDs that I am much smoother on the controls when I look through the HUD than when looking down at the PFD. This may be due to the way the FPM and flight director cue circles work together versus the command bars on the PFD and the difference in the amount of vertical space depicted on the HUD versus the PFD, but this was also the case with the SkyDIsplay HUD in the Cirrus.

The two ILS approaches into Colorado Springs were among the smoothest I have flown, bringing us accurately down to the decision altitude.

On the missed approach after the first ILS, the HUD helped me set the proper climb attitude and fly the correct heading while allowing me to keep looking through the windshield.

After the second ILS, again just as smooth, we returned to Centennial for an RNAV 28 (LPV) approach. The HUD made flying the approach easy, and even though the weather was clear, I could see that the HUD would be a great asset in poor visibility.

Ironically, one drawback to the —HUDand this could apply to any airplane so equipped—is that I had to force myself to look from side to side and up and down occasionally to check for traffic. The HUD is somewhat compelling and held my gaze for long periods of time when I was hand flying the three approaches. Using the autopilot did free up some of my brain bandwidth to look around and away from the HUD.

While the SkyDisplay HUD doesn’t replace primary instruments, it can be helpful in all phases of flight. The MyGoFlight instruction manual explains how to use it for takeoff, climb, cruise, approach, and landing. The company explains that pilots must be properly trained to use the SkyDisplay HUD, especially how to view the outside world while looking at the HUD symbology and also “continuing to scan between the outside world, the HUD, and the head-down displays, just as would be done for a head-down-display-equipped aircraft.”

To gain proficiency and be ready when the weather is IMC, pilots should practice flying with the HUD in all sorts of conditions and all phases of flight, MyGoFlight recommends. “This not only ensures sufficient familiarity when using the system in low visibility but promotes higher levels of HUD proficiency. This is likely to be a risk-mitigating factor in the day-to-day operation of the aircraft. The HUD also mitigates the risk of misidentifying the runway or centerline lights at night.”

Schneider expects the SkyDisplay HUD to retail for $25,000 (not including installation) for a Cirrus-type system and more for a HUD for a single-engine turboprop or larger.

Certification, via an approved model list supplemental type certificate (STC), is expected before the end of this year. “So far, indications are there are no big issues [for the FAA],” he said. The first STC will be for Avidyne-equipped Cirruses, followed by Cessna Caravans equipped with Garmin G600 displays, then later-model Cirrus Perspective (Garmin avionics) models. The Pilatus PC-12 is also a likely candidate, and Schneider has presented the SkyDisplay HUD to two PC-12 fleet operators.

Most pilots don’t learn about HUD until later in their careers, but a low-cost system like the SkyDisplay could be a boon for training providers teaching future commercial pilots. The sooner pilots get used to a HUD, the better use they will make of this safety device later in their careers. And in China, where airlines are required to install HUDs, the SkyDisplay might be a logical choice for the training fleet.