To avoid being driven to the ragged edge of sanity by the profusion of ever-specialized avionics, we like to sort this stuff into bite-sized categories. the latest is one we would have never predicted: the wing-mounted, self-powered AHRS. As if one wasn’t enough, there are actually two of these devices, the Levil Aviation BOM we’re reviewing in this report and a new product called the WingBug, which we’ll examine in a future report.
The BOM appeared as a prototype at Sun ‘n Fun 2017 and in the year since, Levil Aviation has developed the production version, priced at $1995 complete. BOM is actually an acronym meaning Broadcasting Outer Module and Levil pitches it as a self-powered, always-ready backup device to provide complete attitude, airspeed and ADS-B data wirelessly to a tablet. The self-powering part is delivered by a small air-driven generator on the device’s trailing end. It’s reminiscent of the air-powered generators once popular in post-war taildraggers then delivered without electrical systems.
While Levil calls the BOM an AHRS, it’s really closer to an ADAHRS because it has air data capability, including pressure sensing through a small pitot inlet and air temperature which, along with GPS, attitude, speed, altitude and ADS-B In, will feed into a handful of tablet apps that Levil’s other products are also compatible with. That includes ForeFlight, WingX and FlyQ to name three. Levil has its own basic utility app that will display only essential flight data.
Wing Mount
The fact that a gadget like the BOM can be installed without conniptions from the FAA and technicians shows how far we’ve come in just three years. The sole paperwork shipped with the BOM is the so-called NORSEE letter for Non-Required Safety Enhancing Equipment, the FAA’s all-purpose, you’re-on-your-own blessing to bolt stuff to the airplane that might enhance safety without compromising basic airworthiness.
Levil provides an online installation manual that recommends placing the BOM just ahead of the wing leading edge in undisturbed air and giving the GPS antenna a good view of the sky. This may prove challenging on some airplanes because there’s no ready mounting point that far forward, such as a tiedown ring fitting or an inspection plate. Levil provides a small bracket for this purpose, but some minimal metal work might be necessary. On the Cub, I fabricated an angle bracket and attached the BOM to a jury strut.
The manual suggests giving the bracket and anchor a pull test to a resistance of at least 4.8 pounds. I stopped yanking on mine at 10 pounds, so no worries about it coming adrift. Levil says the BOM exerts about 1.5 pounds of drag at its maximum tested speed of 210 knots indicated. The Cub is too slow to suffer any noticeable speed loss, although a faster airplane might.
The BOM is intended to be powered by the air-driven generator, but it also has an internal battery so it can be switched on while stationary or off the airplane. It has a vibration sensor rather than a power switch, so shaking it like a salt shaker will activate it. It shuts off when the vibration ceases or it loses contact with the tablet. To charge the BOM ahead of using it, it has a charging port behind a removable access plate on the side of the device. It’s not convenient, but it works. Levil says 65 knots is sufficient speed to power the device and also charge the battery, but at a speed lower than that in the Cub, the battery appeared to be holding its own.
Aligning the BOM mechanically requires installing it as close to level relative to the wing as possible. But it’s apparently not too picky about this. I used a level and a bevel gauge to find near level on the Cub and in flight, it required no further tweaking, although the utility allows you to do that. (Level flight in a J-3 is a vague experience, but the BOM provides accurate enough vertical speed data to actually nail it fairly accurately.)
In Flight
As claimed, the BOM comes alive as soon as the engine starts and shakes it up. It appears on the tablet as a wireless network and doesn’t require a password. The apps I tried—Levil’s utility and WingX Pro—had no trouble finding and displaying the data.
The Levil app can show actual airspeed—not GPS groundspeed—baro altitude and mag heading with an attitude indicator. There’s also a vertical speed tape and, new with this version, an angle-of-attack indicator. The app has a diagnostics page that offers additional data, such as roll, heading, g-loading, battery state, lat/long and turn rate, to list a few. You can customize the data to use pressure, GPS or both and the heading source can be magnetic or GPS track. Levil warns that magnetic heading may be inaccurate due to ferrous interference. However, the BOM will learn the airplane’s magnetic configuration, says Levil, and will slowly correct any detected errors. This data can be stored so it will be available for the next flight.
Flipping between the apps revealed a few seconds of latency, but I saw no data dropouts and the instrument readouts are fluid and we’ll damped. The angle-of-attack display requires a brief calibration involving setting a “near stall” flight configuration and a max L/D configuration, which is essentially the best rate of climb speed. Since this isn’t published for the J-3, best guessing is in order.
The AoA display has an analog representation that’s essentially a lift-reserve indicator, with green, yellow and red segments. AoA sensing is done by comparative pressure through two tiny ports drilled in the nose of the BOM. And by the way, the pitot circuit is heated to provide some protection against icing, but there’s not a lot of power available to keep it really warm, so how we’ll it would perform in icing is unknown. Levil says the BOM has not been tested in icing conditions, nor is it intended to be used that way.
To get the most out of this device and whatever display you use it with, a semi-permanent mount of some kind for the tablet strikes me as a good idea. Placing the display in a panel mount or a robust yoke mount makes the instruments read almost like a six-pack. In my view, there’s little question of the backup value of the flight data.
As we’ve come to expect, the ADS-B In—there’s no Out function here—is sensitive enough to pick up high-altitude traffic from inside a metal hangar and it provides the usual FIS-B weather data. The standard caution applies on traffic. Without a complementary Out to draw in the local traffic package, you won’t see all of the targets.
Conclusion
At $1995, what’s this thing good for? Levil makes the case that it’s a backup device if all else fails, but these days, even panel-mount avionics are equipped with internal batteries of all sorts. For $1195, Levil’s own iLevil 3 SW can occupy the glareshield and provide battery-powered backup attitude data sufficient for navigation and an emergency letdown. The BOM ups the ante in sophistication by providing real air data and its own power source, if that appeals.
More attractive, in my view, is the convenience. Without fooling with a glareshield-mounted device with the external power cord, the BOM is simply out there. After takeoff, just start the tablet, pick your app and you’re in business. That has value. Unknown is how the thing will behave and survive long term in rain and ice and how durable that little generator will be. Levil says the generator—the aft-most module—is easily replaceable.
The BOM appears well-made, with three individual cylindrical modules fastened by screws and protected against moisture incursion by Buna O-rings. In addition to being heated, the pitot circuit is fitted with drains to keep moisture from accumulating. Before buying, I would recommend noodling the installation a little. The ideal simplest install is to attach it to a bracket on an inspection plate, even if that’s aft of the leading edge. It appears to me that the GPS is more than sensitive enough to receive position data from under a wing. Aerodynamically, that’s not ideal, but for a backup, who cares?
