The Magic of MEMS


These clever little gadgets are why you can buy an impressive portable EFIS for under a grand.

In his incisive biography of Steve Jobs, Walter Isaacson explained Jobs’ genius as having a knack for knowing what consumers wanted to buy before they realized they wanted it. But that’s no less true of any emerging new product line — marketing is the chicken to the egg of sales.

That’s definitely the case for a new class of products that have appeared during the past 18 months — the portable ADS-B receiver, lately coupled with AHRS units that turn an iPad or tablet into a better than fair EFIS. Raising the ante yet again comes Levil Technology with a new portable product that accepts pitot/static input yielding a full-blown, if uncertified, ADAHRS and air data box. It’s called the iLevil AW and we’ll be trying it soon.

What’s going on here? Was the world demanding these gizmos or did the march of technology simply solve a problem that didn’t exist? The answer is a bit of both, but more the latter than the former. A couple of technologies came together, just as pilots were snapping up tablet computers like candy and app writers were on a tear with sophisticated new products. Concurrently, the FAA more or less did what it promised: it built the ADS-B ground network to the point of broad functionality. All of sudden, things came together.

ADS-B’s Driving

Partnered with Sporty’s and ForeFlight, Appareo hit the market a year ago with the Stratus, its first portable ADS-B receiver. SkyRadar’s pioneering portable ADS-B beat the Stratus to market by a year and more competition soon followed.

As Appareo’s Barry Batcheller and Jeff Johnson see it, what ignited the current frenzy of portable ADS-B receivers is public-access development done by the Mitre Corp. Mitre is a consortium of federally funded research and development centers that has done — and continues to do — development work on aviation technology for the FAA. In the ADS-B segment, a marketable reference design it developed proved a pump starter for ADS-B portables. SkyRadar used a licensed version of the Mitre design and got to market with it just as the initial iPad market was building to a fever pitch. SkyRadar is still out there, but it has plenty of competition.

Adding AHRS

The second technology isn’t quite so new, but it’s recently become cheaper and more reliable — virtually a commodity. It’s so-called microeletromechanical systems or MEMS. These remarkable devices — tiny little solid-state gyros based on a microscopic tuning forks — and accelerometers make it practical to produce AHRS units in small, lightweight packages. How light? About a gram for the lightest.

With costs driven down by massive volume in the automotive and consumer electronics industry — especially cellphones– MEMS parts may be the least expensive components in some of the ADS-B/AHRS products. One company told us the MEMS parts for its product cost under $10 and the entire thing fits on your thumbnail.

For as inexpensive and small as they are, these MEMS-based AHRS deliver remarkably good performance, but it’s not necessarily easy getting them to do that. In the world of non-mechanical gyros, inexpensive MEMS systems are at the bottom of the pecking order, behind higher quality MEMS, fiber optics and laser ring gyros of the sort found in transport category aircraft, says Appareo’s Batcheller.

That’s another way of saying you don’t just plant cheap MEMS parts on a board and pump the output into an iPad. The developmental trick is to understand how the components work across a range of temperatures and g-loads and to write software that reliably translates the sensor output to a three-dimensional position in space.

“If there’s any secret sauce to it, that’s it,” says Appareo’s Jeff Johnson. We heard the same thing from Kevin Scribner, whose Sagetech Clarity SV is the only portable AHRS product to also offer rudimentary synthetic vision. (See an exclusive AVweb video on the Clarity SV here.)

Both Appareo and Sagetech have leveraged experience in other technology to produce their downstream products. Appareo’s expertise is in INS and flight monitoring equipment while Sagetech has worked extensively in the UAV industry. In fact, Scribner’s business card depicts a picture of a transponder for a UAV whose actual size is that of a standard business card and the real component is only a quarter-inch thick. Levil is primarily a builder of small CNC equipment, with AHRS as a general aviation sideline.

MEMS technology drives all sorts of things, from anti-lock brakes and traction control to cellphone navigation platforms. “The Segway is a product that MEMS made possible,” says Kelvin Scribner. “The gyros for that were an inch cube; now we get three of those in the size of your small fingernail.”

What They’re Good For

We’ve flown with the MEMS-based AHRS units and we’re impressed with their performance. They’re not quite as smooth and certified systems, but for the price, it’s unrealistic to expect them to be. But they’d certainly keep you upright in the clouds and provide reliable speed and altitude information. Just keep in mind that with these products, you’re looking at GPS groundspeed, not airspeed, and GPS altitude above the spheroid, not MSL altitude. The units are equipped with magnetometers, but being portables susceptible to location errors, they don’t necessarily provide accurate heading. It’s best to think of them as suitable for general reference, not fine-point course flying, although they’ll do in a pinch.

But are they intended to work in a pinch? Not exactly. All of the manufacturers of combined ADS-B/AHRS boxes say they’re intended to be advisory only; no pretentions are made about these gadgets replacing a certified electronic or mechanical backup gyro that you might otherwise install in your aircraft.

“Do I want to release this product for specific functions, as a primary instrument or even a backup where you’re depending on it? The answer to that is no,” says Appareo’s Batcheller. We heard virtually the same explanation in different words from Sagetech’s Scribner. The problem isn’t so much the devices themselves – the MEMS chips are actually considered quite reliable – but the tablets and apps. “We just haven’t done the testing to that degree,” says Jeff Johnson at Appareo.

There are simply too many variables in the operating systems and how the apps relate to them to preclude inflight failures and lockups. Indeed, we experienced some of these when flight testing the products. Using them for critical backup in IMC, while tempting, would thus be nerve wracking. When portable database GPS navigators first appeared, buyers were cautioned not to use them for navigation, but they did it anyway, with minimal risk. With portable EFIS, the risk is probably higher, although no one really has any numbers for it.

“That’s what gets flushed out in certification…the end-to-end process. That comes with a very rigorous standard. If you have a data-enabled iPad, when you change cell towers, it will drop the WiFi connection. So there are little things about the iPad…you could be flying along thinking of it as an adequate backup instrument and you haven’t looked at it for 30 seconds. It might be gone,” says Scribner.

But the MEMS chips themselves? Not much likelihood of failure, he says. “What we’re seeing is that the quality of the sensor is improving now. There’s less drift in them,” he adds. And when sensors get smaller, they also get more reliable. “There’s some cool things that happen when you miniaturize things. They get ultra rugged; they’re just stiffer,” says Scribner of ever-smaller MEMS.

Dynon’s View

And that’s what Dynon is counting on in considering its D1 Pocket Panel as suitable for real-world backup, even though it’s also a battery operated portable. But with one important difference: It has its own dedicated software, not an app, and there’s no underlying third-party operating system to go haywire when a certain company in Cupertino decides it needs an “upgrade.” We reviewed the D1 in this video and found it had impressive performance. Although we only flew it for a few hours, we didn’t encounter any lockups.

Does that mean the MEMS-based D1 is a suitable backup gyro? “That’s essentially the designed-use case,” says Dynon’s Michael Schofield. To the question of whether the D1 should replace an electric or vacuum backup gyro, Schofield says that’s up to the customer.

“Software stability is an issue. That’s why we decided to build a device that doesn’t rely on the iPad. Raw software reliability is going to be problem when you’re working in an ecosystem with wireless and Bluetooth and a lot of other apps,” he says.

None of this means that MEMS-based portable AHRS/EFIS products driving tablets won’t reach the same level of reliability as the D1, but they haven’t yet. We’ll check back in a few years and see what develops.